Thermoset polymers and fiber-reinforced polymer composites possess the chemical, physical, and mechanical properties necessary for energy-efficient vehicles and structures, but their energy-inefficient manufacturing and the lack of end-of-life strategies render these materials unsustainable. Here, we demonstrate end-of-life deconstruction and upcycling of high-performance poly(dicyclopentadiene) (pDCPD) thermosets with a concurrent reduction in the energy demand for curing via frontal copolymerization. Triggered material deconstruction is achieved through cleavage of cyclic silyl ethers and acetals incorporated into pDCPD thermosets. Both solution-state and bulk experiments reveal that seven- and eight-membered cyclic silyl ethers and eight-membered cyclic acetals incorporate efficiently with norbornene-derived monomers, permitting deconstruction at low comonomer loadings. Frontal copolymerization of DCPD with these tailored cleavable comonomers enables energy-efficient manufacturing of sustainable, high-performance thermosets with glass transition temperatures greater than 100 °C and elastic moduli greater than 1 GPa. The polymers are fully deconstructed, yielding hydroxyl-terminated oligomers that are upcycled to polyurethane-containing thermosets with higher glass transition temperatures (ca. 158 °C) than the original polymer upon reaction with diisocyanates. This approach is extended to frontally polymerized fiber-reinforced composites, where high fiber volume fraction composites (Vf = 65%) containing a cleavable comonomer are deconstructed and the reclaimed fibers are used to regenerate composites via frontal polymerization that display near identical properties compared to the original. This work demonstrates that the use of cleavable monomers, in combination with frontal manufacturing, provides a promising strategy to address sustainability challenges for high-performance materials at multiple stages of their lifecycle.