Iron and cobalt ethylene polymerization catalysts bearing 2,6-bis(imino)pyridyl ligands: Synthesis, structures, and polymerization studies

Chelating 2,6-bis(imino)pyridyl ligands, readily prepared via condensation reactions, coordinate to Fe(II), and Fe(III), and Co(II) halide salts to give monomeric, paramagnetic complexes. Activation of these compounds with methylaluminoxane results in highly reactive catalytic species for the production of linear polyethylene. Iron-based catalysts were found to be more active than cobalt-based systems, and ketimine ligands generally imparted enhanced efficiency compared to the analogous aldimine ligands. Bulky substituents at the ortho positions of the aryl groups were found to be crucial for optimal catalytic activity. Investigations of the various reaction conditions, including amount of MAO, ethylene pressure, reaction time and temperature, revealed that chain termination likely proceeds via two competing mechanisms, namely β-H transfer and chain transfer to aluminum