Facile Mechanochemical Synthesis of Malleable Biomass-Derived Network Polyurethanes and Their Shape-Memory Applications

Because of the environmental issues associated with thermoset or network polymers, recyclable polymers are highly in demand, and the use of sustainable biomass-derived ingredients is also becoming increasingly important. In this work, we utilized 2,5-bis(hydroxymethyl)furan as a starting material to produce network polyurethanes (NPUs) under facile, solvent-free (solid-state) ball milling conditions. Urethane bonds may undergo thermally controlled transcarbamoylation, a reversible dynamic covalent bond exchange, enabling reshaping of NPUs. Taking advantage of this chemistry, we demonstrate the self-healing, reprocessing, and shape-memory properties of biomass-derived NPU films using meso-erythritol as a cross-linking agent. Interestingly, in urethane-bond-forming reactions, the relative reactivity of the secondary alcohol group of meso-erythritol over the primary one is remarkably different in the solid state, resulting in NPU films with much enhanced mechanical properties. Dynamic mechanical thermal and stress relaxation analyses indicate that the NPU films possess typical characteristics of vitrimers, such as constant cross-link density and Arrhenius-like reduction in viscosity at elevated temperatures, even though the dissociative exchange of urethane bonds may work here. Our mechanochemical approach is facile and scalable, enabling the preparation of sustainable and recyclable polymers from various biomass-derived chemicals.