PEGylation reagent for "Click" Chemistry. This PEG reagent with a terminal azide functionality reacts with 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. Without a catalyst PEG-azides readily react with strained alkynes such as BCN (bicyclononyne) - or DBCO (dibenzocyclooctyne) derivatives.