Propargyl-PEG4-t-butyl ester is widely used in the field of bioconjugation and chemical biology. It serves as a key building block for the synthesis of complex molecules, particularly in click chemistry reactions. The propargyl group enables efficient coupling with azide-containing compounds through copper-catalyzed azide-alkyne cycloaddition (CuAAC), making it valuable for labeling biomolecules such as proteins, peptides, and nucleic acids. The PEG4 spacer enhances solubility and reduces steric hindrance, improving the efficiency of conjugation processes. Additionally, the t-butyl ester group provides stability during synthesis and can be selectively deprotected to expose a carboxylic acid for further functionalization. This compound is particularly useful in drug discovery, diagnostic probe development, and the creation of biomaterials.