This PEG crosslinking reagent with a terminal azide functionality at one chain end reacts with DBCO, BCN or other alkynes under "Click" Chemistry conditions (for example Cu (I) catalyzed alkyne-azide cycloaddition, CuAAC) to form a stable triazole link between the two reaction partners. The hydroxy group on one side of this PEG crosslinker is able to undergo esterification reactions with acids for reversible pegylation. It also enables further derivatization or replacement with other reactive functional groups.