The copolymerization of ethylene and cycloolefins [cyclopentene (CPE), cyclohexene (CHX), cycloheptene (CHP), cyclooctene (COT), cyclododecene (CDO), norbornene (NB), and 5,6-dihydrodicyclopentadiene HDCPD] and cyclodiolefins [1,3-cyclopentadiene (CPD), 1,4-cyclohexadiene (CHD), 1,5-cyclooetadiene (COD), 2,5-norbornadiene (NBD), and dicyclopentadiene (DCPD)] was investigated with a constrained-geometry catalyst, dimethylsilylene(tetramethylcyclopentadienyl)(N-tertbutyl)titanium dichloride, with methyl isobutyl aluminoxane as a cocatalyst. In the copolymerization with cycloolefins, the olefins, except for CHX, CDO, and HDCPD, were copolymerized via the 1,2-insertion mode with the following reactivity: NB > CHP > COT > CPE. In the copolymerization with cyclodiolefins, corresponding copolymers, except for copolymerization with CHD, were obtained. A crosslinking fraction was detected in the copolymers with COD and NBD. The reactivity of the cyclodiolefins, except for COD, was higher than that of the cycloolefins. CPD was copolymerized via 1,2-insertion, 1,4-insertion or 1,2-insertion of dimerized DCPD. The copolymerization with COD showed peculiar behavior under the copolymerization condition of a high COD concentration in the feed. (C) 2005 Wiley Periodicals, Inc.