Poly(ethylene terephthalate) (PET) copolymers containing 2,6-anthracenedicarboxylate structural units are modified by Diels-Alder reactions with maleimides. Low molecular weight bis(maleimide)s are used to chain extend and cross-link the polymers. The resulting materials were characterized by H-1 NMR, UV-vis, DSC, and dilute solution viscometry. The Diels-Alder reaction of low molecular weight anthracenes and maleimides is thermally reversible at 250 degrees C. However, the reverse reaction is slow, and the PET-anthracene copolymers are prone to thermal decomposition at; these higher temperatures.