Mechanistic Origins of Polydicyclopentadiene Oxidation

Polydicyclopentadiene (pDCPD) is a high-performance thermoset whose unusual oxidative sensitivity has long been recognized but is not mechanistically understood. Here we identify singlet oxygen (¹O₂) as the primary oxidant and the pendant cyclopentene ring as the primary site of reactivity that together drive the intrinsic oxidative degradation of pDCPD. Controlled ¹O₂ generation, model compound studies, and weathering experiments reveal a chemoselective oxy-ene pathway that selectively converts cyclopentene units into cyclopent-2-en-1-ones and initiates allylic-hydroperoxide-driven cross-linking. In contrast, main-chain alkene models and studies on pH₂DCPD─in which the cyclopentene unit is saturated to suppress oxy-ene reactivity─show that the main-chain alkene follows a slower β-scission pathway. These mechanistic distinctions explain the long-observed oxidative instability of pDCPD and clarify why polymers lacking the pendant cyclopentene, such as pH₂DCPD, offer improved resistance to oxidative aging. Guided by this insight, oligo-DCPD fragments are transformed into polar, benzyl-acrylate-miscible, polyfunctional additives whose enone groups enable tunable cross-linking during radical acrylate polymerization. Together, these results connect the molecular origin of pDCPD oxidation to both monomer design principles and selective macromolecular editing strategies.