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)pyr idyl ligands L [L = 2,6-(ArNCR1)(2)C5H3N] is reported. X-ray diffraction st udies show the geometry at the metal centers to be either distorted square pyramidal or distorted trigonal bipyramidal. Treatment of the complexes LMX n with methylaluminoxane (MAO) leads to highly active ethylene polymerizati on 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). Ca talyst 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 p roductivity and molecular weight; in some systems, a bimodal molecular weig ht 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.