Bis-PEG4-PFP ester is a PEG derivative with two terminal PFP ester moieties. PFP esters are activated esters that react with primary amines to form amide bonds. They have been found to be more stable than other amine reactive groups because they are less likely to undergo hydrolysis. The hydrophilic PEG linker increases the water solubility of the compound in aqueous media.