Fine-Tuning Nickel Phenoxyimine Olefin Polymerization Catalysts: Performance Boosting by Alkali Cations

To gain a better understanding of the influence of cationic additives on coordination-insertion polymerization and to leverage this knowledge in the construction of enhanced olefin polymerization catalysts, we have synthesized a new family of nickel phenoxyimine-polyethylene glycol complexes (NiL0, NiL2-NiL4) that form discrete molecular species with alkali metal ions (M+ = Li+, Na+, K+). Metal binding titration studies and structural characterization by X-ray crystallography provide evidence for the self-assembly of both 1:1 and 2:1 NiL:M+ species in solution, except for NiL4/Na+ which form only the 1:1 complex. It was found that upon treatment with a phosphine scavenger, these NiL complexes are active catalysts for ethylene polymerization. We demonstrate that the addition of M+ to NiL can result in up to a 20-fold increase in catalytic efficiency as well as enhancement in polymer molecular weight and branching frequency compared to the use of NiL without coadditives. To the best of our knowledge, this work provides the first systematic study of the effect of secondary metal ions on metal-catalyzed polymerization processes and offers a new general design strategy for developing the next generation of high performance olefin polymerization catalysts.