The amido-bridged zirconocene's reactivity and catalytic behavior for ethylene polymerization

Reaction of the amido-bridged zirconium complex (CpSiMe₂NSiMe₂Cp)ZrCH₃ (1) (Cp = C₅H₄) with half an equivalent of B(C₆F₅)₃ or Ph₃CB(C₆F₅)₄ afforded the binuclear zirconium complexes [(CpSiMe₂NSiMe₂Cp)Zr)₂(μ-CH₃)][RB(C₆F₅)₃] (2a, R = CH₃, 2b, R = C₆F₅) with a methyl group as the bridge between the two zirconium atoms. In the presence of one equivalent of B(C₆F₅)₃ or Ph₃C(C₆F₅)₄, 1 was transformed to the zwitterionic complexes [(CpSiMe₂NSiMe₂Cp)Zr][RB(C₆F₅)₃] (3a, R = CH₃, 3b, R = C₆F₅) which are free of a metal-bound σ-alkyl ligand. 2b is stable with Me₃Al while 3b combined with Me₃Al to form a hetero-binuclear complex [(CpSiMe₂NSiMe₂Cp)Zr(μ-CH₃)]Al(CH₃)₂][B(C₆F₅)₄] (4) as shown by NMR spectroscopy at room temperature. Treatment of 2a or 3a with an excess of Me₃Al led to (CpSiMe₂NSiMe₂Cp)Zr(C₆F₅) (5) through a group exchange process. 2b, 3a and 5 have been characterized by X-ray diffraction studies. 2a, 2b, 3a and 3b were highly active catalysts for ethylene polymerization and copolymerization with 1-octene in the presence of trialkylaluminium, but the binuclear zirconium complexes (2a and 2b) showed higher activities than their mononuclear counterparts 3a and 3b. Polymerization activities varied with the trialkylaluminiums and increased with the trialkylaluminium concentration applied in the system. The product existed mainly in the form of Al(PE)₃ with polymeric chains, and its molecular weight and distribution were greatly influenced by the type and amount of trialkylaluminium applied in the catalytic system.