Chloroacetamido-PEG4-t-Butyl Ester is a PEG derivative containing a chloroacetamide and a t-butyl ester. The chlorine is a good leaving group and can undergo substitution reactions. The t-butyl group is an acid labile protecting group. Upon removal, the carboxylic acid can be further reacted. The hydrophilic PEG chain increases the water solubilty of compounds in aqueous media.