Potassium n-butyl 3-thiolate-1-propionate (K-nBMP) and dipotassium 3-thiolate- 1-propionate (KMPA) initiated anionic polymerization of ethylene oxide (EO) to yield carboxy-functionalized poly(ethylene glycol)s (HOOC-PEG) with predicted number-average molecular weights (M-n up to 18 700) and narrow molecular weight distributions (M-w/M-n <= 1.25, M-w is weight-average molecular weight). Additional thiol-containing anionic initiators such as mercaptoacetic acid and 11-mercaptoundecanoic acid resulted in PEGs having bimodal molecular weight distributions. Evaluation of a series of carboxy-functionalized oxy(gen) anionic initiators revealed that only potassium 12-hydroxydodecanoate (K-HDA) allowed for controlled polymerization of EO and yielded polymers with a narrow molecular weight distribution. The efficiency of the anionic initiators is related to their nucleophilicity. The carboxy-functionalized PEGs were also employed as a macroinitiator for synthesis of alpha-carboxy-PEG-blockpoly(is an element of-caprolactone) copolymers.