Eight different molybdenum compounds were tested in the catalysts of ring-opening polymerization (ROP) of epsilon-caprolactone (CL). All homopolymerizations were conducted in bulk at 150 degrees C using a CL/molybdenum compound molar ratio of 200. Ammonium decamolybdate (NH4)(8)[Mo10O34] comes to be the best catalyst for ROP, based on its selectivity, short reaction times (2 h) and high conversions (98%). Aliphatic copolyesters with acid-funtional asymetric telechelic architecture alpha-hydroxyl-omega-(carboxylic acid) (HA) were synthesized from lactones-such as CL, delta-valerolactone (VL) and gamma-butyrolactone (BL)- by ring-opening copolymerization. HA-copolyesters, namely HA-poly(epsilon-caprolactone-co-gamma-butyrolactone) (HA-PCB), HA-poly(delta-valerolactone-co-gamma-butyrolactone) (HA-PVB) and HA-poly(epsilon-caprolactone-co-delta-valerolactone) (HAPCV), were obtained at 150 degrees C in 2 h using ammonium decamolybdate as catalyt and water as initiator. A control of the degree of polymerization (DP, measured by NMR) can be achieved in the range between 6 and 24 for HA-PCB and HA-PCV, based on the initial monomer/initiator ratio. DP shows a linear dependence with M/H2O ratio (where M = CL + (BL or VL)) in this range. The nature of hydroxyl and carboxylic acid end groups of HA-copolyesters was determined by H-1 and C-13 NMR. Finally, HA-poly(epsilon-caprolactone-block-delta-valerolactone) (HA-PCbV) was successfully prepared by sequential copolymerization of HA-poly(E-caprolactone) with VL and characterized by H-1 and C-13 NMR, GPC and MALDI-TOF. (c) 2005 Published by Elsevier Ltd.