An equimolar cyclopentadiene-maleic anhydride copolymer was formed by the addition of t-butyl hydroperoxide to the molten Diels-Alder adducts at 175 to 285[ddot] C. The same copolymer was formed by the high temperature copolymerization of maleic anhydride, in the presence of the hydroperoxide, with cyclopentadiene per se or as generated in situ from dicyclopentadiene. The copolymer resulted from the homopolymerization of the comonomer complex which underwent excitation under the influence of the catalyst. The copolymer, softening point 290 to 320[ddot] C, was saturated and had a proposed norbornane-2, 3-dicarboxylic anhydride repeating unit with 5, 7 linkage.