UNSYMMETRIC ANSA-ZIRCONOCENE COMPLEXES WITH CHIRAL ETHYLENE BRIDGES -INFLUENCE OF BRIDGE CONFORMATION AND MONOMER CONCENTRATION ON THE STEREOSELECTIVITY OF THE PROPENE POLYMERIZATION REACTION

Two general synthetic routes for the preparation of ethylene-bridged a nsa-zirconocene dichlorides ac-[1-(eta5-Cp1)-1-R1-2-(eta5-Cp2)-2-R2-C2 H2]ZrCl2 (6a-c) bearing different cyclopentadienyl fragments [Cp1,Cp2 = Cp (cyclopentadienyl), Ind (indenyl), Flu (fluorenyl)] and a variabl e bridge substitution pattern (R1, R2= H, Ph) are described, using chi ral epoxides as starting materials. Diastereomeric complexes were sepa rated by crystallization. The solid-state structures of two representa tive examples are reported. In combination with methylalumoxane (MAO), -(eta5-9-fluorenyl)-2-(eta5-1-indenyl)ethane]ZrCl2 (6a), carrying no bulky bridge substituent, and the diastereomeric systems -1(R,S)-pheny l-2-(eta5-1(R,S)-indenyl)ethane]ZrCl2 (6b1 = RR,SS; 6b2 = RS,SR), show ing different bridge conformations, were used for propene polymerizati on. The stereoselectivity depends on monomer concentration for all thr ee unsymmetric catalysts. Under comparable conditions 6a and 6b1,2 pro duce polypropenes with significantly different stereoregularities. Thi s demonstrates the importance of a defined bridge conformation for the design of highly stereoselective catalysts.