Thiol-PEG-Acetic Acid (HS-PEG-CM or SH-PEG-COOH) is a carboxylic acid functionalized PEG polymer commonly used as a crosslinker or two different chemical entities, which can be activated with EDC or HATU to couple with amines. Thiols react with maleimides and disulfide bonds to form stable thioether bonds and new disulfide bonds, respectively. Applied in medical research, drug release, nanotechnology and new material research, cell culture. Active compounds in research ligands, peptide synthesis support, grafted polymer compounds, new materials and polyethylene glycol modified functional coatings.