Critical Role of Free Amine Groups in the Imine Bonds Exchange in Dynamic Covalent Networks

Introducing\ndynamic covalent bonds in polymers is a promising solution\nto improve their recyclability and self-healing abilities minimally\ncompromising the mechanical properties. Imine bonds are exciting candidates\nas dynamic functionalities because of their rapid exchange and easy\nchemical synthesis. Although the exchange mechanisms of imines in\nsmall molecules are well-established, some misconceptions still exist\nregarding their behavior in polymers. Hence, we investigate the dynamic\nand viscoelastic properties of imine-bearing model systems and highlight\nthat imine bond exchange only happens when mediated by unreacted amine\ngroups. Therefore, imine metathesis is unlikely at temperatures below\npolymeric decomposition, and transimination is the major mechanism\nof bond exchange. Surprisingly, the energy barrier for this mechanism\ndepends on the concentration of functional groups forming imine bonds.\nWe hypothesize that the hydrogen bonds between amine and aldehyde\ngroups might affect this process, but further work is needed to solve\nthis puzzle.