Direct synthesis of poly(4′-oxy-4-biphenylcarbonyl) and poly(2-oxy-6-naphthoyl) under nonstoichiometric conditions

Direct synthesis of poly(4′-oxy-4-biphenylcarbonyl) (POBP) and poly(2-oxy-6-naphthoyl) (PON) was examined by polycondensation of 4′-hydroxy-4-biphenylcarboxylic acid (HBPA) and 2-hydroxy-6-naphthoic acid (HNA) in the presence of 4-ethoxybenzoic anhydride or 2-naphthoic anhydride as condensation reagents. Polymerizations were carried out at 320°C in aromatic solvents and liquid paraffin. POBP, having a number-average degree of polymerization (DPn) of 38, was obtained as plate-like crystals at the molar ratio of HBPA and anhydride of 50 mol%. PON was also obtained as plate-like crystals but the DPn was only 13. HBPA and HNA were first converted to reactive acyloxyaromatic acid intermediates. Then the DPn was increased by means of reaction-induced crystallization of oligomers and subsequent solid-state polymerization via an acid-ester exchange under nonstoichiometric conditions caused by the monocarboxylic acid by-product. Even though the DPn of PON was not as high, direct polycondensation of HBPA and HNA proceeded successfully with aromatic anhydrides.