The ring-opening polymerization of epsilon-caprolactone (CL) was induced by using polypropylene glycol (PPG) as an initiator in the presence of the monomer activator HCl center dot Et2O to synthesize triblock copolymers composed of PPG and poly(epsilon-caprolactone) (PCL). The degree of CL conversion and the molecular weight of PCL increased linearly with the polymerization time or with the feed ratio of CL to PPG in the presence of HCl center dot Et2O in CH2Cl2 at 25 degrees C. The PCLs obtained had molecular weights close to the theoretical values calculated from the CL:PPG molar ratios and exhibited monomodal GPC curves with narrow polydispersity indexes. The apparent rate constant (k(app)) for the polymerization of CL activated by HCl center dot Et2O was greatly affected by the ratio of HCl center dot Et2O/PPG. The activation energy for the polymerization of CL in this system was estimated to be 49.8 kJ/mol K. We successfully prepared PPG and PCL triblock copolymers using this activated monomer mechanism. (C) 2009 Elsevier Ltd. All rights reserved.