Efficient Copolymerization of Acrylate and Ethylene with Neutral P, O‑Chelated Nickel Catalysts: Mechanistic Investigations of Monomer Insertion and Chelate Formation

The efficient copolymerization of
acrylates with ethylene using
Ni catalysts remains a challenge. Herein, we report two neutral Ni­(II)
catalysts (POP-Ni-py (1) and PONap-Ni-py (2)) that exhibit high thermal stability and significantly higher incorporation
of polar monomer (for 1) or improved resistance to tert-butylacrylate (tBA)-induced chain transfer (for 2), in comparison to previously reported catalysts. Nickel
alkyl complexes generated after tBA insertion, POP-Ni-CCO­(py) (3) and PONap-Ni-CCO­(py) (4), were isolated and,
for the first time, characterized by crystallography. Weakened lutidine
vs pyridine coordination in 2-lut facilitated the isolation
of a N-donor-free adduct after acrylate insertion PONap-Ni-CCO (5) which represents a novel example of a four-membered chelate
relevant to acrylate polymerization catalysis. Experimental kinetic
studies of six cases of monomer insertion with aforementioned nickel
complexes indicate that pyridine dissociation and monomer coordination
are fast relative to monomer migratory insertion and that monomer
enchainment after tBA insertion is the rate limiting step of copolymerization.
Further evaluation of monomer insertion using density functional theory
studies identified a cis–trans isomerization via Berry-pseudorotation
involving one of the pendant ether groups as the rate-limiting step
for propagation, in the absence of a polar group at the chain end.
The energy profiles for ethylene and tBA enchainments are in qualitative
agreement with experimental measurements.