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 Mn 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 Mn 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.