C2-symmetric ansa-metallocene catalysts for propene polymerization:: Stereoselectivity and enantioselectivity

Important similarities between the new homogeneous catalysts for I-alkene polymerization based on C-2-symmetric ansa-metallocenes of the 4th column and their heterogeneous Ziegler-Natta predecessors are unquestionable; in particular, the factors leading to the remarkable enantioselectivity of both catalyst classes have been proved to be strictly analogous. In the case of metallocene catalysts, however, an insidious side reaction of chain epimerization, unnoticed for a relatively long time, can undermine the stereoselectivity of the polymerization process; this recent finding and its mechanistic implications are discussed in the present paper. (C) 1998 Elsevier Science B.V.