N-(Azido-PEG3)-N-(PEG2-NH-Boc)-PEG3-acid is a branched PEG derivative with a terminal azide group, a terminal carboxylic acid, and a terminal Boc protected amine. The azide group participates in Click Chemistry reactions with alkynes, DBCO and BCN. The terminal carboxylic acid can be reacted with primary amine groups in the presence of activators (e.g. EDC, or DCC) to form a stable amide bond. The Boc protected amine can be removed under acidic conditions.