Thiol-Functionalized Poly(ethylene glycol)-<i>b</i>-polyesters:  Synthesis\nand Characterization

Abstract

This paper describes a novel synthetic strategy for the preparation of thiol end-functionalized poly(ethylene glycol) (PEG)-<i>b</i>-polyesters. Block copolymers containing an internal disulfide bond were prepared\nthrough the ring-opening polymerization of dl-lactide (LA) and ε-caprolactone (CL) employing a PEG disulfide\n[(PEG-S)₂] as the macroinitiator. This initiator was synthesized from α-<i>tert</i>-butanethio-ω-hydroxy-PEG (<i>t</i>Bu-S-PEG) through the deprotection of <i>t</i>Bu and the subsequent formation of a disulfide. The disulfide bond of the\nblock copolymers was cleaved by reduction using tributylphosphine to generate block copolymers bearing a thiol\nat the PEG chain end. Thiolated PEG-<i>b</i>-PLA and PEG-<i>b</i>-PCL with number-average molecular weights (<i>M</i>_n) in\nthe range of 3300−5800 and 3600−4600, respectively, were thereby obtained. The PLA and PCL contents could\nbe varied according to the feed ratio and ranged between 20−47 and 15−30 mol %, respectively. Aqueous\nsolutions of the disulfide block copolymers formed degradable gels at high concentration and underwent a gel−sol transition upon an increase in temperature. The gels were liquefied by treating with dithiothreitol, indicating\nthat the triblock configuration is essential for the gelation.