Masked Primary Amines for a Controlled Plastic Flow of Vitrimers

We present a simple method for increasing the reprocessability of vinylogous urethane (VU) vitrimers while decreasing the possibility of creep deformation at lower temperatures. In particular, varying amounts of triethylenetetramine were added as a comonomer to the curing VU formulation to ensure that all of the primary amines reacted to form enaminone cross-links, resulting in a network without reactive primary amine chain-ends. As a result, transamination was significantly slowed down because secondary amines are much less reactive to VU exchange. On the other hand, at higher temperatures, pendent primary amines can be released via a dynamic, endothermic exchange with a nearby less-reactive secondary amine, thereby (re)activating material flow. As a result, ambivalent viscoelastic behavior could be achieved without depolymerization by dynamically releasing pendent primary amines from vinylogous urethane polymer chains. Through careful comonomer selection, VU vitrimers with low viscosity at processing temperatures and at the same time high viscosity at service temperatures could be prepared without the use of catalysts or additives, leveraging the synergistic effects of mildly reactive functionalities through neighboring group participation.