Dual-side ansa-zirconocene dichlorides for high molecular weight isotactic polypropene elastomers

The four new asymmetric ansa-zirconocene dichlorides rac-[1-(9-eta(5)-fluorenyl)-2-(2-phenyl-1-eta(5)-indenyl)ethane]zirconium dichloride (4c), rac-[(9-eta(5)-fluorenyl)(5,6-cyclopenta -2-methyl-1-eta(5)-indenyl)dimethylsilane]zirconium dichloride (4d), rac-[(9-eta(5)-fluorenyl)(2-methyl-1-eta(5)-indenyl)dimethylsilane]zirconium dichloride (4e), and rac-[(9-eta(5)-fluorenyl)(2-phenyl-1-eta(5)-indenyl)dimethylsilane]zirconium dichloride (4f) have been prepared, and their polymerization behavior was compared to the recently published rac-[1-(9-eta(5)-fluorenyl)-2-(5,6-cyclopenta 2-methyl-1-eta(5)-indenyl)ethane]zirconium dichloride (4a) and rac-[1-(9-eta(5)-fluorenyl)-2-(2-methyl-1-eta(5)-indenyl)ethane]zirconium dichloride (4b). The Si-bridged ligands are easily accessible by the reaction of fluorenyllithium with dimethyldichlorosilane and the subsequent addition of indenyllithium. A similar route using 1-(9-fluorenyl)-2-bromoethane was applied for the synthesis of the ethylene-bridged ligands. The Zr(IV) complexes of all ligands are highly active catalysts for the propene polymerization reaction after activation with MAO. The influence of the bridge and the particular substitution pattern of the indenyl fragments has been studied with respect to monomer concentration and polymerization temperature. The exchange of the ethylene bridge by a dimethylsilane unit results in a strong increase of the molecular weights but also in a decreased polymerization activity deriving from a fast decomposition of the active catalyst species. Interestingly, significantly higher polymer molecular weights could be found for the complexes that contain the 5,6-cyclopentyl substituent on the indenyl moiety. All catalysts showed the effect of a declining stereoselectivity with increasing monomer concentration, leading to the formation of homopolypropene elastomers. The mechanism of stereoerror formation of these C-1-symmetric species was investigated by deuterium labeling studies on the propene monomers and by comparison with Ct-symmetric complexes.