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
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.
Vitrimers can combine the advantageous properties of
cross-linked
materials with thermoplastic processability. For the prominent case
of polyethylene, established post-polymerization introduction of cross-linkable
moieties results in extremely heterogeneous compositions of the chains.
Here, we report the generation of functionalized polyethylenes directly
by catalytic insertion polymerization, with incorporated cross-linkable
aryl boronic esters or alternatively acetal-protected groups suited
for cross-linking with difunctional boronic esters. In addition to
the desired homogeneous in-chain distribution, the reactive cross-linkable
groups are enriched at the chain ends. This enables the incorporation
of all chains in the network, as also supported by simulations of
all chains’ compositions. The uniform molecular composition
of the chains reflects in resulting vitrimers’ material properties,
particularly lack of leaching with solvents. At the same time, cross-linking
is indeed fully reversible and the vitrimers can be recycled.
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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