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 amine functional group at one end of the PEG linker molecule allows for selective and covalent attachment to other molecules with complementary functional groups, such as carboxylic acids or activated esters, aldehydes and epoxides. The newly formed bonds are amides, imines or secondary amines. Applicable for reversible or permanent crosslinking with PEG.