AbstractExperimental data have been obtained evidencing the chain character of cyclo‐olefin ring‐opening polymerization and the carbene nature of the active centers in this reaction. The formation of metal–carbene complexes via the decomposition of organometallic derivatives of tungsten, molybdenum, and tantalum has been investigated. The mechanism of the chain propagation in cycloolefin polymerization and olefin metathesis as well as the mechanism of cyclodestruction of unsaturated polymers are discussed. The possibility of the inhibition of the chain metathesis process by carbene complexes more stable than active centers of the growing chain is