The reaction of endo,endo-5,6-bis(chloromethyl)bicyclo[2.2.1]hept-2-ene in the presence of seven-coordinate tungsten(II) and molybdenum(II) complexes of the type [MCl(M'Cl-3)(CO)(3)(NCMe)(2)] (M'=W, Mo; M=Sn, Ge) leads to ring-opening metathesis polymerization and to the formation of high-molecular-weight soluble polymers with dispersity index in the range 1.4-2.0 and number average molecular weights in the range 300 000-790 000 g mol(-1). The geometric structure of these polymers was determined by means of H-1- and C-13-NMR spectroscopy. Molybdenum catalysts gave polymers with lower cis vinylene content (20-50%), whereas tungsten catalysts gave polymers with higher cis vinylene content (84-95%).