SYNTHESIS AND CHARACTERIZATION OF WELL-DEFINED, AMPHIPHILIC, IONIC COPOLYMERS. A Dissertation. Presented to

Transcription

1 YNTHEI AND CHARACTERIZATION OF WELL-DEFINED, AMPHIPHILIC, IONIC COPOLYMER A Dissertation Presented to The Graduate Faculty of The University of Akron In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy Yuqing Liu December, 2011

2 YNTHEI AND CHARACTERIZATION OF WELL-DEFINED, AMPHIPHILIC, IONIC COPOLYMER Yuqing Liu Dissertation Approved: Accepted: Advisor Dr. Kevin Cavicchi Department Chair Dr. Robert Weiss Committee Chair Dr. Thein Kyu Dean of the College Dr. tephen Cheng Committee Member Dr. Hendrik Heinz Dean of the Graduate chool Committee Member Dr. Abraham Joy Date Committee Member Dr. Wiley Youngs ii

3 ABTRACT Amphiphilic ionic block copolymers are promising materials for the fabrication of ion-exchange membranes in fuel cells, water purification and advanced molecular engineering applications, such as nanotemplating. For example, block copolymer architectures provide a route to fabricate membranes with tunable transport properties through polymer self assembly. A significant challenge in this field is the synthesis of amphiphilic copolymers, where the intrinsic immiscibility of the hydrophobic and hydrophilic monomers complicates polymerization. To address the immiscibility between sodium p-styrenesulfonate and styrene monomers, styrenesulfonate monomers were neutralized by hydrophobic trialkyl ammonium salts via ion-exchange reactions, and synthesized successfully by RAFT polymerization with low polydispersity (PDI). Diblock or triblock copolymers with well-defined architectures were obtained by sequential RAFT polymerization with styrene. These sulfonate groups were then converted to the sodium salt form via ion-exchange to obtain amphiphilic ionic block copolymers. It was observed that dimethyl n-alkyl ammonium salts of polystyrenesulfonate displayed thermo-reversible gelation behavior in low polarity organic solvents. The investigation of the gelation behavior as a function of temperature, concentration, and solvent was consistent with iii

4 gelation driven by the ionic aggregation of the polymer as would be expected for polyelectrolyte surfactant complexes in non-polar solvents. Cationic amphiphilic block/graft copolymers containing quaternary ammonium salts were prepared by the RAFT polymerization of polystyrene-b-poly(vinylbenzyl chloride) (P-b-PVBC) copolymers, and sequential post-polymerization quaternization of the PVBC blocks. P-b-PVBEA-b-P triblock copolymers with well-defined architectures were obtained and the ion conductivity of the corresponding membranes, as well as the morphology of the membranes, was investigated. To improve the mechanical properties of the membranes, different architectures, such as pentablock, heptablock and graft copolymers, were designed and synthesized by RAFT polymerization, and chemical crosslinking was employed to improve the mechanical properties and control the swelling in water. Lastly, a new method to prepare multiblock copolymers via a facile route was developed. Polytrithiocarbonates were prepared by condensation polymerization of a dicarboxylic acid functional trithiocarbonate and a diol, and the trithiocarbonate group was controlled by tuning the amount of acid catalyst and reaction time. The polytrithiocarbonate RAFT agents were used to polymerize P, polystyrene-b-poly (tert-butylstyrene) (P-b-PtB), and P-b-PVBC. The PDI of the polymers toward 2, but the PDI of the polymer blocks between two trithiocarbonate groups was narrow ( for P and P-b-PtB, and 1.46 for P-b-PVBC). The PVBC segments were quaternized to achieve anionic amphiphilic multiblock copolymers. iv

5 ACKNOWLEDGEMENT First, I deeply appreciate my advisor and mentor in my Ph.D. study, Dr. Kevin A. Cavicchi, for his excellent guidance and tremendous support in all direction of my research. He not only offered training in my experimental skills and guidance in research, but also trained my scientific attitude, the ability to analyze and solve problems, and communication skills. Throughout my dissertation writing period, he provided tons of ideas, writing guidance, and good ideas. Without his knowledge and continuous support, it would have been an impossible mission for me to finish my Ph.D study. econd, I would like thank my committee members in my dissertation defense: Dr Thein Kyu, Dr. Hendrik Heinz, Dr. Wiley Youngs, and Dr. Abraham Joy. I am very grateful to them for their assistance, valuable suggestions, and time. Third, I should thank all faculties and staffs in the college of polymer science and engineering, especially Rong Bai, who instructed me in the operation of many instruments. I would also like to thank the other members in Dr. Cavicchi s group and all of people helping me in my PhD study and research. I also thank Chemsultants International Company for ion conductivity measurements during my thesis. Lastly, I deeply appreciate my parents, who always supported me in all areas, without whose sacrifices I can complete nothing. v

6 TABLE CONTENT Page LIT OF TABLE... xiv LIT OF FIGURE... xvi LIT OF CHEME... xxi CHAPTER Ⅰ.INTRODUCTION Motivation ulfonated amphiphilic polymers Quaternary ammonium copolymers... 7 Ⅱ.BACKGROUND Controlled/living polymerization to tailor the macromolecular architectures ynthesis of amphiphilic ionic block copolymers Anionic amphiphilic block copolymers containing sulfonic acid groups Post-polymerization modification Direct synthesis Cationic amphiphilic copolymers containing quaternary ammonium salts22 vi

7 Post-polymerization modification Direct synthesis RAFT polymerization technique Mechanism of RAFT polymerization Choice of RAFT polymerization system Control over macromolecular architectures by RAFT polymerization Well-defined end-functionalized polymers via RAFT polymerization ynthesis of trithiocarbonate RAFT agents Ⅲ. EXPERIMENTAL Materials Chemicals used as received Purification ynthetic procedures of P containing block copolymers RAFT agent synthesis docecyl- -(α,α -dimethyl-α -acetic acid)-trithiocarbonate (RAFT-COOH) synthesis Didodecyl-1,2-phenylene-bis(methylene)bistrithiocarbonate synthesis (bis-trithiocarbonate) Alkylammonium sulfonated monomers synthesis Trialkylammonium p-styrenesulfonate monomers synthesis Dimethyl alkyl ammonium p-styrenesulfonate monomers synthesis vii

8 Dimethyl octadecylammonium 2-Acrylamido-2-methyl -Npropanesulfonate monomer (AMP-DMODA) synthesis RAFT polymerization of sulfonated block copolymers RAFT polymerization of P-TOA homopolymers RAFT polymerization of alkyl ammonium p-styrenesulfonate homopolymers RAFT polymerization of 2-Acrylamido-2-methyl -Npropanesulfonic acid (AMPA) ynthesis of P-TOA-b-P diblock and P-b-P-TOA-b-P triblock copolymers via RAFT polymerization ynthesis of P-b-P-DMODA-b-P triblock copolymer via RAFT polymerization Ion-exchange of P homopolymers and block copolymers From ammonium form to sodium form From sodium form to ammonium form ynthetic procedures of quaternary ammonium containing block copolymers RAFT agent synthesis Dibenzyl trithiocarbonate (DBTC) synthesis RAFT polymerization of P-b-PVBC block copolymers RAFT polymerization of polystyrene P-b-PVBC-b-P triblock copolymer synthesis P-r-PVBC-g-trithiocarbonates synthesis P-r-PVBC-g-PVBC graft polymerization viii

9 3.4.3 Quaternization of P-PVBC block/graft copolymers Crosslink of PVBC Vinylbenzyl thiol synthesis Vinylbenzyl alcohol synthesis Grafting vinylbenzyl thiol on PVBC Grafting vinylbenzyl alcohol on PVBC Thermal induced crosslinking ynthetic procedures of multiblock copolymers Polytrithiocarbonate RAFT agent synthesis Dicarboxylic acid RAFT agent synthesis ynthesis of poly-trithiocarbonate RAFT agent RAFT polymerization of homopolymers and block copolymers using poly(trithiocarbonate) RAFT agent Polystyrene homopolymers Polystyrene-b-poly (tert-butylstyrene) (P-b-PtB) multiblock copolymer Poly (styrene-b-vinylbenzyl chloride) (P-b-PVBC) multiblock copolymer Reduction of Polymers containing trithiocarbonate groups Aminolysis Radical reduction Characterization ix

10 3.6.1 Nuclear magnetic resonance (NMR) characterization ize Exclusion chromatography (EC) characterization mall angle x-ray scattering (AX) characterization Conductivity measurements of membranes Gelation transition temperature measurement Cross polarized optical microscopy canning electron microscope (EM) characterization Thermogravimetric analysis (TGA) characterization Differential scanning calorimetry (DC) characterization Fourier Transform Infrared pectroscopy (FTIR) characterization Ⅳ. RAFT POLYMERIZATION OF IONIC HOMOPOLYMER AND AMPHIPHILIC BLOCK COPOLYMER Introduction Results and discussion Preparation of hydrophobic ionic monomers tri-alkylammonium p-styrenesulfonate monomers ynthesis of dimethyl alkylammonium p-styrenesulfonate monomers ynthesis of P-TOA Determination of polydispersity of P via size exclusion chromatography (EC) RAFT polymerization of other alkylammonium p-styrenesulfonate monomers x

11 4.2.5 Ion-exchange of alkylammonium polystyrenesulfonate ynthesis of P-TOA-b-P diblock and P-b-P-TOA-b-P triblock copolymers P-b-P-TOA diblock copolymer P-b-P-TOA-b-P triblock copolymer Ion-exchange of P-P block copolymers ynthesis of P-b-P-DMODA-b-P triblock copolymers RAFT polymerization of other monomers containing sulfonic acid groups Conclusion Ⅴ. THERMO-REVERIBLE GELATION BEHAVIOR OF DIMETHYL ALKYL AMMONIUM ALT OF POLYTYRENEULFONATE (POLYELECTROLYTE-URFACTANT COMPLEXE) Introduction Result and discussion Dimethyl octadecylammonium polystyrene sulfonate (P-DMODA) organogelator Thermo-reversible gelation behavior Birefringence of the thermo-reversible organogel canning electron microscope (EM) characterization Conclusion Ⅵ. AMPHIPHILIC COPOLYMER CONTAINING QUATERNARY AMMONIUM GROUP PREPARED BY POT-POLYMERIZATION MODIFICATION xi

12 6.1 Introduction Results and discussion Dibenzyl trithiocarbonate (DBTC) synthesis P-b-PVBC triblock copolymers Quaternization of P-b-PVBC P-PVBC heptablock copolymer synthesis Properties of the P-b-quaterized PVBC membranes Preparation of P-g-quaternized PVBC amphiphilic graft copolymers Crosslinking of PVBC Preparation of crosslink agents Grafting crosslink agents on PVBC Thermal induced crosslinking Quaternization of crosslinked PVBC Conclusion Ⅶ. YNTHEI OF MULTIBLOCK COPOLYMER Introduction Results and discussion Poly-trithiocarbonate RAFT agent Polystyrene homopolymerization using poly-raft agent Multiblock copolymers prepared by RAFT polymerization polystyrene-block-poly(tert-butylstyrene) (P-b-PtB) xii

13 poly (styrene-block-vinylbenzyl chloride) (P-b-PVBC) Quaternization of P-b-PVBC multiblock copolymer Conclusion Ⅷ. CONCLUION REFERENCE xiii

14 LIT OF TABLE Table Page 2.1 tructures of Functional RAFT agents The relative peak intensity of trialkylammonium p-styrenesulfonate The relative peak intensity of dimethyl alkylammonium p-styrenesulfonate The molecular weight and distribution measured by EC and 1 H NMR P-TOA-20 (RAFT-COOH) polymerization data The molecular weight characteristics of P-TBA-20 in different polymerization conditions Molecular weight and distribution of dimethyl alkylammonium polystyrenesulfonate Molecular weight characteristic of P-TOA-b-P Molecular weight characteristic of P-b-P-TOA-b-P triblock copolymers The molecular weight characteristics of P-b-P-DMODA triblock copolymers Molecular weight characteristics of PAMP-DMODA The solubility of P-DMODA polymers in different solvents olubility parameters of some organic solvents Characteristics of P-PVBC block copolymers in Figure Molecular weight and distribution of P-r-PVBC and P-r-PVBC-g-RAFT xiv

15 6.3 molecular weight and distribution characteristics of P-r-PVBC-g-PVBC graft copolymers Molecular weight and distribution of poly-raft agent Molecular weight and polydispersity of cleaved polystyrene blocks xv

16 LIT OF FIGURE Figure Page 1.1 Mean-field phase diagram for conformationally symmetric diblock copolymer melts The looping and bridging behavior of multiblock copolymers The relationship of sulfonation level and the molar ratio of sulfonating agent and polymer Guidelines for the selection of Z group substituents for various polymerizations The general approaches for generation of complex macromolecular architectures by RAFT chemistry H-NMR spectrum of -TOA monomer H-NMR spectrum of -TBA monomer H-NMR spectrum of -TEA monomer H NMR spectrum of dimethyl alkylammonium p-styrenesulfonate monomers (a) -DMODA, (b) -DMHDA, (c) -DMDDA, (d) -DMDA, (e) -DMOA H-NMR spectra of P-TOA aliquots (5 kda target molecular weight) at 1, 2, 4 and 8 h polymerization times Pseudo first-order kinetic plots for the P-TOA RAFT polymerizations. Target molecular weights: ( ) 5000 Da ( ) 10,000 Da ( ) 20,000 Da H-NMR spectra of P-TOA 5kDa (left) and 10kDa (right) with bis-trithiocarbonate RAFT agent. (a) Aliquot without purification; (b) After purification xvi

17 4.8 The EC traces of P-TOA-20k (RAFT-COOH) with different eluent or different time. Left: In 2g/100mL, (a) 3h, (b) 5h, (c) 9h, (d) 11h; Right: (a) pure THF, (b) THF+1g/100mL BA.TOA, (c) THF+1g/100mL TOA, (d) THF+2g/100mL TOA EC curves of P-TOA homopolymers (prepared with RAFT-COOH). (a) pure eluent (THF+2g/100mL TOA), (b) P standard samples (VWR) converted to TOA form via ion-exchange, (c) P-TOA-5, (d) P-TOA-10, (e) P-TOA EC curves of P-TOA homopolymers (prepared with bis-trithiocarbonate RAFT agent). (a) pure eluent (THF+2g/100mL TOA), (b) P standard samples (VWR) converted to TOA form via ion-exchange, (c) P-TOA-5, (d) P-TOA EC traces of P-TBA polymerized in benzene solution with a conc. of (a) 2M (b) 1.5M and (c) 1M The NMR spectra of P-C18-50 (8h polymerization) (a) unpurified (monomer was not removed) (b) purified (monomer was removed) EC traces of P-DMODA (a) P-DMODA-10, (b) P-DMDOA-20, (c) P-DMODA EC traces of P-DMHDA (a) P-DMHDA-5, (b) P-DMHDA-10, (c) P-DMHDA-20, (d) P-DMHDA EC traces of P-DMDDA (a) P-DMODA-120, (b) P-DMDOA H NMR spectra of (a) P-DMODA-50 in chloroform-d; and (b) PNa prepared by ion-exchange in D 2 O H NMR spectra of (a) P-DMHDA-50k in chloroform-d; and (b) PNa (ion-exchange) in D 2 O, (c) P-DMHDA (ion-exchange) in chloroform-d Pseudo first order kinetic plots for the P-TOA-b-P polymerizations: ( ) 0.1g P-TOA-M4.8 /1 ml styrene, ( ) 0.2 g P-TOA-M4.8/1 ml styrene EC traces of P-TOA-b-P at different feed ratio and reaction time Mn (GPC) vs. conversion and PDI vs. conversion for P-TOA-b-P block copolymers: ( )the Mn of P-TOA-b-P-1:10, ( ) the Mn of P-TOA-b-P-1:5, ( )the PDI of P-TOA-b-P-1:10, ( ) the PDI of P-TOA-b-P-1: xvii

18 H NMR spectra of (a) P-TOA-M8.4 (bis-raft) and (b) P-b-P-TOA-b-P EC traces of a. P-TOA-M4.7 (bis-raft) and P-b-P-TOA-b-P; b. P-TOA-M8.4 (bis-raft) and P-b-P-TOA-b-P H NMR spectra of (a) P-TOA-b-P, (b) P-Na-b-P and (c) P-A336-b-P EC traces of P-DMODA-M18.3 (solid line) and P-b-P-DMODA-b-P triblock copolymer (dash line) EC traces of PAMP-DMODA homopolymers (a) PAMP-DMNODA- 10, (b) PAMP-DMODA Organogel of P-DMODA-N38: (1) in benzene solutions with different concentrations: (a) 2.5%, (b) 5%, (c) 10%, (d) 20% (w/v); (2) 10% concentration in (e) benzene, (f) styrene, (g) toluene, (h) o-xylene Gel transition temperatures of P-DMODA benzene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v) Gel transition temperatures of P-DMODA toluene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v) Gel transition temperatures of P-DMODA o-xylene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v) Two glass capillaries sealed with (a) 10wt/v% toluene gel of P-C18-N38 and (b) pure toluene. They are characterized by optical microscope (1) under normal lens; (2) under crossed polarizer The birefringence behavior of the P-C18-N90 10wt/v% toluene gel at different temperatures The EM images of the bulk morphologies of the PE-URFs organogels H NMR spectrum of dibenzyl trithiocarbonate The molecular weight and polydispersity (PDI) of P homopolymers with different feeding ratio (RAFT / monomer) xviii

19 6.3 EC traces of (a) P-1 and P-PVBC-P-1, (b) P-2 and P-PVBC-P The 1 H NMR spectra of quaternized P-b-PVBC-b-P with various tertiary amines The EC traces of P homopolymer, P-PVBC triblock, pentablock and heptablock copolymers The membrane of triethylamine quaternized P-b-PVBC heptablock copolymer Conductivity of quaternized P-b-PVBC copolymers: ( ) ABA triblock, ( ) ABABA pentablock, ( ) ABABABA heptablock Azimuthally averaged AX intensity profiles for P-PVBC triblock (34mol%) and heptablock (47mol%) TGA analysis of P-PVBC block copolymer (heptablock 2#) H NMR traces of P-r-PVBC random copolymers and P-r-PVBC-g-RAFT EC traces of P-r-PVBC random copolymers and P-r-PVBC-g-RAFT EC traces of (a) P-r-PVBC (7mol%); (b) P-r-PVBC-g-RAFT; (c) P-r-PVBC-g-PVBC-0.5h; (d) P-r-PVBC-g-PVBC-1h; (e) P-r-PVBC-g-PVBC-2h Pseudo first order kinetic plots for the P-r-PVBC-g-PVBC polymerization and molecular weight distribution (PDI) H NMR spectrum of (a) P-r-PVBC-g-PVBC (120 C, 2h, in chloroform-d) (b) quaternized P-r-PVBC-g-PVBC (in the mixture of chloroform-d and methanol-d with the volume ratio of 2:1) H NMR spectra of vinylbenzyl chloride and vinylbenzyl thiol H NMR spectra of (a) vinylbenzyl chloride, (b) vinylbenzyl acetate and (c) vinylbenzyl alcohol monomers H NMR spectra of (a) PVBC and (b) PVBC--vinylbenzyl thioether (10mol% feeding ratio) xix

20 H NMR spectra of (a) PVBC and (b) PVBC-O-vinylbenzyl ether (20mol%) DC curve of PVBC-O-vinylbenzyl during heating at 1 C/min (a) Crosslinked PVBC-O-vinylbenzyl ether film in dried state (b) uncrosslinked PVBC (left) and crosslinked PVBC-O-vinylbenzyl ether film in chloroform The FT-IR spectra of (a) PVBC-O-vinylbenzyl ether, (b) directly quaternized PVBC-O-vinylbenzyl ether, and (c) post-crosslinking quaternized PVBC-O-vinylbenzyl ether (soak in the mixture of water and TMA aqueous solution) TGA curves of (a) directly quaternized PVBC-O-vinylbenzyl ether, and (b) post-crosslinking quaternized PVBC-O-vinylbenzyl ether (soak in the mixture of water and TMA aqueous solution) EC traces of the poly(trithiocarbonate) RAFT agents, (a) poly-raft-1, (b) poly-raft H-NMR spectra of poly-raft agents. (a) poly-raft-1, (b) poly-raft EC traces of P initiated by poly-raft agent, (a) 5k, (b) 9.8k, (c) 19k, and (d) 38k target molecular weights Pseudo first order kinetic plots of the aminolyzed polystyrene polymers, P target molar masses: ( ) 5kDa ( ) 9.8 kda( ) 19 kda ( ) 38 kda. The solid lines are linear fits to the data EC trace of the P-PtB and aminolyzed P-PtB multiblock copolymer EC traces of (a) P and P-b-PVBC multiblock copolymer, (b) aminolyzed P and P-b-PVBC diblock, (c) radical reduced P and radical reduced P-b-PVBC diblock H NMR spectra of (a) P-b-PVBC and (b) P-b-quaternized PVBC by triethylamine xx

21 LIT OF CHEME cheme Page 1.1 ome examples of anionic polyelectrolytes ome examples of cationic polyelectrolytes Di-, tri-, tetra-, and hexa- functional initiators for ATRP polymerization Three routes to synthesize graft or comb polymers Partial sulfonation of polystyrene block Different architectures of sulfonated amphiphilic copolymers Polystyrenesulfonate ester graft copolymer The neutralized AMPA by TBA Quaternization of PVBC Various kinds of PVBC quaternary ammonium salts via quaternization Radiation-grafting of VBC onto PVDF and FEP and the conversion to alkaline anion-exchange membranes Mechanism of RAFT polymerization The generic structures of RAFT chain transfer agents Examples of RAFT chain transfer agents Mechanism for fragmentation-addition chain transfer Formation of diblock copolymers by the chain extension of macro-raft reagent xxi

22 ynthesis of symmetric and unsymmetrical trithiocarbonate RAFT agents Trithiocarbonation of various alkyl halides ynthesis of trithiocarbonate RAFT agents with TCDI ynthesis of RAFT-COOH ynthesis of didodecyl-1,2-phenylene-bis(methylene)bistrithiocarbonate RAFT agent ynthesis of DBTC ynthesis of, -bis(α,α -dimethyl-acetic acid)-trithiocarbonate (HOOC-RAFT-COOH) Aminolysis of trithiocarbonate groups Radical reduction of trithiocarbonate groups ynthesis of trialkylammonium styrenesulfonate monomers, -TOA, -TBA, and -TEA ynthesis of dimethyl alkylammonium p-styrenesulfonate monomers RAFT polymerization of P-TOA homopolymer with RAFT-COOH Homopolymerization of P-TOA with a bis-trithiocarbonate RAFT agent The typical RAFT polymerization of dimethyl alkylammonium p-styrenesulfonate Ion-exchange reaction of alkylammonium polystyrenesulfonate Polymerization of P-TOA-b-P diblock copolymer RAFT polymerization of P-b-P-TOA-b-P triblock copolymer ynthesis of P-b-P triblock copolymers xxii

23 4.10 ynthesis of AMP-DMODA monomers The structure of P-DMODA organogelator ynthesis of graft polymers via RAFT, employing the Z group approach or the R group approach The RAFT polymerization steps of P-b-PVBC-b-P triblock copolymers Quaternization of PVBC with triethylamine Random copolymerization of P-r-PVBC via RAFT using RAFT-COOH as chain transfer agent ynthesis of P-r-PVBC-g-trithiocarbonate (P-r-PVBC-g-RAFT) Graft reaction of vinylbenzyl thiol onto PVBC Graft reaction of vinylbenzyl alcohol onto PVBC Integrated process of ring-opening and RAFT polymerization involving cyclic trithiocarbonates The structures of some multifunctional RAFT agents Condensation polymerization of poly(trithiocarbonate) RAFT agent RAFT polymerization of polystyrene with poly-raft agent RAFT polymerization of polystyrene-b-poly (tert-butylstyrene) (P-b-PtB) RAFT polymerization of P-b-PVBC Quaternization of P-b-PVBC multiblock copolymer by triethylamine xxiii

24 CHAPTER Ⅰ INTRODUCTION 1.1 Motivation Ionic polymers containing sulfonic acid groups or quaternary ammonium salt groups are strong acid or base polyelectrolytes, which are water soluble and dissociate to macroions and counter ions in aqueous solution 1. cheme1.1 and cheme 1.2 show some examples of anionic and cationic polyelectrolytes 2-4. These kinds of hydrophilic ionic polymers potentially form interesting amphiphilic polymer systems when combined with another hydrophobic component. Two examples of this are, amphiphilic block copolymers comprised of a hydrophilic ionic block and another hydrophobic block and second, amphiphilic complexes of these hydrophilic polyelectrolytes with long alkyl chain surfactants as counter-ions 1

25 n O 3 Na sodium polystyrenesulfonate n COOH polymethylacrylic acid cheme 1.1 ome examples of anionic polyelectrolytes. n Cl N n N n NH CH 3 N CH 3 Poly(vinylbenzyl quaternary ammonium) poly-1-methyl-2-vinylpyridinium polyimidazolium cheme 1.2 ome examples of cationic polyelectrolytes. Recently, there has been increasing interest in preparing amphiphilic ionic block copolymers containing sulfonic acid groups or quaternary ammonium salt groups due to their special properties and application in membranes. As strong electrolytes, sulfonic acid (anionic) groups and quaternary ammonium salt (cationic) groups can transfer and conduct cations and anions, respectively. As block copolymers, they can self-assemble to form periodic ordered nanostructures, as shown in Figure The hydrophilic, ionic domains can form ionic channels for ion-conducting, and the hydrophobic domains can confine the swelling of hydrophilic domains as physical crosslinks. The multiblocks can repeatedly crossover the interface of the two immiscible domains or link different 2

26 domains like bridges. In the former case, the mechanical properties of materials will be enhanced. In the later case, the hydrophilic block will be fixed when there are multiple hydrophobic blocks in the copolymer bridging different domains, and the overall swelling of the hydrophilic domains in aqueous solution will be restricted, as shown in Figure 1.2. o, these polymers may be applied for proton or alkaline direct methanol fuel cell membranes, water desalination reverse osmosis membranes, ordered nano-template, or self-assembled micelles in selective solvents. Figure 1.1 Mean-field phase diagram for conformationally symmetric diblock copolymer melts [Reproduced from reference 5 with permission]. 3

27 Looping Bridging diblock triblock pentablock Figure 1.2 The looping and bridging behavior of multiblock copolymers. For example, one problem that limits the application of methanol fuel cell is the high permeability of methanol in the membrane, which lowers the overall performance of the fuel cell and limits the lifetime of the membrane 6. To improve the efficiency of fuel cell membranes, the challenge is to enhance the ion conductivity and restrict the methanol permeability simultaneously. Generally, the transport of small molecules, such as ions, methanol, water, follows a solution diffusion mechanism, and the transport rate is related with their permeability coefficient respectively 7. Improving the selectivity of ions over methanol in the membrane is an effective way to enhance efficiency of the membrane. Covalent crosslinks can confine the water uptake of the membrane, which will limit both methanol permeability and ion conductivity. The self-assembly of amphiphilic ionic block copolymers may provide another way to confine the water uptake of the membrane. Due to the incompatibility between two unlike blocks, periodic ordered morphologies can be formed, such as sphere, cylinder, lamellar, and gyroid. The glassy hydrophobic 4

28 domains can work as physical crosslinks to restrict the water uptake of the membrane, and the ionic-rich hydrophilic domains can transfer ions effectively. The tunable structure of block copolymer in the nano-scale can adjust the selectivity of the membrane and optimize the transport properties imilar principles are involved in restricting the diffusion of salt compared to water for reverse osmosis membranes, or separation selectivity in pervaporation membranes. Currently, the synthesis of amphiphilic ionic block copolymers with well-defined architectures is a significant challenge. This research developed from the central goal of developing facile methods to synthesize amphiphilic ionic copolymers with well-defined architectures, such as diblock, triblock, multiblock, graft copolymers. Well-defined amphiphilic ionic copolymers were approached via two different methods, direct synthesis and post-polymerization modification. The challenge of the former is the intrinsic immiscibility between hydrophobic and hydrophilic monomers, and the difficulty of the later was to achieve 100% conversion of functional groups and avoid side reactions. 1.2 ulfonated amphiphilic polymers Polystyrenesulfonate acid is a typical water soluble, strongly acidic polyelectrolyte. Currently, there is a lack of methods to prepare amphiphilic block copolymers containing sulfonic acid groups via a facile route. In this research, we tailored the solubility of ionic monomers by a simple method, the ion-exchange with hydrophobic alkylammonium salts 5

29 to form stoichiometric electrolyte-surfactant monomers, which can directly polymerize to form polyelectrolyte-surfactant complexes (PE-URFs), which can then be copolymerized with hydrophobic monomers to form block copolymers. In this research, a model anionic amphiphilic block copolymer, polystyrene-b-sodium polystyrenesulfonate (P-b-P), was studied. odium p-styrenesulfonate (Na) monomers were converted to hydrophobic form by ion-exchange with hydrophobic alkylammonium salts, such as tri-n-octylammonium, dimethyl octadecylammonium etc. Reversible addition fragmentation chain transfer (RAFT) polymerization was employed to prepare the alkylammonium polystyrenesulfonate (P) polymers directly, due to its simple reaction procedure and high tolerance to functional groups. The alkylammonium P polymers were used as macro-raft agents to polymerize styrene to obtain P-b-P diblock and triblock copolymers. The alkylammonium P could be converted back to the sodium salt form by ion-exchange to obtain amphiphilic block copolymers easily. Different kinds of alkylammonium salts were used as counter ions of styrene sulfonate to approach better control of RAFT polymerization. During the study, it was found that the PE-URFs displayed interesting behavior in organic solvents, acting as thermo-reversible organogelators. This class of amphiphilic polymer comprised of stoichiometric complexes of a hydrophilic polyelectrolyte and a hydrophobic surfactant is another interesting material, which could form self-assembled ordered structure in solid states or selective solvents. The self-assembly behavior of PE-URFs in solid states has been well studied 11, 12. The stoichiometric PE-URFs could 6

30 form a variety of highly ordered mesophase in bulk, such as lamellar, cylindrical, and undulating layered structures. The actual structure is determined by the volume fractions of ionic mesophase and the absolute amount of interface, and some other factors of the polyelectrolyte and surfactants. The various combinations of polyelectrolyte and surfactants (backbones, ionic groups, hydrophobicity, chain length of surfactants, etc.) offer a way to finely tailor the phase morphology and related functional properties of these materials. Previous study of self-assembly, aggregation and micellation of polyelectrolyte-surfactant complexes in solution mainly focused on aqueous solution, while the self-assembly behavior in selective organic solvents has not been discussed widely. The PE-URF self-assembly in organic solvents could be potentially used as template or precursor to prepare nanomaterials or porous materials. Therefore, the fundamental thermodynamic behavior in organic solvent was investigated. 1.3 Quaternary ammonium copolymers Polymeric quaternary ammonium compounds are a typical class of cationic polymers with positively charged macromolecular backbones, which are widely used in industry as polymeric surfactants and phase transfer catalysts 13. Recently, polymeric quaternary ammonium compounds have gained attention due to their applications in alkaline fuel cell membranes, such as poly (vinylbenzyl quaternary ammonium salts) (PVBQAM) 14. Generally, they can be prepared by synthesis of a reactive precursor polymer, poly(vinylbenzyl chloride) (PVBC), and sequential post-polymerization 7

31 modification. However, there has been little reported investigation on the preparation of PVBQAM amphiphilic copolymers with well-defined architectures. In this research, synthesis of polystyrene-co-poly (vinylbenzyl chloride) (P-co-PVBC) block copolymers with various well-defined architectures were investigated, such as triblock, pentablock, heptablock, multiblock and graft copolymers. The quaternization of PVBC was studied also to achieve amphiphilic ionic copolymers with full conversion of functional groups. To achieve 100% conversion of PVBC quaternization, proper solvent and various tertiary amines were chosen to prepare the cationic amphiphilic copolymers. The preliminary characterization of the transport properties of the P-b-PVBC quaternary ammonium salts for use in alkaline fuel cell membranes was conducted. 8

32 CHAPTERⅡ BACKGROUND To target well-defined amphiphilic ionic block copolymers, the methods to tailor the architectures of macromolecules will be briefly reviewed at first. The general approaches to prepare amphiphilic ionic block copolymers are discussed, based on polystyrene-b-sodium polystyrenesulfonate (P-b-P) anionic amphiphilic block copolymers and polystyrene-b-poly(vinylbenyl trialkylammonium) (P-b-PVBTA) cationic amphiphilic block copolymers. In this dissertation, reversible addition-fragmentation chain transfer polymerization (RAFT) technique was employed to synthesize well defined amphiphilic ionic copolymers due to its versatility for tailoring architectures and high tolerance to functional groups. The development and mechanism of RAFT polymerization, as well as the approaches to obtain various architectures via RAFT polymerization, are also reviewed. 2.1 Controlled/living polymerization to tailor the macromolecular architectures To develop high performance and novel functional materials, well -defined 9

33 macromolecular architectures have attracted interest to design model macromolecules on the molecular level and tailor the structure of materials on nano-scale. Well defined macromolecular architectures can only be synthesized by controlled/living polymerization methodology, which provides access to uniform polymers with controllable size, component fraction, and shapes 15. The way to design well defined macromolecular architectures has expanded since the exploration of ionic living polymerization techniques, and continued to rapidly with the introduction of other controlled/living polymerization techniques. In 1956, living polymerization was explored first by zwarc through the anionic polymerization of styrene in THF, and block copolymers were synthesized by sequentially adding the second monomer to the living chain ends of the hompolymer 16, 17. But many functional groups may terminate the polymerization by transfer reactions, and protection and deprotection steps of functional groups are necessary to prepare the anionic polymerization of polymers containing functional groups 18, 19. The number of monomers that are not compatible with anionic polymerization and the strict reaction conditions for anionic polymerization limit the application of the anionic polymerization of functional polymers. With the development of other controlled/living polymerization techniques, new methods of controlled/living polymerization were developed for synthesis of more kinds of well defined polymers. The controlled/living polymerization via cationic polymerization was discovered in 1970s and 1980s by Higashimura 20 and Kennedy et.al. 21 In the 1990s, living free radical polymerization techniques made 10

34 important breakthroughs, and several kinds of polymerization methods were well developed, such as nitroxide mediate polymerization (NMP) 22, atom transfer radical polymerization (ATRP) 23, and reversible addition fragmentation transfer polymerization (RAFT) 24. More and more kinds of functional polymers with well defined architectures have been synthesized successfully. With the development of controlled/living polymerization techniques, various functional polymers with well defined macromolecular architecture can be designed and synthesized. Generally, diblock and triblock copolymers can be synthesized via sequential living polymerization. Macromolecules with complex architectures, such as star or multi-arms, can be synthesized by using corresponding multifunctional initiators. For example, Quirk synthesized a tri-functional initiator based on s-buli, which was used in anionic polymerization to prepare three arm polystyrene 25. Mayajaszewski synthesized di-, tri-, tetra- and hexa- functional initiators based on benzyl bromide to prepare corresponding di, tri-, four-, or six-arms polymers via ATRP polymerization, as shown in cheme The number of arms, arm length, composition can be tailored precisely. 11

35 Br O O O O Br Br O O O O Br O Br O Di-functional Tri-functional Cl Cl Br O O O O O O Br O Cl O O P N N O P P O O N O Cl Br O Br Cl Cl Tetra-functional hexa-functional cheme 2.1 Di-, tri-, tetra-, and hexa- functional initiators for ATRP polymerization. Graft and comb polymers composed of a polymer backbone and side chains/ branches, could be synthesized by controlled polymerization via three different routes, including grafting from, grafting to and grafting through, as shown in cheme In a typical grafting from route, styrene and vinylbenyl chloride monomers were copolymerized via NMP polymerization to obtain a random copolymer with narrow polydispersity. The vinylbenzyl chloride units can be grafted with nitroxide initiators to form active sites, and side chains can grow up from these active sites to form graft 12

36 polymers 28. Via the grafting to method, the functional groups of polymer backbones react with the end functionalities of the second kind of polymer chain to form graft copolymers with well defined structures. For example, a styrene and vinylbenzyl chloride (VBC) random copolymer was synthesized via NMP polymerization, and PVBC units were reacted with anionic polymerized polyisoprenes containing diphenylethylene lithium anions to form the graft copolymers 29. Grafting through is direct homopolymerization of macromonomers, which bear a polymer chain per unit. Monomer B Grafting From X X X Backbone with active sites X Y Grafting To X X X Backbone with functional groups x Grafting Through R + backbone monomers cheme 2.2 Three routes to synthesize graft or comb polymers. 2.2 ynthesis of amphiphilic ionic block copolymers Ionic amphiphilic copolymers with well defined structure can be prepared via direct synthesis or post-polymerization modification. Both methods have been reported in the 13

37 literature to prepare amphiphilic ionic copolymers containing sulfonic acid groups or quaternary ammonium groups Anionic amphiphilic block copolymers containing sulfonic acid groups Ionic polymers containing sulfonic acid groups are a typical kind of anionic polyelectrolyte, which are water soluble and have strong ionic dissociation ability in polar solvents. The amphiphilic block copolymers containing sulfonic acid groups have interesting properties due to the ion-exchange ability of sulfonic acid groups and the self 2, 30 assembly ability of the amphiphilic block copolymers Post-polymerization modification The post-polymerization modification method is a facile way to enlarge the library of functional polymers. Various kinds of novel functional polymers could be obtained from some commercial polymer materials via simple and versatile modification process. In recent years, many amphiphilic block copolymers containing sulfonic acid groups were prepared by post-polymerization modification, especially by the sulfonation of the polystyrene block. In 1979, a sodium polystyrene sulfonate based amphiphilic block copolymer was synthesized by anionic polymerization of poly (2-vinyl pyridine-b-styrene) block copolymer, and followed by selective sulfonation of polystyrene block 31. A triethyl phosphate-sulfur trioxide complex was used sulfonating agent in the sulfonation. Many commercial block copolymers synthesized by anionic polymerization, such as 14

38 poly(styrene isobutylene styrene) (IB) 8, 10, poly (styrene-ethylene-butylene-styrene) (EB) 32-34, can be selectively sulfonated to obtain the ionic amphiphilic block copolymers. The aromatic groups can be partly sulfonated by the complex of acetate anhydride and sulfonic acid, as shown in cheme 2.3. Besides these commercial triblock copolymers, hydrogenated poly(styrene-butadiene) (HPB) diblock copolymer 35, P(VDF-co-HFP)-b-P diblock copolymer 36 can be sulfonated to obtain amphiphilic ionic block copolymers. In all these kinds of sulfonated block copolymers, the polystyrene blocks were partly sulfonated rather than fully sulfonated. O O O O H 3 C C O C CH 3 + H2 O 4 H 3 C C OH + H 3 C C OO 3 H O + H 3 C C OO 3 H + O H 3 C C O H O 3 H cheme 2.3 Partial sulfonation of polystyrene block. Varying architectures of amphiphilic ionic copolymers have been synthesized by post-polymerization modification successfully, such as diblock 36, triblock 9, multiblock 37 and graft copolymers, as shown in cheme

39 H 2 C H 2 C H 2 C H 2 C CH x C y CH m H 2 CH n CH 2 CH 3 O 3 Hc (a) Partly sulfonated and hydrogenated poly (styrene-butadiene) CH 3 H 2 H 2 C H C H 2 C H C H 2 C C C H H 2 C C H C m n m CH 3 O 3 H O 3 H (b) Partly sulfonated poly (styrene isobutylene styrene) (-IB) triblock copolymer O O O O MO 3 O 3 M O x O O C O n O y (c) Multiblock copolymers of poly (2,5-benzophenone) and disulfonated poly(arylene ether sulfone H 2 C H H 2 C C m CN H C H 2 C H C n O 3 H (d) Graft copolymer PAN-g-macPA cheme 2.4 Different architectures of sulfonated amphiphilic copolymers. The sulfonation level could be tuned by adjusting the ratio of sulfonating agent and 16

40 polymers and reaction time, but it is difficult to get full conversion. As shown in Figure 2.1, the sulfonation level increased linearly with increasing molar ratio of sulfonating agent and polymer, when the sulfonation level is lower than 60%, but then began to level off. Partly sulfonated polymer rather than fully sulfonated polymer could be obtained by post-polymerization sulfonation 38. Figure 2.1 The relationship of sulfonation level and the molar ratio of sulfonating agent and polymer [reproduced from reference 38 with permission] Direct synthesis Direct synthesis of amphiphilic ionic block copolymers could control the architecture of block copolymers more precisely than post-polymerization, and can avoid some side reactions. The main challenge of direct synthesis of amphiphilic ionic copolymers is the immiscibility between the hydrophilic component and hydrophobic component. The functionalities usually have higher reactivity with the active sites of 17

41 living polymerization, and may terminate the living polymerization by transfer reactions. In 1981, the synthesis of p-styrene sulfonic acid and isoprene block copolymers via anionic living polymerization was studied by Whicher et.al. p-tyrene sulfonic acid is not compatible with anionic polymerization and the sulfonic acid was converted to more hydrophobic form, including sulfonamide and sulfonate esters, such as propyl, isopropyl and methyl styrene sulfonates. ulfonamide is still too acidic to synthesize via anionic polymerization technique, but the sulfonate ester monomers could be polymerized. But, the anionic polymerization is not easy to control and a limited degree of polymerization was achieved 39. Later, N,N -dialkyl-4-vinylbenzenesulfonamides were synthesized via anionic polymerization in THF with low polydispersity, and poly(styrene-b-n,n -diethyl-4-vinylbenzenesulfonamides) and poly( isoprene-b- N,N -diethyl-4- vinylbenzene sulfonamides) block copolymers were synthesized by anionic polymerization successfully 40. The development of living free radical polymerization techniques provided a new route to synthesize well defined polystyrene sulfonate ionic polymers. In 1995, well defined sodium polystyrenesulfonate (PNa) was synthesized in aqueous solution directly via the NMP controlled free radical polymerization technique 41. Later, the diblock copolymer of sodium polystyrenesulfonate and another hydrophilic monomer, sodium 4-vinylbenzoate, was synthesized via sequential NMP polymerization 42. In 2007, various kinds of water-soluble nitroxide compounds were used for the NMP polymerization of PNa with the PDI between 1.11 and The ATRP 18

42 polymerization technique can also be employed to synthesize PNa. In aqueous solution, the polymerization is very rapid but poorly controlled, and better control can be achieved in methanol. For the highly polar monomers, ATRP polymerization was run in mixed aqueous solution, such as methanol/water or pyridine/water to optimize the polymerization, and the PDI of ~1.2 can be achieved 44, 45. Recently, the diblock copolymers of PNa and polymethylmethacrylate (PMMA) were polymerized successfully via ATRP in the mixture of water and DMF, but the PDI was not reported 46. RAFT polymerization has been employed directly in aqueous media to synthesize ionic polymers. In the work of Chiefari et al. sodium 4-styrenesulfonate could be synthesized directly in water at 70 C to yield a sodium salt of polystyrene sulfonate homopolymers with an Mn of 8000 and molecular weight distribution of Then it was reported that the sodium salt of polystyrene sulfonate homopolymers could be used as a macro chain transfer agent to form block copolymers with other hydrophilic monomers in aqueous solution, such as sodium 4-vinylbenzoate 48 and ethylene oxide 49. Via a similar procedure, PNa-b-PEG-b-PNa triblock copolymers with narrow PDIs of 1.28~1.4 and well-defined structures were synthesized in water at 70 C 50. However, the monomers containing sulfonic acid groups generally have been synthesized in aqueous environment only, due to the strong hydrophilicity of the sulfonic acid groups 51, 52. Water soluble RAFT agents should be chosen for the synthesis of monomers containing sulfonic acid groups. In the aqueous environment, their stability against hydrolysis is one concern during polymerization

43 Generally, it is difficult to directly synthesize amphiphilic P-containing block copolymers. The challenge in the direct polymerization of amphiphilic P-containing block copolymers is the strong immiscibility of the hydrophilic -Na and P-Na with most hydrophobic monomers and organic solvents. While block copolymers of P-Na with polystyrene (P) and polymethylmethacrylate (PMMA) have been prepared, the homogeneous copolymerization and subsequent characterization over a wide range of P-Na volume fractions is difficult 54, 55. To enhance the compatibility of -Na or P-Na with the second hydrophobic monomer, p-styrenesulfonate monomer can be modified to a more hydrophobic form, which is amenable to controlled free radical polymerization with hydrophobic monomers. One approach is to convert the sulfonic acid group to a sulfonate ester to synthesize the block copolymer, and the precursor can be hydrolyzed or thermally degraded back to the acid form. Neopentyl p-styrenesulfonate (-np) was prepared by the esterification of p-styrene sulfonyl chloride with neopentyl alcohol, which was used to prepare P-b-P-nP block copolymers via NMP 56. Ethyl p-styrenesulfonate (-E) was prepared by the metathesis reaction between silver p-styrenesulfonate and ethylbromide, and P-co-P-E comb polymers were synthesized by atom transfer radical polymerization, shown in cheme

44 n H k Br m O O R O R= C 2 H 5 = C 12 H 25 cheme 2.5 Polystyrenesulfonate ester graft copolymer. Another more facile way was investigated to modify p-styrenesulfonate acid than esterification of sulfonic acid group, which was to neutralize p-styrene sulfonic acid with trioctylamine to produce a hydrophobic trioctylammonium p-styrenesulfonate monomer. The modified p-styrenesulfonate monomers copolymerize with styrene by suspension polymerization to prepare random sulfonated styrene ionomer 60. Recently, Matyjaszewski neutralized 2-Acrylamido-2-methyl-N-propanesulfonic acid (AMPA) with tri-n-butylamine (TBA), and polymerized the neutralized monomers via ATRP, as shown in cheme 2.6. PAMP-TBA homopolymers were synthesized with low polydispersities, and corresponding block copolymers have been achieved by using polyacrylate macroinitiators to polymerize with AMP-TBA via ATRP polymerization in DMF to obtain diblock or triblock copolymers

45 O O N H O O HN cheme 2.6 The neutralized AMPA by TBA Cationic amphiphilic copolymers containing quaternary ammonium salts Cationic polymers containing quaternary ammonium salts represent a class of polyelectrolytes with unique properties. They can also be synthesized via two different routes: post-polymerization of the precursor polymers or direct synthesis of suitable monomers. Direct synthesis can achieve the cationic polymers with 100% functionality, but the characterization of cationic polymers is difficult. And the immiscibility between the hydrophilic cationic component and hydrophobic component will hinder the preparation of the copolymers. Post-polymerization modification is a much simple and facile way to prepare the amphiphilic block copolymers, but the complete functionality is not easy to achieve Post-polymerization modification Poly(vinylbenzyl chloride) (PVBC) is widely used in synthesis and modification of functional polymers, which contains pendant methyl chloride groups that could react with nucleophiles to introduce functional groups, as shown in cheme Via 22

46 post-polymerization modification, polyelectrolytes containing various kinds quaternary ammonium salts could be obtained by quaternizing same polymer with different quaternization agents, such as amines 3, diamines 63, pyridines 64, as shown in cheme 2.8. The post-polymerization modification can enrich the library of cationic polyelectrolytes easily, based on a reactive precursor polymer. Cl + N Cl N cheme 2.7 Quaternization of PVBC. n n n n N Cl N Cl N Cl NH 2 N Cl PVBC-TMA PVBC-TBA PVBC-DMA PVBC-PYC cheme 2.8 Various kinds of PVBC quaternary ammonium salts via quaternization. Copolymers of PVBC and other hydrophobic polymers could form amphiphilic copolymers by quaternizing the PVBC block. In lade et.al s work, PVBC blocks were grafted onto the surface of PVDF and FEP films via radiation polymerization, which were further quaternized to obtain cationic polyelectrolyte brushes on the surface of the 23

47 films, shown in cheme PVBC can be also grafted on other hydrophobic films surface via the radiation polymerization technique to obtain an ionic surface layer by sequential quaternization 66. The radiation grafting technique is facile way to prepare amphiphilic copolymers, but it can not tailor the architectures of macromolecules precisely, as well as morphology of the films. FEP PVDF gamma-ray VBC n H 2 C trimethylamine water HCl (aq.) Cl n H 2 C KOH (aq.) Cl N n H 2 C OH N cheme 2.9 Radiation-grafting of VBC onto PVDF and FEP and the conversion to alkaline anion-exchange membranes. The combination of controlled polymerization of PVBC and sequential quaternization is the main way to target well-defined cationic amphiphilic block copolymers of quaternary ammonium salts. ave et. al. prepared well-defined P-b-PVBC diblock copolymers via RAFT polymerization, which have the narrow molecular weight distribution between 1.15~1.38. The PVBC block was quaternized to form cationic amphiphilic block copolymers which are good stabilizer for emulsion polymerization 67. The P-b-quaternized PVBC diblock copolymers were also synthesized by reverse iodine transfer polymerization and following quaternization, and the cationic 24

48 block copolymers have polydispersities of 1.4~ NMP polymerization was used to synthesize P-b-PVBC block copolymers with PDIs of 1.28~1.30, which were quaternized by tri-n-ethylamine to obtain the amphiphilic block copolymers. In Jaeger s work, P-b-PVBC diblock copolymers with PDIs between were synthesized by sequential NMP polymerization, and VBC blocks were modified to carry different cationic groups P-r-PVBC random copolymers can be used as backbone to prepare graft/comb polymers with well-defined architectures via controlled radical polymerization. The benzylchloride of VBC blocks can be substituted by dithiobenzoates, and prepare graft/comb polymers by RAFT polymerization via grafting from route. The polymerization was well controlled in the early stage and then lost control due to the steric hindrance Direct synthesis Monomers containing quaternary ammonium salts are also highly hydrophilic and water soluble. These kinds of monomers are not compatible with the anionic living polymerization technique, and the architectures can be controlled by controlled free radical polymerization techniques. The quaternary ammonium salt monomers can be synthesized via controlled living polymerization in aqueous solution directly. In 1999, Armes synthesized PNa in aqueous solution via NMP polymerization, and then polymerized with vinylbenzyl 25

49 trimethyl ammonium chloride (VBTAC) to obtain Poly (acid-block-base) diblock copolymers, which are insoluble in any solvents 42. McCormick et.al. studied RAFT polymerization of vinylbenzyl trimethyl ammonium chloride in aqueous solution. The polymeric ammonium salts were used as macro-raft agent to polymerize another water soluble monomer, N,N -dimethylvinylbenzylamine, to obtain hydrophilic diblock copolymers 48. Later diblock copolymer of VBTAC and N-[3-(dimethylamino) propyl] methacrylamide (DMAPMA) was synthesized by RAFT polymerization in aqueous buffer solution via same strategy 74. imilar procedures were employed to synthesize other kinds of hydrophiphilic diblock copolymers 75. Compared with hydrophilic diblock copolymers, the synthesis of amphiphilic block copolymers faces the challenge of immiscibility between the hydrophilic and hydrophobic components. There is therefore little investigation on the direct synthesis of amphiphilic block copolymers composed of polyvinylbenzyl trimethyl/ethyl ammonium chloride and polystyrene due to solubility issues. 2.3 RAFT polymerization technique A new method of controlled free radical polymerization, called reversible addition fragmental chain transfer (RAFT) polymerization, was first reported by Australian researchers at the Common Wealth cientific and Industrial Research Organization (CIRO) in The radical addition-fragmentation process has been used in organic chemistry in 1970s 76. The typical process examples involve a reaction step with the 26

50 mechanism of N 2, including allyl transfer reactions with allyl sulfides/stannanes and Barton-McCombie deoxygenation process with xanthates 77. Polymerizations with addition-fragmentation chemistry, which showed some characteristics of living polymerization were reported in , and it was applied to control polymerization of unsaturated methyl methacrylate in The similar technique was also reported as macromolecular design via the interchange of Xanthates (MADIX) polymerization by a French group in , it has same mechanism of addition fragmentation chemistry and the only difference is the nature of the chain transfer agents Mechanism of RAFT polymerization A RAFT polymerization system consists of a reversible chain transfer agent, monomer, initiator, and possibly solvent. Compared with conventional free radical polymerization, the main difference is the presence of the reversible addition-fragmentation chain transfer (RAFT) agent. The mechanism of RAFT polymerization is shown in cheme 2.10, which shows the steps of initiation, propagation, reversible chain transfer (or pre-equilibrium), reinitiation, chain equilibration (also known as main equilibrium) and termination 81. The radicals are first initiated and added to monomers to start the propagation of the oligomeric chains. The RAFT agent reacts with the propagating chain to generate an intermediate radical, which can fragment to produce a new propagating chain. This is the pre-equilibrium process, and it should be completed in the early stage of reactions for all chains to come into the main equilibrium 27

51 stage for a controlled/living polymerization. In the main equilibrium stage, the controlled/living behavior of the polymerization is maintained by the reversible transfer of the dithio moiety between active/living species (R, P n, P m ) and dormant species( Z-C-R, Z-C-P n, Z-C-P m ). The chain equilibration is the fundamental step of the whole RAFT polymerization, and polymer chains are in an equilibrium between the active and dormant stages, which keeps the polymer chains quasi- living. To hinder termination reactions in the system, the chain transfer constant should be very high and the radical concentration should be low due to the chain equilibration. It means for different monomers polymerization, the chain transfer agents have to be chosen properly to have very a high chain transfer constant during RAFT polymerization. Then all polymer chains will propagate at similar rates, which leads to good molecular weight and molecular weight distribution control

52 cheme 2.10 Mechanism of RAFT polymerization [reproduced from reference 81 with permission] Choice of RAFT polymerization system The key component in RAFT polymerization is the chain transfer agent, which should have an effective chain transfer constant. The typical chain transfer agents are thiocarbonylthio compounds, which have the general structure of RC(=)Z, including dithioether, trithiocarbonates, xanthates, and dithiocarbamates, as shown in cheme 2.11, while cheme 2.12 shows some examples of chain transfer agents

53 Z C R R' C R R'O C R R 2 'N C R Dithioester Trithiocarbonate Xanthate Dithiocarbomate cheme 2.11 The generic structures of RAFT chain transfer agents. CN CN COOH CTP CPDB BDB COOH HOOC COOH CDB CDB CMP C 12 H 25 COOH COOH COOH EMP DMP CPP COOH HOOC COOH BPA BPA cheme 2.12 Examples of RAFT chain transfer agents. The chain transfer constant of the RAFT agent depends on the structures of the R group and Z group, which is the critical to a controlled RAFT polymerization. R is a homolytic leaving group, and a free radical of R (R ) should be able to reinitiate 30

54 polymerization. The Z group modifies the addition and fragmentation rate of reactions, and can also be an R group. The R group is very important in the pre-equilibrium stage, and it should be a better leaving group than the propagating radical. The mechanism of fragmentation-addition chain transfer is shown in cheme 2.13, and X is mainly CH 2 or 87. cheme 2.13 Mechanism for fragmentation-addition chain transfer [reproduced from reference 87 with permission]. The structure of the R and Z groups determines whether the RAFT polymerization is controlled or not. The leaving/reinitiating ability of an R group is related with its steric factors, radical stability, and polar effects. Higher radical stability should help the R group leaving ability, but if the radical is too stable, it is not easy to add monomers and 31

55 reinitiate polymerization. Increased steric effects will increase the leaving ability of the R group, and will hinder reinitiation. High electron withdrawing substituents within the R group will enhance the reinitiation ability 81. The Z group also influence greatly the reactivity and effectiveness of the chain transfer agent at controlling polymerization, which will activate the C= bonds toward radical polymerization and affect the stability of the adduct radicals. The choice of the Z group must be suitable for mediating the polymerization of a specific monomer. More reactive monomers will form a more stable adduct radical, and a Z group that has great destabilizing effect on the adduct radical is needed 88. Figure 2.2 shows the guideline for selecting chain transfer agents for various monomers 87, 89. Figure 2.2 Guidelines for the selection of Z group substituents for various polymerizations [reproduced from reference 87 with permission]. In Figure 2.2, fragmentation rates increase and addition rates decrease from left to right. Dashed lines indicate partial control over the polymerization (i.e., control over the 32

56 molecular weight evolution but poor control over the PDI>. MMA> methyl methacrylate, ty > styrene, MA > methyl acrylate, AM > acrylamide, VAc > vinyl acetate. Currently RAFT polymerization is among the most successful controlled free radical polymerization techniques due to its application for a wide range of monomers. Most monomers that have been polymerized via conventional free radical polymerization could be synthesized by RAFT polymerization. The typical monomers that could be used in RAFT polymerization include styrene derivatives, acrylates, methacrylates, acryamides, methacryamides, vinyl ethers, isoprene, vinylpyridine and acrylonitrile. Depending on the reactivity of the monomers, various kinds of RAFT chain transfer agents have been designed with proper R and Z groups to obtain effective chain transfer constants. The selection of an initiator system is important to optimize the control of polymerizations. Usually the initiation systems that could be used in conventional free radical polymerization can also be used in RAFT polymerization. A RAFT polymerization can be initiated by thermal autoinitiation for styrene derivatives, thermal radical initiator such as AIBN, photo initiators, etc Control over macromolecular architectures by RAFT polymerization Well-defined homopolymers or copolymers with controlled molecular weight, molecular weight distribution, structure, and compositional homogeneity are only synthesized by controlled polymerization. RAFT polymerizations not only control the molecular weight and distribution of macromolecules, but can also be used to synthesize 33

57 polymers with more complex chain architectures. Block copolymers can be prepared by RAFT polymerization via the sequential addition of monomers using homopolymers generated by RAFT polymerization, as macro-raft agents. The macro-raft agent takes on a similar role as low molecular weight chain transfer agent during the RAFT polymerization. If the homopolymers are generated with RAFT agents with a single R group, AB diblock copolymers are generated. If RAFT agents with two R groups are used, such as trithiocarbonate compounds, A-B-A triblock copolymers can be obtained. The general formation steps of A-B diblock copolymer using macro-raft agent are shown in cheme

58 (1) Initiator 2 I I M 2 P(M 2 ) x P(M 2 ) x (2) P(M2 ) P(M x 1 ) n Z Z P(M 1 ) n P(M 2 ) x Z P(M 1 ) n (3) P(M 1 ) n M 2 P(M 1 ) n P(M 2 ) y Block formation (4) P(M 1 ) n P(M 2 ) y + Z (a) P(M 1 ) n P(M 1 ) n P(M 2 ) y M 2 Z P(M 1 ) n P(M 1 ) n P(M 2 ) y P(M 1 ) n Z (b) P(M 1 ) n P(M 2 ) y P(M 2 ) x P(M 1 ) n P(M 2 ) y P(M 2 ) x Z M 2 Z P(M 1 ) n P(M 2 ) y P(M 2 ) x Z (5) 2 P(M 1 ) n P(M 2 ) y P(M 1 ) n P(M 2 ) y P(M 2 ) y P(M 1 ) n Termination 2 P(M 1 ) n P(M 2 ) y 2 P(M 1 ) n P(M 2 ) y cheme 2.14 Formation of diblock copolymers by the chain extension of macro-raft reagents [reproduced from reference 91 with permission]. Other complex architectures, such as star polymers, graft polymers, are obtainable using macro-raft agents with multiple R groups ( grafting from, R approach) or by grafting RAFT polymerized polymers through the Z group (grafting to, Z approach). The 35

59 general approaches for generation of complex macromolecular architectures by RAFT chemistry are shown in Figure Via the Z-approach or R-approach, various complex architectures are obtained by RAFT polymerization, such as star, comb, brush, dumbbell, dendrimers, and hyperbranched polymers. The architecture approached by the Z group is not always stable due to the thiocarbonylthio groups, which may be cleaved in some cases. Figure 2.3 The general approaches for generation of complex macromolecular architectures by RAFT chemistry [reproduced from reference 91 with permission] Well-defined end-functionalized polymers via RAFT polymerization Another advantage of RAFT polymerization is the ability to produce end functional polymer chains. There are three approaches to introduce functional groups at the 36

60 chain-end according to the structure of the chain transfer agent. (1) α-functional groups could be introduced to polymer chain through the Z group of the chain transfer agent; (2) ω-functional group could be introduced to polymer chain via the R group of chain transfer agent; (3) ω-functional group could be introduced by modification of the thiocarbonylthio group post-polymerization. The introduction of chain-end functionalities via the R group approach or Z group approach of chain transfer agent is very attractive, as the functional group is introduced without any post-polymerization modification. A large range of functional RAFT agents have been described in literatures already, such as hydroxyl groups, allyl groups, and so on 92, 93. Table 2.1 lists the various RAFT agents containing functional groups 94. Various kinds of functional groups can be introduced to the ends of macromolecules facilely by choosing proper chain transfer agents in RAFT polymerizations. Besides using proper RAFT agents containing functional groups, polymers prepared by RAFT polymerization can also be end-functionalized by post-polymerization modification of the thiocarbonylthio groups. For example, thiol groups could be obtained by hydrolysis 95 or aminolysis 96. The thiocarbonylthio groups could also be oxidized to convert the C= groups to C=O groups

61 Table 2.1 tructures of Functional RAFT agents [reproduced from reference 94 with permission]. 38

62 2.3.5 ynthesis of trithiocarbonate RAFT agents Among the specific types of chain transfer agents, the trithiocarbonate RAFT agents stand out due to their high activity. The Z group of the trithiocarbonate RAFT agent can also be replaced by the R group to obtain a symmetric trithiocarbonate RAFT agent. Polymer chains can propagate on both sides of the trithiocarbonate group as the center of the chain, and symmetric triblock copolymers can also be synthesized with the symmetric trithiocarbonate RAFT agent easily. The trithiocarbonate RAFT agents can be prepared by nucleophilic displacement reactions. The general route to synthesize trithiocarbonate RAFT agents is to prepare a trithiocarboxylic acid salt, which is reacted with an alkyl halide to produce the RAFT agent. The important factors in this synthesis depends on the salt counterions (Na +, K +, Mg 2+, NH + 4 ) and the appropriate alkyl halide 98, as shown in cheme

63 symmetric Na 2 + C 2 Catalyst Na 2 C 3 Na 2 C 3 + 2RX Catalyst R C R unsymmetric R 1 H + C 2 + NaOH Catalyst R 1 C Na + H 2O R 1 C Na + R 2 X Catalyst R 1 C R 2 + NaX cheme 2.15 ynthesis of symmetric and unsymmetrical trithiocarbonate RAFT agents. There are several methods to perform the route to synthesize trithiocarbonate RAFT agents. The thiol can react with carbon disulfide in the presence of alkoxide or tertiary amine, followed by addition of an alkyl halide 100. The reaction can also be performed in a two phase system of an alkaline aqueous solution and an organic solution with the presence of a phase transfer catalyst 82, 101. The yield of trithiocarbonate RAFT agent is very high in this system. The symmetric trithiocarbonate RAFT agents can also synthesized by trithiocarbonation of alkyl halides by catalysis with cesium carbonate under mild conditions in one pot, as shown in cheme The yield of this reaction is very high. 40

64 2 R X X=Cl, Br 2 C 2 (1 equiv) Cs 2 CO 3 R C R 1M DMAc or MeCN 25, air R= allyl, 3 R= Et 4 R= n-bu, 5 R=CH 3 (CH 2 ) 7 6 R= i-pr, 7 R= 1-phenylethyl cheme 2.16 Trithiocarbonation of various alkyl halides. Trithiocarbonate RAFT agents can also be synthesized by reaction of 1,1 -thiocarbonyl diimidazole (TCDI), shown in cheme This method avoids the use of carbon disulfide, and can be achieved in one pot, which provides another way to synthesize symmetric and asymmetric trithiocarbonate RAFT agents. N N N N + R H R R TCDI primary thiol N N TCDI N R' H N N secondary thiol N R' R H Primary thiol R R' cheme 2.17 ynthesis of trithiocarbonate RAFT agents with TCDI. 41

65 CHAPTER Ⅲ EXPERIMENTAL 3.1 Materials Chemicals used as received Tri-n-ethylamine (Acros, HPLC grade), tri-n-octylamine (Alfa Aesar, 98%), tri-n-propylamine (Aldrich, GC), tri-n-butylamine (Aldrich, 98.5%), 4-styrenesulfonic acid sodium salt (Alfa Aesar), benzene (Alfa Aesar, HPLC grade), o-xylene (Aldrich, reagent grade), N,N -dimethyl n-octadecylamine (TCI), N,N -dimethyl n-hexadecylamine (TCI), N,N -dimethyl n-octylamine (TCI), N,N -dimethyl n-decylamine (TCI), hydrochloric acid (EMD, AC), chloroform (Fisher cientific), hexane (EMD), toluene (EMD, AC grade), methanol (MeOH, Fisher cientific, reagent grade), tetrahydrofuran (THF, EMD, AC grade), methylene chloride (EMD, AC grade), carbon disulfide (C2, Aldrich, AC reagent, > 99.9%), 1-dodecanethiol (Aldrich, 98%), p-toluenesulfonic acid monohydrate (Aldrich, AC reagent, > 98.5%) o-xylylene dibromide (TCI, GC) 42

66 N,N-dimethylformamide (DMF, Aldrich 99.9%), benzyl peroxide (Aldrich, reagent grade), Aliquat 336 (Aldrich), 2-propanol (EMD, HPLC grade), cyclohexane (Fisher cientific, AC) Purification tyrene (99%, stabilized, ACRO), tert-butylstyrene (94%, stabilized, Alfa Aesar), and vinylbenzyl chloride (97%, Aldrich) were purified by passing through a column of basic alumina. Azobisisobutyronitrile (AIBN) was purified by recrystallization in methanol, dried in a vacuum oven at room temperature and stored in a refrigerator. 3.2 ynthetic procedures of P containing block copolymers RAFT agent synthesis docecyl- -(α,α -dimethyl-α -acetic acid)-trithiocarbonate (RAFT-COOH) synthesis -1-docecyl- -(α,α -dimethyl-α -acetic acid)-trithiocarbonate (RAFT-COOH) was synthesized according to a previous reported procedure 101, as shown in cheme 3.1. The RAFT agent was purified by passing through a silica gel column using hexane as eluent. 43

67 C 2 + C 12 H 25 H + (CH 3 ) 2 CO + NaOH PTC H + C 12 H 25 COOH cheme 3.1 ynthesis of RAFT-COOH Didodecyl-1,2-phenylene-bis(methylene)bistrithiocarbonate synthesis (bis-trithiocarbonate) The bis-trithiocarbonate RAFT agent (bis-raft) was synthesized via two step phase transfer catalyzed reaction, as shown in cheme mol 1-dodecanthiol (9.05g), mol Aliquat 336 (0.72g) and 60mL toluene were added into a flask. The solution was stirred under nitrogen gas in an ice bath. 15min later, 0.046mol 50% sodium hydroxide aqueous solution (3.63g) was injected into the flask. After 15min 0.045mol carbon disulfide (3.44g, dissolved in 20mL toluene) was injected into the solution, and the color of the solution changed to yellow quickly. After stirring of 15min, 0.023mol toluene solution of o-xylylene dibromide (5.94g, dissolved in 30mL toluene) was added to the reactor. The solution was stirred at room temperature under nitrogen for 12h, and terminated by adding 100mL deionized water and stirring for 30min. The mixture was poured into a separation funnel, and yellow toluene layer was collected and washed with deionized water 3 times. The product was recovered by rotational evaporation (14g, yellow solid), and further purified by recrystallization in hexane (9.5g, yellow powder). 44

68 The 1 H NMR spectrum: 7.38ppm and 7.28ppm (4H, m, CH 2 C 6 H 4 CH 2 ), 4.70ppm (4H, s, CH 2 C 6 H 4 CH 2 ); 3.39ppm (4H, t, CH 2 CH 2 (CH 2 ) 9 CH 3 ), 1.72ppm (4H, m, CH 2 CH 2 (CH 2 ) 9 CH 3 ), 1.28ppm (36H, s, CH 2 CH 2 (CH 2 ) 9 CH 3 ), 0.90ppm (6H, t, CH 2 CH 2 (CH 2 ) 9 CH 3 ). The 13 C NMR spectrum: ppm (2C, s, 2C 3 ), ppm, ppm, and ppm (6C, s, CH 2 C 6 H 4 CH 2 ), 38.89ppm (2C,, CH 2 C 6 H 4 CH 2 ), 37.13ppm (2C, s, 2C 3 CH 2 CH 2 (CH 2 ) 9 CH 3 ), ppm (18C, m, 2C 3 CH 2 (CH 2 ) 9 CH 2 CH 3 ), 22.66ppm (2C, s, 2C 3 CH 2 (CH 2 ) 9 CH 2 CH 3 ), 14.09ppm (2C, s, 2C 3 CH 2 (CH 2 ) 9 CH 2 CH 3 ). C 12 H 25 H+ C 2 NaOH Br C 12 H 25 C Na Br C 12 H 25 C 12H 25 cheme 3.2 ynthesis of didodecyl-1,2-phenylene-bis(methylene)bistrithiocarbonate RAFT agent Alkylammonium sulfonated monomers synthesis Generally, trialkylammonium p-styrenesulfonate and dimethyl alkylammonium p-styrenesulfonate monomers were synthesized via a two step procedure. The tertiary 45

69 amine was reacted with a molar equivalent amount of hydrochloric acid to obtain the corresponding tertiary ammonium hydrochloride salt, followed by reaction with excessive sodium p-styrenesulfonate (1.2 molar ratio) in an oil-water two phase system. The alkylammonium p-styrenesulfonate monomers were located in the organic phase, and recovered after washing and drying Trialkylammonium p-styrenesulfonate monomers synthesis In a typical synthesis of trialkylammonium p-styrenesulfonate, trioctylammonium p-styrenesulfonate (-TOA) was synthesized via a two step procedure, first the synthesis of the trioctylammonium hydrochloride salt and then the synthesis of -TOA. Trioctylammonium hydrochloride was prepared based on a previously reported procedure mL trioctylamine (TOA, mol) was dissolved in 40mL hexane and placed in an ice bath followed by the dropwise addition of 5mL of concentrated HCl (0.0605mol). The solution was stored in a freezer for 4~5h and a white precipitate (trioctylammonium hydrochloride) formed. The precipitate was filtered and washed with cold hexane to remove excess HCl. The product (TOA-HCl) was dried in a vacuum oven over night (16h) at room temperature. White solid; yield 15.8 g (89%). 1 H-NMR: δ 2.95(6H, s, N-CH 2 -), 1.80 (6H, s, N-CH 2 -CH 2 -), 1.27 (30H, s, -CH 2 -), 0.89 (9H, s, CH 3 ). 10gTOA-HCl (0.0256mol) was dissolved in 50mL benzene, and 5.8g sodium p-styrenesulfonate (0.0282mol) was dissolved in 40mL deionized water. Both solutions were added to a separation funnel, mixed and allowed to settle. The top layer (benzene) 46

70 was collected and washed 3 times with deionized water. The solution was concentrated on a rotary evaporator, frozen in a freezer and then dried in a vacuum oven overnight at room temperature to yield a white solid. White solid: yield 11.5g (84%). 1 H-NMR: δ 6.70 (1H, d, CH 2 =CH-), 5.81 (H, d, CH 2 =CH-), 5.31(H, d, CH 2 =CH-), 3.02(6H, s, N-CH 2 -), 1.71 (6H, s, N-CH 2 -CH 2 -), 1.27 (30H, s, -CH 2 -), 0.89 (9H, s, CH 3 ) Dimethyl alkyl ammonium p-styrenesulfonate monomers synthesis In a typical synthesis of dimethyl alkylammonium p-styrenesulfonate monomers, dimethyl octadecylammonium p-styrenesulfonate monomer (-DMODA) was prepared by neutralization of dimethyl octadecylamine and hydrochloride acid and followed by ion-exchange with sodium p-styrenesulfonate according to a similar procedure for the preparation of trioctylammonium p-styrenesulfonate. 20g dimethyl octadecylamine was dissolved in 100mL chloroform, and 5.55mL (12.1M, 1:1 molar ratio) of a concentrated HCl aqueous solution was added dropwise to the solution. The chloroform layer was collected, and concentrated on a rotary evaporator. The dimethyl octadecylamine hydrochloride salt was recovered after drying in a vacuum oven, and a white powder was obtained (20.1g, yield 89.6%). The 1 H NMR spectrum: 2.92ppm (2H, t, NCH 2 CH 2 -), 2.74ppm (6H, s, N-(CH 3 ) 2,), 1.73ppm (2H, m NCH 2 CH 2 CH 2 -), 1.14ppm (30H, m, NCH 2 CH 2 (CH 2 ) 15 CH 3 ), 0.80ppm (3H, t, NCH 2 CH 2 (CH 2 ) 15 CH 3 ). 200mL 0.1g/mL chloroform solution of the ammonium hydrochloride salt was combined with 100mL 0.17g/mL aqueous solution of p-styrenesulfonic acid sodium salt 47

71 and stirred for 30min. The mixture was poured into a separation funnel and allowed to phase separate for 1 d and the bottom layer became transparent. The chloroform layer was collected, and dried by anhydrous sodium sulfate to obtain a clear solution. The solution was concentrated by a rotary evaporator, and dimethyl octadecylammonium p-styrenesulfonate (-DMODA) monomer was recovered by drying in a vacuum oven at room temperature, and a white powder was obtained (27.3g, yield 94.9%). The 1 H NMR spectrum: 7.86ppm and 7.43ppm (4H, d, C 6 H 4 ), 6.72ppm (1H, q, C 6 H 4 -CHCH 2,), 5.81ppm and 5.31ppm (2H, d, C 6 H 4 -CHCH 2 ), 3.03ppm (2H, t, NCH 2 CH 2 -), 2.87ppm (6H, s, N(CH 3 ) 2 ), 1.78ppm (2H, m, NCH 2 CH 2 CH 2 -), 1.26ppm (30H, m, NCH 2 CH 2 (CH 2 ) 15 CH 3 ), 0.88ppm (3H, t, NCH 2 CH 2 (CH 2 ) 15 CH 3 ) Dimethyl octadecylammonium 2-Acrylamido-2-methyl -N- propanesulfonate monomer (AMP-DMODA) synthesis AMP-DMDOA was prepared by the same method as the synthesis of -DMODA, but AMPA was neutralized with sodium hydroxide first. AMP and equal molar amount of NaOH were dissolved in deionized water and stirred for 2h to complete neutralization. Then DMODA.HCl chloroform solution was poured into the aqueous solution of AMP, the molar ratio of DMODA.HCl to AMP is 5:6. The mixture was stirred at room temperature for 1h and stayed in a separation funnel for phase separation. The chloroform layer was collected and dried with anhydrous sodium sulfate to obtain a clear solution. The solution was concentrated by a rotary evaporator, and dried in a vacuum oven at 48

72 room temperature to get AMP-DMODA RAFT polymerization of sulfonated block copolymers RAFT polymerization of P-TOA homopolymers 105 In a typical polymerization of P-TOA, 2g -TOA monomer, RAFT agent (RAFT-COOH or bis-raft agent), AIBN (1:5 molar ratio to RAFT-COOH) and 1.86 ml benzene were added to make a 2 M monomer solution. The solution and a stir bar were added to a round bottom flask sealed with a rubber septum. The solution was sparged with nitrogen for 15min in an ice bath and heated to 80 C. During the polymerization, aliquots were collected by a gas-tight syringe under nitrogen pressure. The aliquots were concentrated on a rotary evaporator and dried under vacuum at room temperature for 24h. The remaining polymer was precipitated from cyclohexane. Three polymerizations were run with RAFT agent concentrations of g/mL, g/mL, and g/mL (g RAFT/ ml benzene) to target P-TOA molecular weights of 5 kda, 10 kda, and 20 kda RAFT polymerization of alkyl ammonium p-styrenesulfonate homopolymers In a typical polymerization of P-DMODA, 10g -DMODA, bis-raft-2, AIBN (1:5 molar ratio to RAFT agent) and 21mL chlorobenzene were added to make a 1 M monomer solution. The solution was sparged with nitrogen gas for 20min and then heated 49

73 to 80 C. 8 hours later, the reaction was terminated by quenching in a water bath, and an aliquot was collected to characterize the conversion. The product was precipitated from hexane. To remove unreacted monomers from the polymer, the polymer was dissolved in 50mL of tetrahydrofuran, and 100mL deionized water was slowly added into the THF solution to precipitate the polymer. 3 polymerizations were run with RAFT agent concentrations of 1mmol/mL, 0.5mmol/mL, and 0.2mmol/mL (mmol RAFT/mL chlorobenzene) to target P-DMODA molecular weight of 10kDa, 20kDa and 50kDa. Other types of alkyl ammonium p-styrenesulfonate polymers were synthesized according to the same procedure. Dimethyl hexadecylammonium p-styrenesulfonate (-DMHDA), dimethyl dodecylammonium p-styrenesulfonate (-DMDDA), dimethyl decylammonium p-styrenesulfonate (-DMDA) and dimethyl octylammonium p-styrenesulfonate (-DMOA) were synthesized via RAFT polymerization. In a typical polymerization of the monomers, bis-raft, AIBN (1:5 molar ratio to RAFT agent) and 21mL chlorobenzene were added to make a 1 M monomer solution. The solution was sparged with nitrogen gas for 20min and then heated to 80 C. 8 hours later, the reaction was terminated by quenching in a water bath, and an aliquot was collected to characterize the conversion. The product was precipitated from hexane. To remove unreacted monomers from the polymer, the polymer was dissolved in a 50mL mixture of tetrahydrofuran and methanol (1:1 volume ratio), and 100mL deionized water was slowly added into the mixture solution to precipitate the polymer. The target molecular weight of the alkyl ammonium p-styrenesulfonate is 10kDa, 20kDa and 50kDa. 50

74 RAFT polymerization of 2-Acrylamido-2-methyl -N- propanesulfonic acid (AMPA) In a typical polymerization of PAMP-DMODA, 2g -DMODA, various amount of bis-raft, AIBN (1:3 molar ratio to RAFT agent) and 4.5mL chlorobenzene were added to make a 1 M monomer solution. The solution was sparged with nitrogen gas for 20min and then heated to 80 C. 8 hours later, the reaction was terminated by quenching in water bath, and the aliquots were collected during polymerization under positive nitrogen pressure to characterize the conversion. The product was precipitated from hexane. To remove unreacted monomers from the polymer, the polymer was dissolved in 50mL of tetrahydrofuran, and 100mL deionized water was slowly added into the THF solution to precipitate the polymer. The product was dried in a vacuum oven at room temperature for 16h ynthesis of P-TOA-b-P diblock and P-b-P-TOA-b-P triblock copolymers via RAFT polymerization P-TOA-M4.8 (Mw=4.8kDa, PDI=1.09) prepared with RAFT-COOH was used as a macro RAFT agent to synthesize P-TOA-b-P diblock copolymers. Two polymerizations were run with different P-TOA concentrations of 0.2g/mL and 0.1g/mL (g P-TOA/ ml styrene). The solutions were sparged with nitrogen for 15 min and heated to 120 C for 6h. During the polymerizations, aliquots were removed by a 51

75 gas-tight syringe under nitrogen pressure and precipitated in hexane. The samples were dried in vacuum oven at 80 C overnight. P-TOA-M4.7 (MW=4.7kDa, PDI=1.15) and P-TOA-M8.4 (MW=8.4kDa, PDI=1.25) prepared with bis-trithiocarbonate RAFT agent was used as macro RAFT agent to synthesize P-b-P-TOA-b-P triblock copolymers. Purified P-TOA was dissolved in styrene with the concentration of 0.2g/mL. The solution was sparged with nitrogen gas for 15min and heated to 120 C for 5h. During the polymerizations, aliquots were removed by a gas-tight syringe under nitrogen pressure and precipitated in hexane. The samples were dried in vacuum oven at 80 C overnight ynthesis of P-b-P-DMODA-b-P triblock copolymer via RAFT polymerization P-DMODA-10kDa and 50kDa were used as macro-raft agent to polymerize styrene in bulk to obtain P-b-P-DMODA-b-P triblock copolymers. Generally, polymer was dissolved in styrene at a concentration of 0.2g/mL, and the solution was sparged with nitrogen gas for 15mins and stirred at 120 C for 2h. The product was precipitated out of hexane and dried in a vacuum oven at room temperature for 16h Ion-exchange of P homopolymers and block copolymers From ammonium form to sodium form Alkyl ammonium polystyrenesulfonate can be converted to the sodium salt form or 52

76 other kinds of counterion forms by ion-exchange. In a typical ion-exchange reaction, 0.2g P-DMODA-N90 was dissolved in 2mL chloroform, and mixed with a 1mL 10w/v% sodium hydroxide aqueous solution for 5min. After layer separation, the color of the chloroform layer faded from yellow to colorless, and the color of the water layer changed from colorless to yellow. The aqueous layer was collected, and the polymer was precipitated by adding methanol into the aqueous solution From sodium form to ammonium form The sodium p-styrenesulfonate could also be converted to alkyl ammonium salt form. In a typical ion-exchange reaction, PNa and equal amount of DMHDA.HCl salt were mixed and stirred in the mixture of chloroform/water. The chloroform layer was collected and the polymer precipitated from hexane. 3.4 ynthetic procedures of quaternary ammonium containing block copolymers RAFT agent synthesis Dibenzyl trithiocarbonate (DBTC) synthesis Dibenzyl trithiocarbonate (DBTC) was synthesized via the reaction of equal amount of alkyl halide and carbon disulfide with cesium carbonate in polar aprotic solvents according to previously reported procedure 102, as shown in cheme 3.3. The product was 53

77 purified by passing through silica gel column eluted with hexane. 2 C 2 (1 equiv.) 2 Cl Cs 2 CO 3 1 M aprotic solvent cheme 3.3 ynthesis of DBTC RAFT polymerization of P-b-PVBC block copolymers RAFT polymerization of polystyrene Polystyrene homopolymer was synthesized by bulk RAFT polymerization. The styrene monomer and DBTC were added into a reactor with varied molar ratios to target different the molecular weights of P. The reactor was degassed three times and refilled with argon. The bulk polymerization was run at 130 C for 6hrs. The product was precipitated from methanol and dried at 80 C under vacuum P-b-PVBC-b-P triblock copolymer synthesis P-PVBC-P triblock copolymer was prepared using P as a macro-raft agent. P was dissolved in vinylbenzyl chloride at the concentration of 1g/5mL. The reactor was degassed three times and refilled with argon, after which polymerization was run at 110 C for different times to control the molecular weight of the PVBC block. 54

78 P-r-PVBC-g-trithiocarbonates synthesis Poly (styrene-r-vinylbenzyl chloride) random copolymers were synthesized via RAFT polymerization using RAFT-COOH agent. tyrene, vinylbenzyl chloride and RAFT-COOH were added into a round bottle flask, which was sealed by a rubber septum cap. The amount of monomers and RAFT agent were varied to target different molecular weight and graft densities. The mixture was sparged with nitrogen gas for 15min and stirred at 130 C for 6h. The product was recovered from hexane and dried in a vacuum oven at 80 C overnight P-r-PVBC-g-PVBC graft polymerization P-r-PVBC-g-RAFT macro-g-raft agent was dissolved in vinylbenzyl chloride at the concentration of 0.2g/1mL. The solution was sparged with nitrogen gas for 15min, and stirred at 120 C for 2h. Aliquots were collected under positive nitrogen pressure at 0.5h and 1h. The products were precipitated out of methanol and dried in the vacuum oven at room temperature Quaternization of P-PVBC block/graft copolymers The P-PVBC block copolymer was dissolved in THF (10vol %), and a 5 times excessive TEA to VBC units was added into solution. The solution was stirred at 50 C for 8hrs and the solubility of polymer decreased. Methanol (30vol %) was added to obtain a transparent solution, and the reaction continued for 8 more hrs. The product was 55

79 precipitated from hexane and dried in a vacuum oven at room temperature Crosslink of PVBC PVBC was crosslinked by grafting crosslinking agents, vinylbenzyl thiol or vinylbenzyl alcohol, onto the polymer, which will be crosslinked under heating Vinylbenzyl thiol synthesis In a typical reaction, 1g vinylbenzyl chloride, 0.61g HAc and 2.2g K 2 CO 3 (molar ratio is 1:1.1, HAc is a little excessive) were dissolved in 15mL THF, and was stirred under nitrogen for 30mins. Then 15mL methanol was injected into the solution. After stirring another 30mins, 2mL dilute HCl was injected to neutralize the solution. THF and methanol were removed by rotational evaporation. The product was extracted by chloroform. The solution layer was washed by water 3 times. The solvent was removed by rotational evaporation, and the product was recovered by drying in a vacuum oven Vinylbenzyl alcohol synthesis (1) KAc salt was prepared by neutralization of equal amount of acetic acid and K 2 CO 3. The reaction is irreversible, and the reaction was done when there is no more bubbles (CO 2 ) formed. The product KAc was purified by recrystallization in aqueous solution. (2) Vinylbenzyl acetate (VBAc) was synthesized by mixing and stirring VBC 56

80 monomer (unpurified), KAc (molar ratio 1:1.1) and DMO at 40 C for 2d. The reaction was terminated by adding deionized water, the product was extracted by ethyl acetate and dried by anhydrous Na 2 O 3. (3) VBAc (2.4g) and NaOH (1g) were mixed together in the mixture of ethanol and water (6mL: 1mL). The reactor was sparged by nitrogen gas for 20min and heated to 80 C for 2h. The reaction was terminated by cooling to room temperature, and 6mL water was poured into the reactor. The product was extracted by ethyl acetate and dried anhydrous Na 2 O 3. The brown oil was recovered by drying in vacuum oven at room temperature Grafting vinylbenzyl thiol on PVBC In a typical reaction, 0.05g vinylbenzyl thiol, 0.122g TBAI, 0.108g CsCO 3 and 2mL DMF were added into a flask, and sparged with nitrogen gas for 20mins in an ice bath. The mixture was stirred in nitrogen atmosphere for 1h, and 1mL DMF solution of PVBC (0.5gPVBC/1mL DMF) was injected, and the reaction was stirred at room temperature for 8hrs. The product was precipitated in methanol, and purified by reprecipitating in methanol. The product was dried in vacuum oven for 2h at room temperature and stored in a freezer Grafting vinylbenzyl alcohol on PVBC Generally, 0.06g vinylbenzyl alcohol monomer and 0.1g Aliquat 336 were dissolved 57

81 in 1mL toluene, and mixed with 1mL 50% sodium hydroxide aqueous solution under nitrogen. 20mins later, a 2mL toluene solution of 0.3g PVBC was injected into the reactor, and the mixture was stirred under nitrogen at room temperature overnight. The toluene layer was collected and precipitated in methanol. To purify the product, the product was redissolved in chloroform and precipitated in methanol again. The product was dried in a vacuum oven for 2h at room temperature and stored in the freezer Thermal induced crosslinking The PVBC--vinylbenzyl thioether and PVBC-O-vinylbenzyl ether were dissolved in chloroform and cast into a Teflon dash to obtain films. The film was dried in the hood by slow evaporation at room temperature, and heated to 140 C for 2h. 3.5 ynthetic procedures of multiblock copolymers Polytrithiocarbonate RAFT agent synthesis Dicarboxylic acid RAFT agent synthesis -1-docecyl- -(α,α -dimethyl-α -acetic acid)-trithiocarbonate (RAFT-COOH) was synthesized according to a previous reported procedure 101, as shown in cheme 3.4. The RAFT agent was purified by passing through the silicon column using hexane as eluent. 58

82 C 2 + CHCl 3 + (CH 3 ) 2 CO + NaOH PTC H + HOOC COOH cheme 3.4 ynthesis of, -bis(α,α -dimethyl-acetic acid)-trithiocarbonate (HOOC-RAFT-COOH) ynthesis of poly-trithiocarbonate RAFT agent The poly(trithiocarbonate) RAFT agent was synthesized by esterification of trithiocarbonate compounds containing two carboxylic acid groups and 1,6-hexanediol at a stoichiometric ratio of carboxylic acid to hydroxyl groups g, -Bis(α,α - dimethyl-α -acetic acid)-trithiocarbonate (DMATC, 7.1mmol) and 0.837g 1,6-hexanediol (7.1mmol), 3.7mL cyclohexane and varying amount of p-toluenesulfonic acid (1.42mmol (reaction 1) or 2.84mmol (reaction 2) ) were added into a round bottom flask fitted with a Dean-tark trap and condenser column, which is filled with cyclohexane to remove water produced in the reaction. The two phase mixture was refluxed at 80 C under nitrogen for 24h (reaction 1) or 36h (reaction 2). The reaction was cooled to room temperature and a brown solid was recovered, which was not soluble in cyclohexane. The product was washed with methanol three times and dried in a vacuum oven at room temperature overnight. Two polytrithiocarbonates were obtained labeled as poly-raft-1 and poly-raft-2. 59

83 3.5.2 RAFT polymerization of homopolymers and block copolymers using poly(trithiocarbonate) RAFT agent Polystyrene homopolymers Polystyrene (P) homopolymer was synthesized via bulk RAFT polymerization. In a typical polymerization, the poly-raft agent and styrene monomer were added in a round bottom flask sealed with a rubber septum. The solution was sparged with nitrogen gas for 15min and heated to 130 C. Aliquots were removed using a gas tight syringe under nitrogen. The polymerization was terminated by cooling to room temperature and the polymer was precipitated in methanol and dried in a vacuum at 80 C for 16h. Five polymerizations were run. Polymerizations with the poly-raft-1 were run at concentrations of g/mL, g/mL, g/mL, and g/mL (g poly-raft-1/ml styrene) to target polystyrene molecular weights of 5, 9.8, 19, 38 kda between the trithiocarbonate groups at 100% conversion. A polymerization with poly-raft-2 was run at a concentration of p-raft-2- in styrene of g/ml to target a polystyrene molecular weight of 9.8 kda between trithiocarbonate groups Polystyrene-b-poly (tert-butylstyrene) (P-b-PtB) multiblock copolymer For the polymerization of the polystyrene-block-poly(tert-butylstyrene) (P-b-PtB) 0.2g polystyrene homopolymer (P-20k-6h, Mn 44000) was dissolved in 1 ml tert-butylstyrene in a round bottom flask. The solution was sparged with nitrogen gas for 60

84 15 min, and then heated to 130 C for 2h. The flask was cooled to room temperature and the polymer was precipitated in methanol and dried in the vacuum oven at 80 C for 16h Poly (styrene-b-vinylbenzyl chloride) (P-b-PVBC) multiblock copolymer For the polymerization of the poly (styrene-b-vinylbenzyl chloride) (P-b-PVBC) 0.2g of polystyrene homopolymer (P-10k-6h) was dissolved in 1 ml vinylbenzyl chloride in a round bottom flask. The solution was sparged with nitrogen gas for 15 min, and then heated to 130 C for 6h. The flask was cooled to room temperature and the polymer was precipitated in methanol and dried in a vacuum oven at 80 C for 16h Reduction of Polymers containing trithiocarbonate groups Aminolysis The P homopolymers and P-b-PtB multiblock copolymer were cleaved to individual P or P-b-PtB blocks by aminolysis of the trithiocarbonate groups, as shown in cheme g P or P-b-PtB was dissolved in 0.5 ml benzene in a chlenk tube and degassed three times followed by the addition of 0.05 ml n-butylamine under positive argon pressure. The reaction was stirred for 30 minutes at room temperature. During this time the color of solution faded from yellow to colorless. The product was recovered as a white powder by freezing drying. 61

85 room temperature 30min benzene n-butylamine H cheme 3.5 Aminolysis of trithiocarbonate groups Radical reduction P-b-PVBC multiblock copolymer was cleaved to P-b-PVBC diblocks by a radical reduction reaction, shown in cheme g P-b-PVBC multiblock copolymer and 0.2 g benzyl peroxide (BPO) was added to a round bottom flask and dissolved in a mixture of 6mL toluene and 3mL 2-propanol. The flask was sealed by a rubber septum cap and the solution was sparged with nitrogen gas for 15min. The reactor was put into an oil bath at 100 C for 3h and the color of the solution faded to colorless during the reaction. The reaction was terminated by cooling to room temperature and the product was precipitated in methanol and dried in vacuum for 16h at room temperature. 100 o C 2-propanol: toluene (1:2) BPO H cheme 3.6 Radical reduction of trithiocarbonate groups 62

86 3.6 Characterization Nuclear magnetic resonance (NMR) characterization 1 H NMR spectra were measured on a Varian Mercury-300MHz spectrometer. amples were dissolved in deuterated chloroform (CDCl 3, 99.8%D, Cambridge Isotope laboratories), deuterated water (D 2 O, 99%D, Cambridge Isotope laboratories), or the mixture of deuterated chloroform and deuterated methanol (99%D, Cambridge Isotope laboratories) at concentrations of 10 mg/ml. The 1 H NMR spectra were referenced to the residual protons peak of CDCl 3 at 7.27ppm, or the residual protons peak of D 2 O at 4.75ppm. The relaxation time was 5s. 13 C-NMR spectra were measured on a Varian Mercury-300MHz spectrometer. amples were dissolved in deuterated chloroform (99.8%D, Cambridge Isotope laboratories) at the concentration of 100mg/mL ize Exclusion chromatography (EC) characterization The molecular weight and molecular weight distribution were obtained using size exclusion chromatography (EC) with THF as the eluent at a flow rate of 0.5mL/min at 35 C. These measurements were taken on a Waters 1515 HPLC instrument equipped with a differential refractometer. The molecular weights were determined by the universal calibration method, which was calibrated by polystyrene standards. For the ionic polymers containing sulfonic acid groups, a mixture of 63

87 tri-n-octylamine and THF at the concentration of 2g/100mL was used as eluent, and the column was equilibrated for 24h at the flow rate of 0.5mL/min before measurement mall angle x-ray scattering (AX) characterization AX data were collected with a Rigaku MicroMax-002+ sealed tube X-ray generator coupled with multiwire position sensitive X-ray detector at room temperature. Calibration was performed using silver behenate. The exposure time for each AX pattern was 15 min Conductivity measurements of membranes Films were prepared and the conductivity was measured by Chemsultants International company (Mentor, Ohio). The polymers were dissolved in DMF. The membranes were cast on glass panels and dried at 80 C for 2hrs, following vacuum for 30min to remove solvent. The membranes were then immersed in 1M NaOH solution for 48hrs to obtain OH - counterions. Ionic conductivity measurements were performed using CH 604 Impedance Analyzer. A voltage is applied with varying frequency 1 to 20,000 Hz and the impedance response was measured. The window panel conductivity measurement cell was immersed in distilled water and the conductivity was measured in liquid water Gelation transition temperature measurement P-DMODA (N20, N38 and N90) was dissolved in chlorobenzene, benzene, toluene or o-xylene at different concentrations, such as 2.5 %, 5%, 10% and 20% (w/v), 64

88 under heating. The hot solution in a 4mL vial was quenched in ice bath for ~10min, and the solution was converted to transparent gel. The gelation transition temperature of the gel was measured by inversion method. The gels were heated slowly in a hot plate at the rate of 2 C/15mins, and the transition temperature was determined if the gel was broken and began to flow when inversed. This measurement was repeated for 7 times to obtain the gel transition temperature Cross polarized optical microscopy The birefringence behavior was measured under transmission mode with a cross polarized optical microscopy (OLYMPU BX51). The light path is light sourcepolarizer- condenser- sample- analyzer- camera. amples were sealed in a glass capillary (diameter is 1mm) and fixed on the hot stage of the optical microscope and heated step by step at an interval of 5 C/10mins canning electron microscope (EM) characterization The bulk morphology of the freeze-dried gel sample was characterized by a scanning electron microscope (EM, JEOL-JM-7401F). The samples were frozen by liquid nitrogen, and freeze-dried under vacuum to obtain a white powder cake. In a typical freeze-drying procedure, the sample was dissolved in benzene at 80 C, and gelled in an ice bath. The benzene gel was frozen in liquid nitrogen. The frozen sample was pull vacuum below -10 C (in the dry ice bath of the mixture of 2-propanol and water). The 65

89 samples were mounted on conductive tape and sputter coated with silver for characterization Thermogravimetric analysis (TGA) characterization Thermogravimetric analysis (TGA) was performed on a TA 2950 instrument at a scan rate of 20 C/min under nitrogen atmosphere Differential scanning calorimetry (DC) characterization Differential scanning calorimetry (DC) was performed on a TA Q50 instrument under nitrogen atmosphere Fourier Transform Infrared pectroscopy (FTIR) characterization Fourier infrared spectroscopy was performed on a NICOLET-380 FTIR spectrophotometer under attenuated total reflectance (ATR) mode. An average of 32 scans was taken for each run. 66

90 CHAPTER Ⅳ RAFT POLYMERIZATION OF IONIC HOMOPOLYMER AND AMPHIPHILIC BLOCK COPOLYMER 4.1 Introduction Amphiphilic ionic block copolymers containing sulfonic acid groups have great potential application in ion-exchange resins, such as proton exchange membranes, water 10, 30, 34, purification and water desalination, and surfactants for emulsion polymerization 109, 110. One way to prepare well defined block copolymers is direct synthesis via controlled polymerization. Controlled polymerization of sodium p-styrenesulfonate (Na) has been achieved via controlled radical polymerization in aqueous solution or polar solvents, such as atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP) and reversible addition-fragmentation chain transfer polymerization (RAFT) 41, 47, 52, 111. Na can also be sequentially polymerized with another hydrophilic monomers to obtain diblock copolymers 42. However, due to the strong immiscibility between the hydrophobic 67

91 monomers and Na, it is difficult to synthesize amphiphilic block copolymers directly. To enhance the compatibility of -Na or P-Na with a second hydrophobic monomer, the p-styrenesulfonate monomer can be modified to a more hydrophobic form, which is amenable to controlled free radical polymerization with hydrophobic monomers. One approach is to convert the sulfonic acid group to sulfonate ester to synthesize the block copolymer, and the precursor can be hydrolyzed or thermally degraded back to the acid form In this chapter, an alternate method to polymerize sulfonated polymers was investigated by neutralizing p-styrenesulfonic acid with trialkylamines to produce a hydrophobic trialkylammonium p-styrenesulfonate monomer. Compared with modification of sulfonate esters, this method is very simple and avoids light sensitive and water sensitive conditions in the synthesis. The alkylammonium p-styrenesulfonate monomers have previously been copolymerized with styrene by suspension polymerization to prepare random sulfonated styrene ionomers 60. Recently, another kind of sulfonated monomer, 2-acrylamido-2-methyl-N- propanesulfonic acid (AMPA), was neutralized by tributylamine and the block copolymers of AMPA and acrylate were synthesized via ATRP polymerization 61. In this research, we prepared hydrophobic stoichiometric alkylammonium p-styrenesulfonate monomers, and investigated the controlled polymerization of the homopolymer and corresponding block copolymers. Alkylammonium p-styrenesulfonate monomers were polymerized in solution via RAFT polymerization with monofunctional trithiocarbonate and bis-functional 68

92 trithiocarbonate RAFT agents. The sulfonated polymers were used as macro-raft agents to sequentially to polymerize styrene monomers in bulk to obtain P-b-P diblock and triblock copolymers. To optimize the polymerization conditions and target well-defined architectures of the amphiphilic ionic block copolymers, varying ammonium salts were used as counterions of p-styrenesulfonate for RAFT polymerization. 4.2 Results and discussion Preparation of hydrophobic ionic monomers tri-alkylammonium p-styrenesulfonate monomers The preparation of the trialkylammonium p-styrenesulfonate monomer is shown in cheme 4.1. Tri-n-octylammonium (TOA), tri-n-butylammonium (TBA) and tri-n-ethylammonium (TEA) salts were used as counterions of p-styrenesulfonate (). With increasing the number of carbons in the alkyl chains the monomer became more hydrophobic. -TOA has good solubility in organic solvents, such as benzene, toluene, tetrahydrofuran (THF), cyclohexane and methanol at room temperature. The 1 H NMR spectrum of -TOA monomer is shown in Figure 4.1. The stoichiometric ratio of styrene sulfonic acid and TOA in the compounds is confirmed by the integration of the corresponding peak intensity. The 1 H NMR spectra of -TBA and -TEA are in Figure 4.2 and 4.3, respectively. The integrated peak intensity of CH 2 =CH and (CH 3 ) 3 of 69

93 trialkylammonium are listed on Table 4.1, in which the peak intensity of methyl groups was normalized to 9, the number of protons of methyl groups per molecule. toichiometric addition of the ammonium salt and p-styrenesulfonate was observed. By this method, p-styrenesulfonate has been ion-exchanged with different kinds of trialkyl ammonium salts to adjust the polarity and solubility of the sulfonated monomer. (a) R R N R benzene, 10 o C R + HCl N R HCl R (b) R R N HCl + R benzene/water + NaCl (aq.) R R= O 3 Na octyl O HN 3 R R butyl ethyl cheme 4.1 ynthesis of trialkylammonium styrenesulfonate monomers, -TOA, -TBA, and -TEA. 70

94 l j k i j b-f i h f d b O 3 HN ( g ) e c a 3 a i j l h g l k Chemical shift (ppm) Figure H-NMR spectrum of -TOA monomer. a b d h c O 3 HN e f cd b g h aa e g f chemical shift (ppm) Figure H-NMR spectrum of -TBA monomer. 71

95 a b d c O 3 N e f f cd b aa e chemical shift (ppm) Figure H-NMR spectrum of -TEA monomer. Table 4.1 The relative peak intensity of trialkylammonium p-styrenesulfonate. CH 2 =CH CH 2 =CH (cis-) CH 2 =CH (trans-) (CH 3 ) 3 a Molar ratio b -TOA TBA TEA a. The peak intensity was divided by 9 during calculation, which is the number of protons of methyl groups, to obtain the relative amount of methyl group per molecule. b. The ratio of monomer to alkylammonium was calculated using the average value of the three vinyl peak intensities. 72

96 ynthesis of dimethyl alkylammonium p-styrenesulfonate monomers Besides trialkyl ammonium salts other kinds of ammonium salts can be used as counterions of sulfonic acid groups also, such as dimethyl alkylammonium salts, which have two short methyl groups and a long alkyl chain. The preparation scheme of dimethyl alkylammonium p-styrenesulfonate monomers is shown in scheme 4.2. Various kinds of dimethyl alkylammonium p-styrenesulfonates, such as dimethyl n-octadecylammonium (DMODA), dimethyl n-hexadecylammonium (DMHDA), dimethyl n-dodecylammonium (DMDDA), dimethyl n-decylammonium (DMDA), dimethyl n-octylammonium (DMOA), were prepared according to the same procedures as the -DMODA synthesis. Again, the monomer became more hydrophobic with increasing alkyl chain length. These monomers can be dissolved in toluene and benzene under heating, and they could be dissolved in chlorobenzene, chloroform, THF, and methanol at room temperature. N x HCl CHCl 3 NH x Cl + O 3 Na NH Cl x CHCl 3 / H 2 O O 3 HN + NaCl (aq.) x x=6,8,10,12,14 cheme 4.2. ynthesis of dimethyl alkylammonium p-styrenesulfonate monomers. 73

97 The structure of these dimethyl alkylammonium p-styrenesulfonate monomers was characterized by 1 H NMR, shown in Figure 4.4. Due to the similar structure of the compounds, the chemical shift of the dimethyl alkylammonium p-styrenesulfonate monomers are similar, but the intensity of (-CH 2 ) x - varied due to the differences in alkyl chain length. Peak a is assigned to the protons on the unsaturated C=C bond of p-styrenesulfonate, and peak i is assigned to the protons of the methyl group at the end of alkyl chain. The stoichiometric ratio of styrene sulfonic acid and DMODA in the compounds is confirmed by the integration of intensity of peak a and peak i. From the NMR spectrum, the dimethyl alkylammonium salt was also added to p-styrenesulfonate monomer stoichiometrically. Table 4.2 shows the relative peak intensity of different kinds of dimethyl alkylammonium p-styrenesulfonate monomers. The peak intensity of i is normalized to 3, the number of protons. And the calculated molar ratio of monomer to alkylammonium is ~1, which indicates the stoichiometric addition of the complexes. 74

98 a H 2 C b c d f CH 3 g i O 3 HN CH 3 CH e h x 3 x= 15, 13, 9, 7, 5 (e) (d) (c) (b) d c (a) b a a e f h g i chemical shift (ppm) Figure H NMR spectrum of dimethyl alkylammonium p-styrenesulfonate monomers (a) -DMODA, (b) -DMHDA, (c) -DMDDA, (d) -DMDA, (e) -DMOA. 75

99 Table 4.2 The relative peak intensity of dimethyl alkylammonium p-styrenesulfonate Peak b Peak a (cis-) Peak a (trans-) Peak i a Molar ratio b -DMODA DMHDA DMDDA DMDA DMOA a. The peak intensity was 3 during calculation, which is the number of protons of methyl groups, to obtain the relative amount of methyl group per molecule. b. The ratio of monomer to alkylammonium was calculated using the average value of the three vinyl peak intensities ynthesis of P-TOA The non-polar solvent benzene was chosen as the solvent and P-TOA was synthesized at 2M monomer concentration at 80 C using -1-dodecyl- -(α,α -dimethyl-α -acetic acid)trithiocarbonate (RAFT-COOH) as the RAFT agent, and AIBN as the initiator, shown in cheme 4.3. The molecular weights of 5 kda, 10 kda and 20 kda were targeted by varying the feed ratio of monomer to RAFT agent. The amount of RAFT agent used in polymerization can be calculated as following equation: 76

100 W(RAFT) = W (monomer)/m (monomer)* M (RAFT). W is the amount of the chemical used in the reaction; M is molecular weight of the chemical. O 3 HN + C 12 H 25 C C COOH AIBN Benzene, 2M 80 o C, 8h C 12 H 25 C O 3 HN C n COOH 3 3 cheme 4.3 RAFT polymerization of P-TOA homopolymer with RAFT-COOH During the polymerization, aliquots were taken out to monitor the conversion of the polymerization, which were dried in a vacuum oven at room temperature and characterized by 1 H NMR. Because the -TOA monomer is non-volatile, the conversion of the polymerization could be determined by the ratio of residual -TOA to P-TOA. The 1 H NMR spectra for the 5kDa target molecular weight polymerizations are shown in Figure 4.5. The conversion was determined by comparing the intensity of the peak at 0.9 ppm from methyl groups of trioctylamine, which is present in both the monomer and polymer to the peaks at 5.2 and 5.8 ppm from CH 2 =CH- (unreacted unsaturated 77

101 monomers). It is obvious that the peaks from the monomers disappeared during the process of polymerization. Therefore, the actual molecular weight of P-TOA polymers can be calculated by the conversion of the polymerization according to the 1 H NMR spectrum and the target molecular weight. The calculated molecular weight of P-TOA are listed in Table 4.3. Figure 4.6 is the pseudo first-order kinetic plots for the P-TOA RAFT polymerizations. The solid lines are a linear fit to the data. The linearity of the data implies that the radical concentration is constant with time as expected for a controlled polymerization 112. There is a small induction time and retardation of the polymerization rate when the concentration of RAFT agent increased, which has been observed in other RAFT homopolymerizations h 4h 2h 1h Chemical shift (ppm) Figure H-NMR spectra of P-TOA aliquots (5 kda target molecular weight) at 1, 2, 4 and 8 h polymerization times. 78

102 ln([m]o/[m]) time (h) Figure 4.6 Pseudo first-order kinetic plots for the P-TOA RAFT polymerizations. Target molecular weights: ( ) 5000 Da ( ) 10,000 Da ( ) 20,000 Da. A bis-trithiocarbonate RAFT agent, didodecyl-1,2-phenylene-bis(methylene) bistrithiocarbonate, was also used in the RAFT polymerization of P-TOA homopolymers, which could be further used as macro-raft agent to synthesize triblock copolymers. The reaction procedure is as same as homopolymerization of P-TOA with RAFT-COOH, as shown in cheme 4.4. Molecular weights of 5kDa and 10kDa were targeted by varying the feed ratio of monomer and RAFT agent. The 1 H-NMR spectra of P-TOA 5kDa and 10kDa are shown in Figure 4.7. The peaks at 5.76ppm and 5.30ppm are from the protons on the residual monomers, and the peak at 0.90ppm is from the methyl group on the end of the alkylammonium salt, which never changed during the polymerization. o, the amount of unreacted monomer and reaction conversion can be 79

103 calculated by comparing the ratio of the integration of the peak areas. From the NMR spectra, the reaction conversion of P-TOA-5 (bis-raft) is 81mol%; the reaction conversion of P-TOA-10 (bis-raft) is 77.5mol%. The real molecular weight of the polymers could be calculated by Mw (target)* conversion% + Mw (RAFT). By calculation, the real molecular weight of two samples is 4700Da and 8400Da respectively. O 3 HN + C 12 H 25 C C H 2 H 2 C C C 12 H 25 AIBN 3 Benzene, 2M 80 o C, 8h C 12 H 25 C H 2 C n H 2 C n C C 12 H 25 O 3 HN O 3 NH 3 3 cheme 4.4 Homopolymerization of P-TOA with a bis-trithiocarbonate RAFT agent. For the NMR and EC characterization of the homopolymer polymerization products are identified by their target molecular weight as P-TOA-x, where x is the 80

104 target molecular weight in kda. b a chemical shift (ppm) b a chemical shift (ppm) Figure H-NMR spectra of P-TOA-5 (left) and P-TOA-10 (right) with bis-trithiocarbonate RAFT agent. (a) Aliquot without purification; (b) After purification Determination of polydispersity of P via size exclusion chromatography (EC) P-TOA homopolymers were dissolved in tetrahydrofuran (THF) for size exclusion chromatography (EC) characterization, but no intensity was observed from the refractive index detector during EC using THF as the eluent. This phenomenon is likely due to strong interaction between the EC column and ionic groups. In previous literature 114, a similar effect has been reported for the EC characterization of sulfonated polystyrene ionomers in THF. The interaction between the column and ionic polymers has been mediated by the addition of a small amount of low molecular weight additives, such as inorganic or quaternary ammonium salts. Different small molecule additives/thf solutions were prepared and used as eluents for the EC characterization, such as lithium 81

105 nitrate, tetrabutylammonium bromide, triethylamine (TEA) and trioctylamine (TOA) 115, 116. everal different low molecule weight additives were added into THF as eluent of the EC, and recycled through the column for 12h before measurement. The EC traces of P-TOA-20k (RAFT-COOH) changed with time and with different eluents are shown in Figure 4.8. From Figure 4.8 a, sufficient equilibration time of the column with the eluent mixture is necessary to obtain a steady system state for measurement. Different small molecule additives were added to THF to prepare the mobile phase. The P-TOA precipitated out of some eluents, such as lithium nitrate+ THF, tetrabutylammonium bromide + THF, triethylamine + THF, due to the ion-exchange of P-TOA with the additives. 1g trioctylammonium benzenesulfonate (BA.TOA)/ 100mL THF, 1g trioctylamine (TOA)/100mL THF, 2g TOA /100mL THF were used as the eluents for EC. The column was equilibrated with the eluent mixture for 12h to arrive at the steady state, and P-TOA-20k sample solutions at same concentration were measured as shown in Figure 4.13 B. The column interaction was minimized in the system of THF+2g/100mL TOA, which was chosen as eluent in the EC characterization of the P polymers. 82

106 d A B d R.I. c b R.I. (a.u.) c b a a elution time (mins) Elution Time (min) Figure 4.8 The EC traces of P-TOA-20 (RAFT-COOH) with different eluent or different time. Left: In 2g/100mL, (a) 3h, (b) 5h, (c) 9h, (d) 11h; Right: (a) pure THF, (b) THF+1g/100mL BA.TOA, (c) THF+1g/100mL TOA, (d) THF+2g/100mL TOA. Figure 4.9 and Figure 4.10 show the EC traces of P-TOA homopolymers (RAFT-COOH), and P-TOA homopolymers (bis-trithiocarbonate-raft) respectively. Table 4.3 shows the molecular weight and distribution measured by EC, as well as molecular weight measured and calculated by 1 H NMR. The standard PNa was obtained from VWR (Mn=5440, PDI is ~1.2), which was converted to TOA salt via ion-exchange. The molecular weight and distribution of P-TOA homopolymers was compared with the standard sample. The molecular weight of standard P-TOA measured by EC with P standards did not match with the molecular weight of the sample, because the hydrodynamic radius of P-TOA and P is different in THF. o, the molecular weight of P-TOA is more accurately measured by 1 H NMR. From the EC 83

107 traces, the P-TOA-5 and P-TOA-10 (RAFT-COOH) have narrow molecular weight distribution, but the polydispersity of P-TOA-20k is broader. The P-TOA-5 and P-TOA-10 prepared with bis-trithiocarbonate RAFT agent have a narrow molecular weight distribution also, which is a little higher than that of corresponding P-TOA with mono-functional RAFT agent. The polydispersity of the P-TOA-20 (RAFT-COOH) was lowered by optimizing the polymerization condition. A polydispersity of 1.34 was obtained by polymerization at 80 C in 1M benzene solution for 5h. Different polymerization conditions are listed in Table 4.4. R.I. Intensity (a.u.) (e) P-20 (d) P-10 (c) P-5 (b) P TD (a)toa Elution Time (mins) Figure 4.9 EC curves of P-TOA homopolymers (prepared with RAFT-COOH). (a) pure eluent (THF+2g/100mL TOA), (b) P standard samples (VWR) converted to TOA form via ion-exchange, (c) P-TOA-5, (d) P-TOA-10, (e) P-TOA

108 R.I. Intensity (a.u.) (d) P 10k (c) P 5k (b) P TD (a)toa Elution Time (mins) Figure 4.10 EC curves of P-TOA homopolymers (prepared with bis-trithiocarbonate RAFT agent). (a) pure eluent (THF+2g/100mL TOA), (b) P standard samples (VWR) converted to TOA form via ion-exchange, (c) P-TOA-5, (d) P-TOA-10. Table 4.3 The molecular weight and distribution measured by EC and 1 H NMR. amples Mn (EC) Polydispersity Mn (NMR) P-TOA (std) (VWR) a P-5 (RAFT-COOH) P-10 (RAFT-COOH) P-20 (RAFT-COOH) P-5 (bis-trithiocarbonate RAFT) P-10 (bis-trithiocarbonate RAFT) a. based on manufacturer reported molecular weight. 85

109 Table 4.4 P-TOA-20 (RAFT-COOH) polymerization data. amples olvent T( C) Conc AIBN:RAFT Time Mn(EC) PDI (molar raito) (h) kda 1 Benzene 80 2M 1: Benzene 80 1M 1: Benzene 80 1M 3: Benzene 80 2M 1: Benzene 80 1M 1: Benzene 65 2M 1: Chlorobenzene 80 2M 1: Chlorobenzene 100 2M 1: Chlorobenzene 100 1M 1: Another factor that may have an influence on the polydispersity of P ammonium is the ammonium counterions. TOA has three long side alkyl chains, which may hinder the addition of monomers to polymer chains due to steric effects. Other trialkylammonium salts, such as TBA and TEA, were used as counterions of p-styrenesulfonate for RAFT polymerization, which has shorter side chains and less steric hindrance but the corresponding monomers are more hydrophilic. -TBA and -TEA can be dissolved in benzene under heating. -TBA was also polymerized in benzene at 86

110 different concentrations at 80 C to target a molecular weight of 20kDa, and the EC traces of the P-TBA products are shown in Figure The molecular weight characteristics of the P-TBA polymers are listed on Table 4.5. The polymerization was terminated when the viscosity of the solution increased and the stirring of the stir bar became difficult. From Table 4.5, the reactions with higher concentration were stopped early due to the high viscosity, which decreased the conversion of polymerization and broadened the molecular weight distribution of the polymers. The polymer synthesized at 1M benzene solution has narrower polydispersity and higher conversion. Comparing the polymerizations of P-TOA-20 and P-TBA-20, the later had better control and obtained polymers with lower polydispersity, and have longer backbone length. To decrease the steric effect of counterions, polymerization of -TEA was studied also, while -TEA is more hydrophilic and has poorer solubility in organic solutions. -TEA was also polymerized in benzene solution at concentration of 1M at 80 C to target a molecular weight of 20kDa also, and the polymer came out of benzene solution after 1h due to the lower solubility, and then the polymerization was terminated by cooling to room temperature. The conversion of polymerization is 73.5%, and polydispersity of P-TEA is 1.53, which is higher than P-TBA-20kDa. o, steric effect of counterions and polarity of the monomers are two important factors for RAFT polymerization of alkylammonium p-styrenesulfonate in non-polar solvents. 87

111 (c) (b) (a) Elution Time (mins) Figure 4.11 EC traces of P-TBA polymerized in benzene solution with a conc. of (a) 2M (b) 1.5M and (c) 1M. Table 4.5 The molecular weight characteristics of P-TBA-20 in different polymerization conditions. Concentration Reaction Conversion Mn (NMR) Mn (GPC) PDI (M) time a (h) (%) 2M M M a. the reaction time depends on the viscosity of the solution RAFT polymerization of other alkylammonium p-styrenesulfonate monomers Trioctylammonium p-styrenesulfonate polymers were synthesized in non-polar 88

112 solvent with good molecular weight distribution. With the increase of molecular weight, the polydispersity became broader due to the steric hindrance effect of the TOA counterion. To optimize the polymerization condition, other kinds of ammonium salts, dimethyl alkylammoniums, were chosen as the counter ions of p-styrenesulfonate. Dimethyl octadecylammonium, dimethyl hexadecylammonium, dimethyl dodecylammonium, dimethyl decylammonium and dimethyl octylammonium were chosen. Chlorobenzene was chosen as solvent and the monomers were synthesized at 80 C at the concentration of 1M for 8h, and didodecyl-1,2-phenylene-bis(methylene) bistrithiocarbonate (bis-raft) was used as the RAFT agent. The scheme of RAFT polymerization of -DMODA is shown in cheme 4.5. The polymerization was terminated by cooling in a water bath, and an aliquot was collected and dried at room temperature in vacuum oven to characterize the conversion. Because the monomers are non-volatile, the conversion of polymerization could be determined by the ratio of residual monomers to corresponding polymers. The NMR spectra and EC traces in this section all correspond to polymers polymerized for 8h. 89

113 + C 12 H 25 C C H 2 H 2 C C C 12 H 25 O 3 HN x 80 o C, AIBN, chlorobenzene 8hrs C 12 H 25 C n n C C 12 H 25 O 3 HN O 3 HN x x X=16, 14, 10, 8, 6 cheme 4.5 The typical RAFT polymerization of dimethyl alkylammonium p-styrenesulfonate The 1 H NMR spectra of P-DMODA-50 is shown in Figure The peak at 5.76ppm and 5.30ppm are from the protons on the residual monomers, and the peak at 0.90ppm is from the methyl group on the end of the DMODA salt, which never changed during the polymerization. o the amount of unreacted monomer and reaction conversion can be calculated by comparing the ratio of the integration of the peak areas. From the NMR spectra, the reaction conversion of P-DMODA-50 is 87mol%, and the corresponding molecular weight of the P-DMODA could be calculated. The molecular 90

114 weights of the alkylammonium p-styrenesulfonate samples are listed in Table 4.6. The molecular weight distribution of the P samples were measured by EC with 2g TOA /100mL THF as eluent. The EC traces of the P-DMODA, P-DMHDA, and P-DMDDA polymers are shown in Figure 4.13, 4.14 and The corresponding polydispersity (PDI) of the dimethyl alkylammonium polystyrenesulfonate polymers are listed in Table 4.6. The steric effect of dimethyl alkylammonium salt is not as obvious as that of trialkyl ammonium salt according to Table 4.6. The polydispersity of P-DMODA, P-DMHDA, and P-DMDDA is between 1.1 and 1.26, and the RAFT polymerization is well controlled with increasing of molecular weight. The polarity of the monomers and polymers can be adjusted by choosing different dimethyl ammonium salts as counterions of p-styrenesulfonate. Intensity (a.u.) b a chemical shift (ppm) Figure 4.12 The NMR spectra of P-C18-50 (8h polymerization) (a) unpurified (monomer was not removed) (b) purified (monomer was removed). 91

115 (c) P-DMODA-50 (b) P-DMODA-20 (a) P-DMODA Elution Time (mins) Figure 4.13 EC traces of P-DMODA (a) P-DMODA-10, (b) P-DMDOA-20, (c) P-DMODA-50. (d) P-DMHDA-50 (c) P-DMHDA-20 (b) P-DMHDA-10 (a) P-DMHDA Elution Time (mins) Figure 4.14 EC traces of P-DMHDA (a) P-DMHDA-5, (b) P-DMHDA-10, (c) P-DMHDA-20, (d) P-DMHDA

116 (b) P-C12-20k (a) P-C12-10k Elution Time (mins) Figure 4.15 EC traces of P-DMDDA (a) P-DMODA-120, (b) P-DMDOA

117 Table 4.6 Molecular weight and distribution of dimethyl alkylammonium polystyrenesulfonate. amples Target degree of Target Mn Measured Mn PDI polymerization, Mn (NMR) degree of (EC) (N) (kda) (kda) polymerization (kda) P-DMODA P-DMODA P-DMODA P-DMHDA P-DMHDA P-DMHDA P-DMHDA P-DMDDA P-DMDDA P-DMDA a a P-DMOA a a a. Not measured Ion-exchange of alkylammonium polystyrenesulfonate After polymerization, alkylammonium polystyrenesulfonates can be converted to 94

118 the sodium salt form or other kinds of counterion forms by ion-exchange at room temperature to obtain hydrophilic polymers, as shown in cheme 4.5. The 1 H NMR spectrum of the ion-exchanged polymer was characterized using CDCl 3 and D 2 O as solvent respectively, shown in Figure NaOH aqueous solution/ chloroform O 3 HN R R R R.T. O 3 Na cheme 4.6 Ion-exchange reaction of alkylammonium polystyrenesulfonate. b Intensity (a.u.) a chemical shift (ppm) Figure H NMR spectra of (a) P-DMODA-50 in chloroform-d; and (b) PNa prepared by ion-exchange in D 2 O. 95

119 PNa was also converted to other alkylammonium salt forms. PNa and equal amount of DMHDA.HCl salt were mixed and stirred in the mixture of chloroform/water. The alkylammonium salts associated with P polymers, and extracted P polymers from water layer to the organic layer, and the sodium salts stayed in aqueous solution. The 1 H NMR spectrum of the ion-exchanged polymer was characterized using CDCl 3 and D 2 O as solvent respectively, shown in Figure P-DMHDA Intensity (a.u.) P-Na P-DMHDA chemical shift (ppm) Figure H NMR spectra of (a) P-DMHDA-50 in chloroform-d; and (b) PNa (ion-exchange) in D 2 O, (c) P-DMHDA (ion-exchange) in chloroform-d. 96

120 4.2.6 ynthesis of P-TOA-b-P diblock and P-b-P-TOA-b-P triblock copolymers P-b-P-TOA diblock copolymer The alkylammonium polystyrenesulfonates can be dissolved in styrene and heated directly to obtain P-b-P block copolymers. In these sections the P polymers are identified as P-XX-MY, where XX indicates the counter-ion and Y indicates the actual molecular weight in kda. The P-TOA-M4.8 (MW=4.8kDa, PDI=1.09) initiated by monofunctional RAFT-COOH RAFT agent was used as the macro chain transfer agent for the polymerization of P-TOA-b-P diblock copolymers, and polystyrene was added to P-TOA via sequential polymerization. P-TOA was dissolved in styrene monomer, and the polymerization was run at 120 C in bulk by thermal auto-initiation of the styrene monomer, shown in cheme 4.6. Aliquots were removed periodically and precipitated in hexane to obtain polymers, which were dried under vacuum and characterized by 1 H NMR spectroscopy to monitor the conversion of the polymerization. The mole fraction of polystyrene block in the block copolymers was determined by comparing the peak intensity of the terminal methyl groups of the TOA to the peak intensity of benzyl groups of styrene and styrenesulfonate. Figure 4.18 shows the pseudo first order kinetic plots at both P-TOA: styrene ratios. The data deviates from linearity after 4h, which could be due to the increased viscosity of the system, giving rise to increased irreversible termination reactions 117. The weight fraction of P was tuned by adjusting the feed ratio of P-TOA and styrene, as well as reaction time. The molecular 97

121 weight of P block in the diblock copolymers increased from 20wt% to 75wt%. C 12 H 25 C C n COOH + O 3 HN 120 o C 3 C 12 H 25 C m C COOH n O 3 HN 3 cheme 4.7 Polymerization of P-TOA-b-P diblock copolymer. Ln([M0]/[M]) P-TOA-b-P-(0.1g:1ml) P-TOA-b-P-(0.2g:1ml) Time (h) Figure 4.18 Pseudo first order kinetic plots for the P-TOA-b-P polymerizations: ( ) 0.1g P-TOA-M4.8 /1 ml styrene, ( ) 0.2 g P-TOA-M4.8/1 ml styrene. 98

122 The molecular weight and distribution of P-TOA-b-P block copolymers were characterized by EC with 2gTOA/100mL THF as eluent. The EC traces of the block copolymers are shown in Figure 4.19 a -b. The elution peaks shifted to left with increasing of reaction time, which indicates increasing molecular weights. The polydispersity of the polymers is for the P TOA:styrene ratio of 0.2 g:1 ml and for the P TOA:styrene ratio of 0.1 g:1 ml. The higher polydispersity at the lower P TOA:styrene ratio is attributed to the increased tendency towards irreversible termination reactions broadening the molecular weight distribution. The molecular weight characteristics of the block copolymers are listed in Table 4.7. The plot of molecular weight (EC) vs. conversion is shown in Figure The values of molecular weights are not quantitative, because they are determined from the universal calibration curve of P standards. But good linear fits of molecular weight (EC) vs. conversion are obtained. This linear increase of the molecular weight with time demonstrates that the P TOA macro-raft agent is able to control the polymerization of the styrene. 99

123 R.I. Intensity (a.u.) 6h 4h 3h 2h 1h 0.5h 0.25h THF+2wt%TOA 0.1gP-TOA/1ml styrene Elution Time (mins) (a) 0.1gP-TOA-M4.8 /1mL styrene P-TOA 0.2g/ml styrene R.I. Intensity (a.u.) 6h 4h 2h 1h 0.5h 0.25h THF+2wt%TOA Elution Time (mins) (b) 0.2gP-TOA-M4.8 /ml styrene Figure 4.19 EC traces of P-TOA-b-P at different feed ratio and reaction time. 100

124 EC Mn Conversion PDI Figure Mn (GPC) vs. conversion and PDI vs. conversion for P-TOA-b-P block copolymers: ( )the Mn of P-TOA-b-P-1:10, ( ) the Mn of P-TOA-b-P-1:5, ( )the PDI of P-TOA-b-P-1:10, ( ) the PDI of P-TOA-b-P-1:5. 101

125 Table 4.7 Molecular weight characteristic of P-TOA-b-P. Mn Mn(NMR) a mol% P PDI (GPC) P-TOA-b-P-0.25h (0.1g/1mL) % 1.27 P-TOA-b-P-0.5h (0.1g/1mL) % 1.30 P-TOA-b-P-1h (0.1g/1mL) % 1.23 P-TOA-b-P-2h (0.1g/1mL) % 1.25 P-TOA-b-P-3h (0.1g/1mL) % 1.28 P-TOA-b-P-4h (0.1g/1mL) % 1.29 P-TOA-b-P-6h (0.1g/1mL) % 1.28 P-TOA-b-P-0.25h (0.1g/0.5mL) % 1.15 P-TOA-b-P-0.5h (0.1g/0.5mL) % 1.17 P-TOA-b-P-1h (0.1g/0.5mL) % 1.18 P-TOA-b-P-2h (0.1g/0.5mL) % 1.16 P-TOA-b-P-4h (0.1g/0.5mL) % 1.19 P-TOA-b-P-6h (0.1g/0.5mL) % 1.19 a. The molecular weights of block copolymers are based on 4.8kDa P-TOA P-b-P-TOA-b-P triblock copolymer The P-TOA-M4.7 (MW=4.7kDa, PDI=1.15) and P-TOA-M8.4 (MW=8.4kDa, 102

126 PDI=1.25) polymerized with the bis-raft agent were also used as macro chain transfer agent for the polymerization of P-b-P-TOA-b-P triblock copolymers. P-TOA was dissolved in styrene monomer with the concentration of 0.2g P-TOA/1mL styrene, and the polymerization was run at 120 C for 5h in bulk, as shown in cheme 4.7 The mole fraction of the polystyrene block in the block copolymers was determined by comparing the peak intensity of the terminal methyl groups of TOA to the peak intensity of benzyl groups of styrene and styrenesulfonate. The 1 H NMR spectra of P-b-P-TOA-b-P with different reaction times are shown in Figure The molecular weight and distribution of the triblock copolymers were characterized by EC with 2g TOA/100mL THF eluent. The EC traces of the P-b-P-TOA-b-P are shown in Figure 4.22, which have a narrow molecular weight distribution. The molecular weight characteristics are listed in Table 4.8. P-b-P-TOA(4.7kDa)-b-P has a PDI of 1.24 and P-b-P-TOA(8.4kDa)-b-P has a PDI of 1.27, which shows the polymerization is under control and well defined P-b-P triblock copolymers were obtained. The molecular weight measured by EC is different with that calculated by NMR, due to the difference of hydrodynamic volume of P triblock copolymer and P standards. 103

127 C 12 H 25 C n C C 12 H 25 n O 3 HN O 3 HN o C 3 C 12 H 25 C m n n C C 12 H 25 m O 3 HN O 3 NH 3 3 cheme 4.8 RAFT polymerization of P-b-P-TOA-b-P triblock copolymer. (b) (a) chemical shift (ppm) Figure H NMR spectra of (a) P-TOA-M8.4 (bis-raft) and (b) P-b-P-TOA-b-P. 104

128 a b R.I. Intensity (a.u.) (b) (a) R.I. Intensity (a.u.) (b) (a) Elution Time (mins) Elution Time (mins) Figure 4.22 EC traces of a. P-TOA-M4.7 (bis-raft) and P-b-P-TOA-b-P; b. P-TOA-M8.4 (bis-raft) and P-b-P-TOA-b-P. Table 4.8 Molecular weight characteristic of P-b-P-TOA-b-P triblock copolymers. amples Macro-RAFT Reaction P P Mn a Mn PDI time (h) mol% wt% (NMR) (EC) 1 P-TOA-M % 61% P-TOA-M % 57% a. The molecular weights of block copolymers are based on P-TOA MW Ion-exchange of P-P block copolymers A P-P block copolymer was dissolved in chloroform, and the ion-exchange reaction was carried out with aqueous sodium hydroxide to convert the P-TOA block to the P-Na block to obtain an amphiphilic block copolymer. The P-Na could be 105

129 converted back to ammonium salt form also by ion-exchange. PNa-P was reacted with Aliquat 336(mixture of trioctyl- and tricaprylmethylammonium chloride) to convert the P-Na block back to the hydrophobic P-A336. Figure 4.23 shows the 1 H-NMR spectra of the P-b-P in the trioctyammonium, sodium and Aliquat 336 salt forms. Both the ammonium salts formed clear solutions in deuterated chloroform while the sodium salt form was translucent. ince P-Na is insoluble in chloroform these blocks could aggregate in chloroform. After ion-exchange to the sodium salt form the peak from the methylene protons on the alpha carbon of the trioctylamine disappears (3.5 ppm). After ion-exchanging back to the A336 salt form this peak reappears accompanied by the methylammonium peak ( ppm). Therefore, the ionic groups in these polymers can be exchanged to different forms from the starting TOA salt form. c b a chemical shift (ppm) Figure H NMR spectra of (a) P-TOA-b-P, (b) P-Na-b-P and (c) P-A336-b-P. 106

130 4.2.7 ynthesis of P-b-P-DMODA-b-P triblock copolymers P-DMODA homopolymers synthesized with a bisfunctional trithiocarbonate were used as maro-raft agents to polymerize with styrene to obtain P-b-P-DMODA-b-P triblock copolymers, as shown in cheme 4.8. In a typical polymerization, P-DMODA-M43.5 (MW=43.5kDa, PDI=1.26) was dissolved in styrene at the concentration of 0.1g/mL or 0.2g/mL, and polymerized in bulk at 120 C. The EC traces of P-DMODA homopolymers and corresponding triblock copolymers are shown in Figure The molecular weights of the triblock copolymers are listed in Table 4.9. The polydispersity of P-b-P-DMODA triblock copolymers is broader, but the EC curve of the triblock copolymer is a single peak shifted to lower elution time region, indicating the extension of P-DMODA chain after polymerization with styrene. There is a tail on the right side the peak, which should be due to retardation in RAFT polymerization, which increased the polydispersity of the triblock copolymer. The RAFT polymerization lost control to some extent for higher molecular weight macro-raft agent, while polystyrene propagated on P-DMODA chain and the triblock copolymer was obtained. 107

131 C 12 H 25 C n n C C 12 H 25 O 3 H N O 3 H N x 120 o C x C 12 H 25 C m n n m C C 12 H 25 O 3 H N O 3 H N x=16, 14, 10 x x cheme 4.9 ynthesis of P-b-P triblock copolymers Elution Time (mins) Figure 4.24 EC traces of P-DMODA-M18.3 (solid line) and P-b-P-DMODA-b-P triblock copolymer (dash line). 108

132 Table 4.9 The molecular weight characteristics of P-b-P-DMODA triblock copolymers. s Macro-RAFT Reaction Reaction Feed Mn a Mn PDI temperature time ratio (NMR) (EC) ( C) (h) (g/ml) 1 P-DMODA-M P-DMODA-M P-DMODA-M a. The molecular weights of block copolymers are based on P-DMODA MW RAFT polymerization of other monomers containing sulfonic acid groups. The method of neutralizing the sulfonic acid with ammonium salt to obtain hydrophobic monomers is not only suited to p-styrenesulfonate, but also other kinds of sulfonated monomers. 2-Acrylamido-2-methyl-N-propanesulfonic acid (AMPA) was neutralized by the same way to obtain the ammonium salt form and synthesized via RAFT polymerization. AMPA is the acid form, and was converted to sodium salt form by reacting with sodium hydroxide in aqueous solution, and converted to ammonium salt form via ion-exchange. The reaction scheme is shown in cheme 4.9. The 1 H NMR spectrum of the AMP-DOMDA is shown in Figure

133 HN O NaOH aq. HN O HO 3 NaO 3 HN NaO 3 O + chloroform/water HN NH Cl 16 O 3 H N O + NaCl 16 cheme 4.10 ynthesis of AMP-DMODA monomers. a b O N H d O 3 HN c e g 15 h a b c d f e g h chemical shift (ppm) Figure H NMR spectrum of the AMP-DOMDA. Didodecyl-1,2-phenylene-bis(methylene) bistrithiocarbonate (bisfunctional 110

134 trithiocarbonate) was used as RAFT agent to synthesize PAMP-DMODA, and AIBN was used as initiator. AMP-DMODA was dissolved in chlorobenzene at the concentration of 1M, and stirred at 80 C for 8h. The ratio of monomer and RAFT agent were varied to target molecular weights of 10kDa and 20kDa. AIBN was used as the initiator, and the molar ratio of AIBN to RAFT agent was 1:3. Figure 4.26 shows the EC traces of the PAMP-DMODA homopolymers, and molecular weights and polydispersities are listed in Table It is obvious that the RAFT polymerization is well controlled for low molecular weight PAMP-DMODA (10kDa target MW), the polydispersity of which is But with increase of molecular weight, the polymerization lost control to some extent, and the polydispersity of PAMP-DMODA (20kDa target MW) increased to (b) (a) Elution time (min) Figure 4.26 EC traces of PAMP-DMODA homopolymers (a) PAMP-DMNODA- 10, (b) PAMP-DMODA

135 Table 4.10 Molecular weight characteristics of PAMP-DMODA. amples Target Mn Polymerizati Mn Polydispersity Mn (kda) (NMR) on degree (EC) PAMP-DMODA PAMP-DMODA Conclusion In this chapter, ionic homopolymers and block copolymers of polystyrene and polystyrenesulfonate ammonium salts were synthesized successfully. The sodium p-styrenesulfonate monomers were neutralized by alkyl ammonium salts to obtain hydrophobic sulfonated monomers, which have good solubility in non-polar solvents and good miscibility with hydrophobic monomers, such as styrene. Polystyrenesulfonate ammonium salts were synthesized by RAFT polymerization with controlled molecular weight and distribution, and further were used as macro-raft agents to polymerize styrene to obtain P-b-P diblock and triblock copolymers. Well defined diblock and triblock polymers were obtained with controlled molecular weight and mole fraction. The P ammonium salts can be converted to the sodium salt form by ion-exchanging after polymerization to obtain amphiphilic block copolymers. This is a simple and facile way to obtain amphiphilic ionic block copolymers. Different kinds of alkyl ammonium salts were chosen as counterions of 112

136 p-styrenesulfonate for RAFT polymerization in non-polar solvents, such as tri-n-octylammonium (TOA), tri-n-butylammonium (TBA), tri-n-ethylammonium (TEA), dimethyl octadodecylammonium (DMODA), and dimethyl hexadecylammonium (DMHDA). RAFT polymerization of P-TOA began to lose control when the target molecular weight of the polymer increased, which may be due to the steric effect of the counterions. Polystyrenesulfonate homopolymers with narrow polydispersities and higher molecular weight can be obtained by using TBA and dimethyl alkylammonium salts as counterions. The method to modify the polarity of the sulfonated monomers by ion-exchanging with alkylammonium salts can be applied for other kinds of sulfonated monomers, such as AMP. Therefore, the approach can be generally applied for preparation of amphiphilic ionic block polymers containing sulfonic acid groups. 113

137 CHAPTER Ⅴ THERMO-REVERIBLE GELATION BEHAVIOR OF DIMETHYL ALKYL AMMONIUM ALT OF POLYTYRENEULFONATE (POLYELECTROLYTE-URFACTANT COMPLEXE) 5.1 Introduction Polyelectrolyte-surfactant complexes (PE-URFs) composed of a polyelectrolyte backbone and an oppositely charged low molecular weight surfactant have attracted considerable interest during the last several decades as self-assembling materials Their self-assembly, driven by simultaneous electrostatic and hydrophobic interactions, offers a facile way to tailor the morphology of these materials and fabricate multifunctional materials for industrial and biological applications, such as micro-templates, separation membranes, micro-reactors, and drug carriers 124, 125. Depending on the ratio of ionic groups of polyelectrolyte to surfactants, the complexes could be stoichiometric or nonstoichiometric. The strictly stoichiometric PE-URFs exhibited especially interesting materials properties 126,

138 In general, the preparation of stoichiometric PE-URFs was accomplished by the precipitation of polyelectrolytes and surfactants in aqueous solution. For example, sodium polystyrenesulfonate and 1.5 excess of surfactant (e.g. cectyltrimethylammonium chloride salt) was mixed and precipitated out of water 127, 128. The stoichiometric PE-URFs can form a variety of highly ordered mesophases in bulk, such as lamellar, cylindrical, and undulating layered structures 127, 129. The actual structure is primarily determined by the volume fractions of the ionic mesophase and the absolute amount of interface, temperature, and some other factors of polyelectrolytes and surfactants, such as the cross-section areas of the paraffinic chains, the hydrated polar head groups and conformation of the alkyl tails 11. The various combinations of polyelectrolyte and surfactants systems (backbones, ionic groups, hydrophobicity, chain length of surfactant, and etc.) offers a way to finely tailor the nanostructure and related functional properties of these materials 130, 131. The self-organization behavior of PE-URFs to form ordered mesophases also drives the formation ordered mesophases in solution, which were applied in a wide areas, from paints and detergents to drug and gene carriers 132. As early the 1950 s, the self-association behavior in aqueous solution of an amphiphilic polyelectrolyte of poly(-2-vinylpyridine) quaternized by dodecyl bromide was studied 133. Up to now, the main study of the behavior of PE-URFs in solution has mainly focused on aqueous solutions 134. The interactions between water-soluble polymer and surfactants in aqueous solution have been focused on also, as well as association, micellation and the 115

139 viscoelastic properties of these systems 135, 136. The water-soluble PE-URFs are usually at nonstoichiometric charge ratios, which contain excessive amount of polyelectrolyte or surfactants 137. When the concentration is much lower than the critical micelle concentration (CMC) 138, the complexes will not form micelles. While at higher concentration, micelles with a hydrophobic core will form to lower the entropy due to the driving force of the hydrophobic effect 139, 140. For PE-URFs, the hydrophobic chains of polymers and surfactants form mixed micelles together. The mixed micelles could work as physical crosslinks to obtain intrachain aggregation 141, and a gel may form under the proper conditions by thermal reorganization of the surfactants and polyelectrolytes 142, 143. With increasing concentration, the polyelectrolyte-surfactant complexes may change from isolated chains to micelles, to aggregates, to extended networks, and to macroscopic percolated structures at very high concentration 144. Iliopoulos et.al. studied the aqueous solution of cationic surfactants and anionic hydrophobically modified polyelectrolytes. In aqueous solution, the hydrophobic side chains of polymers and surfactants could aggregate together form mixed micelles due to the hydrophobic effect in aqueous solution. The polymer chains could be crosslinked by the mixed micelles to form aggregates or gels 141. Hansson also described the gelation behavior induced by reorganization of micelles, and the hydrophobic aggregates acted as physical crosslinks 123. It was found that the polyelectrolyte-surfactant complexes formed by a stoichiometric charge ratio of polyelectrolyte and surfactants are water-insoluble, and could be soluble and stable in organic solutions 120, 127, 129, 145. Therefore, it is expected that 116

140 the amphiphilic PE-URFs could also form ordered mesophase structures in non-polar organic solvents. The behavior of polyelectrolyte-surfactant complexes in organic solvents has been studied by some groups, which focused on the viscosity of the solution and the chain conformations in solution. The solubility of the complexes in organic solutions is determined by the hydrophobicity of the complexes, the polarity of the solvents and the conformation of polymer chains in solution 120, 146, 147. The polyelectrolyte-surfactant complexes in polar organic solution has an ionomer-like viscosity curve, and the aggregation of salt groups may occur 148. Lokshin et.al. reported the complexation of polystyrenesulfonate or DNA polyanion and surfactants, dioctadecyldimethylammonium (DODA), in chloroform, which is not protonated in low polarity solvents 149. The compensation of ionic charges by hydrophobic surfactant counterions makes the PE-URFs water-insoluble and soluble in a variety of organic solvents, such as chloroform, THF and benzene 150, 151. Bakeev et.al studied the behavior of nonstoichiometric polyelectrolyte-surfactant complexes in low polarity solvents 151. In the polycation and anionic surfactant complex, the excessive cation groups on the polymer chain aggregate in the low polarity solvents, while hydrophobic segments of the complexes provide the solubility of the complexes in low polarity organic solvents 151. o, the polyelectrolyte-surfactant complexes keep the properties of amphiphilic architectures in low polarity organic solvents, and it is possible for them to form some self-assembled ordered nanostructures in low polarity organic solution driven by electrostatic interactions and hydrophobic interactions. In aqueous solutions, the hydrophobic 117

141 interaction is the main driving force to form ordered structures, however, in low polarity organic solvents, electrostatic interactions should be the main driving force 150. To author s knowledge, there is not any literature discussing the gelation behavior of PE-URFs in non-polar organic solvents. Recently, Wei Wang et.al. chose poly (urethane amide) dendron with long alkyl chains as organogelators to gel toluene by the driving force of hydrogen bonding and hydrophobic interactions. And they found that the complex of poly (urethane amide) dendron with long alkyl chains (containing a carboxylic acid groups, like surfactants) and opposite charged polyelectrolytes shows higher gelation efficiency, including accelerated gelation rate and lower minimum gelation concentration. They attribute the enhanced gelation ability to the dendron bonded on the backbones of polyelectrolytes and influenced by a positive macromolecular effect in the ordering process of aggregate formation and growth This chapter covers the gelation behavior of polyelectrolyte-surfactant complexes in non-polar solvents, where the macromolecular backbone is a strongly hydrophilic polyelectrolyte and the side chains are tertiary ammoniums containing long hydrophobic alkyl chains as the counter ions of the polyelectrolyte. The polyelectrolyte and surfactants are at stoichiometric charge ratio by their preparation method through the polymerization of the electrolyte-surfactant complex monomers. The oppositely charged surfactants are bonded on the polyelectrolyte backbones as side chains by electrostatic interaction, and the polyelectrolyte-surfactant complexes should form comb-shaped, macromolecular amphiphiles in dilute, nonpolar solutions. With increasing concentration, the salt groups 118

142 should aggregate to form core driven by electrostatic interaction, and the side chains should extend in the solution like reverse micelles. 5.2 Result and discussion Dimethyl octadecylammonium polystyrene sulfonate (P-DMODA) organogelator The synthesis and characterization of dimethyl octadecylammonium polystyrene sulfonate (P-DMODA) was discussed on Chapter 4. Well defined P-DMODA polymers with narrow polydispersity of 1.18 ~ 1.26 were successfully prepared via RAFT polymerization. The molecular weight of P-DMODA were tuned by adjusting the ratio of monomer and RAFT agent to target molecular weight, and P-DMODA samples with degree of polymerization of 20, 38, and 90, were obtained respectively. As a kind of polyelectrolyte -surfactant complex, P-DMODA has a hydrophilic backbone (P) and hydrophobic side chains (alkyl chains), as shown in cheme 5.1. The solubility of the P-DMODA polymers was characterized, shown in Table 5.1. With the DMODA counterion, the water-soluble polyelectrolyte backbone will not be dissolved in water, and its solubility in organic solvents varied according to polarity of the organic solvents. P-DMODA has good solubility in the solvents with relatively higher polarity, such as tetrahydrofuran, and chloroform. But in the solvents with relatively lower polarity, such as toluene, benzene, o-xylene, P-DMODA could be dissolved only under heating. When the solution cools, the polymer does not precipitate out but forms an organogel. 119

143 The organogel is transparent and thermo-reversible, which is shown in Figure 5.1, and the heating temperature depends on the solubility in selective solvents and boiling point of the solvents. The light yellow color of the gel is due to the trithiocarbonate groups on the ends of the macromolecular chains. C C 12 H 25 CH 2 m m C C12 H 25 O 3 HN O 3 HN cheme 5.1 The structure of P-DMODA organogelator. 120

144 Table 5.1 The solubility of P-DMODA polymers in different solvents. olvent hexane o-xylene toluene styrene benzene chlorobenzene chloroform tetrahydrofuran methanol water solubility no on heating (140 C) on heating (110 C) on heating (80 C) on heating (80 C) yes, (P-DMODA-N90 on heating, 80 C) yes yes on heating (40 C) no a b c d e f g h Figure 5.1 Organogel of P-DMODA-N38: (1) in benzene solutions with different concentrations: (a) 2.5%, (b) 5%, (c) 10%, (d) 20% (w/v); (2) 10% concentration in (e) benzene, (f) styrene, (g) toluene, (h) o-xylene. 121

145 5.2.2 Thermo-reversible gelation behavior To characterize the gelation behavior of the thermo-reversible gel, the gelation transition temperatures were measured. One way to measure the transition temperatures is determing the temperature where the gel begins to flow. P-DMODA was dissolved in different solvents under heating at the various concentrations from 2.5% to 20% (w/v). The solution was gelled in ice bath, and the gel was heated at the rate of 2 C/15min and the vial was inverted periodically to determine the temperature that the gel began to flow. The measurement for every sample was repeated for 7 times, as shown in Figure 5.2, 5.3 and 5.4. Figure 5.2 shows the gel transition temperatures of P-DMODA benzene gel at different concentrations, 2.5%, 5%, 10%, and 20% (w/v). For all three samples with different polymerization degree, the gel transition temperatures increased with increasing concentration, however, the slope decreased with increasing concentration. With increasing degree of polymerization, the gel transition temperatures also increased. One reason for this molecular weight tendency is that as degree of polymerization (N) increases, there are more sulfonate groups, as well as more dipole interactions and corresponding more physical crosslinks per polymer chain, which leads to higher stability of the gels. It can also be explained by entropy of mixing, the less degree of polymerization is, the more entropy gained by forming homogeneous solution, and the transition temperature decreased. 122

146 T gel ( o C) (a) 0% 5% 10% 15% 20% c (w/v) Figure 5.2 Gel transition temperatures of P-DMODA benzene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v). ( ) P-DMODA-N20, ( ) P-DMODA-N38, ( ) P-DMODA-N90. Figure 5.3 and Figure 5.4 shows the gel transition temperatures of toluene gels and o-xylene gels respectively. It is as same as that of benzene gels that transition temperatures increased with increase of concentration or degree of polymerization. 123

147 T gel ( o C) Figure 5.3 Gel transition temperatures of P-DMODA toluene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v) ( ) P-DMODA-N20, ( ) P-DMODA-N38, ( ) P-DMODA-N90. (b) 5% 10% 15% 20% c (w/v) T gel ( o C) 140 (c) % 5% 10% 15% 20% c (w/v) Figure 5.4 Gel transition temperatures of P-DMODA o-xylene gels at the concentration of 2.5%, 5%, 10%, 20% (w/v) ( ) P-DMODA-N20, ( ) P-DMODA-N38, ( ) P-DMODA-N

148 Under same conditions, o-xylene gels have higher transition temperature, which should be related with polarity of solvents. o-xylene is less polar than toluene and benzene, and the dipole interactions of ionic pairs should be stronger in o-xylene, leading to stronger physical crosslinks and corresponding more stable gels. For more polar organic solvents, it is more difficult to form gels. For example, only P-DMDOA-N90 formed gels in chlorobenzene at the concentration of 10w/v% at the temperature of ~20 C. The solubility parameters of typical organic solvents used in the experiment were listed in Table Table 5.2 olubility parameters of some organic solvents. olvent olubility Parameter (MPa)1/2 Chlorobenzene 19.4 Benzene 18.8 Toluene 18.2 Xylene Birefringence of the thermo-reversible organogel The ordered structure driven by self-assembly of amphiphilic polyelectrolyte-surfactant complexes in organic solvents should exhibit birefringence due to their anisotropic structures. This was demonstrated by polarized optical microscopy. Figure 5.5 shows the images of pure toluene and the corresponding gel under crossed polarizers. Both the solvent and gel were sealed in glass capillaries respectively, and 125

149 Figure 5.6 (1) shows the images of toluene solvent and gel without crossed polarizer, and (2) shows the images with crossed polarizer. Compared with pure toluene solvent, the toluene gel showed obvious birefringence. a b a b (1) (2) Figure 5.5. Two glass capillaries sealed with (a) 10wt/v% toluene gel of P-C18-N38 and (b) pure toluene. They are characterized by optical microscope (1) under normal lens; (2) under crossed polarizer. The P-DMODA-N90 toluene gel (10w/v%) was heated on a hot plate, and the birefringence is always present when the temperature is below 100 C. When the temperature increased to 110 C, the transmitted intensity decreased rapidly, indicating the thermal transition from ordered mesophase to isotropic solution, as shown in Figure 5.6. When the solution was cooled and the temperature dropped lower than 100 C, the sample became birefringent again, indicating the reversible transition from solution to a gel. The birefringence behavior of the gels indicated the ordered self-assembly structure of the polyelectrolyte-surfactant complexes in solution, and could be another way to test the gel 126

150 transition temperatures. Heating 25 C 80 C 90 C 100 C Heating Cooling 25 C 110 C Figure 5.6. The birefringence behavior of the P-C18-N90 10wt/v% toluene gel at different temperatures. 127

151 5.2.4 canning electron microscope (EM) characterization To investigate the bulk structure of the P-DMODA gel, scanning electron microscopy (EM) was employed. The P-DMODA polymers were dissolved in benzene to obtain the gel and freeze-dried to keep the original bulk morphology of the gel. The gel was frozen by liquid nitrogen, and dried under frozen state. A white powder was obtained after freeze-drying, which kept the shape of the gel with a little shrinkage on the surface. There are mainly two kinds of bulk morphology formed in the P-DMDOA benzene gel samples, rod like or plate like networks, as shown in Figure 5.7. The morphology should be related with the concentration of solution and the degree of polymerization of P-DMODA. The polyelectrolyte-surfactant complexes formed micelles or bilayers driven by self-assembly of amphiphilic architectures, and extended to form three dimensional networks. (a) P-DMODA-N38 gel 100mg/ml in benzene ( to be continued) 128

152 (b) P-DMODA-N38 gel 200mg/ml in benzene (c) P-DMODA-N90 gel 100mg/ml in benzene Figure 5.7 The EM images of the bulk morphologies of the PE-URFs organogels. 5.3 Conclusion Polyelectrolyte-surfactant complexes are a class of polymers generated by neutralizing a polyelectrolyte with an oppositely charged surfactant. It has been found that polyelectrolyte-surfactant complexes composed of polystyrene sulfonate and long 129

153 chain alkyl dimethyl amines act as good organogelators for a range of hydrophobic, organic solvents, such as toluene, chlorobenzene, o-xylene, styrene, benzene, and so on. This is because of the amphiphilic characteristic of the polyelectrolyte-surfactant complexes, which have hydrophilic backbones containing ammonium sulfonate ion pairs and comblike hydrophobic long alkyl chains. Thermo-reversible gelation behavior of dimethyl octadecylammonium polystyrenesulfonate (P-DMODA) in non-polar organic solvents was investigated. The organogels were formed by heating and cooling P-DMODA solutions. The gel transition temperature was measured by tilt inversion method, where the transition was dependant on the degree of polymerization, the length of the alkyl side-chain, the solubility parameter of the solvent, and the concentration of the gelator. The birefringence of the organogel due to the anisotropic properties of the gel indicated a self-assembled ordered structure. The bulk morphology of the gel was investigated by scanning electron microscopy characterization of freeze-drying samples. The images show the formation of rod like or plate like three dimensional network morphologies depending on the system parameters. This behavior is consistent with gelation driven by the self-assembly of the amphiphilic polyelectrolyte-surfactant complexes into micellar networks. The gelation behavior should be tailorable by choosing proper counterions, not only alkyl tertiary ammonium salts, but quaternary ammonium salts, alkyl imidazolium salts and so on to vary the solubility and ionic interactions. 130

154 This effect was observed where gels were formed in polyelectrolyte-surfactant complexes dissolved in non-polar aromatic solvents. The gelation ability of the organogelator could be adjusted easily by tailoring the polyelectrolyte backbone or oppositely charged surfactants. o it is a facile way to gel various non-polar organic solvents by choosing the proper PE-URFs. The PE-URFs organogels may be used as templates or nanoreactors to prepare nanomaterials with especial mesophase structures, like the layer-by-layer self-assembled polyelectrolyte multilayer templates 154. For example, silica nanoparticles prepared through sol-gel process can be processed by using PE-URFs as template to obtain a noodle shape 155. They can also be used to prepare nanoporous materials, because the tertiary ammonium salts can be replaced with other smaller counterions by ion-exchange easily. 131

155 CHAPTER Ⅵ AMPHIPHILIC COPOLYMER CONTAINING QUATERNARY AMMONIUM GROUP PREPARED BY POT-POLYMERIZATION MODIFICATION 6.1 Introduction Cationic amphiphilic block/graft copolymers containing quaternary ammonium groups are receiving more and more attention due to their application in anion-exchange resins or alkaline anion-exchange membranes for direct methanol fuel cells (DMFC). Three main challenges that limit the application of DMACs is methanol crossover, expensive catalysts, and slow electrode kinetics 156. Alkaline fuel cells (AFC) have lower methanol permeability than proton exchange membrane fuel cells and faster reactivity at the electrodes, as well as inexpensive catalysts. To avoid the formation of carbonates, as occurs in alkaline aqueous solutions during operation, solid polymer anion exchange membranes are a good candidate for AFCs. However the diffusion coefficient of OH - in ion-conducting membranes is lower than H +, which is the challenge for application of alkaline fuel cell membranes 157, 158. The morphology of membranes has important influence on the transport properties of the membranes, which may be one way 132

156 to enhance the conductivity of OH -. Currently, quaternary ammonium salts are the most commonly reported cations for application of AFC membranes, and poly(vinylbenzyl chloride) (PVBC) is one of the main precursors of the cationic ionomers, which are quaternized by tertiary alkyl amines to form quaternary ammonium groups 159. One method to prepare anion-exchange membranes is grafting PVBC chains onto surface of hydrophobic films via radiation polymerization. In lade et.al s work, PVBC was grafted onto the surface of PVDF and FEP films via radiation-grafting, and further quaternized to quaternary ammonium salts 65, 66. But, radiation-grafting can not control the architectures of the macromolecules and the morphologies of the film finely, and the PVDF backbones show some degradation during the quaternization step. This degradation can be avoided by using fully fluorinated polymer FEP, but it is much more expensive. During the study of ion-conducting block copolymers in proton exchange membranes in the last decade, the benefits of tunable morphologies of block copolymers on the proton conductivity and proton/methanol selectivity improvement have gotten increasing attention 160. It should also be a good strategy to use cationic amphiphilic block/graft copolymers in AFC s membranes to enhance the conductivity and selectivity towards hydroxide ions. The amphiphilic block copolymers will microphase separate to form ionic, hydrophilic domains, which percolate to form ionic channels, and hydrophobic glassy domains, which can confine the swelling of the hydrophilic domains as physical crosslinks. Multiblock copolymers, like triblock copolymer chains, form 133

157 bridging conformations if two end hydrophobic blocks aggregate in two separate domains 161. Currently, there are only a few papers investigating hydroxide ion conducting block copolymers And there are also only a few papers investigating the synthesis of PVBC quaternary ammonium based cationic amphiphilic block copolymers with well-defined architectures In this chapter, the synthesis of quaternized poly (styrene-b-vinylbenzyl chloride) (P-b-PVBC) amphiphilic ionic copolymers with well-defined architectures, such as triblock, pentablock, heptablock, and graft copolymers, was studied by controlled polymerization. P-b-PVBC block copolymer precursors were synthesized by RAFT polymerization, followed by quaternization with tertiary amines. The application of quaternized poly (styrene-b-vinylbenzyl chloride) (P-b-PVBC) amphiphilic ionic copolymers as AFC membranes was also investigated. P-g-PVBC graft copolymers were prepared by using P-r-PVBC random copolymers synthesized via RAFT polymerization as the backbone and grafting trithiocarbonate pendants on the backbone to form macro-g-raft agents. The RAFT agents could be attached via either the R group (R-approach) or the Z group (Z-approach), shown in cheme The side chains from the R group attachment will not be cleaved easily, compared with the graft copolymers formed via Z-approach. But, the termination reactions of grafting via R-approach will result in coupling of different polymer backbones, while polymer backbones will not couple with each other in the synthesis of graft polymers via Z-approach. The increased amount of side chains will lead to increase 134

158 of backbone-backbone coupling termination in graft polymer synthesis. In this project, a branched polystyrene from poly(styrene-co-vinylbenzyl chloride) was synthesized via the R group approach to obtain stable graft copolymers in basic environments. Z group approach Monomers R R R R R group approach Monomers Z Z Z Z cheme 6.1 ynthesis of graft polymers via RAFT, employing the Z group approach or the R group approach. In the preparation of the backbone of graft copolymers, polystyrene-co-poly(vinylbenzyl chloride) (P-co-PVBC) random copolymers were synthesized by RAFT polymerization, and trithiocarbonates were grafted onto the PVBC units by the R-approach. Therefore, the cleaving of trithiocarbonates will not lead to 135

159 degradation of the graft copolymers. The P-co-PVBC-g-RAFT was used as macro-raft agent to synthesize P-co-PVBC-g-PVBC. The graft density could be controlled by adjusting the amount of PVBC in P-co-PVBC random copolymers. The main challenges to obtain amphiphilic ionic block copolymers in this approach are architecture control and full conversion of quaternization reaction. The factors controlling the reactivity of quaternization should be the strength of nucleophile, specific steric effects, and the proper solvent system during modification. To achieve full conversion, varying kinds of tertiary amines were chosen as quaternization agents, including tri-n-ethylamine, tri-n-propylamine, tri-n-butylamine and tri-n-octylamine. And proper solvent system was chosen to be suitable to the solubility change during reaction, which is helpful to enhance reaction conversion also. 6.2 Results and discussion Dibenzyl trithiocarbonate (DBTC) synthesis Dibenzyl trithiocarbonate (DBTC) was synthesized according to the procedure reported previously 102. The 1 H NMR spectrum of DBTC is shown in Figure

160 b a b a chemical shift (ppm) Figure H NMR spectrum of dibenzyl trithiocarbonate P-b-PVBC triblock copolymers Poly (styrene-b-vinylbenzyl chloride-b-styrene) (P-b-PVBC-b-P) triblock copolymers were prepared by a two step RAFT polymerization. Polystyrene (P) homopolymers were synthesized at 130 C in bulk for 6h, using dibenzyl trithiocarbonate (DBTC) as the RAFT agent. Then the polystyrene homopolymer was dissolved in vinylbenzyl chloride at the ratio of 1g:5ml macro-raft agent, and the polymerization was performed at 110 C for varying time, initiated by thermal initiation. cheme 6.2 shows the RAFT polymerization steps of the P-b-PVBC-b-P triblock copolymers. 137

161 + m m n m m n Cl Cl Cl cheme 6.2 The RAFT polymerization steps of P-b-PVBC-b-P triblock copolymers. Figure 6.2 shows the molecular weight and polydispersity of P homopolymers polymerized at 130 C for 6h. The conversion of P homopolymerization is ~50% at 130 C for 6h, and the molecular weight is controlled by varying the ratio of monomer to RAFT agent. The RAFT polymerizations were well controlled with the polydispersity of around 1.2. From size exclusion chromatography (EC) curves, shown in Figure 6.3, P-b-PVBC-b-P triblock copolymers were successfully synthesized via RAFT polymerization. 138

162 M n 3x10 4 2x10 4 1x10 4 M n PDI PDI Conc. (g/ml) Figure 6.2 The molecular weight and polydispersity (PDI) of P homopolymers with different feeding ratio (RAFT / monomer). The P-31kDa and P-20kDa were used as macro-raft agents to polymerize vinylbenzyl chloride to obtain P-b-PVBC-b-P triblock copolymers. The EC traces of the P homopolymers and their block copolymers with PVBC are shown in Figure 6.3. The molecular weight of two P-b-PVBC-b-P triblock copolymer samples is 15.5kDa-14kDa-15.5kDa and 10kDa-11kDa-10kDa respectively. Both of them have a molecular weight distribution of 1.29, which shows the two step RAFT polymerizations were under control and well-defined P-b-PVBC-b-P triblock copolymers were obtained. The PVBC molar fraction in the triblocks is 23mol% and 34mol% separately, which could be controlled by adjusting the feeding ratio of the polystyrene macro chain transfer agent and vinylbenzyl chloride monomer or the polymerization time. 139

163 (b) P-2 P-PVBC-P-2 R.I. (a.u.) (a) P-1 P-PVBC-P Elution time (mins) Figure 6.3 EC traces of (a) P-1 and P-PVBC-P-1, (b) P-2 and P-PVBC-P Quaternization of P-b-PVBC To quaternize the PVBC block fully, various tertiary alkyl amines were chosen, such as tri-n-ethylamine (TEA), tri-n-propylamine (TPA), tri-n-butylamine (TBA) and tri-n-octylamine (TOA). In the general procedure, the P-b-PVBC-b-P triblock copolymer (34mol% PVBC) was dissolved in THF with the concentration of 10%, a 5 molar excess of TEA to VBC units was added to the solution and stirred at 50 C for 8h. The solution became cloudy due to the high polarity of the quaternized PVBC block, and 30vol% methanol was added into the solution and stirred for another 8h. The scheme of reaction is shown in cheme

164 n + N n Cl N Cl cheme 6.3 Quaternization of PVBC with triethylamine. The 1 H NMR spectra of quaternized P-b-PVBC-b-P are shown in Figure 6.4. From the spectra, the peak at 4.5ppm is assigned to protons on the methylchloride groups, and conversion of the quaternization can be calculated by comparing the residual peak at 4.5ppm with the peak of the phenyl groups at 7.1ppm and 6.6ppm. It is obvious that the peak at 4.5ppm disappeared completely for the sample using triethylamine and tripropyamine as quaternizating agents, which means full conversion of quaternization. There is still a small peak left for the quaternization product of tributylamine and trioctylamine. It should be because triethylamine is a stronger nucleophile, compared with other tertiary amine. And the steric effect of tributylamine and trioctylamine is greater, due to the longer alkyl side chains. The reaction temperature and reaction time were increased to 60 C and 20h, but the conversion was not complete yet for quaternization reaction of tributylamine and trioctylamine. 141

165 trioctylamine tributylamine tripropylamine triethylamine P-PVBC-P Chemical hift (ppm) Figure 6.4 The 1 H NMR spectra of quaternized P-b-PVBC-b-P with various tertiary amines P-PVBC heptablock copolymer synthesis P-b-PVBC-b-P triblock copolymers with narrow polydispersity were synthesized by RAFT polymerization successfully, and PVBC blocks were quaternized by triethylamine completely to form well-defined cationic amphiphilic block copolymers. However, the mechanical properties of the quaternized triblock copolymer membranes were poor, forming extremely brittle films when both dry and wet. One way to improve the mechanical properties and prevent the over-swelling of the block copolymer membranes is to increase the number of chain junctions between the P and PVBC 142

166 blocks. There is increasing bridging of the block across different domains, which could lead to better mechanical properties 165. P-b-PVBC heptablock copolymers were synthesized by 4 steps of sequential RAFT polymerization of styrene and vinylbenzyl chloride. The P homopolymer (20k) was synthesized first, and used as a macro-raft agent to synthesize triblock, pentablock and heptablock copolymers. The polymerization was conducted in bulk, and the ratio of macro-raft to monomer was 1g:5ml. The molecular weight of the block copolymer was controlled by varying the time and temperature of the reaction. Figure 6.5 shows the EC traces of P, P-b-PVBC triblock, pentablock, and heptablock copolymers. Table 6.1 lists the characteristics of these polymers. The molecular weight increases with the addition of each block showing the ability of each copolymer to act as a macro-raft agent for subsequent polymerizations. For the PVBC blocks an increase in the polydispersity of the polymer is observed. This is likely due to the higher thermal initiation rate of vinylbenzyl chloride compared to styrene, and the presence of chloromethyl groups lead to higher chain transfer with the monomer 166. This will lead to lower chain transfer efficiency and lose control of the polymerization. 143

167 heptablock R.I. (a.u.) pentablock P-b-PVBC-b-P-2 P elusion time (min) Figure 6.5 The EC traces of P homopolymer, P-PVBC triblock, pentablock and heptablock copolymers. Table 6.1 Characteristics of P-PVBC block copolymers in Figure 6.5. ample Mn (kda) PDI PVBC molar fraction (mol%) Homopolymer Triblock Pentablock Heptablock The heptablock copolymer was fully quaternized by triethylamine, and the membrane was casted from DMF solution. The membrane was much more flexible, as shown in Figure

168 Figure 6.6 The membrane of triethylamine quaternized P-b-PVBC heptablock copolymer Properties of the P-b-quaterized PVBC membranes Different P-b-quaterized PVBC triblock and heptablock cationic amphiphilic copolymers were synthesized by sequential RAFT polymerization. The molecular weight of copolymers was controlling by tuning the reaction temperature and time. Membranes were cast from DMF solutions of the block copolymers, and the ionic conductivity was measured by impedance spectroscopy at Chemsultants International. (Mentor, OH). The membranes were converted to OH - form by immersing in 1M NaOH aqueous solution at room temperature for 48h before measurement. Figure 6.7 shows the ionic conductivity of block copolymer membranes, and the conductivity increased with the increased the mole fraction of the cationic blocks. At the highest PVBC volume fraction, the conductivity of the membranes was 47 mcm

169 σ (m/cm) mol % PVBC Figure 6.7 Conductivity of quaternized P-b-PVBC copolymers: ( ) ABA triblock, ( ) ABABA pentablock, ( ) ABABABA heptablock. mall angle X-ray scattering (AX) was used to examine the morphology of the P-PVBC triblock #2 and P-PVBC heptablock #1 in Table 2. Films of each sample were cast from DMF and annealed in methyl ethyl ketone (MEK), and a diffraction pattern was acquired with the beam parallel to the thick axis of the film. The azimuthally averaged intensity patterns are shown in Figure 6.8. Both profiles show an obvious first order peak. The triblock copolymer has weaker higher order peaks, and the intensity peaks located at the scattering vector positions: q *, 2 q *, and 3q *, where q * is first order peak. The scattering pattern specifically indicates the microphase separation structure, is lamellar. The molar fraction of PVBC is 34mol%, after quaternization the weight fraction of the cationic block is 56wt%. If it is supposed that the density of P and triethylamine quaternized PVBC is same, the triblock amphiphilic copolymer should form the lamellar morphology. The heptablock only has a very weak second order peak at the scattering 146

170 vector position of 2q *, which also indicated the microphase separation. The weight fraction of cationic block in the copolymers is 68wt%, while the block copolymer still has a lamellar morphology. The poor ordering and lack of intense higher order peaks could be due to either kinetic trapping of the morphology during solvent casting or the higher polydispersities of the block copolymers limiting the ability of the system to achieve long-range order. Intensity (a.u.) q * q * 2q * 2q * 3q * heptablock triblock q (A -1 ) Figure 6.8 Azimuthally averaged AX intensity profiles for P-PVBC triblock (34mol%) and heptablock (47mol%) The alkyl quaternary ammonium salt is instable at high temperature 167, and the degradation temperature of triethylamine quaternized P-b-PVBC block copolymer was measured by thermal gravimetric analysis (TGA), as shown in Figure 6.9. The initial weight loss is attributed to absorbed water, which is roughly 10%. Degradation of the polymer begins at 170 C. 147

171 Weight loss (%) Temperature ( o C) Figure 6.9 TGA analysis of P-PVBC block copolymer (heptablock 2#) Preparation of P-g-quaternized PVBC amphiphilic graft copolymers P-g-quaternized PVBC graft copolymers have a branched architecture, which have a hydrophobic backbone and hydrophilic branch chains and may form ordered microphase separation structure also. P-g-PVBC copolymers were synthesized via the graft-from route. P-r-PVBC random copolymers were synthesized via RAFT polymerization first, in which the PVBC units can be grafted with trithiocarbonates. The P-r-PVBC-g-trithiocarbonates was used as a macro-g-raft agent to polymerize vinylbenzyl chloride monomers to form graft copolymers. The graft copolymers were further quaternized to obtain cationic amphiphilic graft copolymers. The graft density can be controlled by adjusting the molar fraction of PVBC units in P-r-PVBC random copolymers. The scheme of random copolymerization of P-r-PVBC is shown in cheme dodecyl- -(α,α -dimethyl-α -acetic acid)-trithiocarbonate (RAFT-COOH) was used as chain transfer agent. Two batches of polymerization were run at 130 C for 6h, and the feeding ratio of vinylbenzyl chloride to styrene monomers was 5mol% and 148

172 10mol% respectively. COOH+ x + y C 12 H o C 6h Cl C 12 H 25 x y COOH Cl cheme 6.4 Random copolymerization of P-r-PVBC via RAFT using RAFT-COOH as chain transfer agent. The dodecyl-trithiocarbonate groups were grafted onto the P-r-PVBC via phase transfer catalyzed reaction, using Aliquat 336 as the phase transfer catalyst. The reaction is shown in cheme 6.5. The actual PVBC molar fraction of P-r-PVBC was determined by 1H NMR, and excessive carbon disulfide and 1-dodecyl-thiol (1.5 excess) were added to achieve full modification of the methylchloride groups. 149

173 NaOH C 12 H 25 H+ C 2 C 12 H 25 C Na C 12 H 25 C Na + x y Cl x y C C 12 H 25 cheme 6.5 ynthesis of P-r-PVBC-g-trithiocarbonate (P-r-PVBC-g-RAFT). The 1 H NMR traces of the P-r-PVBC random copolymers and P-r-PVBC-g-RAFT are shown in Figure According to the spectra (a) in the figure, the molar fraction of PVBC could be calculated by comparing the areas of the peak at 4.50ppm, which is assigned to two protons of the methylchloride groups of PVBC, and the peaks at 7.10ppm and 6.60ppm are assigned to the protons on the phenyl groups of both P and PVBC. After grafting, the peak at 3.30ppm is assigned to -C(=)--CH 2 -C 11 H 23. The peak at 4.50ppm has no obvious shift after the chloride groups were substituted by the trithiocarbonate group. By integration of the peak intensity, 150

174 the ratio of the area of these two peaks is 1:1, which means all PVBC units are grafted with trithiocarbonate pendants. b b a chemical shift (ppm) a chemical shift (ppm) Figure H NMR traces of P-r-PVBC random copolymers and P-r-PVBC-g-RAFT. Left: (a) P-r-PVBC(7%), (b) P-r-PVBC(7%)-g-RAFT; Right: (a) P-r-PVBC(13%), (b) P-r-PVBC(13%)-g-RAFT. The molecular weight and distribution of the P-r-PVBC copolymers and the products grafted with trithiocarbonates were characterized by EC, shown in Figure Table 6.2 shows the corresponding molecular weight and molecular weight distribution of the P-r-PVBC and P-r-PVBC-g-RAFT. Both P-r-PVBC random copolymers were synthesized with the narrow molecular weight distribution of After grafting, the elution peak of P-r-PVBC-g-RAFT shifted to left indicating higher molecular weight. There is a small shoulder on the left side of the peak, and the M peak of the shoulder is around two times of the molecular weight of the main peak, which should be due to the 151

175 side reactions of coupling between a few P-r-PVBC polymer chains. b b a a Elution Time (mins) Elution Time (mins) Figure 6.11 EC traces of P-r-PVBC random copolymers and P-r-PVBC-g-RAFT. Left: (a) P-r-PVBC(7%), (b) P-r-PVBC(7%)-g-RAFT; Right: (a) P-r-PVBC(13%), (b) P-r-PVBC(13%)-g-RAFT. Table 6.2 Molecular weight and distribution of P-r-PVBC and P-r-PVBC-g-RAFT. amples Functionality Random copolymer After grafting Feed mol% Measured mol% a Mn (Da) PDI Mn (Da) PDI P-r-PVBC-g-RAFT-1 5% 7% P-r-PVBC-g-RAFT-2 10% 13% a. characterized by 1 H NMR 152

176 The macro-g-raft agent was dissolved in vinylbenzyl chloride monomer and polymerized in bulk. The length of grafted chains was adjusted by the feed ratio of macro-g-raft to monomer and the reaction time. P-r-PVBC(7%)-g-RAFT was dissolved in vinylbenzyl chloride at the concentration of 0.2g/1mL, and the solution was stirred at 120 C for varying time. The EC traces of P-r-PVBC-g-PVBC polymerized for varying time are shown in Figure Table 6.3 lists the molecular weight and distribution of P-r-PVBC-g-PVBC graft copolymers, as well as the average length of grafted PVBC chains. With increase of polymerization time, the molecular weight of graft copolymers increased, indicating the growth of PVBC branch chains. The polydispersity of the graft copolymers increased also, but is still much lower than that of conventional graft polymerization products. (e) R.I. intensity (d) (c) (b) (a) Elution time (min) Figure 6.12 EC traces of (a) P-r-PVBC (7mol%); (b) P-r-PVBC-g-RAFT; (c) P-r-PVBC-g-PVBC-0.5h; (d) P-r-PVBC-g-PVBC-1h; (e) P-r-PVBC-g-PVBC-2h. 153

177 Table 6.3 molecular weight and distribution characteristics of P-r-PVBC-g-PVBC graft copolymers. Polymerization degree Polymerization Mn PDI PVBC (grafted chains) time (h) (kda) molar fraction (mol%) a of graft chains a. PVBC (side chains) molar fraction is calculated by integration of NMR spectrum, and the molar fraction of PVBC on the backbone is not included. Ln([M]/[M 0 ]) Pseudo first order plots PDI Time PDI Figure 6.13 Pseudo first order kinetic plots for the P-r-PVBC-g-PVBC polymerization and molecular weight distribution (PDI). The pseudo first order kinetic plots of the graft polymerization, as well as the molecular weight distribution, are shown in Figure In the early stage of the graft polymerization, the plots are linear, indicating controlled/living characteristics of the 154

178 polymerization. But with increase of conversion, the polymerization began to lose control. This is due to the steric hindrance of grafted chains, which became more obvious with increased graft chain length 73. Figure 6.14 shows the 1 H NMR spectrum of the graft copolymer and the triethylamine quaternized polymer. The PVBC being quaternized to hydrophilic branches and the molar fraction of PVBC is 51mol% led to poor solubility of the quaternized copolymer in chloroform-d. The mixture of chloroform-d and methanol-d (volume ratio is 2:1) was used as solvent, and the peak of chloroform (7.27ppm) was used as a reference. The sharp peak at 4.25ppm and 3.45ppm is assigned to the residual protons of methanol-d and water. After quaternization, the peak assigned to methyl chloride of the PVBC disappeared completely, indicating the full conversion of the reaction. n (b) Cl N (a) chemical shift (ppm) n 4.50ppm Cl Figure H NMR spectrum of (a) P-r-PVBC-g-PVBC (120 C, 2h, in chloroform-d) (b) quaternized P-r-PVBC-g-PVBC (in the mixture of chloroform-d and methanol-d with the volume ratio of 2:1). 155

179 6.2.7 Crosslinking of PVBC As discussed previously, the P-b-quaternized PVBC block copolymers microphase separated and formed ordered lamellar morphologies. While the swelling ratio increased greatly with the increase of the cationic block molar fraction, the mechanical properties of the block copolymers are not good enough to be applied for preparation of alkaline fuel cell membranes with long lifetime. One way to improve swelling resistance and the mechanical properties of membranes effectively is covalent crosslinking of polymer chains Preparation of crosslink agents The methylchloride groups of PVBC can be displaced by nucleophiles, and nucleophiles containing vinyl bonds can be grafted on PVBC and used as thermal induced crosslinking agents. Two kinds of crosslinking agents were synthesized, including vinylbenzyl thiol or vinylbenzyl alcohol monomers, as shown in cheme , 169. HAc Base olvent (a) Cl H KOAc DMO Hydrolysis NaOH EtOH (b) OAc OH cheme 6.6 The chemical structures of crosslinking agents (a) 4-vinylbenzyl thiol and (b) 156

180 4-vinylbenzyl alcohol. The vinylbenzyl thiol was obtained by reacting vinylbenzyl chloride with thiol acetate according to a previously published procedure 168. Figure 6.15 shows the 1 H NMR spectra of vinylbenzyl chloride and vinylbenzyl thiol. In the spectrum of vinylbenzyl chloride, the peak at 4.6ppm is assigned to the protons on methylchloride groups, which disappears completely in the spectrum of the vinylbenzyl thiol. And there is a new peak appearing at 3.5ppm in the spectrum of vinylbenzyl thiol, assigned to the protons on methyl thiol groups, which indicates vinylbenzyl chloride was converted to vinylbenzyl thiol completely. vinylbenzyl thiol Intensity (a.u.) vinylbenzyl chloride 3.5ppm H 4.6ppm Cl chemical shift (ppm) Figure H NMR spectra of vinylbenzyl chloride and vinylbenzyl thiol. The vinylbenzyl alcohol monomers were synthesized by converting vinylbenzyl chloride to vinylbenzyl acetate, and then vinylbenzyl alcohol monomers were obtained by 157

181 hydrolysis of vinylbenzyl acetate 169. Figure 6.16 shows the 1 H NMR spectra of vinylbenzyl chloride, vinylbenzyl acetate and vinylbenzyl alcohol monomers. In the spectrum of vinylbenzyl chloride, the peak at 4.6ppm disappeared completely, which shifted to 5.0ppm in the spectrum of vinylbenzyl acetate, indicating the full conversion of vinylbenzyl acetate. After hydroxylation, the peak at 5.0ppm disappeared and shifted to 4.6ppm, indicating the complete formation of hydroxyl groups. (c) 4.6ppm OH Intensity (a.u.) (b) 5.0ppm OAc (a) 4.6ppm chemical shift (ppm) Cl Figure H NMR spectra of (a) vinylbenzyl chloride, (b) vinylbenzyl acetate and (c) vinylbenzyl alcohol monomers Grafting crosslink agents on PVBC The vinylbenzyl thiol was grafted on PVBC via the thiolether addition reaction, as shown in cheme

182 n + TBAI DMF Cl H Cl cheme 6.7 Graft reaction of vinylbenzyl thiol onto PVBC. The 1 H NMR spectra of PVBC and PVBC--vinylbenzyl thiolether (10mol% feeding ratio) are shown in Figure After grafting, there are two single peaks appearing at the positions of 5.8ppm and 5.3ppm, which are assigned to the protons of vinyl groups. The graft ratio of the PVBC was calculated by comparing the peak intensity of the them and the peak intensity at 4.5ppm, which are assigned to CH 2 -Cl and CH The result shows that the real graft ratio is 10mol% also, and all vinylbenzyl thiol were grafted on PVBC. 159

183 (b) b a a Cl (a) b a chemical shift (ppm) Figure H NMR spectra of (a) PVBC and (b) PVBC--vinylbenzyl thioether (10mol% feeding ratio). The vinylbenzyl alcohol monomers were grafted onto PVBC by ether linkages, and a phase transfer catalyzed reaction was employed, as shown in cheme 6.8 n + Phase transfer catalysis reaction n Cl OH Aliquat 336 NaOH water/toluene O Cl cheme 6.8 Graft reaction of vinylbenzyl alcohol onto PVBC. 160

184 The 1 H NMR spectra of PVBC and PVBC-O-vinylbenzyl ether (20mol% feeding ratio) was shown in Figure After grafting, there are two single peaks appearing at the positions of 5.8ppm and 5.3ppm, which are assigned to the protons of the vinyl groups. The graft ratio of the PVBC was calculated by comparing the peak intensity of the vinyl group and the peak intensity at 4.5ppm, which are assigned to CH 2 -Cl and CH 2 -O-. The result shows that the actual graft ratio is 20mol%, and the conversion of the phase transfer catalyzed reaction is ~100%. n (b) k i i O Cl (a) k i chemical shift (ppm) Figure H NMR spectra of (a) PVBC and (b) PVBC-O-vinylbenzyl ether (20mol%) Thermal induced crosslinking The samples were heated in a DC under nitrogen flow at the rate of 1 C/min, and the heat flow curve is shown in Figure An exothermic peak appears at 60 C, and the 161

185 exothermic heat flow increased with increasing temperature, indicating faster crosslinking rate. After 110 C, the exothermic heat flow began to drop, and stopped 30mins later, which means the thermal induced crosslinking of the polymer was completed PVBC-O-VB-20% 0.00 heat flow (mw) Temperature Figure 6.19 DC curve of PVBC-O-vinylbenzyl during heating at 1 C/min. The grafted PVBC was dissolved in chloroform and cast into films in a Teflon dish by slowly drying in a fume hood at room temperature. The dried films were heated in the vacuum oven at 140 C for 2h for crosslinking. The film of PVBC-O-vinylbenzyl ether had better mechanical properties and kept good shape in selected solvents, as shown in Figure

186 Figure 6.20 (a) Crosslinked PVBC-O-vinylbenzyl ether film in dried state (b) uncrosslinked PVBC (left) and crosslinked PVBC-O-vinylbenzyl ether film in chloroform Quaternization of crosslinked PVBC The PVBC-O-vinylbenzyl ether was quaternized by an aqueous trimethylamine aqueous solution via two different routes, direct quaternization and post-crosslinking quaternization. (1) In direct quaternization, the reaction was run in THF under nitrogen environment at room temperature for 24h to avoid crosslinking of the polymer during quaternization. The quaternized PVBC was cast from THF solution and crosslinked in vacuum oven at 140 C. (2) To improve the quaternization degree, the crosslinked quaternized PVBC film was further quaternized post-crosslinking. The membrane was soaked in the mixture of methanol and an aqueous trimethylamine solution (5:1 volume ratio) at 80 C for 8h. The FT-IR spectra of the PVBC-O-vinylbenzyl ether and the quaternized polymer 163