Thermal reactions of the binary alkali salts of poly(carboxylic acid)-brominated carboxylic acid such as sodium or potassium poly(4-vinylbenzoate)-2-bromopropanoate [Na or K (PVBA-2-BPA)] in bulk were investigated. A methanol solution of binary acids was prepared by fixing the molar ratio of the repeating unit of polymeric acid to the fraction of brominated carboxylic acid. The binary salts were prepared by the neutralization of the binary acid solution. The product of the thermal reaction followed by esterification was identified as a graft copolymer containing PVBA in the main chain and polylactic acid in the side chain. The reaction of 1/15 K (PVBA-2-BPA) at 120 degreesC for 2 h yielded the highest percentage of grafting (300%). The grafting proceeded gradually for the initial 2 h and then somewhat. Reactivity of the K salt was higher than that of the corresponding Na salt. The thermal reaction of 1/10 K [polymethacrylate-2-BPA (PMA-2-BPA)] at 120 degreesC for 2 h also yielded a graft copolymer, and the percentage of grafting was 300%. However, reaction temperatures higher than 120 degreesC caused homopolycondensation of If. 2-BPA prior to grafting, and homopolycondensation occurred prior to grafting in the reaction with Na (PMA-2-BPA).