Synthesis of poly-p-phenylenebenzobisthiazole used for fabrication of a thermostable, high-modulus fibre

A method was developed for synthesis of poly-p-phenylenebenzobisthiazole (PPBT) from 2,5-diamino-1,4-dimercaptobenzene diphosphate (DADMB) and terephthalic acid (TPA) in a solution of polyphosphoric acid (PPA) with a 12–15 wt. % concentration of PPBT with a molecular weight of 34,000–38,600 ([η] = 24.5–28.8 dl/g). It was shown that a high concentration of P2O5 in PPA, which should be 85.5–86.5 wt. % at the end of the reaction, is one of the basic conditions for obtaining a high-molecular-weight polymer from DADMB diphosphate and TPA. The synthesized liquid-crystalline polymers with a high molecular weight and concentration of 14–15 wt. % in PPA solution were used in the form of polycondensation solutions for spinning high-modulus PPBT fibres.