1. M. A. Emanuel, K.N. James, US Patent No. 3417138, 1968.
2. Y. Okada, US Patent No. 3903152, 1975.
3. Ullmann’s Encyclopedia of Industrial Chemistry, Dicarboxylic Acids, Aliphatic, 10–11.
4. Y. Okada, US Patent No. 3714244, 1973.
5. S.C. Zhang, US Patent No. 6218574, 2001.