Acid-Mediated Autocatalysis in Vinylogous Urethane Vitrimers

Vitrimers are a class of polymeric materials with outstanding properties. Intramolecular substitution reactions lead to a dynamic exchange within the polymer network which enables thermoreversible stress relaxation in yet permanently crosslinked materials. In this paper, the acid-mediated autocatalysis is explored as a rearrangement pathway for vinylogous urethane vitrimers. The autocatalysis enables transimination reactions, resulting in a dynamic exchange among the enamine-one species, without an excess of free amines. Therefore, the enamine-ones are protonated by a Brønsted acid and turn into electrophilic iminium-ones, thus enabling fast backward and substitution reactions with water and free amines. This work provides an in-depth investigation of the mechanism by kinetic studies of selected compounds. In addition, novel elastomeric and thermosetting poly(vinylogous urethane) networks with and without free amine groups and additional para-toluene sulfonic acid as a Brønsted catalyst are prepared by bulk polymerization of hexane-1,6-diylbis(3-oxobutanoate) and tris(2-aminoethyl)amine. The underlying exchange mechanisms are determined by stress-relaxation experiments with stress relaxation times of 0.3-54 000 s at 110 °C.