N-(t-butyl ester-PEG3)-N-bis(PEG3-azide) is a branched PEG linker with two azides and one t-butyl ester linked together at a tertiary amide bond. Each azide is open for click chemistry with alkynes while the t-butyl ester can be deprotected and used to react with nucleophiles like amines. PEG spacer provides aqueous solubility.