Enhanced thermomechanical properties of bisphenol-A based polybenzoxazine (PBA-a) copolymers obtained by reacting bisphenol-A-aniline-type benzoxazine (BA-a) resin with three different aromatic carboxylic dianhydrides, i.e., pyromellitic dianhydride (PMDA), 3,3',4,4' biphenyltetracarboxylic dianhydride (s-BPDA), or 3,3',4,4' benzophenonetetracarboxylic dianhydride (BTDA) were reported. Glass transition temperature (Tg), of the copolymers was found to be in the order of PBA-a:PMDA>PBA-a:s-BPDA>PBA-a:-BTDA. The difference in the Tg of the copolymers is related to the rigidity of the dianhydride components. Furthermore, the Tg of PBA-a:BTDA, PBA-a:s-BPDA, and PBA-a:BTDA films was observed to be significantly higher than that of the neat PBA-a owing to the enhanced crosslink density by the dianhydride addition. This greater crosslink density results from additional ester linkage formation between the hydroxyl group of PBA-a and the anhydride group of dianhydrides formed by thermal curing. Moreover, the copolymers exhibit enhanced thermal stability with thermal degradation temperature (Td) ranging from 410 degrees C to 426 degrees C under nitrogen atmosphere. The char yield at 800 degrees C of the copolymers was found to be remarkably greater than that of the neat PBA-a with a value up to 60% vs. that of about 38% of the PBA-a. Toughness of the copolymer films was greatly improved compared to that of the neat PBA-a. POLYM. ENG. SCI., 2012. (C) 2012 Society of Plastics Engineers