endo-BCN-PEG6-t-butyl ester is a PEG derivative containing a BCN group and a t-butyl protected carboxyl group. The hydrophilic PEG spacer increases solubility in aqueous media. The BCN group can react with azide-tagged biomolecules. The protected carboxyl group (COOH) prevents self coupling or polymerization under standard acid/amine or acid/hydroxyl coupling conditions. The protected acid can be deprotected by acidic or basic hydrolysis.