Synthesis of Branched Polyethylene with “Half-Sandwich” Pyridine-Imine Nickel Complexes

Traditional cationic Pd­(II) and Ni­(II) ethylene polymerization catalysts are supported by ortho-disubstituted aryl diimine ligands. These catalysts are capable of producing high-molecular-weight polyethylene due to positioning of bulk in the two axial sites of the square coordination plane which retards chain transfer. Similar pyridine-imine complexes bearing a single ortho-disubstituted aryl imine moiety were reported to yield very low M n polyethylene. In earlier studies, “sandwich” diimine nickel catalysts incorporating two 8-arylnaphthylimino groups which provide exceptional shielding of the two axial sites were shown to yield ultrahigh-molecular-weight polyethylene. Here we demonstrate that 8-arylnaphthyl groups incorporated into pyridine-imine nickel catalysts that block only a single axial site are highly effective in retarding chain transfer. These catalysts produce branched polyethylene (ca. 30–90 branches per 1000 carbons) with M n values up to 2.6 × 104 g/mol. Effects on the catalyst lifetimes and polymerization behavior as a function of substituent variations at the imine carbon and the aryl group are reported.