Azido-PEG3-S-PEG4-t-butyl ester is a PEG derivative containing one azide group and one t-butyl protected carboxyl group. The azide group enables Click Chemistry. The hydrophilic PEG spacer increases solubility in aqueous media. The t-butyl protected carboxyl group (COOH) prevents self coupling or polymerization under standard acid/amine or acid/hydroxyl coupling conditions.