The chain fragmentation in polycarbonate (PC) and polybenzoxazine blend upon thermal polymerization is investigated by size exclusion chromatography with the aid of monofunctional benzoxazine model compound. Molecular weight reduction of PC polymer via transesterification between the hydroxyl groups of ring-opened benzoxazine and the carbonate groups from PC was observed. In addition, excess heat of reaction compared to the expected value is detected from differential scanning calorimetry and is assigned to the exotherm associated with the exchange reaction.