Controlled Gel Toughening with Tetrafunctional Cyclobutane Mechanophores

Customizing the toughness of single-network polymer gels independently of their composition and topology remains an unsolved challenge. Traditionally, polymer network toughening is achieved by using specialized monomers or solvents, or by adding secondary networks or fillers that substantially alter composition and may limit applications. Here, we report a new class of force-responsive molecules—tetrafunctional cyclobutanes (TCBs)—that enables the simple synthesis of end-linked gels with nearly identical compositions yet substantially decreased or increased toughness. This behavior is shown to arise from force-coupled chemo- and regio-selective TCB reaction pathways that are dependent on subtle changes in TCB substituents and that dictate bulk gel toughness through a topological descriptor we refer to as network strand continuity. This work introduces tetrafunctional mechanophores and the corresponding concepts of regio- and chemoselective force-coupled reactivity to the field of polymer network mechanochemistry, providing a new design concept for tuning the toughness of simple, commonly used single network gels.