Density Functional Theory based computational methods have been applied in the mechanistic studies of the ethylene polymerization processes catalyzed by the half-metalocene titanium (IV) complexes with different ligands. In particular, a special attention has been paid to the catalysts with aryloxo ligands due to their activity in the high-temerature polymerization and co-polymerization experiments. The main goal was to rationalize the changes in activity for the systems with different substituents on the phenoxo ligand. For comparison, a well-known constrained-geometry-catalyst (CGC) was also included in the investigated set of complexes. The results reproduce the experimental trends: it has been found that the activity of systems with a single substituent in the ortho position of the phenyl ring (asymmetric systems) is comparable to that of CGC, while the complexes with two substituents at both ortho-carbon atoms (symmetric systems) are substantially less active. Further, it has been shown that an unique feature of the asymmetric complexes is a preference of the back-side pathway of the ethylene insertion. In contrast, a preference of front-side insertion is observed for the symmetric complexes as well as for CGC. It has been also demonstrated that a rotation of the aryloxo ligand around the Ti-O-C bonds is feasible; it allows for an easy transformation between the non-reactive and reactive isomers/pathways. Results of the static DFT calculations were supported by Molecular Dynamics (MD) simulations, performed at the semiempirical (MSINDO) and the ab initio (CP-MD) level.
KCS