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), Pr-i (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 degrees C), and the same system exhibited relatively high catalytic activity for the ethylene/NBE copolymerization with highly efficient NBE incorporation.