Toward Repairable, Recyclable, Strong, and Flame-Retardant Poly(urethane–urea) Cross-Linked via Dynamic Diels–Alder Reaction

Designing multifunctional polyurethane or polyurea elastomers that are simultaneously strong, recyclable, flame-retardant, and sustainable remains a major challenge in advanced elastomer research. Herein, a phosphorus-containing biobased diol (HMF–DOPO) was synthesized by the reaction between 5-hydroxymethylfurfural (HMF) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). Then, the tough, repairable, recyclable, and flame-retardant poly(urethane–urea) (PUA) elastomers cross-linked via reversible Diels–Alder (DA) bonds were developed. The synthesized elastomers exhibited excellent mechanical properties, while PUA–HMF-20% showed the best performance with a high tensile strength of 51.9 MPa and notable toughness of 167.6 MJ m–3. The thermally reversible DA reaction endowed PUA materials with excellent repair and recycling properties. The cut pieces can be reformed into a complete film during hot pressing and solvent reprocessing. Due to the existence of the phosphorus-containing structure, PUA–HMF elastomers also exhibited excellent self-extinguishing capability, while PUA–HMF-40% showed the highest limiting oxygen index (LOI) value of 27% and reached a V-0 rating in the UL-94 vertical burning test. In the cone test, both the peak heat release rate (PHRR) and total heat release (THR) values of the PUA–HMF samples were significantly reduced. This work provides a promising strategy to expedite high-performance sustainable advanced elastomers and expand the industrial application.