NMR studies of styreneln-butyl acrylate copolymers prepared by atom transfer radical polymerization

Transcription

1 Indian Journal o Chemistry Vol. 43A, November 2004, pp NMR studies o styreneln-butyl acrylate copolymers prepared by atom transer radical polymerization A S Brar* & Puneeta Department o Chemistry, Indian Institute o Technology, Delhi, Hauz Khas, New Delhi , India Received 18 June 2004; revised 4 August 2004 Styrene (S) and II-butyl acrylate (B) have been copolymerized by atom transer radical copolymerization (ATRP) catalyzed by CuBr/N,N,N',N',N"-pentamethyldiethylenetriamine. The composition o copolymers has been determined with IH NMR spectroscopy. The copolymer composition data has been used to determine the reactivity ratios by Kelen-Tudos (KT) and nonlinear error in variables methods (EVM). Molecular weights o the copolymers have been determined by gel permeation chromatography (GPC). Molecular weights o the copolymers increase with increase in percentage conversion. The polydispersity o the copolymers is quite low ( ). Styrene and It-butyl acrylate centered triads concentrations have been determined rom DC { IH} NMR and reactivity ratios and show good agreement with each other. IPC Code: Int. Cl. 7 C08F 120/42; GOIR 33/20 The determination o microstructure o copolymers is o value In establishing structure-properties relationships. NMR spectroscopy, mostly, l3c NMR 1 2, is an important technique or the investigation o the structures o copolymers, particularly, the monomer sequence determination in the copolymers. Radical polymerization has been a subject o keen interest. The versatility o the radical polymerization lies in the act that a large range o the monomers can be polymerized. Nevertheless, ree-radical polymerization has been limited by the inevitable, ast and irreversible termination o the growing radicals by the coupling and disproportionation reactions, leading to the poor control o the reaction and high polydispersity o the resulting polymer. Thus, much research has been devoted to develop a controlled radical polymerization to synthesize well-deined polymers with narrow molecular weight distributions and desired complex architectures 3. One o the most successul methods has been atom transer radical polymerization (A TRP)4.5. This technique is most versatile and has been successully applied to vinyl monomers such as acrylates and styrene(s) to prepare polymers with controlled molecular weights and welldeined structures 6. ATRP is a catalytic process, where a transition metal complex reversibly activates the dormant polymer chains via a halogen atom transer reaction. The control o the polymerization aorded by ATRP is due to a rapid equilibrium between the dormant and active species (Scheme 1). R X + M ~ YI Ligand. k act k deact Scheme 1 Monomer Termination Radicals are generated through a reversible redox reaction catalyzed by a transition metal (Mt -Y) ligand complex, where Y may be ligand or counter ion, which undergoes one electron oxidation with abstraction o a (pseudo) halogen atom (X) rom a dormant species R X. The process occurs with a rate constant o activation (k. ct )7.8 and deactivation (k de ct ). Polymer chains grow by the addition o radicals to monomers similar to conventional radical polymerization with' the rate constant o propagation (k p ). Termination reactions also occur; however, in a well-controlled ATRP not more than a ew percent. A successul ATRP will not only have a small contribution o terminated chains, but also a uniorm growth o all the chains, which is through ast initiation and rapid reversible deactivation. Tllereore, the molecular weight distributions are narrow with the polydispersity index 9 (MwIMn) ranging rom 1.1 to 1.3. The copolymers o styrene and n-butyl acrylate (SIB)JO was prepared by ATRP and their

2 2282 INDIAN J CHEM, SEC A, NOVEMBER 2004 microstructure has been determined by IH and I3C NMR spectroscopy. In this paper, we report the reactivity ratio by KTII and EVM methods. The triad sequence distribution o SIB copolymers were determined rom 13CeH} NMR and rom reactivity ratiosl 4. Materials and Methods n-butyl acrylate (CDH,99%) was washed with aqueous NaOH, dried over used CaCh and distilled under vacuum. Styrene (Merck, 99%) was passed through a column o basic Ah03, and distilled under vacuum. All monomers were stored at O C and purged with nitrogen gas or 30 min beore use. Methyl 2- bromopropionate (Aldrich, 98%) was distilled at reduced pressure. N,N,N',N',N"-pentamethyldiethylenetriamine, PMDET A (Aldrich, 99%), Copper(l) bromide(cubr, Aldrich, 98%) and copper metal powder (Cu(O),CDH, 99.5%) were used as such. Chloroorm and methanol were puriied by general methods. Ten boiling tubes itted with rubber septum were taken and to each tube CuBr (0.8 mmol) and Cu (0) (1.6 mmol) were added. The solids were evacuated and backilled three times with N 2 A solution o styrene (0.04 mol), n-butyl acrylate (0.04 mol), PMDETA (8 mmol) and methyl 2-bromopropionate (2.4 mmol) were added via syringe to each tube. The reaction mixture was heated at 80 C. Tubes were taken out at dierent intervals o time, to observe the change in molecular weight with conversion, and the reaction was quenched by precipitating in methanol. Precipitated polymers were dissolved in chloroorm and passed through the column o neutral alumina to remove the residual copper catalyst (green colour) present in the copolymer. The polymers were dried under vacuum at 78 C or 10 hrs. Percentage conversion was determined gravimetrically. Styrene (0.016, 0.024, 0.056, mol), n-butyl acrylate (0.064, 0.056, 0.024, mol) respectively were taken ineed, then polymerization was carried out as described above. The molecular weights were determined by GPC using polystyrene as narrow standard and THF as eluent. The eluent low was 0.3 ml per min. A reractive index detector was used or detection. It was ound that the molecular weight increases with increase in conversion. IH and I3C NMR spectra were recorded in CDCh on a Bruker DPX-300 MHz spectrometer operating at a requency o and 75.5 MHz or IH and l3c nuclei, respectively at 25 C. DEPT experiments were carried in CDCh using the standard pulse with a J modulation time o 3.7 s (J CH = 135 Hz) with a 2s delay time. All the NMR spectra were recorded in CDCl 3 at 25 C. Results and Discussion Determination o reactivity ratio The copolymer composition o SIB copolymers was determined rom the IH NMR spectrum. The eed mole raction and copolymer composition data are given in Table 1. The initial estimate o the reactivity ratios was done by the Kelen-Tudos method with the help o copolymer composition data. The values o the terminal reactivity ratios obtained rom relevant plots were: rs= 0.87 ± 0.10, rb= 0.25 ± These values along with the copolymer data were used to calculate the reactivity ratios using the nonlinear error invariable method. The values o reactivity ratios obtained rom this method were rs= 0.87, rb = 0.25 respectively. The values o reactivity ratios obtained rom Kelen-Tudos and nonlinear error in variables methods are in complete agreement with each other. Table l--copolymer composition data o the SIB copolymers (<10% conversion) Sample No Styrene mole raction in eed i,) Styrene mole raction in copolymer (Fs) Table 2---Percentage conversion and molecular weight o the SIB copolymers by GPC measurements S.No. Percentage Molecular Polydispersity conversion weight X 10-4 index (MjMII)

3 o BRAR & PUNEETA: NMR STUDIES OF STYRENFJn-BUTYL ACRYLATE COPOLYMERS 2283 Molecular weights detennination The molecular weights were determined by gel permeation chromatography (OPC). It was ound that the molecular weight increases with increase in conversion. The increase in molecular weight with conversion is shown in Table 2. Polydispersity o the copolymers is very low i.e Figure 1 shows the increase o molecular weight with increase in conversion and also OPC chromatograms show the increase in the molecular weight with conversion (Fig. 2). IH NMR studies The complete assignment o the 'H NMR spectrum o the SIB copolymer in CDCh is shown in Fig. 3. The composition o the copolymer was calculated rom the relative intensities o the phenyl (S,) and -OCH 2 - (S2) proton resonances o styrene and n-butyl acrylate units respectively, according to the ollowing equation: I c ::E DO '4 converslon--> Fig. I--Dependence o molecular weight with conversion or the styreneln-butyl acrylate copolymers by ATRP.. " 1\ i \ iii \! V I,: \ : 0!. :\ i i \ I.. \0 \ 0 :..0 \ o I \ 0 j \~ \ /.I!.~: =-=-~ ~../ ~:-= I t I I Elution Tim! X 10 1 (mi l.) (----) 13,200 Mwl Mn= 1.16 (_._) 21,500Mwl Mn = 1.15 (------) 23,200Mw/Mn=I.13 C--) 53,800 Mwl Mn = 1-18 Fig. 2--GPC chromatograms o the styreneln-butyl acrylate copolymers by ATRP. where, F, is the mole raction o styrene monomer in the copolymer. 13C {IH} NMR studies The complete assignment o the resonance signals in the l3c {'H} NMR spectrum o the SIB copolymer in CDCl 3 is shown in Fig. 4. The carbonyl carbon signals or n-butyl acrylate resonate around ppm and the quaternary carbon signals o styrene resonate around ppm and their expanded spectra are shown in Figs 5 and 6. On the basis o variation in the composition o the copolymers, and on comparison with the spectra o Table 3--Triad ractions calculated rom the NMR spectra and reactivity ratios o SIB copolymers «10% conversion) Sample No. 1 Copolymer composition Fs Triads a b om a--triad ractions obtained rom 13C{ IH} NMR spectra o the quarternary and the carbonyl carbon resonance signals o the S- and B- centered monomeric units. b--triad ractions calculated using rs= 0.87, rb

4 2284 INDIAN J CHEM, SEC A, NOVEMBER 2004 CHt($+I) + 'CHt t'01, CH, i i I i i i I I I I I I I I I I I I I s.o ppm.. ( I I I I I I I Fig. 3-- I H NMR spectrum o the styreneln-butyl acrylate copolymer (F, = 0.29) in CDCI 3 at 25 C. -Ctt-ln- CH,- ~H- 2' ~- 0,. Is 0 4 I,~2 7?: CH, C=0(8) r7'\ u I i ". r., I o Fig C (IH) NMR spectrum o the styreneln-butyl acrylate (F, = 0.46) in CDCh at 25 C.

5 BRAR & PUNEETA: NMR STUDIES OF STYRENEIn-BUTYL ACRYLATE COPOLYMERS 2285 ISS ~ ~_~(a) ~s (~ Iii I I i I i I I i i I I I I I I I I I i : I Iii iii i Fig. 5-Expanded Quaternary carbon o the S unit in the l3c {lh} NMR spectra o the styreneln-butyl acrylate copolymer in CDCI 3 at 25 C: a) polystyrene, b) Fs = 0.91, c) F. = 0.79, d) F. = 0.60, e) F, = 0.46, and )F. = t I i lu... iii I I i i I I i I I iii l7i.g iii 174.G iii In:.o i I I I I i I Fig. 6--Expanded carbonyl carbon o the B unit in the l3c {lh} NMR spectra o the styreneln-butyl acrylate copolymer in CDCh at 25 C: a) poly(n-butyl acrylate), b) Fb= 0.71, c) Fb = 0.54, d) Fb= 0.40, e) Fb= 0.21, and ) Fb = 0.09.

6 2286 INDIAN J CHEM, SEC A, NOVEMBER 2004 'C-'C(S) ~ '-Y -OCH,-(S),'-Y CH,(B) 'I--. 'CHr,(S) ppm I ' too 1 50 o Fig. 7-DEPT-135 NMR spectrum o the styrene/n-butyl acrylate copolymer (F. = 0.46) in CDCI 3 at 25 C. respective homopolymers, the various triad sequences in the carbonyl and the quaternary carbon resonance signals were assigned. The resonance signals o quaternary carbon around are assigned to triad, assigned to triad and around are assigned to triad. The resonance signals o carbonyl carbon around ppm are assigned to triad, around ppm assigned to SBB triad and around ppm is assigned to triad. The quantitative calculations o these Sand B centered triads were done rom the areas o the respective resonance signals in \3C CH} NMR o copolymer and rom the reactivity ratios and showed good agreement with each other (Table 3). The spectral region around ppm is quite complex, which can be assigned to aliphatic carbon resonances in the main backbone as well as the side chain o SIB copolymer. Figure 7 shows the DEPT -135 NMR spectrum o the SIB copolymer in CDCI 3 The -OCH 2 - region o the B unit resonate around 863 ppm. The methylene and methine carbon resonances o both Sand B unit o backbone overlap between ppm and they appear as multiplet showing their sensitivity towards compositional sequences. To conclude, the reactivity ratios o monomers have been ound to be rs= 0.87 ± 0.10, rb= 0.25 ± 0.01, rs= 0.87, rb= 0.25 by KT and EVM respectively. The carbonyl and the quarternary carbon resonances o B and S unit respectively were assigned to triad compositional sequences and were used to determine triad concentration. Acknowledgement One o the authors, Puneeta, wishes to thank the CSIR, India or providing the inancial support. Reerences 1 Barron P F, Hill D J T, O'Donell J H & O'Sullivan P W, Macromolecules, 16 (1984) Randall J C, Polymer Sequence Distribution: Carbon J3C NMR Method, (Academic Press, New York), Matyjaszewski K, Ed. Controlled Radical Polymerization, (ACS Symposium Series 685, American Chemical Society, Washington, DC), Wang J S & Matyjaszewski K, JAm Chem Soc, 117 (1995) Kato M, Kamigaito M & Sawamoto M, Macromolecules, 28 (1995) 172l. 6 Karanam S, Goosses H & Klumperman B, Macromolecules, 36(2003) Nanda A K & Matyjaszewski K, Macromolecules, 36 (2003) Nanda A K & Matyjaszewski K, Macromolecules, 36 (2003) Patten T, Xia J, Abernathy T & Matyjaszewski K, Science, 272 (1996) Arehart S V & Matyjaszewski K, Macromolecules, 32 (1999) Kelen T & Tudos F, J Macromolecule Sci Chem, A9 (1975) Harris S H & Gilbert R D, J Polym Sci, Polym Chem Edn, 20 (1982) Schneider H A & Neto H N, Polym Bull, 9 (1983) Harwood H J, J Polym Sci C, 25 (1968) 37.