In this research, processability and some important thermomechanical properties of polybenzoxazine (BA-a) modified with a highly flexible urethane elastomer (PU) are discussed. This copolymer has been reported to show synergy in its glass transition temperature and some mechanical properties thus provides a fascinating group of high temperature polymers with enhanced flexibility. The results reveal that a processing window of the BA-a/PU mixtures is widened with the increasing urethane prepolymer fraction, that is, the liquefying temperature is lowered and the gel point shifted to higher temperature with the amount of the PU. Synergism in glass transition temperature (Tg) of this copolymer was clearly confirmed, i. e., T(g)'s of the BA-a/PU alloys were significantly greater than those of the parent resins, i. e., BA-a (T(g) = 166 degrees C) and PU (T(g) = 70 degrees C). In addition, flexural modulus was found to systemically decrease from 5.4 GPa of the neat polybenzoxazine to 2.1 GPa at 40% by weight of the PU. Flexural strength of the alloys also shows a synergistic behavior at the BA-a/PU ratio of 90/10. Coefficient of thermal expansion of the polymer alloys were also found to show a minimum value at BA-a/PU 90/10. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 36693678, 2011