epsilon-Caprolactone and delta-valerolactone were polymerized in bulk at 150 degrees C using the ruthenium(II) complex RuCl2(PPh3)(3) as initiator in the presence of 1,3-propanediol (PD) with a series of alcohols as coinitiators. Polymerization of lactones proceeds via ruthenium(II) alkoxide active centers. H-1-NMR analysis revealed that the ruthenium complex reacted with the alcohol, generating in situ a ruthenium alkoxide. This species became a more active initiator of ring-opening polymerization than was RuCl2(PPh3)(3). The obtained polylactones were characterized by H-1- and C-13-NMR and matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF). The results showed the formation had occurred of alpha, omega-telechelic PCL and PVL diols, in which PD had been incorporated into the polymer backbone. Depending on the nature of the alcohol used as coinitiator, PCLs with different end groups could be synthesized. Insertion of an alcohol as an end group (benzyl alcohol, n-octanol, or isopropanol) or into the polymeric backbone (propanediol) provided support for the conclusion that a classical coordination-insertion mechanism was operating during lactone polymerization. (c) 2005 Wiley Periodicals, Inc.