Stereoselective polymerization termination

Each of the fluorinated catalysts has an optimum temperature-range for stereoselective polymerization. At the lower temperatures, the rate of propagation and yield of polymer decrease dramatically. At higher temperatures, the molecular weight of the polymer produced becomes lower, presumably because chain transfer or termination processes increase in importance. At still higher temperatures, the stereoregulation is lost, and the low-d.p. polymer produced has a mixed, anomeric configuration. 

Mason, A. F. Tian, J. Hustad, P. D. Lobkovsky, E. B. Coates, G W. Syndiospecific living catalysts for propylene polymerization Effect of fluorination on activity, stereoselectivity, and termination. 

Polymerization of chiral monomers was reviewed recently by Sigwalt430). Two situations may arise. The non-chiral initiator may interact randomly with either enantiomer, but its parity introduces a bias in the subsequent addition. For a homosteric bias, i.e. when the addition of one enantiomer favors the subsequent addition of the same kind, the resulting polymer is composed of sequences of blocks of one enantiomer followed by a block of the other. This is a typical example of stereoselectivity. For an antistericbias, i.e. the addition of, say R enantiomer is favored by the presence of terminal S enantiomer, or vice versa, the resulting polymer shows a bias for a simple alteration. 

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