RAFT Polymerization of a Novel Activated Ester Monomer and Conversion to a Terpyridine-Containing Homopolymer

Transcription

1 RAFT Polymerization of a Novel Activated Ester Monomer and Conversion to a Terpyridine-Containing Homopolymer KHALED A. AAMER, GREGORY N. TEW Department of Polymer Science and Engineering, University of Massachusetts, Amherst, 120 Governors Drive, Amherst, Massachusetts Received 21 June 2007; accepted 17 July 2007 DOI: /pola Published online in Wiley InterScience ( ABSTRACT: Developing new materials with unique properties for nanotechnology applications, in general, and supramolecular polymers, in particular, lie at the heart of much ongoing research. In line with these efforts, we have been exploring polymers containing terpyridine (terpy) in the side chain. Here we report a new monomer that effectively undergoes reversible addition fragmentation chain transfer polymerization (RAFT) to yield high-molecular-weight (M n ) polymers with narrow polydispersity (PDI). The monomer is an N-succinimide activated ester of p-vinyl benzoic acid. Under RAFT conditions, poly(n-succinimide p-vinylbenzoate)s were generated, with M n ranging between 44 and 61 kda and PDI of One of these homopolymers was reacted with an amine functionalized terpy, creating a new homopolymer containing terpy ligands on every monomer. VC 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: , 2007 Keywords: MASI; N-hydroxysuccinimide; RAFT; supramolecular polymer; terpyridine INTRODUCTION The development of novel soft materials that possess unique properties enable enormous opportunities in the areas of nanotechnology, biotechnology, medicine, and advanced materials. 1 The ongoing demand for increased performance and multifunctional materials leads to greater complexity in the macromolecular structure New synthetic methods based on controlled living radical polymerizations offer great promise for generating macromolecular structures with this increased complexity For over a decade now, three major polymerization techniques have received much attention, including atom transfer radical polymerization (ATRP), 24 nitroxide-mediated polymerization (NMP), and reversible Correspondence to: G. N. Tew ( tew@mail.pse. umass.edu), Vol. 45, (2007) VC 2007 Wiley Periodicals, Inc addition fragmentation chain transfer polymerization (RAFT) ,23,25 27 Of these three techniques, each has advantages and disadvantages; ATRP requires optimization of the catalyst and reaction conditions to meet the need of each new monomer, while NMP typically needs higher temperature and is best used for styrenic and acrylate-type monomers. In contrast, RAFT, which requires the chain transfer agent (CTA) to be synthesized, seems to polymerize a larger list of monomers irrespective of their functionalities. RAFT involves controlling the conventional radical polymerization of substituted monomers using a CTA A typical CTA is built around the S ¼C S functional group with substituents R and Z that influence the reaction kinetics and the degree of control over the polymerization. 29,30 A variety of CTAs structures are reported and include dithioesters, trithiocarbonates, dithiocarbamates, xanthates (dithiocarbonates), and phosphoryl-/thiophosphoryldithioformates. 17,18,31 The

2 RAFT POLYMERIZATION OF ACTIVATED ESTER MONOMER 5619 reaction can be initiated by a variety of methods, including conventional thermal, photochemical, redox, or c-irradiation methods. For several years now we have been interested in developing functional materials for supramolecular polymers containing metal complexes in the side chain. During the course of our work, a number of synthetic goals were established to expand the quality and structure of macromolecules generated with metal ligands in their side chain. Some of these goals included the synthesis of macromolecules with predetermined and controllable molecular weights (M n ), narrow polydispersity (PDI), and metal ligands on every monomer. 32 In addition, developing chemistry to enable the synthesis of block copolymers with the metal ligand confined to one or more of the blocks remains an important goal. 32,33 These molecules allow one to couple polymer self-assembly with the rich chemical properties of metal complexes. We have reported several synthetic approaches to these novel macromolecules, including the ATRP polymerization of activated esters based on acrylate monomers such as N-succinimide methacrylate The ATRP method provided poly- (N-succinimide methacrylate) homopolymers, but the highest M n achieved was around 15 kda and the PDI varied between 1.07 and These activated ester homopolymers were efficiently postfunctionalized with amine-functionalized terpyridine (terpy) to give terpy-containing polymer. 33,36,37 Because of the limitation in achievable M n of these homopolymers, we turned our attention to the styrenic version of the monomer, namely N-succinimide p-vinyl benzoate, (NSVB) and explored the possibility of using RAFT methods for polymerization. Here we report that this monomer, NSVB, undergoes RAFT polymerization to produce high molecular-weight homopolymers (M n ¼ 66 and 44 kda) with narrow PDI (1.07 and 1.03). The RAFT polymerization was conducted with a novel CTA based on 4-cyano, 4-(thiobenzoylthio) N-succinimide valerate, (1) so that the CTA contains the same activated ester as in the monomer. One activated ester homopolymer was postfunctionalized to create a homopolymer containing terpy on every monomer. This demonstrates the versatility of these activated ester polymers for generating narrow PDI macromolecules with control over M n. Although not demonstrated here, RAFT methods are well known to yield block copolymers, and so extension of these methods to generate macromolecules with metal ligands, like terpy, confined to one block can easily be envisaged. EXPERIMENTAL Materials All chemical reagents were obtained from Alfa Aesar, Fisher, Cambridge isotopes, Aldrich Chemical Co., Avocado Research Chemicals, or Acros Organics, and used without further purification unless otherwise stated. 2,2 0 -Azobis(2-methylpropionitrile) (AIBN) was purified by recrystallization from methanol and stored in the freezer. The amine-functionalized molecule, terpy-nh 2, was synthesized according to literature procedure. 32 Measurements 1 H NMR spectra were recorded using 300 MHz DPX-300, and 13 C NMR spectra operated at 100 MHz were recorded using 400 MHz Avance Bruker spectrometers in either CDCl 3 or CD 3 CN. The gel permeation chromatography (GPC) to measure molecular weight and PDI index were recorded in DMF as eluant against narrow PS standards using a differential refractometer and three PLgel columns (105, 104, and 103 Å) kept at 35 8C. Synthesis of RAFT Transfer Agent, 1 In a 250 ml RBF equipped with a stirring bar, sulfur powder (10.8 g, mmol) was mixed with a solution of sodium methoxide NaOCH 3 (71 ml, 311 mmol of 25 wt % in methanol; prepared by mixing 7.36 g of small sodium metal pieces with 13 ml methanol followed by dilution with 64.5 ml of methanol). The flask was then attached to a dropping funnel containing benzyl bromide (18 ml, mmol). The benzyl bromide was added to the sulfur/naoch 3 mixture dropwise for over an hour to form a deep red mixture, with vigorous stirring under a stream of N 2 gas. After complete addition of benzyl bromide, the dropping funnel was replaced with a condenser equipped with a CaH 2 tube on top, and the deep red mixture was refluxed for 18 h. The mixture was allowed to cool to room temperature, filtered to remove NaBr salt, and the solvent removed by rotary evaporation to give a red viscous mixture. This mixture was dissolved in 250 ml water, and 400 ml of diethylether was added.

3 5620 AAMER AND TEW A solution of dilute HCl was added to the water/ ether layers until the aqueous layer became colorless. The organic layer was separated, washed with water ( ml), and dried over anhydrous MgSO 4. The diethylether red solution was filtered and combined with 300 ml of diethylamine to precipitate a red solid. This red solid, diethylammonium dithiobenzoate, was kept cold ( 5 8C) overnight, filtered, and dried in air under suction, 18.6 g, 49% yield. The dark red diethylammonium dithiobenzoate (18.6 g, 82 mmol) was dissolved in 100 ml ethanol, and a solution of iodine (21 g, 82 mmol, in 250 ml of ethanol) was added at 5 8C over 1.5 h with vigorous stirring. A pink precipitate appeared after 10 min of iodine addition. After the complete addition of the iodine solution, the bis(thiobenzoyl)disulfide) was collected by filtration, washed carefully with ethanol, and dried at room temperature overnight under vacuum to yield a red solid (8.64 g, 69% yield). 1 H NMR data agreed with previously reported literature values. 17,18 This new red solid, bis(thiobenzoyl)disulfide (1.0 g, 3.3 mmol) was mixed with 4,4 0 -azobis(4- cyanovaleric acid) (1.4 g, 5.0 mmol) in 20 ml ethylacetate in a 250 ml RBF. A condenser was fitted onto the RBF and the mixture refluxed for 18 h, at which point the red mixture was allowed to cool to room temperature, solvent was removed by rotary evaporation, and the red oily residue purified by flash chromatography (40 g RediSep flash column) using ethylacetate:n-hexane 40:60 v/v; R f ¼ 0.89, 1.16 g, 64% yield of red solid of 4-cyano, 4-(thiobenzoylthio) valeric acid (CTBTVA). 1 H NMR (CDCl 3 ) d ¼ 7.91 (d, J ¼ 7.97, 2H, 2CH phenyl), 7.56 (t, J ¼ 7.65, 1H, CH phenyl), 7.40 (t, J ¼ 7.97, 2H, 2CH phenyl), (m, 2CH 2 ), and 1.97 (s, 3H, CH 3 ). The carboxylic acid group of the RAFT agent was converted to the activated ester by reaction with N-hydroxysuccinimide (NHS) as follows: CTBTVA (0.5 g, 1.79 mmol) was mixed in a 100 ml RBF with NHS (310 mg, 2.7 mmol), DCC (555 mg, 5.7 mmol), DMAP (11 mg, 0.09 mmol), and ethylacetate (15 ml). The solution was stirred at room temperature for 66 h to give a red solution with white precipitate. The white solid was filtered and the solution concentrated by rotary evaporation. The obtained red oil was dissolved in DCM, extracted with saturated solution of NaHCO 3 ( ml), the combined organic layers were washed with water ( ml), dried over anhydrous MgSO 4, and concentrated. The red solution obtained was purified by flash chromatography using ethylacetate:n-hexane 40:60 v/v to yield 0.6 g of 1 in 90% yield. 1 H NMR (CDCl 3 ) d ¼ 7.91 (d, J ¼ 7.97, 2H, 2CH phenyl), 7.56 (t, J ¼ 7.65, 1H, CH phenyl), 7.40 (t, J ¼ 7.97, 2H, 2CH phenyl), 2.99 (m, 2H, CH 2 ), 2.85 (s, 4H, 2CH 2 of succinimide), 2.72 (m, 2H, CH 2 ), and 1.97 (s, 3H, CH 3 ). Synthesis of NSVB Monomer In 250 ml RBF equipped with a stirring bar, NHS (5.8 g, mmol) and 4-vinyl benzoic acid (5.0 g, mmol) were mixed with ethylacetate (125 ml) and stirred till all solids dissolved. DCC (10.44 g, mmol) and DMAP (206.1 mg, 1.68 mmol) were added and the solution stirred at room temperature for 36 h at which time a white precipitate was formed. The solution was filtered and then concentrated by rotary evaporation to give a yellow oily residue that was dissolved in DCM, extracted with saturated aq. NaHCO 3 solution ( ml), washed with water ( ml), dried over MgSO 4, and concentrated to an oil. This oil was redissolved in DCM (30 ml) and ethylether (170 ml) to give a white solid (6.58 g, 79% yield). 1 H NMR (CDCl 3 ) d ¼ 8.09 ppm (d, J ¼ 7.32 Hz, 2H, 2CH, phenyl), 7.52 (d, J ¼ 8.79 Hz, 2H, 2CH, phenyl), 6.76 (q, J ¼ 5.86, Hz, 1H, CH, vinyl), 5.92 (d, J ¼ Hz, 1H, CH, vinyl), 5.49 (d, J ¼ Hz, 1H, CH vinyl), 2.91 (s, 4H, 2CH 2 of succinimde). Synthesis of Poly(N-succinimide p-vinylbenzoate) The RAFT CTA, 1 (5.64 mg, mmol), NSVB monomer (368.2 mg, 1.5 mmol), and AIBN (0.5 mg, mmol) were dissolved in a mixture of acetonitrile and chlorobenzene (700 ll, 1:2 vol). The mixture was degassed by three freeze pump thaw cycles, sealed under nitrogen, and heated at 60 8C under nitrogen for 28 h. After cooling, the light pink mixture was dissolved in dichloromethane (15 ml) and precipitated twice in MeOH (400 ml). The light pink solid was dried under vacuum to give the desired x-(thiobenzoylthio) poly(n-succinimide p-vinylbenzoate) (PNSVB; 267 mg, 73% conversion), M n ¼ 61 kda, PDI ¼ H NMR (300 MHz, CD 3 CN), d ¼ (m, broad, 2H), (m, broad, 2H), 2.8 (s, 4H).

4 RAFT POLYMERIZATION OF ACTIVATED ESTER MONOMER 5621 Chart 1. Synthesis of the monomer N-succinimide p-vinylbenzoate, NSVB. Postfunctionalization of PNSVB to Form Poly(VB terpy ) To a 50 ml RBF, PNSVB (100 mg, mmol), in terms of NSVB, was dissolved in DMF (3 ml). The flask was cooled in an ice bath at 5 8C followed by dropwise addition of terpy-nh 2 (340.5 mg, 1.02 mmol) dissolved in DMF (5 ml) over a period of 2 h with stirring. The flask was then placed in an oil bath at 50 8C and heating continued for 16 h, after which the polymer was precipitated in a mixture of n-hexane:ethylacetate (500 ml, 1:2 vol) and the off-white solid was collected by filtration and dried under vacuum overnight to give poly(vb terpy ) (80 mg, 34% yield). 1 H NMR (400 MHz, CDCl 3 ), d ¼ 8.5 (m, 4H), 7.87 (s, 2H), 7.71 (s, 2H), 7.19 (s, 2H), 6.23 (s, 1H), 4.07 (s, 2H), 3.43 (s, 2H), (m, 8H). RESULTS AND DISCUSSION Polymers with increased functionality are of great interest due to their potential application in next generation materials. 1,32 Developing efficient synthetic strategies to create polymers containing metal ligands in their side chain will expand the opportunities for these unique macromolecules which have historically suffered from ill-defined chemistry and characterization. In our efforts to prepare such molecules, it appears that the indirect approach, or the synthesis of polymers that can be subsequently functionalized, is very appealing since it overcomes an inherent difficulty associated with the characterization of these metal ligand containing polymers using GPC methods. The presence of multiple ligands along the polymer backbone causes reversible adsorption to the GPC column stationary phase. The difficulty of eluting and thus GPC chromatographic characterization of metal ligand and metal ligand complex containing polymers is well documented. Schubert and coworkers reported a detailed study outlining the challenges of characterizing end-functionalized metal complex containing polymers via chromatography. 38 Even these polymers, containing only two terpys per chain, were immensely difficult to characterize by GPC. However, the appropriately designed indirect approach allows routine GPC characterization of the precursor polymer followed by rather mild postpolymerization methods to attach the metal ligand. This allows for a reasonable assumption that the degree of polymerization and the PDI of the sample remain unchanged. Although we have reported other methods, including the use of t-butyl acrylate precursors, 39 the use of activated esters to accomplish these goals appears to be extremely promising. NSVB represents a novel monomer that undergoes very well-controlled RAFT polymerization to yield styrenic-based precursor polymers. The synthesis of NSVB is shown in Chart 1 and follows directly from the esterification of 4-vinylbenzoic acid with N-hydroxysuccinate using DCC/DMAP in 79% yield. RAFT polymerization allows end functionalization of the macromolecule and we decided to include an activated ester at the chain end. This required the synthesis of a novel CTA agent, 1, outlined in Chart 2. One con- C1 C2

5 5622 AAMER AND TEW Chart 2. Synthesis of the RAFT CTA 1. cern with using a new CTA is that its ability to control the polymer s M n and PDI is not known in advance; however, the structure in Chart 2 was designed based on well-known CTAs and the activated ester was positioned away from the chain transfer functionality. Chart 2 outlines the synthesis of 1 starting with benzyl bromide. Following reaction with sulfur/methoxide, the thioacid was converted to the disulfide (BTD) in 34% overall yield. The disulfide was refluxed in the presence of 4,4 0 -azobis(4-cyanovaleric acid) to give the acid-functionalized CTA, which was subsequently converted to the activated ester using DCC/ DMAP and N-hydroxysuccinate in 58% yield for the two steps. The controlled RAFT homopolymerization of the NSVB monomer (see Chart 3) was achieved under the following conditions: NSVB, 1, and C3 Chart 3. Synthesis of the poly(n-succinimde p-vinylbenzoate), PNSVB, and the subsequent postfunctionalization to form Poly(VB terpy ).

6 RAFT POLYMERIZATION OF ACTIVATED ESTER MONOMER 5623 Table 1. Molecular Weight Characterization of PNSVB Homopolymers Polymer Calculated M n a (kda) Observed DP b PDI b (%) Conversion Initiator Efficiency (%) PNSVB PNSVB a M n values are from DMF GPC. b Degree of polymerization (DP) are calculated from the DMF GPC M n divided by the monomer MW. AIBN were mixed in CH 3 CN/chlorobenzene (Ph- Cl) (1:2 vol:vol) at 60 8C for a specified period using the ratio [NSVB]:[1]:[AIBN]¼100:1:0.2 at 3.75 M monomer concentration. The activated ester CTA 1 was prepared partially to ensure solubility of the CTA in the same solvent as NSVB and the resulting homopolymer. Solubility of all reagents is critical to achieving excellent control over the RAFT polymerization, and the solvent combination of CH 3 CN and chlorobenzene (1:2 vol:vol) was used. Table 1 summarizes the molecular weight characteristics of two PNSVB polymers synthesized by RAFT. PNSVB-1 has a M n by GPC of 61 kda, a PDI of 1.07, and was produced in 73% conversion. Figure 1 shows that the GPC chromatogram of PNSVB-1 is monomodal, narrow, and corresponds to a M n of 61 kda versus the polystyrene standards. Reducing the reaction time from 28 to 25 h decreased the M n to 44 kda and conversion to 60%. It also resulted in a polymer with a narrower PDI of These two examples demonstrate that the RAFT polymerization of NSVB yields narrow PDI polymers and that control over M n can be achieved by controlling the reaction time. Table 1 shows that the observed M n is significantly higher than the calculated M n. Although there is likely some error due to the use of polystyrene standards, this difference between calculated and observed M n is most likely due to low initiator efficiency for the CTA, which was calculated to be in the range of 29 34%. Following the successful controlled polymerization of NSVB, the postfunctionalization reaction was carried out using an amine-functionalized terpy molecule to produce the new homopolymer called poly(vb terpy ). The reaction conditions are outlined in Chart 3 and have been used previously to convert activated esters to their corresponding amide. These reaction conditions were carefully studied by NMR and IR spectroscopy to ensure conversion of the activated ester to amide as well as for the presence of any hydrolysis leading to the carboxylic acid. 39 The NMR and IR Figure 1. GPC chromatogram of PNSVB homopolymer in DMF as eluant. Figure 2. CDCl 3. 1 H NMR spectrum of Poly(VB terpy ) in

7 5624 AAMER AND TEW data indicated very high ( 99%) conversion to the amide with no evidence for hydrolysis. Here, the efficiency of postfunctionalization was monitored by 1 H NMR. The succinimide methylene protons that appear around 2.8 ppm completely disappeared from the 1 H NMR spectrum after reaction as shown in Figure 2. At the same time, there are no signals at higher chemical shift than those of the terpy signals, which might be attributed to the presence of carboxylic acids. In addition, the terpy signals in the NMR occur at chemical shifts expected for terpy, not a protonated form that would result from the presence of carboxylic acids. Based on integration of the backbone aromatic peaks and the terpy-nh signals, the estimated conversion is greater than 95%. CONCLUSIONS The new activated ester monomer NSVB was polymerized under RAFT conditions to yield narrow PDI polymers with control over M n. The RAFT conditions used a functionalized CTA that also places an activated ester at the end of the polymer chain. This CTA was demonstrated to provide polymers with narrow PDI and, although not specifically exploited here, opens a pathway to end-functionalized macromolecules. The efficiency was low compared to typical CTAs; however, the CTA still leads to control over M n and narrow PDI macromolecules. The new monomer, NSVB, yielded a precursor polymer that was easily converted to the terpy-containing macromolecule under mild conditions. The easy synthetic pathway for this monomer and excellent polymerization using RAFT conditions indicated that this chemistry may be suitable for generating highly functionalized styrenic materials including block copolymers. REFERENCES AND NOTES 1. Stupp, S. Chem Rev 2005, 105, ten Brinke, G.; Ikkala, O. Chem Record 2004, 4, Qin, Y.; Cui, C. Z.; Jakle, F. Macromolecules 2007, 40, Serpe, M. J.; Craig, S. L. Langmuir 2007, 23, Zhang, L. W.; Zhang, Y. H.; Chen, Y. M. Eur Polym J 2006, 42, Tang, H. D.; Radosz, M.; Shen, Y. Q. J Polym Sci Part A: Polym Chem 2006, 44, Li, R. C.; Broyer, R. M.; Maynard, H. D. J Polym Sci Part A: Polym Chem 2006, 44, Tzanetos, N. P.; Andreopoulou, A. K.; Kallitsis, J. K. J Polym Sci Part A: Polym Chem 2005, 43, Vicente, J.; Gil-Rubio, J.; Barquero, N. J Polym Sci Part A: Polym Chem 2005, 43, Lohmeijer, B. G. G.; Schubert, U. S. J Polym Sci Part A: Polym Chem 2004, 42, Aamer, K. A.; Tew, G. N. J Polym Sci Part A: Polym Chem 2007, 45, Harth, E.; Van Horn, B.; Hawker, C. J. Chem Commun 2001, Hawker, C. J. Acc Chem Res 1997, 30, Hawker, C. J.; Bosman, A. W.; Harth, E. Chem Rev 2001, 101, Meyer, U.; Svec, F.; Frechet, J. M. J.; Hawker, C. J.; Irgum, K. Macromolecules 2000, 33, Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, Chiefari,J.;Mayadunne,R.T.A.;Moad,C.L.; Moad, G.; Rizzardo, E.; Postma, A.; Skidmore, M. A.; Thang, S. H. Macromolecules 2003, 36, Chong, Y. K.; Krstina, J.; Le, T. P. T.; Moad, G.; Postma, A.; Rizzardo, E.; Thang, S. H. Macromolecules 2003, 36, Goto, A.; Sato, K.; Tsujii, Y.; Fukuda, T.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 2001, 34, Meijs, G. F.; Rizzardo, E. J.; Macromol Sci Rev Macromol Chem Phys 1990, C30, Meijs, G. F.; Rizzardo, E.; Le, T. P. T. Polym Int 1991, 26, Moad, G.; Chiefari, J.; Chong, Y. K.; Krstina, J.; Mayadunne, R. T. A.; Postma, A.; Rizzardo, E.; Thang, S. H. Polym Int 2000, 49, Rizzardo, E.; Meijs, G. F.; Thang, S. H. Macromol Symp 1995, 98, Matyjaszewski, K.; Xia, J. H. Chem Rev 2001, 101, Barner-Kowollik, C.; Buback, M.; Charleux, B.; Coote, M. L.; Drache, M.; Fukuda, T.; Goto, A.; Klumperman, B.; Lowe, A. B.; Mcleary, J. B.; Moa, G.; Monteir, M. J.; Sanderson, R. D.; Tonge, M. P.; Vana, P. J Polym Sci Part A: Polym Chem 2006, 44, Perrier, S.; Takolpuckdee, P. J Polym Sci Part A: Polym Chem 2005, 43, Monteiro, M. J. J Polym Sci Part A: Polym Chem 2005, 43, Prescott, S. W.; Ballard, M. J.; Rizzardo, E.; Gilbert, R. G. Macromol Theory Simul 2006, 15, Moad, G.; Mayadunne, R. T. A.; Rizzardo, E.; Skidmore, M.; Thang, S. H. In Advances in Controlled/Living Radical Polymerization, Oxford University Press, Washington, DC, 2003; pp

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