Cross-linked polymers are ubiquitous in daily life, finding applications as tires, insulation, adhesives, automotive parts, and countless other products. The covalent crosslinks in these materials render them mechanically robust, chemically resistant, and thermally stable, but they also prevent recycling of these materials into similar-value goods. Furthermore, cross-linked polymers are typically produced from petroleumbased feedstocks, and their hydrocarbon backbones render them nondegradable, making them unsustainable in the long term. In recent years, much effort has focused on the development of recycling strategies for cross-linked polymeric materials. In the following Perspective, we discuss many of these approaches, and highlight efforts to sustainably produce recyclable crosslinked polymers. We present our thoughts on future challenges that must be overcome to enable widespread, viable, and more sustainable and practical implementation of these materials, including the sustainable sourcing of feedstocks, long-term environmental stability of inherently dynamic polymers, and moving toward industrially viable synthesis and reprocessing methods.

Vitrimers are cross-linked polymer networks containing linkages that undergo thermally activated, associative exchange reactions, such that the cross-link density and overall network connectivity are preserved. Polycarbonates are industrially relevant polymers that, to our knowledge, have not yet been explored as vitrimers. We developed hydroxylfunctionalized polycarbonate networks that undergo transcarbonation exchange reactions at elevated temperatures in the presence of catalytic Ti(IV) alkoxides. The rate of transcarbonation within the networks, estimated through stress relaxation experiments, was tuned by adjusting the catalyst loading or hydroxyl group concentration in the networks. The polymer networks exhibit recovery of their tensile strength and plateau storage modulus (71−133%) after reprocessing. In addition to being reprocessable, the networks were hydrolyzed and decarboxylated in aqueous acid to recover 80 wt % of the precursor to the bifunctional cyclic carbonate monomer. These observations demonstrate that PC vitrimers are a novel class of strong, repairable polymers with more facile end-of-life degradation compared to other vitrimers and conventional thermosets. These characteristics, along with the high likelihood of deriving their monomers from bio-based sources, make PC vitrimers outstanding candidates for sustainable manufacture and use.

Identifying plastics capable of chemical recycling to monomer (CRM) is the foremost challenge in creating a sustainable circular plastic economy. Polyacetals are promising candidates for CRM but lack useful tensile strengths owing to the low molecular weights produced using current uncontrolled cationic ring-opening polymerization (CROP) methods. Here, we present reversible-deactivation CROP of cyclic acetals using a commercial halomethyl ether initiator and an indium(III) bromide catalyst. Using this method, we synthesize poly(1,3-dioxolane) (PDXL), which demonstrates tensile strength comparable to some commodity polyolefins. Depolymerization of PDXL using strong acid catalysts returns monomer in near-quantitative yield and even proceeds from a commodity plastic waste mixture. Our efficient polymerization method affords a tough thermoplastic that can undergo selective depolymerization to monomer.

Polymer networks that are cross-linked by dynamic covalent bonds often sacrifice the robust mechanical properties of traditional thermosets in exchange for rapid and efficient reprocessability. Polyurethanes are attractive materials for reprocessable cross-linked polymers because of their excellent mechanical properties, widespread use, and ease of synthesis, but their syntheses typically rely on harmful isocyanate precursors. Polyhydroxyurethanes (PHUs), derived from amines and cyclic carbonates, are promising alternatives to traditional polyurethanes. PHU networks are reprocessable via transcarbamoylation reactions even in the absence of external catalysts, but this process occurs over hours at temperatures above 150 °C. We have dramatically shortened the reprocessing times of PHU networks by incorporating dynamic disulfide bonds. Using cystamine as a comonomer gives materials with similar thermal stability and mechanical properties to other rigid cross-linked PHUs. Despite their excellent mechanical properties, these materials show rapid stress relaxation and have characteristic relaxation times as low as 30 s at 150 °C. This property enables reprocessing with quantitative recovery of cross-link density as measured by DMTA after only 30 min of elevated-temperature compression molding. Disulfide incorporation is a promising approach to obtain reprocessable, crosslinked PHU resins that are not derived from isocyanates.