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

The dichloro titanium complexes (OSSO )TiCl₂ (1) and (OSSO_Cum)TiCl₂ (2) bearing o-phenylene-bridged OSSO-type ligands [OSSO -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 C ₂ symmetric structure, whereas 2 shows an equilibrium between C ₂- and C ₁-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.