Mechanically Activated, Catalyst-Free Polyhydroxyurethane Vitrimers
Fortman, D. J.; Brutman, J. P.; Cramer, C. J.; Hillmyer, M. A.; Dichtel, W.
J. Am. Chem. Soc. 2015, 137, 14019 (doi:10.1021/jacs.5b08084).
Vitrimers are polymer networks whose cross-links undergo associative exchange processes at elevated temperature, usually in the presence of an embedded catalyst. This design feature enables the reshaping of materials with mechanical properties similar to thermoset resins. Here we report a new class of vitrimers consisting of polyhydroxyurethanes (PHUs) derived from six-membered cyclic carbonates and amines. PHU networks relax stress and may be molded into arbitrary shapes at elevated temperature and pressure in the absence of an external catalyst. The as-synthesized networks exhibit tensile properties comparable to leading thermosets and recover ca. 75% of their as-synthesized values following reprocessing. Stress relaxation occurs through an associative process involving the nucleophilic addition of free hydroxyl groups to the carbamate link- ages, and is characterized by an Arrhenius activation energy (111 kJ/mol) lower than that observed for molecular model compounds (148 ± 7 kJ/mol). These findings suggest that transcarbamoylation is activated by mechanical stress, which we attribute, based on DFT calculations, to the twisting of N lone pairs out of conjugation with the carbonyl π-orbitals. PHU vitrimers are a promising new class of repairable networks because of their outstanding mechanical properties, avoidance of toxic isocyanate monomers, and catalyst-free repair processes.