PEG Azide or Azido PEG is widely used in Click Chemistry, with this particular example of the Cu(I) catalyzed reaction between the azide and a terminal acetylene and also the Staudingerligation using functionalized aryl phosphines to couple to the azide, forming amides. The attraction of the azide functionality is its high selectivity and its stability under most conditions. These properties are especially useful where other conjugating functionalities have to be used due to their limited stability or when careful control of variables, such as pH, are needed to ensure high yielding reactions.