Synthesis and structure of the p‐hydroxybenzoic acid polymer

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 = 17.8 Å and c = 18.4 Å, 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°C (not a melting point). The transition was characterized by differential thermal analysis, 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.