Ring-opening polymerization of E-caprolactone using a proton exchanged montmorillonite clay as initiator is reported. Effect of reaction temperature, solvent, weight ratio of initiator/monomer and reaction time on the conversion of monomer and the molecular weight are investigated The kinetics indicated that the polymerization rate is first order with respect to monomer concentration and initiator concentration, respectively. Rate constants of bulk and in solution polymerizations are determinated at 80 degrees C. Mechanism studies showed that monomer inserted into the growing chains with the acyl-oxygen bond scission rather than the break of alkyl-oxygen bond. (c) 2006 Elsevier B.V. All rights reserved.