End-functionalized polyesters have been synthesized by ring-opening polymerization (ROP) of E-caprolactone (CL) initiated with five different ruthenium derivatives in the presence of a series of alcohols as transfer agents. Mechanistic studies were performed for ROP of CL with RuCl2(PPh3)(3) (I), TpRuCI(PPh3)(2) (II), and TpRuCl(PHPh2)(PPh3) (III) as catalysts in the presence or absence of benzyl alcohol (BzOH). Obtained molecular weights are proportional to CL/BzOH ratio, but there is not a direct relationship with CL/ruthenium complex ratios. H-1 and C-13 NMR spectroscopy revealed the existence of benzyl ester end-groups. Catalysis involves (a) dissociation of ruthenium complexes, (b) coordination of the lactone CL, (c) coordination of the BzOH with the formation of a metal alkoxide, (d) transfer from the alkoxyl ligand to the coordinated lactone, and (e) ring-opening of CL by oxygen-acyl bond cleavage. The proposed mechanism is supported by H-1, C-13, and P-31 NMR, gel permeation chromatography (GPC), and MALDI-TOF analysis of the polymers. (c) 2006 Wiley Periodicals, Inc.