Copolymerizations of c-caprolactone (ε CL) and glycolide (GL) were conducted in bulk at 120 ° C with variation of the reaction time. Either Sn(II) 2-ethylhexanoate (SnOct(2)) or bismuth(III) sub salicylate (BiSS) were used as initiators combined with tetra(ethylene glycol) as co-initiator. The resulting copolyesters were analyzed by H-1 and C-13 NMR spectroscopy with regard to the total molar composition and to the sequence of the comonomers. Furthermore, two series of copolymerizations (either Sn- or Bi-initiated) were performed at constant time with variation of the temperature. It was found that BiSS favors alternating sequences more than SnOct(2). Time-conversion curves and MALDI-TOF mass spectrometry of homopolymerization suggest that SnOct2 is the more efficient transesterification catalyst. A hypothetical reaction mechanism is discussed.