Metathesis polymerization of norbomene and norbornadiene in the presence of tert-butylacetylene initiated by seven-coordinate bimetallic tungsten-tin and molybdenum-tin compounds of the [MCl (SnCl3)(CO)(3)(NCMe)(2)] (M = Mo, W) type was investigated. The presence of alkyne in the reaction mixture leads to an increase in cycloolefin conversion and to the formation of blocky polymers with high molecular weights and better solubility in CHCl3 than polynorbornene and polynorbornadiene homopolymers. The structure of polymers was determined by H-1 and C-13{H-1) NMR spectroscopy. In addition to polymers small amounts of cooligomers and cyclotrimers were also detected in the above reactions. The reactivity of tungsten(II) and molybdenum(II) complexes towards tert-butylacetylene is higher than towards olefins. Monitoring of reactions of [WCl(SnCl3)(CO)3(NCEt)(2)] with norbornadiene and tert-butylacetylene by 114 NMR spectroscopy made it possible to observe the coordination of alkyne to the metal atom in the step preceding polymerization and copolymerization. In those reactions the alkyne plays the role of an activator. (C) 2004 Elsevier B.V. All rights reserved.