Stereoelective polymerization

The remarkable dependence of the polymerization stereoelectivity on the nature of the catalyst (107) and some aspects of the stereoselective polymerization of racemic a-olefins, which will be discussed later, seem to be more consistent with the second or the third hypothesis than with the first one. 

A stereoelective (252, 299, 300) or asymmetric selective (298) or enantioasymmetric (301) polymerization where one of the enantiomers polymerizes in a preferential way in an ideal case 50% of the monomer is converted into a pure optically active polymer while the remainder is recovered as nonreacted compound also in the pure enantiomeric form (e.g., R -t- nS). ... 

Stereoelective polymerization (type 3) requires the presence of a chiral catalyst with an excess of active centers of a given configuration or with a differential reactivity of the centers that catalyze polymerization of one or the other of the two enantiomers (299). With regard to racemic a-olefins, the best results were obtained with 3,7-dimethyloctene in the presence of TiCl, + Zn[(S)-2-methylbutyl]2 as catalyst (309). The resulting polymer is dextrorotatory, la o = -1-16.1, and the residual monomer is levorotatory, a o = —0.63, (310). These values indicate a rather ihodest degree of stereoelectivity. 

Stereoelective polymerization of enantiomerically unbalanced (i.e., partly resolved) mixtures of oxiranes and thiiranes has given suq)rising results (277, 318, 322). The stereoelectivity ratio, r, is greatly dependent on the enantio-... 

Thus the stereospecific polymerization of a racemic monomer will yield optically active polymers only if it is accomplished stereoelectively, namely in the presence of catalysts capable of polymerizing preferentially one of the two antipodes of the racemic monomer. 

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). 

In the field of the stereospecific heterogeneous polymerization of a-olefins, the stereoselective (106,118) and stereoelective (103) polymerization of racemic monomers, having an asymmetric carbon atom in a. 

On the above experimental basis, it appears that the stereoselectivity and the stereoelectivity of the polymerization of racemic a-olefins mainly depend, though not exclusively, on the type of catalytic system used. 

Further investigations are necessary to clarify the mechanism of stereoregulation in the polymerization of olefins and diolefins probably the stereoelective polymerization of vinyl monomers and the asymmetric polymerization of diolefinic compounds will give further interesting contributions to the future progresses in this field. 

Furthermore, the stereoselectivity and stereoelectivity of some particular types of organometallic catalysts for the racemic a-olefin polymerization were proved. 

A similar scheme was reported by Pino, based on the stereoregulating effect of electron donors and on the stereoelectivity obtained in the polymerization of racemic a-olefins in the presence of chiral bases 109). 

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. 

Sepulchre has reported the use of an easily prepared zinc-binaphthol (1) complex which gives a high degree of stereoelectivity in the polymerization of episulfides. In the polymerization of rac-ethylthiirane, the ee of the unreacted monomer is 66% at 46% conversion (Rsyj =15) (Scheme 1) [30]. Spassky and Sepulchre have previously reported the use of this compound for the highly selective of polymerization of rac-methylthiirane, where at 50% conversion, the optical purity of the unreacted monomer is 80% (Rs/r=20) [31]. 

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. 

Different independent approaches were used to investigate the mechanism of polymerization of a-olefms with heterogeneous catalysts. As a result, it was shown that isotactic polymerization of racemic mixtures of a-olefms are stereoselective. Also, optically active polymers form with optically active catalysts.Stereoelectivity and stereoselectivity are due to the intrinsic asymmetry of the catalytic centers. This conclusion comes in part from knowledge that propylene coordinates... 

T. Tsuruta, Stereoselective and asymmetric-selective (or stereoelective) polymerizations, J. Polym. Sci. 0 6, 119 (1972). 

P. Pino, A. Oschwald, F. Ciardelli, C. Carlini, and E. Chiellini, Stereoselection and stereoelection in a-olefin polymerization, in Coordination Polymerization, J. C. W. Chien, (ed.). Academic Press, New York, 1975. 

Chirality transfer polymerizations leading to an excess of one stereoisomeric unit asymmetric selective, asymmetric transforming, or stereoelective polymerization. 

Spassky, N., Wisniewski, M., Pluta, C., Le Borgne, A., 1996. High stereoelective polymerization of rac-(D,L)-lactide with a chiral Schiffs base/aluminiumalkoxide initiator. Macromolecular Chemistry and Physics 197, 2627—2637. 

Spassky, N. Stereoselective and stereoelective polymerization of oxiranes and thiiranes. ACS Symposium Series 1977, 59, 191-209. 

Sigwalt, P. Stereoelection and stereoselection in the ring-opening polymerization of epoxides and episulfides. Pure Appl. Chem. 1976, 48, 257-266. 

Haubenstock, H. Panchafingam, V. Odian, G Stereoelective polymerization of propylene oxide with a chiral aluminum alkoxide initiator. Makmmol. Chem. 1987,188, 2789-2799. 

Coulon, C. Spassky, N. Sigwalt, P. Stereoelective polymerization of racemic propylene oxide using a diethylzinc-chiral-l,2-diol system as initiator. Polymer 1976, 77, 821-827. 

Tsuruta, T. Stereoselective and stereoelective polymerizations of some oxiranes. Macwmol. Chem. Phys., Suppl. 1981, 5, 230-233. 

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