Block copolymers were prepared by the direct polycondensation of an aqueous lactic acid solution on monomethoxy or dihydroxyl poly(ethylene glycol) (PEG) in the absence of a catalyst. The resulting poly(lactic acid) (PLA)-PEG diblock and PLA-PEG-PLA triblock copolymers were characterized by various analytical techniques, including matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF MS), gel permeation chromatography, and H-1-NMR. The molecular structure between PLA-PEG and PLA-PEG-PLA could be distinguished after the calculation of the repeat unit masses and end-group masses through the MALDI-TOF MS spectra. Interestingly, both copolymers could serve as a hydrophilic emulsifier to stabilize the squalene/water interfaces and yield narrowly distributed oil-in-water nanoparticles. In contrast, the pre-polymer PEG failed to stabilize the squalene/water interface under the same homogenization conditions. These features are of great interest for applications as bioactive agent delivery, especially for candidate vaccine antigens and lipophilic anticancer drugs. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 114: 509-5-16, 2009