摘要:
Two general synthetic routes for the preparation of ethylene-bridged ansa-zirconocene dichlorides rac-[1-(eta5-Cp1)-1-R1-2-(eta5-Cp2)-2-R2-C2H2]ZrCl2 (6a-c) bearing different cyclopentadienyl fragments [Cp1,Cp2 = Cp (cyclopentadienyl), Ind (indenyl), Flu (fluorenyl)] and a variable bridge substitution pattern (R1, R2= H, Ph) are described, using chiral epoxides as starting materials. Diastereomeric complexes were separated by crystallization. The solid-state structures of two representative examples are reported. In combination with methylalumoxane (MAO), rac-[1-(eta5-9-fluorenyl)-2-(eta5-1-indenyl)ethane]ZrCl2 (6a), carrying no bulky bridge substituent, and the diastereomeric systems rac-[1-(eta5-9-fluorenyl)-1(R,S)-phenyl-2-(eta5-1(R,S)-indenyl)ethane]ZrCl2 (6b1 = RR,SS; 6b2 = RS,SR), showing different bridge conformations, were used for propene polymerization. The stereoselectivity depends on monomer concentration for all three unsymmetric catalysts. Under comparable conditions 6a and 6b1,2 produce polypropenes with significantly different stereoregularities. This demonstrates the importance of a defined bridge conformation for the design of highly stereoselective catalysts.