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

The development of next generation soft and recyclable materials prominently features the use of dynamic (reversible) chemistries such as host-guest, supramolecular, and dynamic covalent. The advantages of dynamic systems are their injectability (shear-thinning and self-healing), reprocessability, and dynamic (time-dependent) mechanical properties, sometimes uncovering unique behavior or functions in the desired application. These properties arise from the inherent relationship between the rate and equilibrium constants (RECs) of molecular junctions (cross-links) and the resulting macroscopic behavior of dynamic networks. However, there are few examples of explicitly measured rate and equilibrium constants establishing this connection between RECs and materials properties, particularly for polymeric hydrogel systems. Here we use dynamic covalent imine formation to study how single-point compositional changes in amine nucleophiles affect binding constants and resulting hydrogel mechanical properties. We find that there is over a 3 decade change in RECs, in both model small molecule studies and model polymeric backbones. Based on established relationships in the literature, we then developed a simple model to describe the cross-linking equilibrium and predict changes in hydrogel mechanical properties. This allowed us to uncover a regime where adding crosslinker before saturation can decrease the crosslink density of a hydrogel. Having determined explicit equilibrium constants, we were then able to demonstrate the veracity of this predicted behavior experimentally. Notably this emergent behavior is not accounted for in covalent hydrogel theory. This study expands upon the structure-reactivity relationships for imine formation, highlighting how quantitative determination of rate and equilibrium constants facilitates predicting the macroscopic behavior of soft materials. 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.