Dual-side ansa-zirconocene dichlorides for high molecular weight isotacticpolypropene elastomers

The four new asymmetric ansa-zirconocene dichlorides rac-[1-(9-eta(5)-fluor enyl)-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)dimethylsilan e]zirconium dichloride (4d), rac-[(9-eta(5)-fluorenyl)(2-methyl-1-eta(5)-in denyl)dimethylsilane]zirconium dichloride (4e), and rac-[(9-eta(5)-fluoreny l)(2-phenyl-1-eta(5)-indenyl)dimethylsilane]zirconium dichloride (4f) have been prepared, and their polymerization behavior was compared to the recent ly 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-bridg ed ligands are easily accessible by the reaction of fluorenyllithium with d imethyldichlorosilane and the subsequent addition of indenyllithium. A simi lar 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 h ighly active catalysts for the propene polymerization reaction after activa tion with MAO. The influence of the bridge and the particular substitution pattern of the indenyl fragments has been studied with respect to monomer c oncentration and polymerization temperature. The exchange of the ethylene b ridge by a dimethylsilane unit results in a strong increase of the molecula r weights but also in a decreased polymerization activity deriving from a f ast decomposition of the active catalyst species. Interestingly, significan tly higher polymer molecular weights could be found for the complexes that contain the 5,6-cyclopentyl substituent on the indenyl moiety. All catalyst s showed the effect of a declining stereoselectivity with increasing monome r concentration, leading to the formation of homopolypropene elastomers. Th e mechanism of stereoerror formation of these C-1-symmetric species was inv estigated by deuterium labeling studies on the propene monomers and by comp arison with Ct-symmetric complexes.