New synthesis of heterobifunctional poly(ethylene glycol) possessing a pyridyl disulfide at one end and a carboxylic acid at the other end

A well-defined poly(ethylene glycol) (PEG) possessing a 2-pyridyldithio end group at one end and a carboxyl group at the other end was prepared after chemical modification of the α-allyl-ω-carboxyl PEG, which was synthesized by an anionic ring-opening polymerization of ethylene oxide (EO) using potassium allyl alcoholate as the initiator, followed by the modification of the ω-end-alkoxide group to a carboxyl group using succinic anhydride. The allyl end group was modified by the radical addition reaction of thioacetic acid in the presence of azoisobutyronitrile (AIBN) that led to α-thioacetate-ω-carboxyl PEG without any side reaction. Selective hydrolysis of the thioester end group was accomplished by n-alkylamine in the presence of 2,2′-dithio pyridine (2-PDS) to give α-pyridyldithio- ωcarboxyl PEG with no aminolysis to the oxoester of the carboxyl group of the other end. The functionalities of both ends characterized by NMR spectra, were almost quantitative.