Dynamic Covalent Polymer Networks Based on Degenerative Imine Bond Exchange: Tuning the Malleability and Self-Healing Properties by Solvent

Covalent polymeric networks composed of imine cross-linkages have been prepared by condensation polymerization of poly(ethylene glycol) bis(3-aminopropyl) with 1,3,5-triformylbenzene with an equal molar ratio of amine and aldehyde functionalities in organic solvents with varying polarity and in neat condition. The polymer networks exhibit malleability and self-healing characteristics. Rheological measurements revealed that longer reaction time is required to reach the gel point (i.e., crossover of G' and G'') in polar solvents than in nonpolar solvents. The malleability of the solvent-swelled polymer network is also strongly dependent on the solvent polarity. Polymer gels in polar solvents are more malleable than those in nonpolar solvents, as supported by the dynamic mechanical analysis. These results are consistent with faster dynamic imine bond exchange in the polar solvents relative to the nonpolar solvents, thus requiring higher functionality conversion to form an elastic network in the polar solvent relative to the nonpolar solvent. The imine-imine bond exchange kinetic was also studied by H NMR spectroscopy on model compounds in the presence of varying amount of primary amine and water. The exchange reaction was significantly accelerated by the presence of primary amine than water, suggesting that the imine bond exchange is mainly promoted by the residual primary amino functionalities in the polymeric network. This study revealed the molecular and kinetic basis for the macroscopic and dynamical properties of the polymer networks composed of imine cross-linkages. The polymer networks are potentially useful as adhesives.