Highly efficient synthesis of cyclic olefin (CO) copolymers with high glass transition temperatures (T-g) as well as high catalytic activity have been attained not only by copolymerization of norbomene (NBE) with alpha-olefins (1-hexene, 1-octene, 1-dodecene), but also by copolymerization of tetracyclododecene (TCD) with a-olefins using half-titanocene catalysts, Cp'TiCl2(N=(CBu2)-Bu-t) [Cp' = (BuC5H4)-Bu-t (1), Cp (2)] in the presence of MAO. Linear relationships between the T-g values and the NBE or TCD contents were observed in all cases; Tg values in poly(TCD-co-alpha-olefin)s were higher than those in poly(NBE-co-alpha-olefin)s with the same CO contents. NBE and TCD incorporations in these copolymerization under high NBE or TCD feed conditions (especially by 2) were not affected by a-olefin (number of methylene units in the side chain) employed. An introduction of terminal olefinic double bond into the polymer side chain could be attained, if the copolymerization of NBE with 1-octene by 1 was conducted in the presence of 1,7-octadiene.