N-(Azido-PEG3)-N-(PEG2-amine)-PEG3-acid is a branched PEG derivative containing azide, amine, and carboxylic acid moieties. The azide can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. An amine is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc. 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.