Abstract A study has been made of the molecular weight and distribution changes that take place during the polymerization of cyclopentene initiated by the non-living metathesis catalyst WCl 6 AliBu 3 . The system appears to be characterized by two distinct phases. In the first phase, the molecular weight distribution, which is bimodal, would appear to be kinetically controlled and the bimodal distribution to result from the presence of two kinetically independent metathesis species. The initial molecular weight distribution would, therefore, appear to be dependent upon the relative concentrations of these active species. The second phase of the polymerization is a slow conversion of the molecular weight distribution to that of an equilibrium distribution after a long reaction time.