The incorporation of dynamic covalent chemistries into polymer networks enables capabilities such as self-healing, responsiveness, and adaptability. Hydrogels, a subset of polymer networks that are water-swollen and biocompatible, recapitulate viscoelastic features of natural tissues like stress relaxation when dynamic crosslinks are incorporated. Most hydrogels exchange through dissociative reaction mechanisms, but in these materials, the properties of stiffness and stress relaxation are challenging to decouple. We are interested in developing crosslinkers that exchange through associative pathways and therefore enable tuning of the rate of exchange and stress relaxation independently of crosslink density and stiffness. Towards this effort, we synthesized two series of thioalkylidene and thioester crosslinkers and used them to fabricate poly(ethylene glycol)-based hydrogels. By using principles of molecular reactivity, such as electronic effects, internal catalysis, and pre-organization, we tuned molecular exchange rates over multiple orders of magnitude. In hydrogels, these trends translated to changes in the timescale of stress relaxation. This work offers fundamental insight into how changing the reaction landscape of crosslinker exchange affects macromolecular network flow. We envision that these hydrogels contribute a means to study unexplored biomechanical mechanisms underlying physiology and disease.

Hydrogels with tunable viscoelasticity enabled by dynamic thiol exchange reactions

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ACS Fall 2022

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