Methyl-, stereoelective polymerization

The only case of stereoelective polymerization of vinyl monomers so far known is the polymerization of some racemic a-olefins with the aid of catalysts prepared from TiCl4 or TiCls and bis-[(S)-2-methyl-butyl]-zinc [104,107). 

Optically active lactones are valuable building blocks in organic synthesis (4) and in the preparation of optically active biodegradable polymers (7,5). Several chemical methods for producing these compounds and their corresponding polymers have been explored (6) but unfortunately all of these methods are either experimentally cumbersome or afford the lactones with only modest enantioselectivities. Examples of chemically prepared optically active polyesters include poly(a-phenyl-P-propiolactone) (7), poly(a-ethy(-a-phenyl-P -propiolactone) (S, 9), poly(a-methyl-a-ethyl-P-propiolactone) (70) and poly(lactic acid) (77, 72). Use of enantioselective polymerization catalysts to carry out stereoelective polymerizations of racemic lactones has produced mixed results. For example, stereoelective polymerization of [/ ,S]- P-methyl-P-propiolactone with a catalyst from Zn ( 2115)2 and [7 ]-(-)-3,3-dimethyl-l,2-butanediol showed only a small enantiomeric enrichment in the final polymer (75). Stereoselective copolymerizations of racemic (LL/DD monomers) and meso (LD monomer) lactides using chiral catalyst that gives heterotactic and syndiotactic PLA, respectively have also been studied (77). 

In a previous paper presented by Dr, Leborgne and Spassky, from the Universite Pierre et Marie Curie, in Paris, it has been shown that it is now possible to prepare optically active polylactones by a "stereoelective" polymerization process. It is the purpose of the present paper to indicate important differences in properties between the optically active and the racemic poly(a-methyl-a-n-propyl-3-propiolactone) (PMPPL). Results will be presented concerning the crystallization and melting properties, as well as the dynamic and transient mechanical properties of these polymers. 

From monomers with two chiral centers, optically active or racemic, (both in the heterocycle and in the lateral chain) different diastereoisomeric polymers can be obtained by classical or stereoelective processes. OA polythiiranes with two OA centers have been synthesized by a stereoelective polymerization of (+) or (-)A -methyl,A -sec-butyl,A-thiiranylamine using ZnEt2/( ) dimethyl-3,3-butanediol-l,2. The presence of an R) or (S) lateral chain has no influence on the stereoelection. In this case the thioether chromo-phore near the asymmetric carbon atom of the main chain becomes optically active and its contribution to the ORD curves is preponderant with no special conformational effect their characterization is now in progress [1 lOd]. 

Effect of the solvent in the stereoelective polymerization of methyl thiirane. (Initiator system ZnEtj —/ (-)-tBu—CHOH—CHj OH (1 1) prepared in situ.)... 

Variation of the percentage of the crystalline part as a function of temperature in stereoelective polymerization of methyl oxirane. 

Fig. 13. Stereoelective polymerization of methyl oxirane using ZnEtj —(-)-tBu—CHOH—CHjOH (1 1) as initiator. Variation of optical activity of the polymer laljj (C, C = 1) with conversion.

Fig. 17. Carbon 13 NMR spectra of the amorphous part of poly(methyl oxirane) obtained in stereoelective polymerization. Optical activity of the polymer = 6.3 (C H, C = 1).

Svirkin, Y. Y., Xu, J., Gross, R. A., Kaplan, D. L., and Swift, G., Enzyme-catalyzed stereoelective ring-opening polymerization of a-methyl-P-propiolactone, Macromolecules, 29, 4591 597, 1996. 

The asymmetric selectivity arises from the preferential formation of (S)-elective center at the beginning followed by the formation of (R)-elective center after the consumption of most of the (S)-monomer. The copolymerization of the (RS)-mono-mer and methyl methacrylate by this complex yielded a highly isotactic copolymer in which the (S>monomer predominantly incorporated over the (R)-monomer. On the other hand, in the copolymerization with a,a-dimethylbenzyl methacrylate only the homopolymer of a-methylbenzyl methacrylate was obtained with the same as-i mmetric selectivity as in the homopolymerization of this monomer. The results indicate that the steric interaction between the methyl group at the a- rosition of benzyl ester and the (-)-sparteine moiety of the catalyst plays an important role in the stereoelection of the polymerization. 

Effect of the temperature on stereoelectivity Few results only were heported on the effect of the tiemperature on stereoelection. In the case of monomers of first class the stereoelectivity was not modified by changing the temperature, while the stereoselectivity of the process increased by lowering the temperature of polymerization as demonstrated in the case of methyl oxi-rane (25). 

Effect of solvents and additives The role of solvent may be important in such anionic-coordinated" polymerizations. It was shown for example in the case of methyl thiirane that addition of tetrahydrofuran decreased the stereoelectivity, a competition occuring between the monomer and the solvent for the coordination on the metallic atom (30). 

In a first step the monomer reacts with the initiator to form a full spectrum of sites having different R and S character. Some of these formed species have a complete selectivity and produce crystalline isotactic polymers. The proportion of such selective species for a given initiator is depending on the nature of the monomer. We have seen that for monomers with bulky substituents like t-butyl thiirane almost all the sites are purely selective, while for other monomers like methyl oxirane only 20 % of the active species are selective. If the initiator is optically active there is an unbalanced amount of R type and S type species and therefore stereoelection will occur when polymerizing a racemic monomer mixture. 

With monomers of the first class, methyl oxirane for example, the active sites are formed in an irreversible way after the reaction (or very strong complexation) of the initial monomer with the initiator. As a proof, one finds that the stereoelectivity (r), i.e. the enahtiomorphic distribution of sites, is not modified by a change of the temperature of polymerization, but (r) is strongly depending on the enantiomeric composition of the initial monomer. 

With enantiomerically enriched monomers, the stereoelectivity ratio r, calculated from an equation similar to that proposed for stereoelective process of racemic monomer, is constant during all the course of the polymerization. This result is consistent with the assumption that the active sites are formed in the presence of the monomer in an irreversible way at the first beginning of the reaction. For methyl- and ethyl-thiiranes, the stereoelectigity a linear function of the initial... 

The attempts to obtain stereoelection with a chirally modified achiral stereospecific initiator such as ZnEt -MeOH generally failed except in the case of polymerization or diastereoisomeric mixtures of 3-methyl 1,2-epoxy pentane or 3-methyl 1,2-epithio-pentane (48). Yet in this last case, the observed election is rather due to different consumption rates for the two diastereo-isomers present in the mixture than to a real modification of the spectrum of catalytic sites. 

Other thiiranes that have heen polymerized with stereoselective and/or stereoelective initiator systems include isopropylthiirane, t-hutylthiirane, ° and cis- and tram-2,S-dimethylthiirane. ° ° The polymerization of t-butylthiirane gives pure isotactic chains with most stereospedfic initiators. An exceptionally high stereoelection was observed in the polymerization of methyl- or ethylthiirane with optically active atropisomeric initiator systems. For example, zinc (S)-l,l-hi-2-naphtholate gave an r-value of 15-20. ° ° At 67% conversion a residual monomer with an optical activity of [a]D24 = 51.77° (neat, 1 dm) was obtained (Scheme 28). 

An equilibrium of complexation of the enantiomers with the different sites was suggested as explanation of the stereoelection. However in that case an effect of the temperature on the equilibrium, i.e. ithe stereoelection, should be observed, which was not presently found in the polymerization of methyl oxirane and methyl thiirane. 

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