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 -1,3,3-oxadiazole)s, POD-DPE, were synthesized through solution polycondens ation of 4,4'-diphenylether dicarboxylic acid, DPE, and hydrazine sulphate, HS, in poly(phosphoric acid), PPA, under different reaction conditions. Fi nal polymer molecular structure (average molecular weights, polymer composi tion, amount of hydrazide groups) is shown to depend strongly upon the poly merization 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 degreesC ) are used. Empirical and semi-theoretical models are developed to describe the kinetics of POD-DPE production in PPA solutions, showing that the like ly existence of cross-linking reactions may explain both the relatively hig h values and the large fluctuations of the average molecular weight obtaine d experimentally. (C) 2000 Elsevier Science Ltd. All rights reserved.