Olefins stereoelective polymerization

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. 

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). 

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. 

Stereoselective or stereoelective polymerization of racemic monomers. The stereoselective polymerization of racemic monomers was first investigated in the case of racemic a-olefins (184). When the asymmetric carbon atom was in the a-position with respect to the double bond, essentially stereoregular polymers were obtained, the single macromolecules having isotactic main chain and carbon atoms with mostly the seune configuration in the lateral chains. The stereoselectivity decreases when the asymmetric carbon atom of the monomer is in the 3-position with... 

New results on stereoelective polymerization of i/,/-oxiranes and if,/-thiiranes are reported by Dr Spassky et al in this book, and stereoelective polymerization of asymmetric a-olefins by Dr Ciardelli et al. Here some information about the synthesis will be briefly described. 

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. 

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. 

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). 

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. 

Stereoselectivity and stereoelectivity in the polymerization of racemic a-olefins have probably the same origin as in the polymerization of epoxides (Section 3.2.2.), auid are determined by the chiral character of the catalysts used. Chiral catalytic centers of a given configuration attack prevailingly one face of the double bond of the monomer and centers of the opposite configuration attack prevailingly the other face. In the case of chiral olefins, the two diastereofaces of a monomer molecule have different reactivities, and whether the re-re face or the si-si face of this molecule is more reactive depends on the type of chirality of the asymmetric carbon atom. For instance, on the basis of the investigation on Pt-complexes (179), the si-si face is the more reactive face in an (S)-olefin. Thus chiral catalytic centers which attack the si-si face may prefer the (S)-antipode, whereas those which attack the re-re face may prefer the (R)-antipode (Scheme 23). 

A(a)<- by stereoelection the polymerization of the racemic monomer R,S) gives a predominant R oi S polymer polypropylene oxide polypropylene sulfide poly-a-aminoacid A -carboxylic acid anhydride (initiator, aluminum alkyl, Ni carboxylate phosphine). A(b) by stereoelection one enantiomer of racemic mixture polymerises more easily than the other one racemic a-olefin B Chromatography of poly-racemic RRS/SSR oiRR/SS) just some examples are known (not separation but more exactly enrichment)... 

In the case of chiral cyclic monomers the prepared polymer is isotactic, while this is not a necessity in principle in the case of chiral olefins. In most of the cases however, the choice is not as perfect as defined and, generally, one speaks of stereoelective process when there is a preferential polymerization of one type of enantiomer from a mixture. Moreover the sites controlling the polymerization could be more or less stereospecific, so that heterotactic polymers could in principle be obtained with the predominance of one type of enantiomeric unit. 

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