Bromo-PEG4-t-butyl ester is a PEG derivative containing a bromide group and a t-butyl protected carboxyl group. The hydrophilic PEG spacer increases solubility in aqueous media. The bromide (Br) is a very good leaving group for nucleophilic substitution reactions. The t-butyl protected carboxyl group can be deprotected under acidic conditions.