Reversion of the chain walking ability of α-diimine nickel and palladium catalysts with bulky diarylmethyl substituents

Abstract

In general, α-diimine palladium species are more likely to undergo chain walking than the corresponding nickel species, resulting in more branched and topological polyethylene. Moreover, the ligand steric effects have a significant influence on the chain walking in α-diimine system. In this contribution, a series of acenaphthene-based α-diimine ligands bearing bulky diarylmethyl moieties with various electronic effects and the corresponding Ni(II) and Pd(II) complexes were synthesized and characterized. These Ni(II) complexes exhibit high activities in ethylene polymerization even at 80 °C, generating ultrahigh-molecular-weight polyethylenes with low branching density and high melting temperature. The corresponding palladium complexes display moderate activity, leading to semicrystalline polyethylene with low branching density and high melting temperature. Polar functionalized semicrystalline polyethylene with high melting temperature can also be obtained via copolymerization of ethylene with polar monomers using these palladium complexes. Moreover, the remote nonconjugated electronic substituents exert a great influence on the ethylene (co)polymerization. Most importantly, the chain walking ability of metal species can be controlled by changing the ligand steric environment, and the diarylmethyl substituents can even reverse the chain walking trend of palladium and nickel species.