Revealing the Topological Analogy between End-Linked and Pendant Cross-Linked Polymer Networks for Mechanophore-Enabled Toughening

Recent advances in mechanochemistry have shown that the deliberate incorporation of mechanically weak bonds, specifically scissile mechanophores, can improve the fracture resistance of polymer networks. In pendant cross-linked elastomers (e.g., side-chain cross-linked or vulcanized elastomers), bifunctional mechanophores incorporated into cross-links produce substantial toughening. By contrast, utilizing the same motifs for toughening in end-linked or step-growth networks, which are ubiquitous in industrial thermosets, has remained a challenge. Here, by establishing a topological correspondence between pendant cross-linked and end-linked architectures, we identify a design strategy that enables bifunctional scissile motifs to successfully toughen end-linked networks. We validate this approach using model thermoset polyurethanes and show that the resulting mechanophore-driven toughening is substantial, yielding a 16-fold enhancement in tensile toughness and an 8-fold enhancement in tearing energy. We further show that this strategy can be combined with conventional polyurethane toughening approaches to produce materials with outstanding properties. More broadly, this work establishes strand continuity as a mechanochemical design principle for fracture resistant materials across diverse polymer architectures.