Ethylene Polymerization Behavior of Tris(pyrazolyl)borate Titanium(IV) Complexes

A set of Tp‘TiCl 3 and Tp‘TiCl 2 (OR) complexes containing tris(pyrazolyl)borate ligands with diverse steric properties has been evaluated for ethylene polymerization under MAO activation conditions (Tp‘ = HB(3-mesitylpyrazolyl) 2 (5-mesitylpyrazolyl) - (Tp Ms* ), HB(3-mesitylpyrazolyl) 3 - (Tp Ms ), HB(3,5-Me 2 -pyrazolyl) 3 - (Τp * ), HB(pyrazolyl) 3 - (Tp), BuB(pyrazolyl) 3 - ( Bu Tp)). The activity of Tp‘TiX 3 /MAO varies in the order Tp Ms* TiCl 3 ( 10c ) > Tp Ms TiCl 3 ≫ Τp * TiCl 3, TpTiCl 3, Bu TpTiCl 3, Tp*TiCl 2 (O t Bu), Tp*TiCl 2 (O-2- t Bu-C 6 H 4 ). The activity of 10c /MAO is similar to that of Cp 2 ZrCl 2 /MAO. High MAO levels or addition of AlMe 3 decrease the activity of 10c /MAO, probably due to coordination of AlMe 3 to the active Ti species. The predominant chain transfer mechanism for 10c /MAO is chain transfer to AlMe 3, which results in broad molecular weight distributions at low Al/Ti ratios (Al/Ti = 200−1000). At very high Al levels ( 10c /5000 MAO or 10c /1000 MAO/4000 AlMe 3 ) bimodal molecular weight distributions comprising a major low molecular weight fraction ( M w / M n ca. 3) and a minor high molecular weight fraction are observed, which suggests that several active species are present, only one of which undergoes efficient chain transfer to Al.