Ester or cyano substituted tetracyclo [4.4.0.12,5.17,10]dodec-3-enes (1) were synthesized and their metathesis ring-opening polymerization was examined. The tungsten-based ternary catalyst system polymerized them very well. The polymers showed high glass transition temperatures (Tg) and no evidence of crystallization (e.g., the Tg of the polymer derived from 8-methyl-8-methoxycarbonyl substituted monomer (1a) was 207°C, and colorless transparent films could be casted from the solution of the polymer). The stability of these high Tg polymers were too unstable, so practical thermal molding methods could not be applied to them. The hydrogenation of these polymers with a palladium catalyst decreased Tg and greatly increased thermal stability. The physical and thermal properties of the hydrogenated polymers were thoroughly investigated. Monomer 1 was successfully copolymerized with other cyclic olefins. The resultant copolymers were hydrogenated, giving thermally stable polymers. In all cases examined in this study, a decrease of Tg by hydrogenation was about 35°C, regardless of the monomer structure. These results indicate that the main-chain mobility is the major contribution to the decrease of Tg. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 367–375, 1997