Synthesis and Deconstruction of Polyethylene-type Materials

Abstract

Polyethylene deconstruction to reusable smaller molecules is hindered by the chemical inertness of its hydrocarbon chains. Pyrolysis and related approaches commonly require high temperatures, are energy-intensive, and yield mixtures of multiple classes of compounds. Selective cleavage reactions under mild conditions (<ca. 200 °C) are key to improve the efficacy of chemical recycling and upcycling approaches. These can be enabled by introduction of low densities of predetermined breaking points in the polyethylene chains during the step-growth or chain-growth synthetic construction of designed-for-recycling polyethylene-type materials. Alternatively, they can be accomplished by postpolymerization functionalization of postconsumer polyethylene waste via dehydrogenation and follow-up reactions or through oxidation to long-chain dicarboxylates. Deconstruction of litter under environmental conditions via the aforementioned break points can alleviate plastics' persistency, as a backstop to closed-loop recycling.

Balancing Degradability and Mechanical Strength in Keto Modified Polyethylene through Hydrogen Bonds

“…High keto group concentrations likely hinder PE’s crystallization, leading to the observed deterioration in mechanical properties. It is worth noting that an optimized synthesis method could improve the CO incorporation threshold without significantly altering the crystallinity. , The difference in crystallinity observed in this study compared to one recent report could explain the variations in mechanical properties.…”

“…The current concern about expanding the keto group content comes from the risk that high keto group content might disturb the PE crystalline order. It might alter the physical properties of these materials so that they cannot be used as substitutes for PE. ,, Mecking et al’s work reports that incorporating 0.3–3% CO does not perturb the crystallinity structure of PE . A higher CO content might not alter the crystallinity structure, as recently reported, indicating that this threshold could increase with an optimized synthesis process.…”

“…These modified PE materials can be generated by incorporation of carbon monoxide (CO) comonomer. , In the process of catalytic chain growth, both the free CO and the newly formed keto groups pose significant hindrance to the coordination sites, thus limiting further expansion of the chain. Moreover, CO’s heightened reactivity prompts the formation of alternating polyketones, diverging from the desired production of PE materials. , In contrast, within the context of free-radical growth, the relatively subdued reactivity of acyl radicals, which form upon the incorporation of CO, hampers additional chain growth, favoring oligomer production over the synthesis of PE materials. , Efforts have been made to develop methods for synthesizing PE with keto groups in an attempt to retain the physical properties of PE while adding degradability. − Several reports have surfaced in the past few years of successful synthesis of PE materials with low-content in-chain keto groups. ,,− For example, a modified PE material that incorporates a low density of keto groups in PE chains is reported to be photodegradable while also maintaining similar tensile properties with those of standard high-density PE . To avoid the formation of large pieces during degradation, a highly selective method is proposed to synthesize high-density PE with low-content isolated in-chain keto groups spread out, which maximizes the breakdown of the polymer chains into smaller pieces .…”

Electrochemical recycling of polymeric materials

“…Recent work, pioneered by the group of Mecking, has demonstrated that synthesizing novel polymers consisting of isolated ester or ketone linkages embedded within an aliphatic C–C backbone affords recyclable/biodegradable polyolefin-like polymers. 117–119 The insertion of weak linkages into backbones could also be used to convert polyolefin waste into recyclable counterparts while keeping the original properties. For instance, thermochemical oxidation of the polyolefin backbone enables the introduction of various functional groups such as alcohols, ketones, esters and oximes, 56,57,60,120 which could serve as chemical handles for depolymerization reactions.…”