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A new class of catalysts for polymerization of cycloalkenes was developed, based on the interaction of these hydrocarbons with π‐allylic transition metal complexes. The reaction route (decyclization or double‐bond opening) and the microstructure of polyalkenamers obtained is determined primarily by the nature of the transition metal and also by the nature and the number of ligands bound to the metal. π‐Allylic complexes of zirconium and chromium catalyze the polymerization of cycloalkenes in both directions. For complexes of the group VI metals the substitution of molybdenum or tungsten for chromium results in complete suppression of the double‐bond opening. In the presence of Group VIII metal compounds the polymerization of cycloolefins occurs exclusively at the double bond. The introduction of acidic ligands into the internal sphere of the π‐allylic compound does not affect the reaction route but results in a significant rise of the yield of polyalkenamers and enriches the content of trans‐1,4‐units. Lewis acids act as activators for complexes of all the metals, the halides of molybdenum and tungsten affecting favorably the decyclization. It is assumed that the ring‐opening polymerization of cycloolefins proceeds via π‐allylic complex formation.

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