Substituent Effects of Pyridine-amine Nickel Catalyst Precursors on Ethylene Polymerization

A series of pyridine-amine nickel complexes with various
substituents
were synthesized and used to evaluate substituent effects of catalyst
precursors on the reactivity of ethylene polymerization. Substituent
effects, including the steric effect of the pyridine moiety, steric
effect of the bridge carbon, and steric and electronic effects of
the amine moiety, were investigated systematically. Introduction of
bulky aryls onto the pyridine moiety on amine pyridine nickel leads
to a significant decrease in the activity and molecular weight of
polyethylene, whereas an increase in bulk of substituents on the bridge
carbon causes an increase in the polymerization activity and molecular
weight of polyethylene. For the amine moiety, increasing the steric
hindrance results in decreasing activity and affords a higher molecular
weight polyethylene with a narrower polydispersity, and introduction
of an electron-donating group on the amine moiety leads to formation
of a high molecular weight polyethylene with enhanced activity. By
optimizing ligand frameworks and reaction conditions, two bulky pyridine-amine
nickel complexes are also developed successfully as catalyst precursors
for living polymerization of ethylene.