The trichloro-bridged tungsten(II) dimer [(CO)(4)W(mu-Cl)(3)W(GeCl3)(CO)(3)](1) has been demonstrated to be a very effective catalyst for the ring-opening metathesis polymerization (ROMP) of 5-vinyl-2-norbornene (V-NBE). The selectivity of ROMP reaction depends very strongly on properties of solvent in which the reaction is carried out. In CCl4 solution the ROMP reaction is accompanied by the formation of small amounts of (V-NBE)-(CCl4) adducts, but in benzene solution 2,2'-bi(vinylnorbornylidene) (bi-(V-NBE)) and (V-NBE)-(C6H6) adducts were also detected. In dichloromethane or chloroform-d(1) solution the selectivity of ROMP reaction is low because it is accompanied by formation of bi-(V-NBE). The tungsten(II) compound activates the more strained cyclic double bond of V-NBE. As was shown by H-1 and C-13 NMR spectroscopy, the pendant vinyl group is left intact in poly-(vinylcyclopentylenevinylene) as well as in bi-(V-NBE). A mechanism involving C-H bond activation of olefin and the formation of tungsta-vinylnorbornylidene is proposed to explain those results. In dichloromethane and chloroform-d(1) solution a tungsta-vinylnorbomylidene species undergoes a carbene-carbene coupling reaction to give bi-(V-NBE). The reaction products are identified by means of chromatography (GC-MS) and H-1 and C-13 NMR spectroscopy. (c) 2006 Elsevier B.V. All rights reserved.