Living radical polymerization 4. Stereospecific living radical polymerization

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1 Nippon Gomu Kyokaishi, No. 2, 2010, pp. 35 xx Living radical polymerization 4. Stereospecific living radical polymerization S Yamago and Y Nakamura Kyoto University Institute for Chemical Research, Gokasho, Uji, Kyoto Selected from International Polymer Science and Technology, 37, No. 5, 2010, reference NG 10/02/35; transl. serial no Translated by K. Halpin 1. INTRODUCTION Previous reviews have looked at leading methods of living radical polymerisation (LRP) [1, 2], block copolymer synthesis utilising the growing chain end of polymers obtained by LRP, and chain-end functional group conversion [3]. While the combination of these methods has afforded control over molecular weight, molecular weight distribution and terminal structure, it gives no control over stereoregularity, a key structural factor in polymer synthesis. However, recent years have seen progress in the development of stereospecific radical polymerisation. The principle of stereoregulation in most of these polymerisations evolved independently of LRP, and recently the development of stereospecific LRP combining this principle with LRP has been keenly pursued, chiefly by the groups of Kamigaito and Okamoto. This article reviews recent advances in the higher order structural control of living polymers by stereospecific LRP, i.e. simultaneous control of molecular weight, molecular weight distribution, and stereoregularity. The reader is also referred to an existing excellent review of the field by Kamigaito et al. [4, 5]. 2. CONTROL OF STEREOREGULARITY Many anionic polymerisation or coordination polymerisation systems realising a high degree of stereoregularity (stereospecific polymerisation systems) have already been developed by skilfully controlling the coordination of the metal counter-ion or metal catalyst with monomer [6-8]. Methods that simultaneously control both the length and stereochemistry of the polymer formed have also been developed by applying living polymerisation to these systems. In contrast, stereoregularity is generally difficult to control in radical polymerisation. This is because free radicals are neutral active species, ruling out ionic or coordinating interaction between the radicals and the monomer or metals added. The stereospecific radical polymerisations hitherto documented realise very high stereoregularity by recourse to special monomers or polymerisation reaction platforms, including monomers with bulky substituents [9], polymerisation in the crystalline state [10] and template polymerisation [11]. At the same time, Okamoto et al. have shown that the stereoregularity of general-purpose monomers improves on addition of a Lewis acid or fluorinated bulky alcohol to the polymerisation system, and we review this below [12]. More recently reported methods include the use of multiple hydrogen-bonding to increase monomer bulk [13, 14] and the use of salts of methacrylic acid as monomer [15, 16], and future developments are awaited with interest. LRP offers the possibility of modifying stereoregularity by means of terminal functional groups deriving from the dormant species or catalysts functioning as activators. In practice, however, whichever method of living radical polymerisation is used, the polymer formed shows no difference in stereoregularity from conventional radical polymerisation [17, 18]. This is because the stereochemistry is determined when the terminal polymer radical reacts with the monomer, when catalysts or substituents deriving from the dormant species make absolutely no contribution to bond formation. In practice, achiral environments have no effect on stereoselectivity, even in nitroxide-mediated polymerisation (NMP) [19, 2010 Smithers Rapra Technology T/51

2 20] and atom-transfer radical polymerisation (ATRP) [21-23] using optically active nitroxyl radical or optically active metal ligands. The above stereospecific polymerisations using Lewis acids or additives such as fluoroalcohols each operate independently of the molecular weight and control of its distribution in LRP. This means that, by combining the two controlled polymerisation techniques, it should in principle be possible to realise stereospecific LRP in which stereoregularity and molecular weight/molecular weight distribution are simultaneous controlled. The approach is outlined below. 2.1 Use of Lewis acids It was well known from studies of alternating copolymerisation that Lewis acids interact with the carbonyl group of a (meth)acrylate monomer, enhancing the electrophilicity of the monomer [24, 25]. Moreover, in 2001 Okamoto et al. showed that Lewis acids were effective in improving stereoregularity [26, 27]. Thus, when the triflate of a rare earth metal such as Y(OTf) 3, Yb(OTf) 3 or Sc(OTf) 3 was added in polymerisation of a (meth)acrylamide or meth acrylate such as N-isopropylacrylamide (NIPAM) or methyl methacrylate (MMA), an isotactic polymer in which the meso dyad content (m) increased from 17-46% to 45-84% was obtained (Table 1). The key to improving stereoregularity has been multi-site interaction of Lewis acid with the carbonyl groups of the monomer [27]. Rare earth Lewis acids can generally take large coordination numbers, and as well as interacting with a plurality of carbonyl groups, the interaction is stronger for the monomer than the polymer. This means that the Lewis acid interacts with the carbonyl group substituted by the growing chain-end radical and the carbonyl group of the penultimate group, favouring the conformation with carbonyl groups located on the same side in the polymer chain; addition to the monomer from this conformation explains why the growing polymer has increased isotacticity (Figure 1). Stereospecific LRP by RAFT (reversible additionfragmentation chain transfer) polymerisation in the presence of a Lewis acid in this way was reported by Okamoto s group in 2003 [28, 29]. Thus, addition of a Lewis acid such as Y(OTf) 3 in polymerisation of NIPAM with the chain transfer agent 1 resulted in the formation of an isotactic poly(n-isopropylamide) (PNIPAM) in which m had improved from 47% to around 80%. However, there was less control over molecular weight distribution: whereas the polydispersity was no more than PDI 1.5 without a Lewis acid, polydispersity rose to PDI in polymerisation with a Lewis acid (Figure 2a). Since the Lewis acid increases polymerisation reactivity by complexing with NIPAM, the reduced control may be attributed to relatively slow inactivation of the chain end due to chain shuttling reaction. Again, Lewis acids interact with the thiocarbonyl group of the RAFT reagent, and this could have the effect of slowing the chain shuttling reaction or inactivating the polymer chain end owing to side-reaction [30]. Matyjaszewski et al. have reported that isotactic polymer forms when a rare earth triflate is added in the polymerisation of N,N-dimethylacrylamide (DMAM) using the chain transfer agent 2 (Figure 2b) [30]. However, the results are not entirely satisfactory since the monomer conversion is low. Figure 1. Proposed mechanism of the stereospecific polymerization in the presence of Lewis acid Table 1. Effect of Lewis acid on tacticity (m) in free radical polymerization of (meth)acrylamides and MMA a Monomer b ([Monomer] 0 /M) c Lewis acid m (%) ([Lewis acid]/m) d In the absence of Lewis acid In the presence of Lewis acid AM (1.0) Yb(OTf) 3 (0.1) NIPAM (2.4) Y(OTf) 3 (0.2) DMAM (1.0) Y(OTf) 3 (0.1) MAM (2.0) Yb(OTf) 3 (0.2) MMA (2.4) Sc(OTf) 3 (0.9) a In CH 3 OH at 60 C b Monomer abbreviation. AM: acrylamide, MAM: methacrylamide, and for others, see text c Initial monomer concentration d Lewis acid concentration T/52 International Polymer Science and Technology, Vol. 37, No. 6, 2010

3 Figure 2. Stereospecific polymerization with a RAFT chain transfer agent in the presence of Lewis acid. The numbers in parentheses are M n, PDI and m for polymerization in the absence of Lewis acid. a M n of polymer obtained in the absence of Lewis acid is not given Stereospecific LRP by ATRP in the presence of a Lewis acid has also been investigated [30,31]. However, despite the observation of improved stereoregularity from addition of Lewis acid, problems remain in that reaction fails to achieve high monomer conversion and there is inadequate control of molecular weight and molecular weight distribution. This is because it is difficult for the catalyst used in ATRP to coexist with Lewis acid. The combination of NMP with Lewis acids has also been investigated but has not proven effective since as well as offering less stereoregularity than other methods, the control over molecular weight and molecular weight distribution is lost [30]. This may be because, along with the effect of interaction between nitroxyl radical and Lewis acid, a high temperature is needed for polymerisation. The authors group has recently investigated the possibility of conducting TERP (telluride mediated radical polymerisation) in the presence of a Lewis acid. It was found that when PNIPAM of M n roughly 20,000-25,000 was synthesised by polymerising NIPAM in the presence of an organo-tellurium chain transfer agent and Lewis acid like Y(OTf) 3 or Sm(OTf) 3, the polymer formed was highly isotactic, m = 73-82%, and had a narrow molecular weight distribution (PDI = ). This was attributed to the high tolerance of the tellurium compound towards the Lewis acid [32]. 2.2 Use of fl uoroalcohol as solvent In 1998 Okamoto s group reported that the polymer formed when vinyl acetate (VAc) was polymerised in (CF 3 ) 3 COH (4) had higher syndiotacticity than when polymerised in bulk or in methanol [33]. Kamigaito s group has reported that 5 was also useful for improving syndiotacticity [34]. On the other hand, the fluoroalcohol CF 3 CH 2 OH (3) had almost no effect (Table 2). Since the hydroxyl group in fluoroalcohols is highly acidic owing to substitution with fluorine, 4 interacts with the carbonyl group in VAc to form a 1:1 complex. This would explain why increase in the apparent bulk of the acetoxy site in VAc is effective in modifying tacticity. Thus, the anti-conformation which avoids mutual steric hindrance between the terminal substituent and substituent of the penultimate group is thermodynamically more stable than the syn-conformation; syndiotacticity therefore increases (Figure 3). Compound 4 is not only bulkier than 3, its hydroxyl group has higher acidity, making it effective in improving syndiotacticity. Table 2. Effect of polar solvent on tacticity (r) in free radical polymerization of VAc a Solvent r (%) None 53 CH 3 OH b a Polymerization conditions: [VAc] = 2.0 M, [V-70] = 200 mm, at 20 C. b [VAc] M, AIBN was used instead of V-70 (AIBN] = 150 mm), at 20 C 2010 Smithers Rapra Technology T/53

4 LRP with conventional LRP. Using RAFT and ATRP, Matyjaszewski et al. synthesised poly(n,n-dimethylacrylamide) containing atactic (before addition of Lewis acid, m=55%) and isotactic (after addition, m=86%) blocks by adding the Lewis acid Y(OTf) 3 during polymerisation of DMAM [30]. Also reported is a similar method for polymerising NIPAM by a combination of RAFT and Lewis acid [29], and synthesis of the corresponding stereoblock polymer of 2-hydroxyethyl methacrylate (HEMA) by polymerisation with a combination of fluoroalcohol solvent and LRP using ruthenium catalyst [38]. Figure 3. Proposed mechanism of the stereospecific polymerization in the presence of fluoroalcohol Since fluoroalcohols are essentially neutral, the method is highly compatible with various LRP techniques. In developing a stereospecific LRP of VAc in 5 using iodine transfer radical polymerisation (IRP), Kamigaito s group showed that polymerisation to syndiotactic poly(vinyl acetate) (PVAc) of M n approximately 8,000 was possible without sacrificing narrow molecular weight distribution (PDI = 1.2) [34]. As noted in our previous review [2], it had generally been impossible to control high molecular weight synthesis in VAc polymerisation because dormant species deriving from head-to-head linkage formed and accumulated in the polymerisation system. However, formation of head-to-head linkages decreases in polymerisation in fluoroalcohol, enabling control of molecular weight up to high M n, since the electronic state at the olefinic site experiences a perturbation due to coordination of the fluoroalcohol to VAc. The polymerisation of MMA [35,36] and N-vinylpyrrolidone (NVP) [37] by ATRP or RAFT in a fluoroalcohol solvent has also been described, though the effect on stereoregularity is limited. However, it is very interesting to note that despite only a modest improvement in syndiotacticity, the properties of the poly(n-vinylpyrrolidone) product change considerably, the glass transition temperature falling by about 10 C. 4. STEREOGRADIENT POLYMERS Although not directly related to stereospecific polymerisation, stereogradient polymers in which the tacticity varies stepwise along a single polymer chain have been synthesised by clever exploitation of differences in the reactivity and stereospecificity of protected and depro tected monomer. The properties of these polymers are of interest. Comparing HEMA and SiHEMA, i.e. HEMA in which the alcohol site has been protected with a t-butyldimethylsilyl (TBS) group, the rate of polymerisation is twice as fast for HEMA than SiHEMA. Furthermore, polymerised in a fluoroalcohol solvent, HEMA has atactic selectivity (syndiotactic triads rr = 59%) whereas SiHEMA has syndiotactic selectivity (rr = 77%). Hence, when HEMA and SiHEMA are copolymerised by ATRP using a ruthenium catalyst and the TBS groups are then deprotected, a stereogradient poly(2-hydroxyethyl methacrylate) is obtained in which there are more atactic chains at the α-terminal end and the proportion of syndiotactic chains gradually increases towards the ω-terminus, as reported by Kamigaito (Figure 4) [39]. Furthermore, the same group has recently reported that stereogradient polymer can be synthesised by polymerising bulky methacrylate monomer near the ceiling temperature. The synthesis exploits the principle that although stereoregularity is dictated by the kinetics 3. STEREOBLOCK POLYMERS Stereoblock polymers combining different tacticities in a single polymer chain have been synthesised by combining the above stereospecific Figure 4. Stereogradient polymer synthesis by Ru-catalyzed ATRP of HEMA and SiHEMA T/54 International Polymer Science and Technology, Vol. 37, No. 6, 2010

5 in the initial phase of polymerisation, depolymerisation becomes prominent in the latter half of polymerisation and stereoregularity is then controlled by thermodynamic preference [40]. Further developments in this field are awaited with interest. 5. CONCLUSIONS We have reviewed recent advances in the simultaneous control of molecular weight, molecular weight distribution and stereoregularity by stereospecifi c LRP. It must be admitted that, at the present stage of development, simultaneous control by stereospecific LRP is still unsatisfactory compared with the level of control documented for stereospecific coordination polymerisation or anionic polymerisation. However, given the superiority of radical polymerisation in its wide monomer range and tolerance of functional groups, we may look forward to continuing research directed towards the ultimate goal of developing methods that enable tacticity to be freely controlled from atactic to isotactic and syndiotactic as the same time as controlling molecular weight in polymers rich in functionality. On the other hand, elucidation of the function of the stereoregular polymers obtained by stereospecific LRP has only just begun. Much may be anticipated from future progress in this area. The next review will look at the creation of novel polymer materials exploiting LRP. REFERENCES 1. Yamago S., Nakamura Y., Nippon Gomu Kyokaishi, 82, (2009). 2. Yamago S., Nakamura Y., Nippon Gomu Kyokaishi, 82, (2009). 3. Yamago S., Nakamura Y., Nippon Gomu Kyokaishi, 82, (2009). 4. Kamigaito M., Satoh K., J. Syn. Org. Chem. Jpn., 66, (2008). 5. Kamigaito M., Satoh K., Macromolecules, 41, (2008). 6. Hatada K., Kitayama Y., Ute K., Prog. Polym. Sci., 13, (1988). 7. Resconi L., Cavallo L., Fait A., Piemontesi P., Chem. Rev., 100, (2000). 8. Coates G. W., Chem. Rev., 100, (2000). 9. Nakano T., Mori M., Okamoto Y., Macromolecules, 26, (1993). 10. Matsumoto A., Matsumura T., Aoki S., Macromolecules, 29, (1996). 11. Serizawa T., Hamada K., Akashi M., Nature, 429, (2004). 12. Habaue S., Okamoto Y., The Chemical Record, 1, (2001). 13. Wan D., Satoh K., Kamigaito M., Macromolecules, 39, (2006). 14. Hirano T., Masuda S., Nasu S., Ute K., Sato T., J. Polym. Sci. A. Polym. Chem., 47, (2008). 15. Ishigaki Y., Takahashi K., Fukuda H., Macromol. Rapid. Commun., 15, (2001). 16. Kaneko Y., Iwakiri N., Sato S., Kadokawa J.-i., Macromolecules, 41, (2008). 17. Ando T., Kamigaito M., Sawamoto M., Macromolecules, 30, (1997). 18. Wang J.-S., Matyjaszewski K., J. Am. Chem. Soc., 117, (1995). 19. Puts R. D., Sogah D. Y., Macromolecules, 29, (1996). 20. Ananchenko G., Matyjaszewski K., Macromolecules, 35, (2002) 21. Haddleton D. M., Duncalf D, J., Kukulj D., Heming A. M., Shooter A. J., Dark A. J., J. Mater. Chem., 8, (1998). 22. Johnson R. M., Ng C., Samson C. C. M., Fraser C. L., Macromolecules, 33, (2000). 23. lizuka Y., Li Z., Satoh K., Kamigaito M., Okamoto Y., Ito J.-i., Nishiyama H., Eur. J. Org. Chem., (2007). 24. Hirooka M., Yabuuchi H., Morita S., Kawasumi S., Nakaguchi K., J. Poly. Sci. C Polym. Lett., 5, (1967). 25. Gotoh Y., lihara T., Kanai N., Toshima N., Hirai H., Chem. Lett., 20, (1990) 26. Isobe Y., Fujioka D., Habaue S., Okamoto Y., J. Am. Chem. Soc., 123, (2001). 27. Isobe Y., Nakano T., Okamoto Y., J. Polym. Sci. A. Polym. Chem., 39, (2001). 28. Ray B., Isobe Y., Morioka K., Habaue S., Okamoto Y., Kamigaito M., Sawamoto M., Macromolecules, 36, (2003). 29. Ray B., Isobe Y., Matsumoto K., Habaue S., Okamoto Y., Kamigaito M., Sawamoto M., Macromolecules, 37, (2004). 30. Lutz J.-F., Neugebauer D., Matyjaszewski K., J. Am. Chem. Soc., 125, (2003). 31. Sugiyama Y., Satoh K., Kamigaito M., Okamoto Y., J. Polym. Sci. A. Polym. Chem., 44, (2006). 32. Kobayashi Y., Yamago S., Polym. Prep. Jpn., 55, 153 (2006). 33. Yamada K., Nakano T., Okamoto Y., Macromolecules, 21, (1998) Smithers Rapra Technology T/55

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