endo-BCN-PEG12-acid is amonodisperse PEG reagent containing a BCN group and a terminal carboxylic acid. 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. The BCN group enable click chemistry with azide -tagged molecules. The hydrophilic PEG spacer increases solubility in aqueous media.