The cationic copolymerization of e-caprolactone with ethylene oxide (EO) under the conditions of activated monomer polymerization, that is, with a low-molecular-weight diol as an initiator and BF3 etherate as a catalyst, was studied. To ensure the uniform composition of the resulting copolymers (telechelic oligodiols), the copolymerization was conducted with incremental feeding of the EO comonomer, which was more reactive in the cationic process. H-1 NMR analysis of samples isolated at different stages of the copolymerization indicated that the average composition of the copolymer was indeed nearly constant over the course of the copolymerization. Matrix-assisted laser desorption/ionization time-of-flight spectra of the products revealed, however, that for the same degree of polymerization, macromolecules containing different numbers of EO units were present. The observed distribution was compared with the distribution simulated under the assumption that the probability of incorporating a given unit depended only on the feed composition (nearly constant during the copolymerization). With this assumption, a good agreement between the observed and simulated spectra was obtained. This indicated that, even when the optimum conditions for the formation of a uniform copolymer were created, the individual macromolecules differed in composition because of the statistical character of the copolymerization. The results of differential scanning calorimetry analysis were compatible with such a conclusion; two melting peaks appeared on differential scanning calorimetry curves when a sample was heated immediately after fast cooling, and this may indicate the presence of different types of crystallites. (c) 2005 Wiley Periodicals, Inc.