Thiophenol and p-nitrothiophenol were evaluated as promoters for the ring opening polymerization of benzoxazine. The ring-opening polymerization of p-cresol type monofunctional N-phenyl benzoxazine 1a with 10 mol % of thiophenols proceeded at 150 degrees C, leading to the high conversion of 1a more than 95% within 5 h, whereas the polymerization of 1a without thiophenols did not proceed under the same conditions. The promotion effect of the thiophenols on curing of bisphenol-A type N-phenyl benzoxazine 1b was also investigated. In the differential scanning calorimetric (DSC) analysis of the polymerization of 1b at 150 degrees C without using any promoters, an exothermic peak attributable to the ring-opening reaction of benzoxazine was observed after 8 h. In contrast, in the DSC analysis of the polymerization of 1b with addition 20 mol % of p-nitrothiophenol, an exothermic peak was observed within 2 h, to clarify the significant promoting effect of p-nitrothiophenol. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 2523-2527