High‐strength–high‐modulus polyimide fibers I. One‐step synthesis of spinnable polyimides

Polyimides with aromatic links are synthesized usually in two steps, but a series of aromatic polyimides with large molecular weights sufficient to produce strong fibers were prepared in one step by the reaction of 3,3′, 4,4′‐biphenyltetracarboxylic dianhydride (BPDA) with various aromatic diamines. The one‐step polycondensation was possible even when BPDA was replaced to some extent by pyromellitic dianhydride, which is a basic component in the conventional two‐step method. The copolyimides based on BPDA and mixtures of aromatic diamines were also synthesized in similar manner. For the one‐step polycondensation, phenol and some of its derivatives were found to be suitable solvents, while the typical solvent employed in the two‐step method, such as N‐methyl‐2‐pyrrolidone, could not be used because of the precipitation of low‐molecular weight polyimides. p‐Hydroxybenzoic acid proved to be a very efficient accelerator for the conversion of polyamic acids to polyimides.