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

The dichloro titanium complexes (OSSOtBu)­TiCl2 (1) and
(OSSOCum)­TiCl2 (2) bearing o-phenylene-bridged
OSSO-type ligands [OSSOtBu-H = 6,6′-((1,2-phenylenebis­(sulfanediyl))­bis­(methylene))­bis­(2,4-di-tert-butyphenol) and OSSOCum-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 kgPE·molcat–1·bar–1·h–1; linear polyethylene with
a Tm of 122 °C and Mw of 107 kDa were obtained under these conditions. Catalyst 1 displayed the moderate activity of 59 kgPE·molcat–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.