The hydrolytic polymerization of epsilon-caprolactam (CLa) was carried out in bulk (in absence of solvent) at 250 degrees C in the presence of carboxylic esters and aqueous H3PO2. It turned out that by conducting the ring-opening polymerization (ROP) of CLa in the presence of PEO-C(O)-O-C5H11, a selected model ester (PEO = poly(ethylene oxide)), a remarkable activating effect of the ester function on the hydrolytic polymerization of the lactam was observed yielding PEO-b-PCLa diblock copolymers. The comparison of the CLa monomer conversions obtained with or without the model ester activated by H3PO2, as determined by H-1 NMR spectroscopy, has enabled to propose a multi-step mechanism in which three major reactions occurred: (i) ester and lactam hydrolysis, (ii) aminolysis of the carboxylic ester by the resulting primary amine of the hydrolyzed/opened lactam ring and (iii) condensation reactions between carboxylic acids and both amine/hydroxyl functions. The overall result of this multi-step mechanism can be assimilated as an "insertion" of the opened lactam into the ester function. By conducting the hydrolytic polymerization of CLa in the presence of an aliphatic polyester chain, such as poly(epsilon-caprolactone) (PCLo), polyesteramides were recovered with high yields and random distributions of the CLa and CLo repetitive units as determined by C-13 NMR. (C) 2008 Elsevier Ltd. All rights reserved.