The ring-opening metathesis polymerization (ROMP) of norbornene catalyzed by bis(acetonitrile) molybdenum and tungsten complexes, [M(eta(3)-C3H5)Cl(CO)(2)(NCMe)(2)] (1-Mo: M=Mo, 1-W: M=W), which have two labile acetonitrile ligands. has been investigated. These complexes catalyzed the ROMP of norbornene as a single-component initiator. The highly vis-selective polymerization proceeded in a THF solution (95% for 1-Mo and 96% for 1-W), whereas polymerization in CH2Cl2 or toluene resulted in lower cis selectivity. The polymerization of terminal acetylenes using these complexes was also examined. The tungsten complex I-W showed a high catalytic activity for the polymerization of terminal acetylenes, such as phenyl- and tert-butylacetylene. A highly active catalytic system for the ROMP of norbornene was achieved by the activation of the tungsten complex, I-W, with one equivalent of phenylacetylene. giving poly(norbornene) with a high molecular weight (M-n = 391 x 10(4)) and a high cis selectivity (cis similar to 89%). (c) 2005 Elsevier B.V. All rights reserved.