Copolyesters of hydroxyphenylalkanoic acids: Synthesis,properties, and in vitro degradation of poly(4‐oxybenzoate‐co‐4‐oxyphenylacetate)

Hydrolytically degradable copolyesters of the naturally occurring monomer 4‐hydroxyphenylacetic acid (HPAA) with 4‐hydroxybenzoic acid (HBA) were synthesized for the first time by the acidolysis melt polymerization of their acetoxy derivatives. The HPAA/HBA copolyesters prepared by acidolysis melt polycondensation had higher yields and molecular weights than those obtained by a one‐pot method. The high‐temperature solvent Dowtherm® improved the color of the polyester. Although catalysts did not affect the inherent viscosity and yield of the polymer, they did reduce the polymerization time. A higher degree of polymerization was achieved with postpolymerization and annealing techniques. Copolyesters prepared in different molar ratios were analyzed by elemental analysis, IR, NMR, and inherent viscosity and were further characterized for their thermal and phase properties by thermogravimetric analysis, differential scanning calorimetry, wide‐angle X‐ray diffraction, and polarized light microscopy. The composition of the copolyesters affected the yield, solubility, and inherent viscosity. The NMR data indicated comparatively high randomization for the copolyester obtained by acidolysis melt polymerization. The 60/40 HPAA/HBA copolyester formed a birefringent melt with a grainy texture above 175 °C with isotropization at 297 °C and thermal stability above 350 °C. The occurrence of birefringence with a grainy texture in the melt indicates a layered smectic phase; this was supported by wide‐angle X‐ray diffraction powder patterns. The in vitro hydrolytic degradability of the copolyester was studied by the measurement of the water absorption of the film samples in buffer solutions of pH 7 and 10 at 30 and 60 °C. The copolyester showed considerable hydrolytic degradation, enough to be called biodegradable, compared with the commercial polyester Vectra®, thereby demonstrating prospects for syntheses of copolyesters with tailor‐made degradability. The degradation of the copolyester was identified by Fourier transform infrared, differential scanning calorimetry, thermogravimetric analysis, and scanning electron microscopy. These polyesters with controlled crystallinity and degradability should be considered for possible applications in biomedical areas (e.g., bone fixation devices in fracture treatment) in which high strength with biodegradability is an essential requirement. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 693–705, 2001