Study on the polymerization of aspartic acid catalyzed by phosphoric acid

The dehydration kinetics of DL- and L-aspartic acid, either in the absence or presence of phosphoric acid, from 323 to 573K were studied with thermogravimetric analysis (TGA), and the polymerization of DL-aspartic acid was studied by a thin-layer polymerization method. The results revealed that the dehydration of both type aspartic acids proceeds in two steps: the first step might be the loss of one water molecule through the reaction of an amino group of one aspartic acid molecule and a hydroxyl of carboxyl group of another aspartic acid molecule, forming amide bonds, and the second step might be the loss of another water molecule through the amide hydrogen and another hydroxyl group, leading to the formation of succinimide ring. The kinetic parameters of the extrapolated onset temperatures of dehydration, the first and the second maximum were obtained by a method similar to the Ozawa - Flynn - Wall method. The results further revealed that the presence of phosphoric acid effectively catalyzed the dehydration of aspartic acid. In the synthesis of polyaspartate, only the product with molecular weight below 5000 was obtained in the absence of phosphoric acid. However, in the presence of phosphoric acid, the product with molecular weight up to 14,000 can be obtained.