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

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). Stereoselective and stereoelective polymerizations (the latter also called asymmetric selective) are polymerizations involving a steric control for the incorporation of monomer units into the polymer chains when the monomer is a mixture of at least two enantiomers. The individual active sites, or the whole catalytic system, may choose preferentially one enantiomer, giving polymer chains with a different composition from the starting mixture of R and S monomers. Most of the work done in this field has been made using racemic mixtures of R and S that are obtained more easily, but one may also start from a relatively pure enantiomer or from optically active mixtures of various compositions. Three families of monomers have been studied and the results have been described in recent review:1'2 asymmetric a olefins, N carboxyanhydrides, substituted oxiranes and thiiranes (see Fig. 1). In the case of olefins, the chiral group is a substituent to the double bond and becomes after polymerization a pendant group to the polymer chain, but in the other cases, the chiral carbon is included into the heterocycle and later into the main polymer chain. If no Asymmetric olef ins CH2 =CH_R*_._ (—CH2— CH—) R* 0 '1' * Ncarboxyanhydrides R —CH—C — (—NH—CO —CH—) / R HN—C Substituted oxiranes and itiiiranes