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 titanium and zirconium sulfur-bridged catalysts, respectively. The Zr/Ti CH2- and directly-bridged catalysts have higher insertion barriers of 10-15 kcal/mol. For the S-bridged systems, there is a strong interaction between the metal and the sulfur bridge, which leads to a less stabilized 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 titanium are lower than for zirconium. Flexible ligands with the ability to donate may be one method for developing more active alternative catalysts as the flexible ligand competes with the approaching olefin. The more weakly bound olefin leads to a fewer activation barrier.