Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

In various nickel(II) salicylaldiminato ethylene polymerization catalysts, which are a versatile mechanistic probe for substituent effects, longer perfluoroalkyl groups exert a strong effect on catalytic activities and polymer microstructures compared to the trifluoromethyl group. This effect is accounted for by a reduced electron density on the active sites, and is also supported by electrochemical studies. Thus, β-hydride elimination, the key step of chain transfer and branching pathways, is disfavored while chain-growth rates are enhanced. This enhancement occurs to an extent that enables living polymerizations in aqueous systems to afford ultra-high-molecular-weight polyethylene for various chelating salicylaldimine motifs. These findings are mechanistically instructive as well as practically useful for illustrating the potential of perfluoroalkyl groups in catalyst design.