Complex OsHCl(CO)((PPr3)-Pr-i)(2) catalyzes the ring-opening metathesis polymerization (ROMP) of norbomene and 2,5-norbomadiene to give poly(norbomene) and poly(norbornadiene), respectively. In both cases the resulting polymers have a high cis (74-95%) content. The stereoregularity or tacticity of the cyclopentane and cyclopentene ring sequences in poly(norbornene) and poly(norbornadiene) estimated from the C-13(H-1) NMR spectra of the hydrogenated derivatives was found to be syndiotactic. Complex OsHCl(CO)((PPr3)-Pr-i)(2) is also active in tandem ROMP-hydrogenation of norbornene and 2,5-norbornadiene. At 40degreesC and 3 atm of H-2, poly(norbomene) is fully hydrogenated in 48 h, while poly (norbornadiene) is fully hydrogenated in 48 h at 75 degreesC and 3 atm of H-2. The complex RuHCl(CO)((PPr3)-Pr-i)(2) is also active in ROMP and tandem ROMP-hydrogenation of norbomene, obtaining trans-poly(norbornene) and hydrogenated poly(norbornene), respectively. (C) 2004 Elsevier Inc. All rights reserved.