Ring-opening polymerization of cycloolefins has led to a variety of new plastics and elastomers with a great range in physical properties. Ring-opening polymerization of cycloolefins, employing organoaluminum-tungsten salt catalysts, is a special case of olefin metathesis, a new reaction. The ring-opening reaction occurs at the double bond and allows making either large ring macro-cyclics or high molecular weight linear chains. Under certain conditions ringchain equilibrium can be established in these systems.