Unreacted amine groups: Indispensable keys to unlock imine bonds in dynamic networks

Several strategic areas rely on the development of polymers, but the growth of plastic waste urges for new materials combining desired mechanical properties and recyclability. A promising solution is the partial/total replacement of permanent crosslinks by dynamic bonds, which allows for designing easily recyclable polymers without loss of their mechanical properties in a desired temperature range.
Among the possible dynamic bonds, imine functionalities are promising candidates as they undergo rapid exchange and can be formed in the absence of catalysts with water as the only byproduct of the reaction (Fig.1). Imines are formed via condensation of primary amines with carbonyl compounds and can undergo hydrolysis, transamination, and metathesis reactions depending on the concentration of water and unreacted amines. While these mechanisms are well-established in chemistry and biology literature, the utilization of imine bonds as a dynamic moiety in long-chain polymers is recent. Hence, the crucial role of unreacted amines, even in residual amounts, as a mediator of metathesis exchange, is often overlooked in the design and description of these networks. Therefore, we seek to provide a mechanistic description of the dynamic and viscoelastic behavior of telechelic and pendant functionalized model systems based on polydimethylsiloxane (PDMS) bearing imine bonds. We combine linear rheology and broadband dielectric spectroscopy to systematically show that in the absence of unreacted amines, network rearrangement based on imine exchange does not occur at temperatures reasonably lower than polymer degradation. Also, we show that the energy barrier of bond rearrangement is dictated by the theoretical availability of amine groups in the system and not by their cross-link density.