ansa-Titanocene Catalysts for α-Olefin Polymerization. Syntheses, Structures, and Reactions with Methylaluminoxane and Boron-Based Activators

The racemic isomers of C 2 H 4 (1-Ind) 2 TiCl 2 and Me 2 Si(2-Me-Benzind) 2 TiMe 2 were synthesized and structurally characterized. Reactions of these and related ansa -titanocene complexes with methylaluminoxane or with the cationizing reagent [CPh 3 ] + [B(C 6 F 5 ) 4 ] - /Al 2 Me 6 were monitored by UV/vis, 1 H NMR, and EPR methods. Under conditions approaching those typically used in olefin polymerizations, heterobinuclear cations of the type [DianTi IV (μ-Me) 2 AlMe 2 ] + (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 DianTi III (μ-Me) 2 AlMe 2, 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 Al 2 Me 6 . Addition of monomer increases the rate of reduction rather drastically, probably via formation of titanium hydrides.