Synthesis of Heterotelechelic Poly(ethylene glycol) Derivatives Having α-Benzaldehyde and ω-Pyridyl Disulfide Groups by Ring Opening Polymerization of Ethylene Oxide Using 4-(Diethoxymethyl)benzyl Alkoxide as a Novel Initiator

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Abstract
New heterotelechelic PEG-containing benzaldehyde and 2-pyridyldithio endgroup (CHO-Bz-PEG-SSpyl) was synthesized with high efficiency and high selectivity. An α-benzylacetal-ω-methansulfonyl PEG was prepared as the first step to CHO-Bz-PEG-SSpyl through the ring-opening polymerization of ethylene oxide (EO) initiated by potassium 4-(diethoxymethyl)benzyl alkoxide (PDA), followed by the successive conversion of the end-alkoxide group to a methanesulfonyl group and then to dithiocarbonate derivative. Further, deprotection of the dithiocarbonate derivative and subsequent conversion to the 2-pyridyldithio group at the ω-end was successfully performed through a one-step reaction to form α-benzylacetal-ω-2-pyridyldithio PEG (aceBz-PEG-SSpyl). The aceBz-PEG-SSpyl was then treated with an aqueous HCl solution (pH 5.0) to generate the benzaldehyde group at the α-end. Molecular functionalities of the benzaldehyde and the 2-pyridyldithio end group of the heterotelechelic PEG (CHO-Bz-PEG-SSpyl) thus prepared were characterized by 1H and 13C NMR, showing that the reaction proceeded almost quantitatively. The benzaldehyde end group is available to conjugate various ligands having a primary amino group by forming the pH-sensitive imine linkage (−NCHC6H4−).

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