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 = Cl, n = 2), bearing
chelating 2,6-bis(imino)pyridyl ligands L [L = 2,6-(ArNCR1)2C5H3N] 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 (R1 = Me) are approximately an order of magnitude more active than precatalysts with aldimine ligands
(R1 = 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 (Mw)
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.