The synthesis and structure of the p-hydroxybenzoic acid polymer is described. The polymer was successfully prepared from either the phenyl ester of p-hydroxybenzoic acid or from p-acetoxybenzoic acid. With highly purified acetoxybenzoic acid, single crystals of the polymer could be prepared. The structure of the polymer was determined and shown to consist of a double helix where the two chains are in a reversed head-to-tail order. The unit cell dimensions are: a equals 17. 8 A and c equals 18. 4 A, where c corresponds to the chain length with a repeat distance of three units. The mechanism of polymerization and formation of the single crystal is discussed. The polymer displays a reversible high-temperature crystalline transition at 325-360 degree C (not a melting point). The transition was characterized by DTA, differential calorimetry, thermal expansion coefficient measurements, high-temperature x-ray scans, and dielectric constant determinations. Orientation of the polymer chains during fabrication and changes in the mechanism of oxidative degradation above the crystal transition are described.
