The effect of the copolymerization temperature and amount of comonomer in the copolymerization of ethylene with 1,3-cyclopentadiene, dicyclopentadiene, and 4-vinyl-1-cyclohexene and the rac-Et[Ind](2)ZrCl2-methylaluminoxane metallocene system was studied. The amount of comonomer present in the reaction media influenced the catalytic activity. Dicyclopentadiene was the most reactive comonomer among the cyclic dienes studied. In general, copolymers synthesized at 60 degreesC showed higher catalytic activities. Ethylene-dicyclopentadiene copolymers with high comonomer contents (>9%) did not show melting temperatures. 1,3-Cyclopentadiene dimerized into dicyclopentadiene during the copolymerization, giving a terpolymer of ethylene, cyclopentadiene, and dicyclopentadiene. A complete characterization of the products was carried out with H-1 NMR, C-13 NMR, heteronuclear chemical shift correlation, differential scanning calorimetry, and gel permeation chromatography.