A side reaction giving rise to cross‐linking in the preparation of poly pyromellitimides

The paper deals with difficulties experienced according to the patent literature in the preparation of linear soluble polymers of poly pyromellitimides from pyromellitic acid and diamines. It is suggested that diamidation which will, on theoretical grounds, always occur as a side reaction in imide formation, is probably the main cause of these difficulties as it gives rise to branching and cross‐linking. The equilibrium reactions involved are discussed and it is shown that a slight excess of basic groups will promote, and a slight excess of acid groups supress the side reaction. These conclusions are underlined by experiment. The magnitude of the equilibirum constants of the elementary reactions in question are determined approximatively from the investigation of non‐polymeric model systems in which pyromellitic acid is substituted by ortho‐phthalic acid.