This study demonstrates the method to distinguish between the active chain end (ACE) and the activated monomer mechanism (AMM) in polymerization of cyclic esters using kinetic analysis. The kinetic equation for the AMM was derived and applied for analysis of the polymerization of epsilon-caprolactone (CL) catalyzed by heteropolyacids (HPA). In contrast to other claims, polymerization of CL catalyzed by HPA proceeds by the AMM and not by ACE. The derived kinetic equation was also used to determine the rate constants of propagation involving activated monomer. Poly(epsilon-caprolactone) with a molar mass (M-n) greater than 10(5) g/mol was prepared with HPA.