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.

Figure 1

Fate of collected plastic waste in Western Europe.

Figure 2

Schematic representation of a bimodal molecular weight distribution of polyethylene, and role of different molecular weight regimes in processing and materials properties (Reprinted with permission from Tailor-Made Polymers Via Immobilization of Alpha-Olefin Polymerization Catalyst2008, 1–42. Copyright © 2008, Wiley-VCH).

Figure 3

Polyethylene crystallinity arises from van der Waals interactions between adjacent stretched hydrocarbon segments, illustrated here for a folded chain crystallite.

Figure 4

Access to PE-type materials from polyethylene waste, natural oil, and petroleum- or renewable-based ethylene. Chemical recycling is enabled via low densities of in-chain functional groups in a PE chain, and biodegradation acts as a backstop that prevents environmental accumulation of mismanaged plastic waste.

Figure 5

LDPE-like materials containing in-chain keto and ester groups from controlled free-radical terpolymerization of ethylene with CO and 2-methylene-1,3-dioxepane.

Figure 6

Routes to HDPE-like materials with in-chain keto and ester groups accessible by catalytic polymerizations. The postpolymerization oxidation route of polyethylene materials to similar materials is shown in comparison.

Figure 7

Reported catalysts for the nonalternating copolymerization of ethylene and CO.^,,,,

Figure 8

Mechanistic steps and activation barriers of nonalternating vs alternating ethylene/CO copolymerization for Pd-phosphinosulfonate (red) and Ni-phosphinophenolate catalysts (blue) as calculated by DFT.

Figure 9

(a) Concept of hydroesterificative polymerization to linear polyesters^, and competitive alkene copolymerization. (b) Formation of branched polyketoesters by combining compatible pathways of linear hydroesterificative and carbonylative alkene polymerization. (c) Catalytic cycles of hydroesterificative and carbonylative alkene polymerization with common acyl intermediate, which allows for switching of the polymerization pathways (Figure 9c: Reprinted with permission from ACS Catal. 2022, 12, 14629–14636. Copyright © 2022, American Chemical Society).

Figure 10

General overview of different approaches to polyethylene with a low degree of in-chain unsaturation via chain-growth polymerization and post-polymerization functionalization.

Figure 11

Synthesis concept of long-chain α,ω-carboxy telechelics from CTA-ROMP as PE-like building blocks.^,

Figure 12

Schematic diagram for the synthesis of long-chain monomers from fatty acids via self-metathesis, isomerizing alkoxycarbonylation, and ω-oxidation.

Figure 13

(a) Polymer synthesis of polyesters and polycarbonates via polycondensation. (b) Comparison of differential scanning calorimetry (DSC) traces of polyesters with different chain lengths (C₁₈, C₂₆, C₄₈) between functional groups. (c) Comparison of DSC traces of polycarbonates with different chain lengths (C₁₈, C₂₆, C₄₈) between functional groups.^,,,

Figure 14

(a) WAXS of PC-18, PE-18,18, and HDPE, reflexes correspond to the orthorhombic unit cell. (b) SEC traces of PE-18,18, PC-18 in comparison to commercial HDPE. (c) Stress–strain curves of PE-18,18, PC-18, and HDPE. (d) Comparison of Young’s moduli and stress at yield values for PE-18,18, PC-18, and HDPE. (e) Schematic representation of the solid-state structure of HDPE (top) and PE-like polymer (bottom) crystallites (Adapted with permission from Nature2021, 590, 423–437. Copyright © 2021, Springer Nature).

Figure 15

(a) Melting points of randomly long-spaced polyketones (red), polyesters (green), and polycarbonates (blue) vs their density of functional groups (reprinted with permission from ACS Macro Lett. 2015, 4, 704–707. Copyright © 2015, American Chemical Society). (b) Molecular and supramolecular structure of PE-22,4 according to SAXS and NMR spectroscopy (reprinted with permission from Macromolecules2007, 40, 8714–8725. Copyright © 2007, American Chemical Society).

Figure 16

Chemical recycling of PC-18 (Adapted with permission from Nature2021, 590, 423–437. Copyright © 2021, Springer Nature).

Figure 17

Mineralization of PE-2,18 and cellulose based on CO₂ evolution under industrial composting conditions at 58 °C following ISO 14855 (reprinted with permission from Angew. Chem., Int. Ed. 2023, 62, e202213438. Copyright © 2023, Wiley-VCH Verlag GmbH & Co. KGaA).

Figure 18

Deconstruction of polyethylene materials via in-chain unsaturation.

Figure 19

Transforming waste polyethylene to recyclable materials with HDPE-like mechanical properties via consecutive dehydrogenation and cross metathesis (Adapted from J. Am. Chem. Soc. 2022, 144, 51, 23280–23285. Copyright © 2022, American Chemical Society).

Figure 20

(a) Concept of transforming waste polyethylene to propylene as a valuable chemical building block (Reprinted with permission from J. Am. Chem. Soc. 2022, 144, 18526–18531. Copyright © 2022, American Chemical Society). (b) Converting waste polyethylene to propylene by dehydrogenation and subsequent tandem isomerizing ethenolysis (Reprinted with permission from Science2022, 377, 1561–1566. Copyright © 2022, The American Association for the Advancement of Science).

Figure 21

Header of H. Alter’s 1960 article proposing waste PE as a valuable resource (reprinted with permission from Ind. Eng. Chem. 1960, 52, 121–124. Copyright 2022, American Chemical Society).

Figure 22

Recent developments and applications of catalytic oxidation processes for the conversion of high-density polyethylene to mixtures of dicarboxylic acids.