Azido-PEG13-acid is a PEG-based PROTAC linker useful in the synthesis of PROTAC molecules. It acts as a click chemistry reagent containing an azide group, allowing it to participate in copper-catalyzed azide-alkyne cycloaddition reactions (CuAAC) with molecules featuring alkyne groups. Additionally, it can engage in strain-promoted azide-alkyne cycloaddition reaction (SPAAC) with molecules that have DBCO or BCN groups.