Theoretical studies of the factors controlling insertion barriers for olefin polymerization by the titanium-chelating bridged catalysts. A search formore 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 d onating electron density to the cationic metal center decreases the inserti on barriers. For [(C6H4O)X(C6H4O)]TiCH3+, both the olefin coordination ener gy, X = S(21.4 kcal/mol) > Se(19.2) >Te(16.6), and migratory insertion barr ier, 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 repla cing it by C2H2 and C2H4 moieties. Having conjugation through the X-CC-Y mo iety in these complexes turns out to be very important, allowing the deloca lization of electron density from the incoming ethylene molecule through al l atoms of the X-[(CC)Y](2) ligand, which in turn makes the bridging atom l ess positively charged and, consequently, the M-X interaction weaker and th e 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.