Azidoethyl-PEG2-t-Butyl ester is a PEG derivative that contains a cleavable disulfide bond, an azide group and a t-butyl ester group. The disulfide bond can be cleaved via reduction reactions. The azide group can be reacted with an alkyne, DBCO or BCN moiety to undergo copper-catalyzed Click Chemistry. The t-butyl ester group can be removed under acidic conditions.