Olefin Polymerization by Half-Titanocenes Containing η2-Pyrazolato Ligands−MAO Catalyst Systems

A series of half-titanocenes containing pyrazolato ligands, Cp′TiCl2(3,5-R2C3HN2) (1a−d, 2a−c, 3a−c) [Cp′ = Cp (1), Cp* (2), tert-BuC5H4 (tert-BuCp, 3); R = H (a), Me (b), iPr (c), Ph (d)], have been employed as the catalyst precursors for ethylene polymerization, syndiospecific styrene polymerization, and copolymerization of ethylene with 1-hexene, styrene, and norbornene (NBE) in the presence of MAO cocatalyst. The Cp* analogues, especially 2a,b, exhibited high catalytic activities for ethylene polymerization affording the polymer with unimodal molecular weight distributions, whereas the Cp analogue (especially 1b,c) and the tert-BuCp analogues (3a−c) exhibited high catalytic activities for syndiospecific styrene polymerization. The ethylene/styrene copolymerization using the tert-BuCp−diisopropyl analogue (3c) −MAO catalyst system proceeds in a living manner, irrespective of the styrene concentration employed (in toluene at 25 °C), and the same system exhibited relatively high catalytic activity for th...