Important Factors Controlling Synthesis of Imides in Water

Abstract

Four amine/anhydride models have been studied in order to understand the critical factors which control the synthesis of imides and polyimides in water. This synthesis method has been proposed recently as a convenient, environmentally friendly method of producing both thermosetting and thermoplastic polyimides. The key starting step in the synthesis is the formation of a salt/complex between the hydrolysed anhydride and the amine. The results from a combination of quantitative ¹H NMR, HPLC and DSC analysis have shown that the reactions are reversible, and that the important factor in driving the reaction to completion in water is the insolubility of the imide product. Analysis of the precipitated imides shows them to be free of significant amounts of the considerably more water soluble amic acids (i.e. the uncyclized amide acids), which often contaminate imides made in conventional aprotic solvents. Of the models investigated, only the imidization of the second amino group in 2, 4, 6-trimethyl-1, 3-phenylene diamine (TMPDA) was subject to kinetic control.

A general conclusion from these experiments is that where a different result is observed in water synthesis of imides compared with conventional aprotic solvent synthesis, it is likely to reflect the vastly different solvation powers of the two solvents rather than a difference in reaction mechanism.