Azido-PEG36-acid is a aqueous soluble, monodisperse PEG reagent comprising an azide group with a terminal carboxylic acid. The azide group can react with alkyne, BCN, DBCO via Click Chemistry. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond.