Phenylene-Bridged OSSO-Type Titanium Complexes in the Polymerization of Ethylene and Propylene

The dichloro titanium complexes (OSSO_tBu)­TiCl₂ (1) and (OSSO_Cum)­TiCl₂ (2) bearing o-phenylene-bridged OSSO-type ligands [OSSO_tBu-H = 6,6′-((1,2-phenylenebis­(sulfanediyl))­bis­(methylene))­bis­(2,4-di-tert-butyphenol) and OSSO_Cum-H = 6,6′-((1,2-phenylenebis­(sulfanediyl))­bis­(methylene))­bis­(2,4-bis­(2-phenylpropan-2-yl)­phenol)] were prepared and characterized. The X-ray structure of 1 revealed that Ti atom has an octahedral coordination geometry with an fac–fac wrapping of the [OSSO] ligand. In solution at 25 °C, 1 mainly retains the C₂ symmetric structure, whereas 2 shows an equilibrium between C₂- and C₁-symmetric stereoisomers. Activation of 2 with (Ph₃C)­[B­(C₆F₅)₄] led to a highly active catalytic system with an activity of 238 kg_PE·mol_cat^–1·bar^–1·h^–1; linear polyethylene with a T_m of 122 °C and M_w of 107 kDa were obtained under these conditions. Catalyst 1 displayed the moderate activity of 59 kg_PE·mol_cat^–1·bar^–1·h^–1. Gel permeation chromatography analysis revealed the formation of high-molecular-weight polyethylenes with very large distributions of the molecular weights, indicating a low control of the polymerization process, probably becaue of the presence of different active species in solution. Density functional theory investigation provides a rational for the relative high-molecular-weight polymers obtained with these complexes. The precatalyst 2 was also active in propylene polymerization producing atactic oligomers terminated with unsaturated end groups.