Kinetic resolution of racemic alpha-olefins with ansa-zirconocene polymerization catalysts: Enantiomorphic site vs. chain end control.

Copolymerization of racemic alpha-olefins with ethylene and propylene was carried out in the presence of enantiopure C1-symmetric ansa metallocene, {1,2-(SiMe2)2(eta5-C5H-3,5-(CHMe2)2)(eta5-C5H3)}ZrCl2 to probe the effect of the polymer chain end on enantioselection for the R- or S-alpha-olefin during the kinetic resolution by polymerization catalysis. Copolymerizations with ethylene revealed that the polymer chain end is an important factor in the enantioselection of the reaction and that for homopolymerization, chain end control generally works cooperatively with enantiomorphic site control. Results from propylene copolymerizations suggested that chain end control arising from a methyl group at the beta carbon along the main chain can drastically affect selectivity, but its importance as a stereo-directing element depends on the identity of the olefin.