A bifunctional cross-linker for PEGylation. PEG polymers offer numerous favorable characteristics including high water solubility, high mobility in solution, lack of toxicity and immunogenicity, and ready clearance from the body. Thus, the chemical modification of biologically active compounds, such as peptides, antibody fragments, enzymes, or small molecules with polyethylene glycol chains (referred to as “PEGylation”), is an effective method to tailor molecular properties to particular applications.