Azide-PEG2-Ms is a PEG derivative containing an azide group and a mesyl group. The hydrophilic PEG spacer increases solubility in aqueous media. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. The mesyl group is good leaving group and can also be used as protecting groups for primary alcohols. It can be deprotected in the presence of sodium amalgam.