Precise anionic polymerization of methyl methacrylate: simultaneous control of molecular weight, stereoregularity and end-structure

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1 Nippon Gomu Kyokaishi, 88, No. 3, 2015, pp Precise anionic polymerization of methyl methacrylate: simultaneous control of molecular weight, stereoregularity and end-structure Yasuhiro Kohsaka and Tatsuki Kitayama Osaka University Graduate School of Engineering Science, 1-3 Machikaneyama-cho, Toyonaka, Osaka Selected from International Polymer Science and Technology, 42, No. 6, 2015, reference NG 15/03/80; transl. serial no Translated by K. Halpin INTRODUCTION Precision polymer chemistry has made astonishing progress in the last 50 years and polymers of controlled molecular weight and terminal structure can now be synthesised by a variety of polymerisation processes. At the same time, the effect of the primary structure of polymers on polymer properties has been clarified and the control of primary structure by precision polymerisation is nowadays recognised as a key industrial technology. Taking styrene-butadiene rubber as an example, the introduction of functional groups at the polymer chain ends improves the dispersibility of fillers and is known to be effective in reducing hysteresis loss [1-3]. The tacticity of a polymer strongly affects its properties: the glass transition point (T g ) of poly(methyl meth acrylate) (PMMA), for example, is around 50 C for isotactic (it-) PMMA but rises to approximately 120 C for syndiotactic (st-) PMMA. This property is utilised in the industrial manufacture [4] of acrylic thermoplastic elastomer (TPE), which has hard segments of st-pmma and soft segments of butyl polyacrylate (T g = -40 C to -50 C). In keeping with the growth in demand, the control of primary structure by precision polymerisation has frequently been reviewed in this journal [5-11]. In regard to the precision anionic polymerisation of acrylic monomers typified by methyl methacrylate (MMA), the authors have accomplished stereospecific living polymerisation that allows simultaneous control of molecular weight and tacticity [12-17]. More recently, we have successfully achieved quantitative termination enabling reactive groups to be introduced at the chain end, allowing control of all three structural factors simultaneously [18,19]. This review looks first at the background to these developments. Methods for the end-group modification of stereoregular polymers, and analysis of polymerisation reactions exploiting this methodology, are then reviewed. LIVING ANIONIC POLYMERISATION IN POLAR SOLVENTS It is known that when conducted in a polar solvent such as tetrahydrofuran (THF) in the presence of lithium chloride, the anionic polymerisation of MMA with an anionic species obtained from alkyl lithium and 1,1-diphenylethylene (DPE) proceeds in living mode and PMMA of molecular weight polydispersity index M w / M n 1.05 is obtained (Figure 1) [20]. Although PMMA of high syndiotacticity forms in this polymerisation, the tacticity (rr content about 80%) is not particularly high compared with polymerisation in non-polar solvents (q.v.). However, polymerisation in a polar solvent has the merit that end groups are relatively easy to insert. For example, if an alkyl lithium [21-23] or DPE derivative [7] modified with a functional group or functional group precursor is used, the functional group can be introduced at the initiating end (initiator method). On the other hand, it is also possible to modify the terminating end by reacting the growing anion with an electrophile, for example, end group modification with allyl halide [21], aryl halide [24] or acid chloride [24] has been reported (terminator method). Takada et al. [25] recently synthesised PMMA terminated with a nitrile oxide group by reacting the PMMA living anion polymer with transb-nitrostyrene and subjecting the anion formed to acid treatment [25]. The terminal nitrile oxide group reacts 2015 Smithers Information Ltd. T/45

2 Figure 1. Synthesis of end-functional PMMA via the living anionic polymerisation of MMA in THF whereas growing species with a meso chain end have very high meso selectivities (P m/m ), the selectivity (P r/m ) of racemo-terminated growing species is low. Isotactic specific living polymerisation with Li-iPrIB/Me 3 SiOLi initiator is effective for primary esters of methacrylic acid other than MMA: an isotactic polymer is obtained from silyl protected propargyl methacrylate, for example. It has been shown that, when a copper catalysed azide-alkyne addition-cyclisation (CuAAC) of some of the propargyl groups of the polymer is performed with organic azide after deprotection, a copolymer of different chain sequence is formed depending on the species of amine ligand used with the copper catalyst [27]. efficiently with olefin, affording polymer derivatives in which PMMA is grafted to natural rubber [25]. STEREOSPECIFIC LIVING ANIONIC POLYMERISATION IN NON-POLAR SOLVENTS Isotactic specific polymerisation Initiator or growing end anionic species and their counterions associate in a complex fashion in non-polar solvents such as toluene or dichloromethane. Their behaviour therefore differs greatly from polymerisation in polar solvents. The species of anion obtained by reacting an alkyl lithium and DPE, for example, is not initiated smoothly in non-polar solvents, giving rise to PMMA of wide molecular weight distribution. When a bulky Grignard reagent (t-bumgbr) is used as initiator, on the other hand, polymerisation proceeds in living fashion to produce polymer of low molecular weight dispersity (M w /M n ca. 1.08) [12]. In contrast with polymer isation in polar solvents, this polymerisation produces it-pmma (mm content ca. 97%). However, the polymerisation is relatively slow and unsuited to the synthesis of polymers of molecular weight above 20,000, it is also ineffective for polymerising methacrylate esters other than MMA. Teyssiê et al. [26] have reported that polymerisation in toluene using the reaction product of s-butyl lithium and cyclic siloxane as initiator affords it-pmma of molecular weight upward of 30,000 (M w /M n ca. 1.1, mm content 90-93%). Referring to this polymerisation system, we discovered that it-pmma (M w /M n = 1.2) was obtained when excess lithium trimethylsilanolate (Me 3 SiOLi) was added in anionic polymerisation with an initiator of a-lithioisopropylbutyrate (Li-iPrIB) (Figure 2) [13]. The tacticity of the polymer improves with decrease in polymerisation temperature and at -95 C the mm content reaches approx imately 99.5%. Statistical analysis has shown that stereoregular chains obey first-order Markov statistics, and Syndiotactic specific polymerisation Polymerisation with a binary initiator of trialkyl aluminium (R 3 Al) [14, 15] or ethylaluminium bis(2,6-di-t-butylphenoxide) [EtAl(ODBP) 2 ] [16] added to t-butyl lithium affords st-pmma (rr content ca. 96%) (Figure 2). The aluminium compound in polymerisation systems of this kind forms a complex with the anion species at the growing chain end, acting as an active species stabiliser that controls side-reactions, at the same time, it coordinates with the carbonyl group in the monomer, lowering the electron density of the C=C double bond, and so acts also as an activator enhancing the electrophilicity of the monomer [16]. It may be surmised that this dual function of the aluminium compound plays a key role in controlling tacticity. Terminal modification of stereoregular polymers Initiator method The choice of initiators applicable to stereo specific polymerisation systems is limited, and this generally makes it difficult to modify the polymer chain ends with an initiator containing functional groups. One of the few examples reported 28) is the binary initiator system combining silyl-protected lithium N-benzylamide (initiator 1) with EtAl(ODBP) 2, which affords terminally aminated st-pmma (Figure 3a). Terminator method: direct functionalisation In nonpolar solvents the growing anion is stabilised by interacting strongly with the counterion and additive, in polar solvents, therefore, termination with an effective electrophile like allyl halide is difficult to achieve and terminal modification requires some ingenuity. Take for example isotactic specific polymerisation using a Grignard reagent or syndiotactic specific polymerisation T/46 International Polymer Science and Technology, Vol. 42, No. 10, 2015

3 with t-buli/r 3 Al binary initiator, termination reaction with an electrophile of allyl halide or benzyl halide proceeds almost quantitatively in hours if excess strong amine base or a multidentate ligand is added and the temperature is then raised to 0 C to weaken inter action with the Li + cation or R 3 Al [29] (Figure 3b). Kamigaito et al. recently discovered that a bromine atom can be introduced at the chain end using carbon tetrabromide as terminator and exploited this to provide a macroinitiator for atom transfer radical polymer isation (ATRP) [30]. In highly stereospecific polymerisation systems using an additive such as Me 3 SiOLi or EtAl(ODBP) 2 that acts more strongly on the growing anion, termination with an electrophile barely proceeds at all irrespective of addition of amine or increase in temperature. We have recently discovered a novel means of end-functionalisation overcoming this problem. Details are given below. Figure 2. Stereospecific living anionic polymerisation in nonpolar solvents END-GROUP MODIFICATION OF STEREOREGULAR POLYMERS BY CLICK REACTION Termination of polymerisation with a-(chloromethyl)acrylate ester The authors have recently conducted an extended study of the polymerisation of a-substituted acrylate esters [18, 19, 31, 32]. Part of this work explored block copolymerisation with an a-(alkoxymethyl)acrylate ester 2 added to a living anion of stereoregular PMMA. However, almost no polymerisation of 2 occurred, instead, it was found that a termination reaction occurred in which the alkoxide ion detached itself from the growing anion, yielding stereoregular PMMA with a terminal C=C double bond (Figure 4) [32]. When the esters 3 and 4 containing a more readily eliminated halogen atom were used, termination alone proceeded, with satisfactory effici ency [18, 19]. Termination with 3 dispensed with the need to add amine or raise the temperature, proceeding more or less quantitatively simply on addition at polymerisation temperature. The same approach is effective in polymerisation with Me 3 SiOLi and EtAl(ODBP) 2, affording end-functionalised polymer of extremely high tacticity (Table 1). Termination also proceeded smoothly in the polymerisation of vinyl methacrylate (VMA) and trimethyl silyl methacrylate (TMSMA) [19]. Both polymers yielded terminally modified poly(methacrylic acid) on hydrolysis. In the polymerisation of butyl acrylate (nba), on the other hand, the growing anion was less reactive and at best only 80% of the terminal group was introduced, reflecting the characteristic feature of anionic polymerisation whereby slight differences in monomer structure greatly affect reactivity [33]. Quantitative Figure 3. Synthesis of end-functional steroregular PMMA: (a) initiator method, and (b) terminator method Figure 4. Synthesis of end-functional stereoregular PMMA via terminating reaction with a-substituted acrylate and the subsequent thio-ene click reaction catalysed by weak base 2015 Smithers Information Ltd. T/47

4 Table 1. Terminating reaction of stereospecific lviing anionic polymeristion of various monomers Monomer Additive Mn/10 3 M w /M n Tacticity F b /% (equiv.) a mm mr rr MMA (20) VMA (20) TMSMA (50) Me 3 SiOLi 7.0 EtAl(ODBP) Me 3 SiOLi 7.4 EtAl(ODBP) Me 3 SiOLi 14.0 EtAl(ODBP) nba (50) EtAl(ODBP) a Equivalent to the initiator (Li-iPrIB) b End-functionalisation ratio introduction of end-groups needs further investigation. If unreacted monomer persists at the stage where terminator is added, termination will compete with chain growth and the molecular weight distribution could widen. To examine selectivity for the two competing reactions, a mixture of 3 and 4.2 equivalents of ethyl methacrylate (EMA) was added to st-pmma living anion prepared in the presence of EtAl(ODBP) 2 (Figure 5). Analysis of the 1 H NMR spectrum of the resulting polymer revealed that almost none of the EMA polymerised (degree of polymerisation of EMA m = 0.14) as a result of selective termination [18]. The reactivity of 3 was hence estimated to be about 30 times that of EMA, showing that polymerisation is rapidly terminated despite of the presence of unreacted monomer. Modification of end-functional group by click reaction The terminal C=C double bond introduced by termination based on 3 is activated by the carbonyl group, permitting Michael addition of thiols with a base catalyst. The reaction proceeds quantitatively and is functional group selective (a click reaction), enabling quantitative modification by thiols containing various functional groups including hydroxy, amino and carboxy groups (Figure 4) [18, 19]. Furthermore, when initiation with the initiator 1 mentioned earlier is terminated with the terminator 3, the polymerisation product is a heteroteleclick polymer with an amino group at the initiating end and C=C double bond at the terminating end, enabling modification with thiol at the terminating end and modification at the initiating end using urea formation via isocyanate (Figure 6a) [19]. A series of one-pot reactions is possible, and stereoregular polymers functionalised with different groups at each end are readily obtained. Use of the terminator 5 containing a propargyl group as ester substituent affords stereoregular PMMA containing a C=C double bond and C C triple bond at the chain end (Figure 6b) [34]. The C=C double bond permitted Michael addition of thiol while the C C triple bond permitted CuAAC click reaction, one end of the polymer thus being successfully modified twice with different functional groups. The click reactions can be accomplished simultaneously and quantitatively simply by adding the reagents at the same time. A terminal azido Figure 5. Comptetitive reactions between propagation of 2nd monomer and termination with 3 against PMMA living anion Figure 6. Double end-functionalisation of stereoregular PMMA via double click reactions T/48 International Polymer Science and Technology, Vol. 42, No. 10, 2015

5 group can be introduced if polymerisation is terminated using the chloroethyl ester 6 as terminator and the polymer is then treated with sodium azide [34]. This polymer too is capable of double end-functionalisation by Michael addition of thiol and CuAAC reaction. Evaluation of monomer reactivity utilizing termination reactions As noted in 4.1, EMA and 3 can be added simultaneously to the living anion of st-pmma and the reactivity of EMA relative to 3 evaluated from the degree of polymerisation of EMA in the polymer product (Figure 5). A similar experiment with the acrylate ester showed that the 1 H NMR signals of the terminal vinylidene group appeared at different positions according as the living anion reacted with 3 and polymerisation terminated directly, or polymerisation terminated after the anion had first reacted with nba [36]. The proportion of living anion whose polymerisation terminated without reaction with nba was estimated from the integrated signal intensity as 4.5% of the total polymer, and the relative reactivity of BA taking into account the charging ratio ([3] 0 :[BA] 0 = 4.1:1) was found to be 88 times that of 3. The experiment was repeated with different monomers and the relative reactivities were estimated (Table 2) [33]. The chemical shift of the b-carbon in 13 C NMR and the e-value in the Q-e scheme have traditionally been used as indices of the relative reactivity of the monomer in anionic polymerisation systems. In the present case, however, the values of these indices are not consistent with the order of relative reactivity (Table 2). The e-value and 13 C NMR chemical shift of the b-carbon represent the electron density of the vinyl group in the isolated monomer but in the anionic polymerisation system under examination here the EtAl(ODBP) 2 used as additive coordinates with the monomer carbonyl group and evidently exerts an activating effect that enhances the reactivity of the vinyl group. Thus, the relative reactivities obtained in the present experiment should reflect the contribution of the activating effect of EtAl(ODBP) 2 and offer a more faithful representation of the reactivity in the actual polymerisation system. Comparing the relative reactivities of the monomers, it is clear that t-butyl acrylate (tba) is about one ninth as reactive as nba. The difference in reactivity presumably arises because steric hindrance from the ester substituent in tba is unfavourable toward coordination of EtAl(ODBP) 2 with the carbonyl group. Again, although acrylate esters are on the whole more reactive than methacrylates in anionic polymerisation, coordination activity towards acrylates takes precedence in polymerisations using EtAl(ODBP) 2 as auxiliary, further emphasising differences in reactivity. The results of the present technique using terminator indicate that nba has approximately 3000 times the reactivity of MMA. In practice, it has been found that when nba and MMA are copolymerised, nba reacts selectively in the initial phase of polymerisation and when all the nba has been consumed the MMA polymerises, with spontaneous formation of block copolymer [35]. Analysis of the stereoregulation mechanism in polymerisation utilising termination reactions When the polymer product from anionic polymerisation of MMA and termination with 3 under a number of different conditions was subjected to detailed 1 H NMR analysis, the termination reaction was found to have the same stereo specificity as the growth reaction [34]. The termination reaction is presumably a two-stage process of Michael addition to the vinyl group of the living anion followed by elimination of the chlorine atom, and since the first stage of Michael addition proceeds by the same mechanism as the growth reaction, it would be expected to have a similar stereospecificity. Knowledge of the stereospecificity of termination is useful for examining the stereoregulation mechanism in living anions. By way of example, we added EtAl(ODBP) 2 to the it-pmma living anion and discovered that, on further polymerisation of MMA, a stereo-block polymer formed with st-pmma coupled to it-pmma (Figure 7a) [36]. It had been anticipated that, because EtAl(ODBP) 2 was added, the active species in this reaction would transform to a syndiotactic-specific species, but when the terminator 3 was added immediately after addition of EtAl(ODBP) 2, termination proceeded in meso specific fashion, i.e. addition of EtAl(ODBP) 2 did not in itself result in syndiotactic specificity [33]. The inferred mechanism of stereo specific transformation is shown in Figure 7b. Thus, although EtAl(ODBP) 2 is known to coordinate with the MMA carbonyl group, forming an associated species of higher reactivity, it appears that the living Table 2. Relative reactivity of various (meth)acrylates Monomer Relative reactivity e-value d (b- 13 C)/ppm nba tba 3 EMA MMA TMSMA (standard) Smithers Information Ltd. T/49

6 The terminator 3 has very much higher reactivity than MMA and terminal reaction proceeds selectively despite the presence of unreacted monomer. If, conversely, a termin ator of markedly lower reactivity than monomer could be developed, a polymerisation system would materialise in which polymerisation proceeds selectively in the initial stage and termination occurs when virtually all the monomer has been consumed. The initiation, growth and termination reactions in such a polymerisation system are spontaneously controlled once all the reactants are mixed together, allowing control of molecular weight and terminal structure in one simple operation. Our research aims to realise this new system (the Atropos polymerisation system). REFERENCES Figure 7. (a) Sterospecificity-transformation in living anionic polymerisation of MMA and (b) it's proposed mechanism anion reacts exclusively with the associated species, and is transformed to a syndiotactic active species because the action of EtAl(ODBP) 2 does not extend to MMA units newly formed at the chain end. CONCLUSIONs As remarked at the outset, the tacticity of a polymer frequently affects its properties. More precise control of tacticity is being sought, not least in the application of terminally reactive polymers to materials innovation. While the end-group functionalisation of stereoregular polymers has been a major problem in that context, click reaction following termination with the terminator 3 has enabled three-way control of molecular weight, tacticity and terminal structure. These develop ments prompt expectations of materials design including polymer composites and the surface modification of stereoregular polymers exploiting end-group reactivity. 1. Hayashi M., Inagaki K., Imai A, Hirao A.: Nippon Gomu Kyokaishi, 78, 91 (2005). 2. Hattori I., Tadaki T.: Nippon Gomu Kyokaishi, 80, 140 (2007). 3. Sone T., Yuasa T.: Nippon Gomu Kyokaishi, 83, 103 (2010). 4. Morishita Y.: Nippon Gomu Kyokaishi, 86, 321 (2013). 5. Yamago S.: Nakamura Y.: Nippon Gomu Kyokaishi, 82, 522 (2009). 6. Yamago S., Nakamura Y.: Nippon Gomu Kyokaishi, 83, 35 (2010). 7. Hayashi M., Hirao A.: Nippon Gomu Kyokaishi, 80, 8 (2007). 8. Sugiyama K.: Hirao A.: Nippon Gomu Kyokaishi, 80, 59 (2007). 9. Sugiyama K.: Hirao A.: Nippon Gomu Kyokaishi, 80, 100 (2007). 10. Kanaoka S., Aoshima K: Nippon Gomu Kyokaishi, 83, 151 (2010). 11. Kanaoka S., Aoshima K: Nippon Gomu Kyokaishi, 84, 287 (2011). 12. Hatada K., Ute K., Tanaka K., Okamoto Y., Kitayama T.: Polym. J., 18, 1037 (16). 13. Kitaura T., Kitayama T.: Macromol. Rapid Commun., 28, 1889 (2007). 14. Kitayama T., Shinozaki T., Sakamoto T., Yamamoto M., Hatada K.: Makromol. Chem. Suppl., 15, 167 (19). 15. Nishiura T., Abe Y., Kitayama T.: Polym. J., 42, 868 (2010). 16. Kitayama T., Hirano T, Hatada K.: Tetrahedron, 53, (1997). 17. Hirano T., Yamaguchi H., Kitayama T., Hatada K.: Polym. J., 30, 767 (19). T/50 International Polymer Science and Technology, Vol. 42, No. 10, 2015

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