Circular Nonisocyanate Polyhydroxyurethane Networks with High Stiffness and Thermal Reprocessability through Furan–Maleimide Diels–Alder Chemistry

Abstract

High-performance nonisocyanate polyurethanes with circular end-of-life options remain difficult to obtain because polyhydroxyurethane networks often combine limited mechanical performance with poor reprocessability. Here, furan-functionalized polyhydroxyurethane oligomers were crosslinked with a trifunctional maleimide through thermoreversible furan–maleimide Diels–Alder chemistry to obtain stiff, thermally reprocessable networks. Two precursor architectures were compared: a fossil-based diepoxide-derived oligomer and a partially biobased epoxidized soybean oil-derived oligomer. Nuclear magnetic resonance and Fourier-transform infrared spectroscopy confirmed cyclic carbonate formation, hydroxyurethane formation, and Diels–Alder crosslinking. Differential scanning calorimetry, in situ infrared spectroscopy, rheology, swelling experiments, and three-point bending tests were used to define the thermal reprocessing window and assess network recovery. The resulting networks reached flexural moduli of 3.9 and 1.7 GPa, respectively, and retained most of their mechanical performance after compression-molding recycling. Reprocessing was governed by a narrow temperature window where retro-Diels–Alder dissociation enables flow while irreversible maleimide side reactions remain limited. These results establish precursor architecture as a key parameter for balancing stiffness, reprocessability, and topology preservation in circular polyhydroxyurethane thermosets.