CHIRAL AUXILIARY CONTROL OF TACTICITY IN FREE-RADICAL POLYMERIZATION

Chiral oxazolidine acrylamides undergo stereocontrolled free radical polymerization. Remarkably high degrees of tacticity have been demonstrated in the polymerization of acrylamides with these chiral auxiliaries that are derived from valine, phenylglycine, and tert-leucine. The polyacrylamides formed in these polymerizations can be converted to poly(acrylic acid), P(AA), and poly(methyl acrylate), P(MA), by hydrolysis and esterification. Tacticities as high as 92% have been measured for P(AA) and P(MA) resulting from these reactions. NMR studies of the microstructure of the polymers formed using chiral auxiliaries to control the stereochemistry of the reaction showed that the distribution of stereochemical sequences could be predicted by a statistical analysis based upon the mechanism of stereoselection. Specifically, these studies confirm that the stereocontrol is the result of facial selectivity in the addition of the monomer to the growing polymer radical. This face selectivity is thought to be the result of steric hindrance to approach of the monomer to one face of the radical caused by the chiral auxiliary. Further NMR studies were used to propose an analysis of the stereochemistry of P(MA) at the hexad level. Using HETCOR techniques, eight of the twenty hexads were conclusively identified, and a further six were narrowed to two possible choices.