Ethylene Polymerization with Sila‐Bridged Dinuclear Zirconocene Catalysts

A series of sila‐bridged dinuclear zirconocenes E(C₅H₄)₂]Cp′ZrCl₂]₂ (Cp′=C₅H₅, E = Me₂Si ( 1 ), Me₂SiSiMe₂ ( 2 ), Me₂SiOSiMe₂ ( 3 ), Me₂SiOSiMe₂OSiMe₂ ( 4 ); Cp′=C₅HMe₄, E = Me₂SiOSiMe₂ ( 5 ), Me₂SiOSiMe₂OSiMe₂ ( 6 )) have been synthesized. These complexes have been studied as catalysts for ethylene polymerization in the presence of MAO. Polymerization results indicated that the polymerization activity of the zirconocenes increased as the bridging ligand became longer for the same type of bridging ligands. Complex 4 , holding trisiloxane bridging, exhibited greater activity than the mononuclear zirconocene Cp₂ZrCl₂]. All the siloxane‐bridged dinuclear zirconocenes showed the highest activity at higher temperature (60 or 70°C) in contrast to the corresponding siloxane‐bridged dinuclear cyclopentadienyl and indenyl zirconocenes which showed a maximum activity at 40°C. The molecular weight of polyethylene with 4 was very high at low temperature (13.43×10⁵ g·mol^–1 at 20°C) but decreased significantly with increasing temperature. The molecular weight distributions obtained for polyethylene were similar with the mononuclear metallocene catalysts at low temperatures and slightly broad at higher temperatures. The tetramethylcyclopentadienyl complexes 5 and 6 showed lower polymerization activities than the corresponding cyclopentadienyl complexes 3 and 4 as a result of steric effects. The relationship between structures and catalytic properties of catalysts has been discussed.