AbstractIn this investigation, chitosan (CS) is modified by propane sultone via a sulfonation reaction to create various degrees of sulfonation. The sulfonated chitosan (SCS) has a pendant alkyl sulfonic group dangling at the side chain, which can improve its hydrophilicity and water solubility. Elemental analysis, Fourier transform infrared spectroscopy (FTIR) and 13C nuclear magnetic resonance (13C NMR) were applied to identify the structure by determining the distribution of the substituents in the product. The degree of sulfonation in the SCS can be controlled and the hydrogen bonding interaction can be reduced by varying the degree of sulfonation. A solubility test proved that solubility increased with degree of substitution at over a wide range of pH values. X‐ray diffraction patterns of SCS samples demonstrated that the crystallinity declined as the degree of sulfonation increased. Thermogravimetric analysis and modulated differential scanning calorimetry (MDSC) results indicated that thermal stability fell but water absorbance increased with the degree of sulfonation. More water‐soluble SCS is thus obtained. The controllable different sulfonation degree of the CS polymer suggests new possibilities for the application of CS‐based materials. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010