PHYSICAL-PROPERTIES AND ENZYMATIC DEGRADABILITY OF COPOLYMERS OF (R)-3-HYDROXYBUTYRIC AND 6-HYDROXYHEXANOIC ACIDS

Copolymers of (R)-3-hydroxybutyric acid ((R)-3HB) and 6-hydroxyhexanoic acid (6HH) with a wide range of compositions varying from 11 to 91 mol % 6HH were synthesized by the ring-opening polymerization of (R)-beta-butyrolactone with epsilon-caprolactone at various feed ratios in the presence of 1-ethoxy-3-chlorotetrabutyldistannoxane as a catalyst. The structure and physical properties of P[(R)-3HB-co-6HH] were characterized by H-1 and C-13 NMR spectroscopy, X-ray diffraction, differential scanning calorimetry, and optical microscopy. The copolyesters were shown to have a random sequence distribution of(R)-3HB and 6HH monomeric: units. The glass-transition temperature of P[(R)-3HB-co-6HH] decreased Linearly from +4 to -67 degrees C as the 6HH composition was increased from 0 to 100 mol %. The melting temperature of P[(R)-3HB-co-6HH] samples decreased from 179 to 48 degrees C as the 6HH fraction was increased from 0 to 43 mol %. Then, the melting temperature of the copolyester increased from 36 to 60 degrees C with an increase in the 6HH fraction from 81 to 100 mol %. The degree of X-ray crystallinity of solvent-cast copolyester films decreased from 62 to 18% as the 6HH composition was increased from 0 to 43 mol %, and the copolyesters showed a P[(R)-3HB] crystal lattice. On the other hand, the X-ray crystallinities of copolymers with 6HH fractions of 81-100 mol % increased from 31 to 55% with the 6HH fraction, and those samples showed a P(6HH) crystal lattice. The P[(R)-3HB-co-6HH] samples with 53-70 mol % 6HH were amorphous polymers. Enzymatic degradations of P[(R)-3HB-co-6HH] films were carried out at 25 degrees C in 0.1 M potassium phosphate buffer (pH 7.4) in the presence of PHB depolymerase purified from Alcaligenes faecalis or of lipase from Rhizopus delemar. The rates of enzymatic degradation by PHB depolymerase of copolymer films ranging from 11 to 43 mol % 6HH were higher than that of bacterial P[(R)-3HB] film. The highest rate of enzymatic hydrolysis by PHB depolymerase was observed at 11 mol % 6HH. Little erosion was observed for the copolyester films ranging in 6HH fractions from 53 to 100 mol %. In contrast, the weight loss profile of P[(R)-3HB-co-6HH] films with a lipase showed a trend in the copolymer composition opposite to that of a PHB depolymerase. The highest rate of enzymatic hydrolysis by Lipase was observed at 91 mol % 6HH. HPLC analysis of the water-soluble products liberated during the enzymatic degradation of copolymer films showed a mixture of monomers, dimers, and trimer of (R)-3HB and 6HH units. A model for the enzymatic hydrolysis of the polyester chain by PHB depolymerase was proposed on the basis of the results.