Analysis of the kinetics of soluble polyimide formation by the thermal imidization of polyamic acids in amide solvents with allowance for the side reaction of anhydride group hydrolysis

A kinetic analysis of the process aimed at preparing soluble polyimide by the thermal imidization of a prepolymer, polyamic acid (PAA), in a dimethylacetamide solution in a temperature range of 120–160 °C was carried out. The scheme including elementary reactions of imidization, synthesis, and decomposition of PAA, as well as the reversible side reaction of hydrolysis of anhydride groups with water released during imidization was used in the analysis. The PAA was synthesized by the low-temperature polycondensation of 2,2-bis[(3,4-dicarboxyphenoxy)phenyl]propane dianhydride (BPADA) and 2,2-bis(4-aminophenoxyphenyl)propane (BAPP). The rate constants of hydrolysis of anhydride groups and dehydration of phthalic acid fragments were determined experimentally for model compounds: phthalic anhydride and phthalic acid. The experimentally found rate constants of elementary reactions of imidization, decay, and synthesis of PAA of the same chemical structure and in the same temperature range determined by the authors earlier were also used in the calculations. The solution of the system of kinetic equations was performed by the numerical integration using the Maple 17 software. The developed model makes it possible to calculate the dependence of the change in the number average molecular weight of the formed polyimide. According to the calculation results, the initial PAA concentration affects the character of the dependence: the higher the initial concentration of PAA, the more noticeable the role of the side reaction of hydrolysis of terminal anhydride groups, which reduces the rate of increasing average degree of polymerization.