The Catalyst Review June 2024 - 13

High-Density Polyethylene with In-Chain Photolyzable and Hydrolyzable Groups Enabling
Recycling and Degradation
The growing presence of persistent plastic waste is of grave environmental concern, and polyethylene, the most abundant and stable of these
contaminants, poses a particular challenge concerning its resistance to chemical recycling and biodegradation. Efforts are underway to modify
polyethylene's structure by introducing in-chain ester groups, which will not compromise its crystal structure and materials properties but rather endow
these HDPE-like materials with closed-loop recyclability and biodegradability. Herein, the authors describe the preparation of linear polyethylenes with
in-chain ester and keto groups and show how these impact the properties and degradability of these materials.
These workers began with models of mixed keto-esterPEs
with variable functional group densities generated
by ADMET copolymerization of tricosa-1,22-dien-12one
and undec-10-en-1-yl undec-10-enoate, followed
by exhaustive hydrogenation. Melting temperatures
determined by differential scanning calorimetry (DSC)
of these materials with an equimolar amount of ester and
keto groups correlate linearly with the functional group
density (Figure 1). No adverse effect of the simultaneous
presence of ester and keto groups on thermal properties
was observed, but instead, their contributions are additive,
and the data allows for the prediction of melting points of
targeted compositions.
The simultaneous presence of ester and keto-groups
has no adverse effect on solid structures, which, like
mechanical properties, remain HDPE-like. The functional
groups' impact on thermal properties is an additive of
those observed in neat keto-PEs and ester-PEs with low
densities of functional groups. In-chain esters provide
opportunities for chemical recycling by solvolysis under
mild conditions compared to polyethylene recycling to
monomer.
To obtain high molecular weight keto-PEs, the authors
employed the recently reported Ni(II)-catalyzed nonalternating
copolymerization of ethylene with carbon
monoxide. The resulting materials were then subjected to
Baeyer-Villiger oxidation (Scheme 1). Virtually complete
conversion to in-chain ester groups was achieved. Notably,
the oxidation proceeded with little or no chain scission,
thus yielding satisfactory high molecular weight ketoester-PEs
(e.g., Mn 48.000 gmol- 1
, Mw 77.000 gmol- 1
with 64% conversion of initial 1.3% keto groups.
Keto-PEs from chain growth copolymerization are
capable of undergoing photodegradation, which reduces
their environmental persistency. In order to evaluate
the degradability of keto-ester-PEs as well as the
contribution of the different functional groups, samples
with systematically varied compositions (from ADMET/
hydrogenation) were exposed to simulated sunlight. SEC
analysis revealed a substantial reduction of molecular
weight upon prolonged exposure under these conditions.
Baur M, Mast NK, Brahm JP, et al. (2023). Angew. Chem.
Int. Ed., doi.org/10.1002/anie.202310990.
The Catalyst Review
June 2024
13
),
Figure 1. Correlation of melting temperature vs. density of in-chain functional groups for
keto-ester-PEs with equimolar amounts of in -chain keto and ester groups (grey triangles),
and reported data for keto-PEs (red squares)[20]
and ester-PEs (blue circles).[21]
Scheme 1. Generation of in-chain ester groups via non-alternating ethylene-CO
copolymerization and postpolymerization Baeyer-Villiger oxidation (R = m-CIC6
H4
).

http://www.doi.org/10.1002/anie.202310990

The Catalyst Review June 2024

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