Study of the synthesis of poly(4,4′-diphenylether-1,3,4-oxadiazole) in solutions of poly(phosphoric acid)

Using statistical design of experiments, samples of poly(4,4′-diphenylether-1,3,4-oxadiazole)s, POD–DPE, were synthesized through solution polycondensation of 4,4′-diphenylether dicarboxylic acid, DPE, and hydrazine sulphate, HS, in poly(phosphoric acid), PPA, under different reaction conditions. Final polymer molecular structure (average molecular weights, polymer composition, amount of hydrazide groups) is shown to depend strongly upon the polymerization conditions and that adequate properties for membrane synthesis (high average molecular weight, low amounts of hydrazide groups and low gel contents) can only be obtained if high synthesis temperatures (160°C) are used. Empirical and semi-theoretical models are developed to describe the kinetics of POD–DPE production in PPA solutions, showing that the likely existence of cross-linking reactions may explain both the relatively high values and the large fluctuations of the average molecular weight obtained experimentally.