ansa-titanocene catalysts for α-olefin polymerization : syntheses, structures, and reactions with methylaluminoxane and boron-based activators

The racemic isomers of C2H4(1-Ind)2TiCl2 and Me2Si(2-Me-Benzind)2TiMe2 were synthesized and structurally characterized. Reactions of these and related ansa-titanocene complexes with methylaluminoxane or with the cationizing reagent [CPh3]+[B(C6F5)4]-/Al2Me6 were monitored by UV/vis, 1H NMR, and EPR methods. Under conditions approaching those typically used in olefin polymerizations, heterobinuclear cations of the type [DianTiIV(μ-Me)2AlMe2]+ (with Dian = bridged ligand dianion) dominate in each of these reaction systems, which catalyze the polymerization of propene at temperatures below ambient. Above 0 °C, these cations are reduced, mainly to neutral heterobinuclear species DianTiIII(μ-Me)2AlMe2, which appear to be inactive with regard to polymerization catalysis. In the absence of monomer, the main reducing agent appears to be ethyl aluminum admixtures in Al2Me6. Addition of monomer increases the rate of reduction rather drastically, probably via formation of titanium hydrides.