Polyethylene with a nickel’s worth of CO
The biggest problem with polyethylene, the most abundantly manufactured plastic, is that it doesn’t break down easily once it is discarded. Chemists have long sought to introduce small quantities of carbon monoxide (CO) into polyethylene chains to promote photodegradation, but too much CO tends to jump in and spoil the plastic’s other properties. Baur
et al
. report that nickel catalysts coordinated by bulky phosphinophenolate ligands can catalyze ethylene polymerization with approximately 1% CO incorporation, preserving tensile strength while promoting degradation under ultraviolet exposure (see the Perspective by Sobkowicz). —JSY
Small amounts of in-chain keto groups render polyethylene (PE) photodegradable, a desirable feature in view of environmental plastics pollution. Free-radical copolymerization of CO and ethylene is challenging due to the formation of stable acyl radicals which hinders further chain growth. Here, we report that copolymerization to polyethylenes with desirable low ketone content is enabled in dimethyl carbonate organic solvent or under aqueous conditions at comparatively moderate pressures <350 atm that compare favorable to typical ethylene polymerization at 2000 atm. Hereby, thermoplastic processable materials can be obtained as demonstrated by injection molding and tensile testing. Colloidally stable dipersions from aqueous polymerizations form continuous thin films upon drying at ambient conditions. Extensive spectroscopic investigation including 13 C labeling provides an understanding of the branching microstructures associated with keto groups. Exposure of injection molded materials or thin films to simulated sunlight under sea-like conditions results in photodegradation.
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.
Linear polyethylenes
with a combination of incorporated in-chain
keto as well as side-chain ester groups are formed by Ni(II)-catalyzed
terpolymerization of ethylene, carbon monoxide, and methyl acrylate.
These possess a random structure, with largely isolated nonalternating
in-chain keto groups as well as ester-substituted units adjacent to
the polyethylene chain, whereas the solid-state structure of polyethylene
is retained. Molecular weights of the terpolymers (M
n ∼ 20.000 g mol–1) are predominantly
determined by chain transfer after acrylate incorporation.
The world’s most important plastic, polyethylene, consists of inert hydrocarbon chains. An introduction of reactive polar groups in these chains is much sought-after, to overcome the problematic environmental persistency and enhance compatibility with other materials. However, with state of the art catalytic polymerization processes this has not been possible. Here, we show how a low density of individual in-chain keto groups can be generated in the high molecular weight polyethylene chains by catalytic copolymerization with carbon monoxide. Most importantly, the desirable materials’ properties of high density polyethylene (HDPE) are retained. Processing can be performed by common injection molding and mechanical characteristics are on a par.<br><br>
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