Synthesis of Carboxy-Functionalized Heterobifunctional Poly(ethylene glycol) by a Thiol-Anionic Polymerization Method

Potassium n -butyl 3-thiolate-1-propionate (K−nBMP) and dipotassium 3-thiolate-1-propionate (K−MPA) 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 α-carboxy−PEG- block -poly(ε-caprolactone) copolymers.