Mechanistic Insights into Ni(II)-Catalyzed Nonalternating Ethylene–Carbon Monoxide Copolymerization

Abstract

Polyethylene materials with in-chain-incorporated keto groups were recently enabled by nonalternating copolymerization of ethylene with carbon monoxide in the presence of Ni(II) phosphinephenolate catalysts. We elucidate the mechanism of this long-sought-for reaction by a combined theoretical DFT study of catalytically active species and the experimental study of polymer microstructures formed in pressure-reactor copolymerizations with different catalysts. The pathway leading to the desired nonalternating incorporation proceeds via the cis/trans isomerization of an alkyl-olefin intermediate as the rate-determining step. The formation of alternating motifs is determined by the barrier for the opening of the six-membered C,O-chelate by ethylene binding as the decisive step. An η 2 -coordination of a P-bound aromatic moiety axially oriented to the metal center is a crucial feature of these Ni(II) catalysts, which also modulates the competition between the two pathways. The conformational constraints imposed in a 2′,6′-dimethoxybiphenyl moiety overall result in a desirable combination of disfavoring ethylene coordination along the alternating incorporation pathway, which is primarily governed by electronics, while not overly penalizing the nonalternating chain growth, which is primarily governed by sterics.

Cationic Palladium Catalyzed Nonalternating Copolymerization of Ethylene with Carbon Monoxide: Microstructure Analysis and Computational Study†

“…Regardless of CO or ethylene insertions, the cis coordination complex is lower in energy than the corresponding trans one, but the insertion at the trans site features a lower energy barrier relative to insertion at the cis site, even if the energy barrier is calculated based on more stable cis coordination complex (Table S5, CO insertion, 9.6 versus 19.4 kcal/mol; Table S6, ethylene insertion, 16.6 versus 28.6 kcal/mol). These results underline that the monomer prefers to coordinate at the cis cite and isomerize to the trans site, then the subsequent insertion occurs, which is in line with the previous literatures reported by Mecking, [ 43‐45 ] Nozaki, [ 46 ] Ziegler, [ 47 ] and Luo. [ 48 ] We also illustrated this cis ‐to‐ trans isomerization process in PNPO‐Pd system with ethylene insertion as the model reaction (Figure S3).…”

Origin of Suppressed Chain Transfer in Phosphinephenolato Ni(II)-Catalyzed Ethylene Polymerization

“…4−14 Most recently, state-of-the-art Ni(II) phosphinephenolate catalysts have been found to enable the long-sought nonalternating copolymerization 15,16 of ethylene and carbon monoxide to high-molecularweight keto-polyethylenes with largely isolated in-chain keto groups. 17,18 These do not compromise the desirable processing and materials properties of polyethylene while enabling photolytic degradation that could alleviate the problematic environmental persistency of polyethylene litter.…”

Polyethylenes with Combined In-Chain and Side-Chain Functional Groups from Catalytic Terpolymerization of Carbon Monoxide and Acrylate

“…propensity for adjacent keto and ester incorporation or other effects of incorporated functional groups on the incorporation of the other polar comonomer is observed. This suggests that the chelates, 9,15,16,19 which are formed by reversible coordination of the functional groups incorporated in the chain to the active sites, are opened much the same way as in corresponding copolymerizations of ethylene with only CO or acrylate, respectively. Therefore, no evidence for a conceivable strong preference for opening of chelates from acrylate incorporation by CO is observed.…”

“…Among others, small amounts of such keto-units can impart the material with desirable photodegradability, to reduce the problematic environmental persistency of mismanaged polyethylene waste . An access to linear HDPE-type polyethylenes with in-chain keto units (Keto-PEs) by incorporation of small amounts of carbon monoxide during ethylene polymerization had been long sought for, as usually ethylene–CO copolymerization results in alternating polyketones due to the preference for CO incorporations. , Such Keto-PE materials have been enabled only recently by nonalternating copolymerization − catalyzed by advanced phosphinophenolato − Ni­(II) complexes. Among others, due to their high molecular weights (up to M w 400.000 g mol –1 ; M n 200.000 g mol –1 ), these polymers are processable and on par in their mechanical properties with commercial high density polyethylene (HDPE) .…”