This chemical, N-(Tos-PEG4)-N-bis(PEG4-t-butyl ester), is a branched PEG linker primarily utilized in bioconjugation and drug delivery systems. Its structure features a central nitrogen atom connected to one tosyl-activated PEG4 chain and two PEG4 chains terminated with t-butyl ester groups. The PEG chains enhance solubility, stability, and biocompatibility, while the tosyl group serves as a reactive leaving group for selective and efficient conjugation reactions with nucleophiles like thiols or amines on biomolecules such as proteins, peptides, or antibodies. The t-butyl ester groups act as protected carboxylic acids, allowing for further functionalization or deprotection in synthetic routes. It is often employed in the development of targeted therapies for precise drug delivery to specific cells or tissues. Additionally, its use in modifying surfaces or nanoparticles improves biocompatibility and reduces immunogenicity, making it valuable in biomedical research and therapeutic applications.