Azido-PEG4-Amido-tri-(carboxyethoxymethyl)-methane is a branched PEG derivative with a terminal azide group and three terminal carboxylic acids. The azide group enables PEGylation via Click Chemistry. The terminal carboxylic acids can be reacted with primary amino groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond.