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

Abstract

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