Morphology of Poly(p-oxybenzoyl) Crystal Prepared by Direct Polycondensation of p-Hydroxybenzoic Acid with p-Toluenesulfonyl Chloride in Pyridine

Poly(p-oxybenzoyl) (POB) crystals were prepared with the reaction-induced crystallization of oligomers during the direct polycondensation of p-hydroxybenzoic acid (HBA) with p-toluenesulfonyl chloride (TsCl) and N,N-dimethylformamide in pyridine. Sheaflike lozenge-shaped POB crystals were obtained, of which the longer diagonal was 7.0-8.0 μm. The influence of the polymerization condition on the morphology was examined to optimize the preparative condition for the crystals exhibiting the clearest habit, and the favorable condition was determined as the molar ratio of TsCl to HBA of 1.3 and polymerization concentration of 3.0%. The crystals possessed extremely high crystallinity and outstanding thermal stability. The formation mechanism of the crystal was proposed as follows. When the number-average degree of polymerization of the oligomers exceeded a critical value of about 4, they were precipitated to form the hexagonal lamellae. The crystals were grown very quickly to lozenge-shaped crystal through screw dislocation with the continuous precipitation of oligomers from the solution. Finally, the further polymerization occurred in the precipitated crystal with developing polymer-chain packing.