A difunctional benzoxazine monomer, 6,6'-bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazinyl) sulfone (BS-a), was synthesized via a solution method from bisphenol-S, aniline and formaldehyde. The chemical structure of the benzoxazine was confirmed by H-1 and C-13 nuclear magnetic resonance spectroscopy, Fourier transform infrared (FIR) spectroscopy, elemental analysis, and size exclusion chromatography. The ring-opening polymerization of BS-a monomer was investigated with FTIR under air and nitrogen atmospheres, and with differential scanning calorimetry (DSC) in both dynamic and isothermal conditions. The FTIR results show that the absorption intensities of C-O-C, C-N-C, and oxazine ring decrease gradually with temperature and time rising during the polymerization reaction. The change rates of some absorption intensities of oxazine ring are affected by different atmospheric environments, and a higher degree of conversion is obtained in nitrogen than that in air at the same reaction temperature and in equal time. Kinetic parameters of the dynamic polymerization DSC results were evaluated with Kissinger and Ozawa methods, respectively. The isothermal DSC results show that the polymerization reaction of BS-a monomer follows an autocatalytic mechanism. (C) 2010 Elsevier Ltd. All rights reserved.