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Block Copolymer Synthesis by Living Free Radical Polymerization Process

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Block Copolymer Synthesis by Living Free Radical Polymerization Process BLOCK COPOLYMER SYNTHESIS BY LIVING FREE RADICAL POLYMERIZATION PROCESS José Carlos Moreira and Shu Hui Wang* Department of Metallurgical and Materials Engineering, University of São Paulo, Brazil – [email protected] Av. Prof. Mello Moraes 2463, Cidade Universitária, São Paulo, SP, 05508-900 In this communication, we report the synthesis of homo (2) and diblock copolymers of p-acetoxystyene with isoprene (3), using a TEMPO-based initiator (1) by a “living” free radical polymerization process. The GPC analyses indicated the block copolymers formation with higher molecular weight and polydispersity, compared to the correspondent parent homopolymers. The poly(p-acetoxystyrene-b-isoprene) copolymers were prepared with good control of composition and molecular weight up to 94500 amu. DSC analyses showed that the p-acetoxystyrene block glass transition was 5 to 18ºC lower than those determined for the parent homopolymers. The isoprene blocks’ glass transition had similar values as those of the polyisoprene homopolymer (-67 to –55ºC). 1. Introduction 2. Experimental Section In general block copolymers are synthesized by ionic, Details of the block copolymers’ synthesis and ring-opening and coordination polymerizations because characterization can be found elsewhere [14]. of the living nature of the polymer chain end [1-2]. Recent advances in the past decade have led to the 3. Results and Discussion development of “living” free radical polymerization 3.1 Conversion Studies techniques and these techniques have been used to Figure 2 shows a plot of conversion versus time for the prepare polymers with narrow molecular weight polymerization of styrene (Hawker et al. 1996) [15] distribution and block copolymers [3-6]. Block and p-acetoxystyrene at 125ºC using benzyl-TEMPO copolymers are a unique class of polymeric materials initiator (1). that undergo nanophase separation and have a variety of applications including dispersants, compatibilizers 100 and viscosity modifiers. Substituted styrene monomers ) 80 % ( are very singular due to the possibility of preparation n o of polymers with photochemical and photophysical 60 nversi o properties [7-8]. The controlled homo and 40 copolymerization of p-acetoxystyrene resulting in age C 20 Hawker et al. 1996 block copolymers remain relatively unexplored. There Percent Polymerization of p-acetoxystyrene are very few reports on controlled free radical 0 0 20406080 polymerization of p-acetoxystyrene [9-13]. In this Time (hrs) communication, we report the synthesis of homo (2) Figure 2: Evolution of conversion with time for the polymerization and diblock copolymers of p-acetoxystyene with of styrene and p-acetoxystyrene at 125oC using the initiator benzyl- isoprene (3), using a TEMPO-based initiator (1) by a TEMPO. “living” free radical polymerization process. As can be seen from Figure 2, the insertion of the O N acetoxy group on the benzene ring increases the polymerization rate, compared to styrene. The O N accelerating effect of electron-withdrawing substituent n 2 in nitroxide-mediated “living” free radical 1 OAc polymerization was previously reported by Barclay et al. [11]. ON Figure 3 shows the polymers’ polydispersity as a function of conversion for styrene (Hawker et al. 1996) [15] and p-acetoxystyrene at 125oC, using benzyl- n m 3 OAc TEMPO initiator (1). Figure 1: Chemical structures of (1) benzyl-TEMPO initiator, (2) poly(p-acetoxystyrene) macroinitiator and (3) poly(p-acetoxystyrene- b-isoprene). 238 Anais do 7o Congresso Brasileiro de Polímeros transition had similar value as that of the polyisoprene o 2.4 homopolymer (-67 to –55 C) [16]. No other transitions 2.2 were observed in the DSC. sity 2.0 4. Conclusions disper ly 1.8 The synthesis of various poly(p-acetoxystyrene) Po (PASt) via a ”living” radical polymerization procedure 1.6 Hawker et al. 1996 has been demonstrated. Moderate molecular weight Polymerization of p-acetoxystyrene 1.4 distribution was maintained throughout the 0 20406080100 Conversion (%) polymerization of the p-acetoxystyrene. The controlled nature of this system was demonstrated by formation of Figura 3: Polydispersity of the polymers as a function of well-defined diblock copolymers. Future work will polymerization conversion of styrene and p-acetoxystyrene at 125oC using benzyl-TEMPO initiator. entail the evaluation of the hydrolysis of these homopolymers and diblock copolymers. Therefore, the polymers polydispersity, in both cases, obtained from benzyl-TEMPO initiator Acknowledgements increased gradually as the conversion increased, The authors thank FAPESP (99/01783-0, 99/08444-6 although most of them are under the value expected for and 01/12849-3) and CNPq (Millenium Project) for the bimolecular radical termination. In general, the financial support. acetoxy monomer exhibited slightly higher dispersities throughout the “living” polymerization compared to Bibliography styrene. The molecular weights of the p-acetoxystyrene 1. Rempp, P.; Franta, E.; Herz, J. Adv. Polym. Sci. polymers increased in an approximately linear fashion 1988, 86, 145. with conversion, indicative of a “living” 2. Webster, O. W. Science 1991, 251, 887. polymerization. Furthermore, the moderate control of 3. Hawker, C. J; Bosman, A. W.; Harth, E. Chem. polydispersity and the incremental increase in Rev. 2001, 101, 3661. molecular weight with time indicates that the “living” 4. Benoit, D.; Harth, E.; Fox, P.; Waymouth, R. M.; nature of the polymerization is unaffected by the Hawker, C. J. Macromolecules 2000, 33, 363. presence of the electron-withdrawing acetoxy moiety 5. Malmström, E. E.; Hawker, C. J. Macromol. in the para position of styrene. The results corroborate Chem. Phys. 1998, 199, 923. those obtained by Barclay et al. [11]. 6. Georges, M. K.; Veregin, R. P. N.; Kasmaier, P. M.; Halmer, G. K. Macromolecules 1993, 26, 3.2 Diblock Copolymers 2987. In the Table I is summarized the characterization of the 7. Aguiar, M; Hu, B; Karasz, F.E.; Akcelrud, L. samples by gel permeation chromatography (GPC) and Macromolecules 1996, 29, 3161. differential scanning calorimetry (DSC) along with the 8. Barclay, G. G.; King, M.; Sinta, R.; Malmström, chemical composition of block copolymers. E.; Ito, H.; Hawker, C. J. Polym Prep 1997 38(1) 902. Table I: Molecular weight, polydispersity, and glass transition for poly(p-acetoxystyrene) (PASt) and poly(p-acetoxystyrene-b- 9. Puskas, J. E.; Kaszas, G. Prog. Polym. Sci. 2000, isoprene) (PASt-b-PIP). 25, 403. Sample Poly(p-acetoxystyrene) Tg (oC)b Compositionc PASt-b-PIP block Tg (oC)b starting blocka copolymera 10. Chen, X.; Jankova, K.; Kops, J.; Batsberg, W. J. Polymer Mw PD PASt ASt/IP Mw PD PASt-b-PIP I 17400 1.34 122.0 46.8/53.2 30200 1.83 114.7/ -64.4 Polym. Sci. Polym. Chem. Ed. 1999, 37, 627. II 17400 1.34 122.0 36.8/63.2 34700 1.72 117.4/ -62.9 III 17400 1.34 122.0 42.7/57.3 33200 1.87 104.6/ -65.0 11. Barclay, G. G.; Hawker, C, J.; Orellana, A.; IV 34600 1.59 126.0 45.2/54.8 42300 1.96 113.0/ -64.4 V 34600 1.59 126.0 43.7/56.3 45000 1.75 107.7/ -62.2 Malefant, P. R. L.; Sinta, R. F. Macromolecules VI 79200 1.69 126.3 53.3/46.7 85600 2.34 116.1/ -60.4 VII 79200 1.69 126.3 30.8/69.2 94500 1.82 109.1/ -63.6 1998, 31, 1024. Determined by aGPC, bDSC, and cFTIR analysis; PD represents the polydispersity. 12. Chen, X. Y.; Iván, B.; Kops, J; Batsberg, W. Macromol. Rapid Comm. 1998, 19, 585. The GPC analyses indicated the block copolymers 13. Gao, B.; Chen, X. Y.; Iván, B.; Kops, J.; Batsberg, formation with higher molecular weight and W. Macromol. Rapid Commun. 1997, 18, 1095. polydispersity, compared to the correspondent parent 14. Moreira, J. C.; Wang, S. H.; Proceedings of the homopolymers. The poly(p-acetoxystyrene-b-isoprene) American Chemical Society – Division of copolymers were prepared with good control of Polymeric Materials: Science & Engineering, composition and molecular weight up to 94500 amu 2003, 88, 154. and the molecular weight distribution typically in the 15. Hawker, C. J.; Barclay, G. G.; Orellana, A.; Dao, range under 2.0. DSC analyses showed that the p- J.; Devonport, W. Macromolecules 1996, 29, acetoxystyrene block glass transition had the same 5245. value as that of the poly(p-acetoxystyrene) 16. Brandrup, J.; Immergut, E. H. “Polymer homopolymer (100-120oC) reported in literature [11], Handbook”, Third Edition, 1989. although 5 to 18ºC lower than those determined for the parent homopolymers. The isoprene block glass Anais do 7o Congresso Brasileiro de Polímeros 239.
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