Some features of the synthesis of polyimides by single-stage high-temperature polycyclization

Abstract

The synthesis of aromatic polyimides from the anhydrides of tetracarboxylic acids and diamines is carried out in two stages with the precipitation of an intermediate product (a polyamido-acid) in the first stage and its subsequent intramolecular molecular cyclodehydration which is carried out by heating or under the action of chemical agents [1]. Data exist about the synthesis of aromatic polyimides in phenols at elevated temperatures in the presence of tertiary amines [2], and about the synthesis of polyimides from 2,3,5,6-pyridinetetracarboxylic acid in polyphosphoric acid at a temperature below 180°C [3]. With soluble polyimides of anilinephthalein as an example, we have previously [4, 5] developed a new method of synthesizing polyimides, by single-stage high-temperature polycyclization in an organic solvent; the method consists of interaction between a diamine and the dianhydride of a tetracarboxylic acid in a high-boiling solvent at 190–210°C. Subsequently, this method was extended to the synthesis of other polyides with different chemical structures [5–7]. The comparison which we made between the single-stage method and the two-stage method widely used at the present time brought to light certain advantages of single-stage polycyclization. The present communication is devoted to the study of certain rules governing the synthesis of polyimides by single-stage polycyclization, the reaction between pyromellitic dianhydride (I) and 3,3-bis-(4-aminophenyl) phthalide (anilinephthalein) (II) in nitrobenzene being used as the example. Moreover, the possibility of adapting this method to polymers which are insoluble or swell in the reaction medium, by varying the nature of the organic solvent and the chemical structure of the polyimide, has been discussed.