Molecular Motion and Connectivity Govern Mechanophore Activation in High- <i>T</i> _g Glassy Polymers

Mechanochemical activation has been extensively investigated in solutions and soft polymeric materials, where force transmission along stretched chains dominates the response. In contrast, activation behavior in higher-modulus, glassy thermosets remains poorly understood, and the role of the material environment itself has not been systematically quantified. Here, we investigate spiropyran (SP) mechanophores incorporated into a high-performance poly(dicyclopentadiene) (pDCPD) network fabricated directly by polymerization into its final form. This chemically faithful synthesis preserves network integrity and enables the quantitative analysis of mechanophore activation under tensile and compressive loading. We find that activation is absent in the elastic regime and emerges only beyond the macroscopic yield point. These results demonstrate that the mechanochemical response in this glassy network is governed by deformation and irreversible segmental mobility rather than tension magnitude alone, establishing motion-induced activation as distinct from the behavior previously observed in soft materials. The SP-pDCPD system thus provides a quantitative framework for understanding mechanochemical activation in high-performance glassy polymers.