Structure-reactivity relationships in a small library of imine-type dynamic covalent materials: Determination of rate and equilibrium constants enables model prediction and validation of a unique mechanical softening in dynamic hydrogels

Leveraging established relationships in the literature, we then developed a simple model to describe the cross-linking equilibrium and predict changes in hydrogel mechanical properties. Interestingly, we observed that a narrow ≈2-decade range of Keq’s determine the bound fraction of imines. Our model allowed us to uncover a regime where adding cross-linker before saturation can decrease the cross-link density of a hydrogel.

We then demonstrated the veracity of this predicted behavior experimentally. Notably this emergent behavior is not accounted for in covalent hydrogel theory. This study expands upon structure-reactivity relationships for imine formation, highlighting how quantitative determination of RECs facilitates predicting macroscopic behavior.

Furthermore, while the present study focuses on dynamic covalent imine formation, the underlying principles of this work are applicable to the general bottom-up design of soft and recyclable dynamic materials.