Remote Perfluoroalkyl Substituents are Key to Living Aqueous Ethylene Polymerization

Abstract

Perfluoroalkyl groups are well established substituents to enhance the solubility of molecular catalysts in apolar and fluorous media. However, they have been little studied to control the catalytic properties of active sites. 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 trifluoromethyl groups as a reference. This can be accounted for by a reduced electron density on the active sites, also supported by electrochemical studies. Hereby β-hydride elimination as the key step of chain transfer and branching pathways is disfavored while chain growth rates are enhanced. This 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 on a mechanistically instructive as well as practically useful probe illustrates the potential of perfluoroalkyl groups for catalyst design beyond solubility, which benefits from the synthetic accessibility and stability of these electron-withdrawing groups.