Promoted random orientation of the phenyl substituent of phenylhydroquinone–terephthalic acid polyesters prepared with a diphenyl chlorophosphate/pyridine condensing agent

The reaction of sterically hindered hydroxyl groups at the 2‐position of methylhydroquinone and phenylhydroquinone (PhHQ) to form esters was largely promoted by their slow addition to benzoic acid activated by diphenyl chlorophosphate in pyridine. A modification of this reaction was applied to the preparation of thermotropic terephthalic acid/PhHQ and 2,5‐dichloroterephthalic acid/PhHQ polymers with randomly oriented phenyl substituents, and the properties of the polymers were studied in terms of their transition temperatures, which were determined by differential scanning calorimetry and microscopic observation. The melting points were lowered by about 30–50 °C by the dropwise addition of PhHQ over 10–30 min. The molecular structures of the 2,5‐dichloroterephthalic acid/PhHQ polymers were studied by 13C NMR.