Iron and Cobalt Ethylene Polymerization Catalysts Bearing 2,6-Bis(Imino)Pyridyl Ligands: Synthesis, Structures, and Polymerization Studies

Abstract

The synthesis, characterization, and ethylene polymerization behavior of a series of iron and cobalt halide complexes, LMX n (M = Fe, X = Cl, n = 2, 3, X = Br, n = 2; M = Co, X = Cl, n = 2), bearing chelating 2,6-bis(imino)pyridyl ligands L [L = 2,6-(ArNCR 1 ) 2 C 5 H 3 N] 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 LMX n with methylaluminoxane (MAO) leads to highly active ethylene polymerization catalysts converting ethylene to highly linear polyethylene (PE). LFeX 2 precatalysts with ketimine ligands (R 1 = Me) are 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 β -H transfer and some chain transfer to aluminum.