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Stereoelection and Stereoselection in the Ring-Opening Polymerization of Epoxides and Episulfides

Abstract Polymerizations of racemic mixtures of epoxides or episulfides may lead to stereoregular polymers through a stereoselective process, and to optically active polymers through a stereoelective process that gives either atactic or isotactic polymers. Monomers that have been most studied are propylene oxide, propylene sulfide and t-butyl thiirane, and their polymerization with insoluble initiators such as alcoholates of Zn is discussed, and also the polymerization of thiiranes in homogeneous phase using cadmium thiolates. Polymers obtained in some conditions may be fully isotactic, and in the case of t-butylthiirane, optically active polymers could be fractionated, giving a racemic and a pure enantiomeric fraction. Stereoelection laws differ according to monomers, being either first or second order in enantiomers concentration, and showing or not a temperature effect and a “memory effect” of the initial enantiomeric composition of the monomer mixture. An interpretation is proposed for these various behaviours, that may lead to high optical yields for some systems (with stereoelectivity ratios up to 8).