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.