Thermooxidatively stable articulated benzobisoxazole and benzobisthiazole polymers

Articulated all‐para polymers with 2,6‐benzobisoxazole and 2,6‐benzobisthiazole units in the backbone weree synthesized by copolycondensation in polyphosphoric acid of 4,6‐diamino‐1,3‐benzenediol dihydrochloride and 2,5‐diamino‐1,4‐benzenedithiol dihydrochloride, respectively, with terephthalic acid and reactive 3,3′‐biphenyl or 4,4′‐(2,2′‐bipyridyl) monomers. Inherent viscosities of up to 13.18 dL/g (CH3SO3H, 25°C, 0.2 g/dL) were achieved. The average length of the rodlike all‐para segments between the relatively flexible biphenyl or bipyridyl units was controlled by the stoichiometry of the copolycondensation reactions. Dependent upon the solids content of the polymerization mixture and the mole proportion of the flexible units in the polymer backbone, copolycondensation proceeded in the liquid‐crystalline state to give polymerization mixtures which exhibited lower bulk viscosities than comparable copolycondensation reactions that remained in the isotropic state. Films which exhibited optical birefringence under crossed polars could be cast from methanesulfonic acid solutions of the polymers. Thermooxidative stability of the articulated polymers was evaluated by isothermal aging in air at 371°C. Stability for the polymers was found to decrease slightly with increased content of the flexible biphenyl structure in the polymer backbone. The biphenyl structure was found to be more thermooxidatively stable than the bipyridyl structure.