Mechanisms of stereocontrol for doubly silylene-bridged C-s- and C-1-symmetric zirconocene catalysts for propylene polymerization. Synthesis and molecular structure of Li-2[(1,2-Me2Si)(2){C5H2-4-(1R,2S,5R-menthyl)}{C5H-3,5-(CHMe2)(2)}].3THF and [(1,2-Me2Si)(2){eta(5)-C5H2-4-(1R,2S,5R-menthyl)}{eta(5)-C5H-3,5-(CHMe2)(2)}]ZrCl2

Doubly [SiMe2]-bridged metallocenes (1,2-SiMe2)(2){eta(5)-C5H2-4-R}{eta(5)- C5H-3,5-(CHMe2)(2)}ZrCl2 (R = H (1a), CHMe2 (1b), SiMe3 (1c), CHMe(CMe3) (1 d), (+)-menthyl (1e)), when activated by methylaluminoxane (MAO), catalyze propylene polymerization with high activities. The preparations and X-ray s tructures of the dilithio salt of an enantiopure, doubly silylene-bridged b is(cyclopentadienyl) ligand, Li-2[(1,2-Me2Si)(2)-{C5H2-4-(1R,2S,5R-menthyl) }{(C5H-3,5-(CH as well as the corresponding zirconocene dichloride, [(1,2-M e2Si)(2){eta(5)-C5H2-4-( 1R,2S,5R-menthyl)}{eta(5)-C5H-3,5-(CHMe2)(2)}]ZrCl 2 (1e), are reported. The C-s,symmetric systems la-e are highly regiospecif ic and syndiospecific (>99.5%) in neat propylene. At lower propylene concen trations, polymers with lower molecular weights and tacticity (mostly m-typ e stereoerrors) are obtained. The microstructures of polymers produced unde r differing reaction conditions are consistent with stereocontrol dominated by a site epimerization process, an inversion of configuration at zirconiu m resulting from the polymer chain swinging from one side of the metallocen e wedge to the other without monomer insertion. The relative importance of chain epimerization (at the beta carbon) has been established by parallel p olymerization of 2-d(1)-propylene and d(0)-propylene with 1b/MAO at low pro pylene concentrations. The C-1-symmetric systems 1d,e/MAO display an unusua l dependence of stereospecificity on propylene concentration, switching fro m isospecific to syndiospecific with increasing propylene pressure, consist ent with a competitive unimolecular site epimerization process and a bimole cular chain propagation. The microstructures of the polypropylenes produced by 1d/MAO and 1e/MAO with [r] approximate to 50% resemble the hemiisotacti c microstructure produced by Me2C(eta(5)-C5H3-3-Me)(eta(5)-C13H8)ZrCl2 (2b) /MAO. Contrastingly, the hemiisotactic polypropylene microstructure obtaine d with 2b/MAO is found to be maintained at all propylene concentrations exa mined.