Azide-PEG-Azide (N3-PEG-N3) is a linear bifunctional PEG reagent with two azides. The azide groups at both ends can react with copper-catalyzed alkynes in aqueous solution and are easily Reduction to amine. Hydrophilic polyPEG increases the water solubility of compounds in aqueous media. It can modify proteins, peptides and other small molecular materials.