Azido-PEG5-S-methyl ethanethioate is widely used in bioconjugation and chemical biology due to its reactive azide group and polyethylene glycol (PEG) spacer. The azide group allows for efficient click chemistry reactions, particularly copper-catalyzed azide-alkyne cycloaddition (CuAAC), enabling the attachment of biomolecules, polymers, or other functional groups. The PEG spacer enhances solubility, reduces immunogenicity, and improves biocompatibility, making it suitable for applications in drug delivery, protein modification, and surface functionalization. Additionally, the thioester group provides a handle for further chemical modifications or conjugation with thiol-containing molecules, expanding its utility in synthesizing complex molecular architectures. This compound is particularly valuable in developing targeted therapeutics, diagnostic probes, and biomaterials.