The four new asymmetric ansa-zirconocene dichlorides rac-[1-(9-η 5 -fluorenyl)-2-(2-phenyl-1-η 5 -indenyl)ethane]zirconium dichloride (4c), rac-[(9-η 5 -fluorenyl)(5,6-cyclopenta-2-methyl-1-η 5 -indenyl)dimethylsilane]zirconium dichloride (4d), rac-[(9-η 5 -fluorenyl)(2-methyl-1-η 5indenyl)dimethylsilane]zirconium dichloride (4e), and rac-[(9-η 5 -fluorenyl)(2-phenyl-1-η 5indenyl)dimethylsilane]zirconium dichloride (4f) have been prepared, and their polymerization behavior was compared to the recently published rac- 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 C 2 -symmetric complexes.

Synthesis and polymerization data of a series of new sulfur-containing C 1 -symmetric metallocenes,rac-[1-(9-η 5 -fluorenyl)-2- (8), are reported. The characteristic structural feature of these complexes is the 2-methyl-1H-benzo[b]indeno[4,5-d]thiophene moiety (3: MBIT), which was derived from dibenzothiophene. In the case of 7a-c the MBIT unit shows a "back" orientation, whereas in 8 it is directed to the "front" of the tetrahedral metallocene dichloride species, which was unambiguously illustrated by X-ray structure analysis. Propylene polymerization experiments revealed higher activity and stereoselectivity for 7a/MAO compared to 8/MAO. The catalysts 7a,8/MAO were found to operate under different polymerization mechanisms indicated by a diverse correlation of tacticity and polymerization temperature. Crystalline, flexible polypropylenes with tacticities ranging from 65 e [mmmm] e 90 and molecular weights up to 2 × 10 5 g mol -1 were obtained for 7a/MAO, whereas experiments using 8/MAO produced low tacticity, waxy materials. To achieve ultrahigh molecular weight isotactic polypropylene plastomers (M w up to 1.5 × 10 6 g mol -1 ), further investigations on the hafnium dimethyl complex 7c/[(C 6 H 5 ) 3 C + ][(C 6 F 5 ) 4 B -] in liquid propylene were performed.