Thiol-functionalized poly(ethylene glycol)-b-polyesters:: Synthesis and characterization

This paper describes a novel synthetic strategy for the preparation of thiol end-functionalized poly(ethylene glycol) (PEG)-b-polyesters. Block copolymers containing an internal disulfide bond were prepared through the ring-opening polymerization of DL-lactide (LA) and epsilon-caprolactone (CL) employing a PEG disulfide [(PEG-S)(2)] as the macroinitiator. This initiator was synthesized from alpha-tert-butanethio-omega-hydroxy-PEG (tBu-S-PEG) through the deprotection of tBu and the subsequent formation of a disulfide. The disulfide bond of the block copolymers was cleaved by reduction using tributylphosphine to generate block copolymers bearing a thiol at the PEG chain end. Thiolated PEG-b-PLA and PEG-b-PCL with number-average molecular weights (M-n) in the range of 3300-5800 and 3600-4600, respectively, were thereby obtained. The PLA and PCL contents could be varied according to the feed ratio and ranged between 20-47 and 15-30 mol %, respectively. Aqueous solutions 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 that the triblock configuration is essential for the gelation.