The mechanical properties of covalent polymer networks often arise from the permanent end-linking or cross-linking of polymer strands and molecular linkers that break more easily would likely produce materials that require less energy to tear. We report that cyclobutane-based mechanophore cross-linkers that break via force-triggered cycloreversion lead to networks that are up to 9 times tougher than conventional analogs. The response is attributed to a combination of long strong primary polymer strands and cross-linker scission forces that are ~ 5-fold smaller than control cross-linkers at the same timescales. The enhanced toughness comes without the hysteresis associated with noncovalent cross-linking and it is observed in two different acrylate elastomers in fatigue as well as constant displacement rate tension and in a gel as well as elastomers.
