THEORETICAL-STUDIES OF ETHYLENE POLYMERIZATION REACTIONS CATALYZED BYZIRCONIUM AND TITANIUM CHELATING ALKOXIDE COMPLEXES

Ethylene polymerization catalyzed by zirconium and titanium chelating alkoxide complexes has been studied using a density functional method. Activation barriers of 6.4 and 7.4 kcal/mol are found for the titaniu m and zirconium sulfur-bridged catalysts, respectively. The Zr/Ti CH2- and directly-bridged catalysts have higher insertion barriers of 10-1 5 kcal/mol. For the S-bridged systems, there is a strong interaction b etween the metal and the sulfur bridge, which leads to a less stabiliz ed cc-complex and a lower activation energy. The larger exothermicity to the gamma-agostic direct product by 5 kcal/mol in the titanium case compared to zirconium for all three catalysts (S-, CH2-, and directly -bridged) is suggested to be one of the reasons why the barriers for t itanium are lower than for zirconium. Flexible ligands with the abilit y to donate may be one method for developing more active alternative c atalysts as the flexible ligand competes with the approaching olefin. The more weakly bound olefin leads to a fewer activation barrier.