Theoretical studies of the factors controlling insertion barriers for olefin polymerization by the titanium-chelating bridged catalysts. A search for more active new catalysts

In search of more active new catalysts, density functional theory was used to predict insertion barriers for ethylene polymerization for a variety of unknown Ti-chelating bridged alkoxide catalysts, [YR'XR'Y]TiCH3+, where X, Y = O, S, Se, Te, and R' = C6H4, C2H2, C2H4 with and without substituents. The use of ligands having donating and bridging atoms that are capable of donating electron density to the cationic metal center decreases the insertion barriers. For [(C6H4O)X(C6H4O)]TiCH3+, both the olefin coordination energy, X = S(21.4 kcal/mol) > Se(19.2) >Te(16.6), and migratory insertion barrier, X = S(6.4) > Se(5.9) > Te(5.7), decrease with the increasing donating capability of the bridging atom X to the metal center, i.e., via X = S < Se < Te. The oxygen bridge, however, gives the lowest insertion barrier (4.5 kcal/mol) in this group. The role of the phenyl group was explored by replacing it by C2H2 and C2H4 moieties. Having conjugation through the X-CC-Y moiety in these complexes turns out to be very important, allowing the delocalization of electron density from the incoming ethylene molecule through all atoms of the X-[(CC)Y](2) ligand, which in turn makes the bridging atom less positively charged and, consequently, the M-X interaction weaker and the insertion barrier smaller. The increase in the electron density in the X-[(CC)Y](2) ligand, as well as having chelating atoms (like O and S) with p-lone pair electrons, also reduces the insertion barrier. The complexes with the Y(C2H2)X(C2H2)Y ligand where X = Y = O and S are predicted to have the lowest insertion barriers.