New copolyesters with designed architecture were obtained via anionic ring-opening copolymerization of beta-butyrolactone with beta-ethoxymethyl-beta-propiolactone. The resulting copolyesters contained predominantly linear macromolecular chains terminated by acetate end groups derived from the used initiator regardless of the type or the composition. Similar behavior of beta-alkoxy-substituted-beta-lactone and beta-alkyl-substituted-beta-lactone (including side reactions leading to unsaturated end groups) was observed under the conditions of anionic ring-opening copolymerization. The application of ion trap ESI-MS/MS analysis for structural studies and investigation of the fragmentation product patterns of the individual molecular ions showed significant differences in the molecular structure at the level of individual molecular chains of random and diblock copolyesters. Thus, the sequence distribution of diblock and random copolyester was determined based on the investigation and comparison of the fragmentation product patterns of the individual molecular ions selected from the both types of studied copolyesters.