Kinetics of dehydration-polymerization of aspartic acid and synthesis of polyaspartate catalyzed by potassium bisulfate

The dehydration-polymerization kinetics of DL- and L-aspartic acid, either in the absence or presence of KHSO₄, from 323 to 573 K, was studied by thermogravimetric analysis (TGA), and the synthesis of polyaspartate through the polymerization of L-aspartic acid was investigated by a thin-layer polymerization method. The TGA results revealed that the dehydration-polymerization of both type of aspartic acids proceeds in two steps: First, the loss of one water molecule through the reaction of an amino group of one aspartic acid molecule and a hydroxyl group of another aspartic acid molecule, forming amide bonds, and secondly, the loss of another water molecule through the amide hydrogen and another hydroxyl group, leading to the formation of a succinimide ring. The kinetic parameters of the extrapolated onset temperatures of dehydration-the first and the second maxima-were obtained by a method similar to that of Ozawa-Flynn-Wall. The kinetic results indicate that the dehydration of L-aspartic acid is slightly more difficult than for DL-aspartic acid, and that the presence of potassium bisulfate effectively catalyzes the dehydration-polymerization of aspartic acid. In the synthesis of polyaspartate, the product with a weight-average molecular mass (M_w) of 5000 g mol^-1 was obtained in the absence of catalyst. However, in the presence of potassium bisulfate, the products obtained had M_ws of up to 7000 g mol^-1