Projects

Polyethylenes endowed with low densities of in‐chain hydrolyzable and photocleavable groups can improve their circularity and potentially reduce their environmental persistency. We show with model polymers derived from acyclic diene metathesis polymerization that the simultaneous presence of both groups has no adverse effect on the polyethylene crystal structure and thermal properties. Post‐polymerization Baeyer–Villiger oxidation of keto‐polyethylenes from non‐alternating catalytic ethylene‐CO chain growth copolymerization yield high molecular weight in‐chain keto‐ester polyethylenes ( M_n ≈50.000 g mol⁻¹ ). Oxidation can proceed without chain scission and consequently the desirable materials properties of HDPE are retained. At the same time we demonstrate the suitability of the in‐chain ester groups for chemical recycling by methanolysis, and show that photolytic degradation by extended exposure to simulated sunlight occurs via the keto groups.

Ionic groups can endow apolar polymers like polyethylene with desirable traits like adhesion with polar compounds. While ethylene copolymers provide a wide range of tunability via the carboxylate content and neutralization with different cations, they lack degradability or suitability for chemical recycling due to their all-carbon backbones. Here, we report ion-containing long-chain polyesters with low amounts of ionic groups (M_n = 50–60 kg/mol, <0.5 mol % of ionic monomers) which can be synthesized from plant oils and exhibit HDPE-like character in their structural and mechanical properties. In the sulfonic acid as well as neutralized sulfonate-containing polyesters, the nature of the cation counterions (Mg²⁺, Ca²⁺, and Zn²⁺) significantly impacts the mechanical properties and melt rheology. Acid-containing polyesters exhibit a relatively high capability to absorb water and are susceptible to abiotic degradation. Enhanced surface wettability is reflected by facilitation of printing on films of these polymers. Depolymerization by methanolysis to afford the neat long-chain monomers demonstrates the suitability for chemical recycling. The surface properties of the neutralized sulfonate-containing polyesters are enhanced, showing a higher adsorption capability. Our findings allow for tuning the properties of recyclable polyethylene-like polymers and widen the scope of these promising materials.

We report a novel polyester material generated from readily available biobased 1,18-octadecanedicarboxylic acid and ethylene glycol possesses a polyethylene-like solid-state structure and also tensile properties similar to high density polyethylene (HDPE). Despite its crystallinity, high melting point (T_m=96 °C) and hydrophobic nature, polyester-2,18 is subject to rapid and complete hydrolytic degradation in in vitro assays with isolated naturally occurring enzymes. Under industrial composting conditions (ISO standard 14855-1) the material is biodegraded with mineralization above 95 % within two months. Reference studies with polyester-18,18 (T_m=99 °C) reveal a strong impact of the nature of the diol repeating unit on degradation rates, possibly related to the density of ester groups in the amorphous phase. Depolymerization by methanolysis indicates suitability for closed-loop recycling.

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 η2-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.

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dc.contributor.author: Häußler, Manuel; Eck, Marcel; Rothauer, Dario; Mecking, Stefan

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 ¹³C 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.