Poly(oxyethylene)s terminated at both ends with 2-bromopropionate end-groups were prepared and characterized by means of MALDI TOF mass spectrometry. It was shown, that atom transfer radical polymerization (ATRP) of methyl methacrylate with a poly(oxyethylene) macroinitiator in bulk proceeds with low initiation efficiency while polymerization of tert-butyl acrylate proceeds with practically quantitative initiation, leading to ABA block copolymers. Originally formed tert-butyl acrylate blocks contain terminal bromine, as expected for the ATRP mechanism. MALDI TOF analysis indicates, however, that in the later stages of polymerization side reactions lead to elimination of terminal bromine.