The nature of the equilibrium in the reaction of aromatic anhydrides with aromatic amines and its role in synthesis of polyimides

Abstract

The preparation of several highly heat-resistant heterocyclic polymers, polyimides in particular, is based on the reaction of anhydrides with amines.The first stage, preparation of polyamido-acids, is normally carried out in highly polar solvents such as dimethylformamide, dimethylacetamide, dimethylsulphoxide, etc. The molecular weight of polyamido-acid (PAA) determines to a considerable extent the properties of the end product prepared by cyclization. Thus, with an increase in [η] of PAA from 0·7 to 4·87 the tensile strength and breaking elongation of the polyimide film increase from 141 and 0·5% to 1477 kg/cm2 and 200% respectively [1]. Several factors contribute to the high molecular weight of PAA, including reaction temperature, purity of the initial reagents and solvent and the order in which the reagents are dissolved [2, 3]. These parameters are derived experimentally, since the kinetic and thermodynamic relations of the reaction of anhydrides with amines have not been examined. Furthermore, the causes of instability of polyamido-acids in solution are not clear, thus complicating the preparation of high-grade polyimides. The value of ηlog of PAA prepared from pyromellitic dianhydride (PDA) and 4,4′-diaminodiphenyl ester in dimethylformamide (DMFA) at 20° decreases from 2·1 to 0·7 in 30 days [3, 4]. This effect increases considerably if water is present in the solution [4]. Hence there is a need for studying the factors which influence the molecular weight of PAA.