N-(Azido-PEG3)-N-Boc-PEG4-acid is a branched PEG derivative with a terminal azide group, Boc protected amino group, and a terminal carboxylic acids. The azide group enables PEGylation via Click Chemistry. The protected amine can be deprotected by acidic conditions. The terminal carboxylic acid can be reacted with primary amino groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond.