The effect of imine-carbon substituents in bis(imino)pyridine-based ethylene polymerisation catalysts across the transition series

Abstract

The synthesis, characterization and ethylene polymerisation behaviour of a series of first row transition metal complexes of the general formula LMX_n (M = Fe, Co, Mn, n = 2, X = Cl; M = V, Cr, Ti, n = 3, X = Cl; M = Ni, n = 2, X = Br) with bis(imino)pyridine ligands L are reported, whereby the ligands contain heteroatom substituents DR_m at the imine carbon (D = O, S, m = 1, R = Me, Ph, 2,6-Me₂C₆H₃ or D = N, m = 2, R = Me, Ph). Only the O- and S-substituted complexes show catalytic activity for the polymerisation of ethylene, upon activation with methylaluminoxane (MAO). The Mn- and Ni-based catalysts were found to be inactive under these conditions. The S-substituted complexes are generally more active than the O-substituted complexes and the catalytic activities increase with the size of the substituents. Iron-, vanadium- and chromium-based catalysts give highly active catalyst systems, which in some cases are more active than the well-known ketimine catalysts. All catalysts produce highly linear polyethylene with molecular weights affected by M, D and R. O-substituted catalyst systems are generally less active and produce lower molecular weight polyethylene compared to S-substituted systems.