From Furfural to Reprocessable Thermosets: Diels–Alder Monomers for Dynamic Polyurethane Networks

Covalent adaptable networks (CANs) offer a promising route toward sustainable polymers by addressing the inherent conflict between thermoset performance and thermoplastic reprocessability through exchangeable covalent bonds. This review highlights a bio-based synthetic approach utilizing functionalized furan-maleimide Diels-Alder adducts, derived from furfural, as preformed monomers for polyurethane CANs. Unlike conventional postpolymerization crosslinking, this adduct-based strategy allows predictable incorporation of dynamic motifs into the backbone. This enables precise control over network topology and crosslink density while advancing sustainability through renewable feedstocks and recyclable network design. We demonstrate the advantages of the adduct-based approach and establish structure-property relationships that yield materials combining thermoset-like mechanical properties with stimuli-triggered self-healing, shape memory, and recyclability. Emerging applications in 3D/4D printing, UV-curable and waterborne coatings, and debondable adhesives are surveyed. Persistent challenges, including thermal and chemical instability of furanic and maleimide components, high activation barriers for network rearrangement, reliance on nonrenewable comonomers, and potential toxicity concerns, are also discussed. Future directions emphasize rationally designed multidynamic networks, robust dienophiles, and nonisocyanate routes to bridge academic innovation and industrial viability toward fully circular polymer systems.