Ethylene polymerization behavior of tris(pyrazolyl)borate titanium(IV) complexes

A set of Tp'TiCl3 and Tp'TiCl2(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)(-) (Tp(+)), HB(pyrazolyl)(3)(-) (Tp), BuB(pyrazoly)(3)(-) ((Bu)Tp)). The activity of Tp'TiX3/MAO varies in the order Tp(Ms*)TiCl(3) (10c) > Tp(Ms)TiCl(3) much greater than Tp*TiCl3, TpTiCl(3), (Bu)TpTiCl(3), Tp*TiCl2((OBu)-Bu-t), Tp*TiCl2(O-2-Bu-t-C6H4). The activity of 10c/MAO is similar to that of Cp2ZrCl2/MAO. High MAO levels or addition of AlMe3 decrease the activity of 10c/MAO, probably due to coordination of AlMe3 to the active Ti species. The predominant chain transfer mechanism for 10c/MAO is chain transfer to AlMe3, 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 AlMe3) bimodal molecular weight distributions comprising a major low molecular weight fraction (M-w/N-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.