The role of bulky substituents in the polymerization of ethylene using late transition metal catalysts: a comparative study of nickel and iron catalyst systems

A series of nickel(II) and iron(II) complexes of the general formula [LMX2] containing bidentate (for M = Ni) and tridentate (for M = Fe) heterocycle-imine ligands L have been synthesized and characterized. Compared to the well-known alpha-diimine nickel and bis(imino)pyridine iron catalysts, these systems contain a bulky imine substituent on one side and a non-bulky N-heterocycle on the other. Depending on the ligand and the conditions used, either four- or five-coordinate complexes are obtained in the case of nickel. Iron complexes are generally five-coordinate, even with potentially tetradentate ligands. Activation of these precatalysts with MAO affords active catalyst systems for the oligomerization/polymerization of ethylene. Compared to alpha-diimine nickel and bis(imino)pyridine iron catalysts, both- metal systems provide only half of the steric protection and consequently the catalytic activities and the degree of polymerization are significantly lower. Lower activities are attributed to a reduced stability of the active species under polymerization conditions, whereas the lower molecular weights are a result of increased beta-H transfer rates. Variations within the heterocyclic component of the ligand reveal that both steric and electronic factors influence the polymerization behavior of these catalysts. (C) 2002 Elsevier Science B.V. All rights reserved.