Oxime-Based and Catalyst-Free Dynamic Covalent Polyurethanes

Polyurethanes (PUs) have many applications resulting from their preeminent properties, but being commonly used toxic catalysts, and the lack of processability for PU thermosets cause limitations. Herein, we report a new class of the PU-like dynamic covalent polymers, poly(oxime-urethanes) (POUs), which are prepared from the uncatalyzed polyaddition of multifunctional oximes and hexamethylene diisocyanate (HDI) at ambient temperature. Kinetics studies reveal that almost complete polymerization (∼99% conversion) can be achieved in 3 h at 30 °C in dichloromethane (DCM), the most effective among the solvents evaluated, producing linear POUs with comparable molecular weights to the catalyzed PUs. We find that the oxime-carbamate structures are reversible at about 100 °C through oxime-enabled transcarbamoylation via a thermally dissociative mechanism. The cross-linked POUs based on oxime-carbamate bonds show efficient catalyst-free healable/recyclable properties. Density functional theory (DFT) calculations suggest that the fast oxime-urethanation and the mild thermoreversible nature are mediated by the characteristic nitrone tautomer of the oxime. Given widespread urethane-containing materials, POUs are of promising potential in applications because of the excellent mechanical performances, facile preparation, and dynamic property without using catalysts.