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

The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes, LMXn (M = Fe, X = Cl, n = 2, 3, X = Br, n = 2; M = Co, X = CI, n = 2), bearing chelating 2,6-bis(imino)pyridyl ligands L [L = 2,6-(ArNCR1)(2)C5H3N] is reported. X-ray diffraction studies show the geometry at the metal centers to be either distorted square pyramidal or distorted trigonal bipyramidal. Treatment of the complexes LMXn with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts converting ethylene to highly linear polyethylene (PE). LFeX2 precatalysts with ketimine ligands (R-1 = Me) an approximately an order of magnitude more active than precatalysts with aldimine ligands (R-1 = H). Catalyst productivities in the range 3750-20600 g/mmol.h.bar are observed for Fe-based ketimine catalysts, while Co ketimine systems display activities of 450- 1740 g/mmol.h.bar. Molecular weights (M-w) of the polymers produced are in the range 14000-611000. Changing reaction conditions also affects productivity and molecular weight; in some systems, a bimodal molecular weight distribution is observed. On the basis of evidence gathered to date, the lower molecular weight fraction is a result of chain transfer to aluminum while the higher molecular weight fraction is produced by a combination of mainly beta-H transfer and some chain transfer to aluminum.