Investigation of Secondary Amine-Derived Aminal Bond Exchange toward the Development of Covalent Adaptable Networks

Secondary amine-derived aminal bonds represent an attractive class of dynamic covalent bonds that undergo rapid exchange reaction under mild conditions without the need of exogenous catalysts. The aminal bond exchange occurs by a dissociative transamination mechanism in organic media with a low activation barrier. Kinetic studies using model compounds have revealed that the transamination pathway is catalyzed by a protic source and involves the formation of iminium ion intermediate. Polyaminal polymers can be readily synthesized by condensation of multifunctional secondary amines and paraformaldehyde with a 2:1 amine-to-formaldehyde stoichiometry. The rapid aminal exchange by transamination has given rise to the stress relaxation and self-healing behaviors of polyaminal thermosets. Thermodynamic measurements revealed that the aminal–iminium equilibrium is weakly dependent on temperature, thus rendering the polyaminal thermosets to remain as a network (above the gel points) in a wide temperature range. This allows for remolding and reprocessing of the fractured polyaminal networks as a solid by heating at high temperatures, resulting in the restoration the mechanical properties that are comparable to the undamaged polymers.