In the present article, we describe the synthesis and characterization of conjugates based on pyroglutamyl-S-glutamic acid and bisoligo-[R,S]-3-hydroxybutyrates (PyGlu-S_-Glu _bisOHB) using anionic ring opening polymerization of beta-butyrolactone with a dipeptide bearing two carboxylate groups as potassium salt. The results indicated that the above-mentioned reaction is accompanied of oligomerization of beta-butyrolactone yielding (3-hydroxybutyrates) oligomers with crotonate and carboxyl end groups. We report also the end group analysis of the synthesized conjugates using electrospray ionization tandem mass spectrometry (ESI-MS), the latter confirmed the presence of a mixture of dipeptide conjugate with beta-butyrolactone oligomer chain and P-butyrolactone homopolymer. (c) 2008 Wiley Periodicals, Inc.