Degradable thermosets with tailored properties using a single monomer
Crosslinked polymers, or thermosets, represent 15%–20% of global polymer production. These materials offer high mechanical strength, thermal stability, and chemical resistance from their crosslinked networks, making them vital across industrial and consumer applications. However, conventional thermosets such as epoxies and phenolic resins possess high depolymerization energy barriers due to dense crosslinking, preventing melting or reshaping after cure (Figure 1B). Globally, because approximately 25 million metric tons of thermoset waste are generated annually worldwide, recycling has become an urgent environmental priority—underscored by the EU Circular Economy Action Plan, which targets a 55% plastic recycling rate by 2030. These factors emphasize the growing need for sustainable recycling strategies for thermosetting materials.
Thermoset recycling via multiple pathways
The evolution of thermoset recycling has advanced through three phases: first-generation (1980–2000) used mechanical grinding to recover fillers, second-generation (2000–2015) employed pyrolysis to reclaim energy and chemicals, and current third-generation (2015–present) focuses on chemical depolymerization to regenerate monomers (Figure 1A). In recyclable thermosets, introducing different monomers and crosslinkers complicates recycling (Figure 1C). Using a single substance as both monomer and crosslinker simplifies recycling but requires high functionality (e.g., trifunctional sites), which limits property adjustability after curing. Stepwise curing with orthogonal monomers is thus essential for tuning material properties. However, challenges remain in controlling polymer synthesis, including precise management of monomer concentration, purification, and crosslinking during sequential reactions. Even reported systems using orthogonal monomers still depend on chlorinated solvents, high temperatures, or long reaction times (typically 2–12 h).
Single monomer design for degradable thermosets
Recently reported in Nature, the Fors group developed a degradable thermoset using an orthogonal polymerization strategy based on the bio-based, commercially available monomer 2,3-dihydrofuran (DHF) (Figure 1D).5 By controlling catalyst loading and light exposure, they obtained materials with broadly tunable mechanical properties and spatial patterning within a single material. The thermosets also exhibited multiple degradation pathways, offering a simplified route to produce bio-based materials while enabling controlled degradation and monomer recycling.
The critical aspect of this strategy lies in the elaborately designed orthogonal polymerization, which allows two distinct polymerization mechanisms to proceed independently within a single pot. This approach requires precise kinetic control over both pathways and ensures mutual exclusivity between the catalytic systems. Specifically, the ring-opening metathesis polymerization (ROMP) of DHF must proceed sufficiently slowly to afford initial poly(r-DHF) while leaving residual DHF monomers available for subsequent cationic polymerization to form poly(c-DHF). This slow kinetics was achieved by employing the Grubbs second-generation catalyst (G2), which exhibits characteristically slow propagation rates. On the other hand, the cationic polymerization must remain dormant until externally activated, enabling spatiotemporal control over the crosslinking process, yet propagate rapidly once initiated. To meet these requirements, a photoacid generator (PAG), [4-(octyloxy)phenyl]phenyliodonium hexafluoroantimonate, coupled with o-dimethoxybenzene as a photosensitizer, was introduced. Upon irradiation with blue light, this initiating system generates the superacid HSbF6, which rapidly initiates the cationic polymerization of the vinyl ether groups, achieving near-quantitative conversion within minutes.
