In this work, an efficient and simple method to graft a vinylsulfonic acid sodium salt on a poorly water- soluble chitosan is described. Commercially available low molecular weight chitosan is converted to water-soluble chitosan containing hyperbranched-vinylsulfonic acid sodium salt groups. The process comprises the following steps: Michael addition of methyl acrylate, amidation with ethylenediamine, and Michael addition of methyl acrylate, amidation with ethylenediamine, salt. A variety of chitosans containing vinylsulfonic acid sodium salt, with improved water solubility, is synthesiszed by repeating these three steps. The new chitosan derivatives show better antimicrobial activity against Micrococcus luteus ATCC 10240 and Achromobacter xylosoxidans ATCC 2706. In addition, they display better chelating behavior with heavy metals, like cadmium(II), copper(II), and nickel(II), than the starting chitosan. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116:2074-2082, 2010