C-2-SYMMETRICAL ANSA-METALLOCENE CATALYSTS FOR PROPENE POLYMERIZATION- STEREOSELECTIVITY AND ENANTIOSELECTIVITY

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