Chain termination stereoselective polymerization

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. 

To develop better stereocontrol agents, it is crucial to understand stereoselection at a fundamental mechanistic level and appreciate the subtle but crucial differences between tacticity determination in ionic and coordination versus radical processes. As previously outlined, the propagation step of Hving radical polymerization is identical to that of free-radical polymerization and so the mechanism of tacticity determination in these processes is also identical. In radical polymerization, the planar (sp hybridized) carbon radical of the polymer terminus is prochiral, and its stereochemistry is determined during subsequent monomer addition. The addition of monomer occurs exclusively to the anti-h.ce of the radical, as the syn-face is shielded by the steric bulk of the penultimate group (Scheme 2). Hence, the prochiral polymer terminus can adopt either pro-meso or pro-racemo arrangements defined by the relative orientation of the terminal and penultimate side-chains with respect to the macromolecular C—C backbone. 

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