Enantioselectivity of Cs- and C2-symmetric ansa-metallocene catalysts in the styrene insertion

The correlation between relative reactivity of the two styrene enantiofaces and geometry of ansa-zirconocene systems has been determined by C-13 NMR microstructural analysis of the poly(propylene-co-styrene-ethylene) (P/S-E) obtained in the presence of C-2- and C-s-symmetric zirconocene catalysts. The chemical shifts assignment accomplished by using configurational additivity rules suggests that secondary styrene insertion with C-2-symmetric metallocene occurs with the opposite enantioface with respect to primary propylene insertion. On the contrary the analysis of C-13 enriched copolymer obtained in the presence of C-s-symmetric catalyst seems to indicate that the secondary styrene insertion occurs with same enantioface with respect to propylene. This stereochemistry is what one expects for a simple olefin inserting in secondary fashion despite the peculiar behavior of the aromatic monomer toward zirconocene-based catalysts.