We report a rare occurrence in polymers where a photoreactive group has been incorporated into the backbone of an engineering thermoplastic polyester resulting in a polymer stable at 250 degrees C that is highly UV crosslinkable at room temperature. Poly(1,4-cyclohexylenedimethylene-1,4-cyclohexane dicarboxylate) (PCCD) copolymers containing trans-4,4'-stilbene dicarboxylate structural units were synthesized, and their thermal stability was confirmed by parallel plate rheometry and the absence of crosslinks upon melt processing (melt extrusion and melt-blowing processes) at 250-300 degrees C. The glass transition temperature of the copolymers increased linearly with trans-4,4'-stilbene dicarboxylate units in the backbone which further enhanced with the crosslinking of films resulting from exposure to UV light. The fluorescence emission spectrum revealed that the majority of stilbene units formed ground-state dimers or aggregates, and dynamic mechanical analysis indicates that most of the stilbene units can form crosslinks upon UV irradiation. The group contribution method predicts that the I,4-cyclohexane dicarboxylate repeating unit and the trans-4,4'-stilbene dicarboxylate repeating unit should have a strong tendency toward dimerization or aggregate formation. The crosslinking. mechanism is attributed to the face-to-face dimerizalion and subsequent [2 + 2] cycloaddition of the stilbene moieties in the copolymers.