The synthesis of a water-soluble ionic methacrylate monomer containing a sodium 4-hydroxybenzenesulfonate reactive blocked isocyanate is described. The optimized synthetic procedure is fast and easy to use, yielding a monomer of high purity and in good yields. An investigation of the hydrolytic stability of this electrophilic reagent indicated that it could be stored under reasonable conditions. A polyelectrolyte exhibiting the pendant reactive functionalities as side groups was obtained by free-radical polymerization. The obtained polymer exhibited excellent reactivity toward a primary aliphatic amine in water at room temperature, supporting the possible use of this monomer in the bioconjugation area. (C) 2009 Elsevier Ltd. All rights reserved.