Copolymers of polybenzoxazine (BA-a) and urethane elastomer (PU) with three different structures of isocyanates [i.e., toluene diisocyanate (TDI), diphenylmethane diisocyanate, and isophorone diisocyanate 1, were examined. The experimental results reveal that the enhancement in glass transition temperature (T,) of BA-a/PU copolymers was clearly observed [i.e., T, of the BA-a/PU copolymers in 60 : 40 BA-a : PU system for all isocyanate types (T, beyond 230 degrees C) was higher than those of the parent resinsL (165 degrees C for BA-a and -70 degrees C for PU)]. It was reported that the degradation temperature increased from 321 degrees C to about 330 degrees C with increasing urethane content. Furthermore, the flexural strength synergism was found at the BA-a : PU ratio of 90 : 1.0 for all types of isocyanates. The effect of urethane prepolymer based on TDI rendered the highest T-g, flexural modulus, and flexural strength of the copolymers among the three isocyanates used. The preferable isocyanate of the binary systems for making high processable carbon fiber composites was based on TDI. The flexural strength of the carbon fiber-reinforced BA-a : PU based on TDI at 80 wt % of the fiber in cross-ply orientation provided relatively high values of about 490 MPa. The flexural modulus slightly decreased from 51 GPa for polybenzoxazine to 48 GPa in the 60 : 40 BA-a : PU system. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113: 3823-3830, 2009