NH2-PEG4-Lys(t-Boc)-NH-m-PEG24 is a PEG derivative containing a terminal primary amine and a t-Boc protected lysine. The primary amine can react with carboxylic acids, activated NHS esters and other carbonyl groups. The t-Boc protecting group can the removed under acidic conditions. The hydrophilic PEG chains increase the water solubility of a compound in aqueous enviornments. Longer PEG chains have improved solubility relative to shorter PEG chains.