PHYSICAL-PROPERTIES AND ENZYMATIC DEGRADABILITY OF POLY(3-HYDROXYBUTYRATE) STEREOISOMERS WITH DIFFERENT STEREOREGULARITIES

Poly(3-hydroxybutyrate) (P(3HB)) stereoisomers of high molecular weights (M(n) > 10(5)) were synthesized by the ring-opening stereocopolymerization of (R)-beta-butyrolactone with (S)-beta-butyrolactone at various feed ratios (R/S = 96/4 to 50/50) in the presence of 1-ethoxy-3-chlorotetrabutyldistannoxane as a catalyst. The isotactic diad fractions [i] of P(3HB) stereoisomer samples were varied from 0.92 to 0.30. A syndiotactic P(3HB) sample ([i] = 0.30) was obtained by polymerizing a racemic mixture of beta-butyrolactone. The glass-transition temperatures of P(3HB) samples were 5 +/- 2 degrees C, independent of the stereoregularity. The melting temperature of P(3HB) samples decreased from 177 to 92 degrees C as the [i] value was decreased from 1.00 to 0.68. The predominantly syndiotactic P(3HB) sample ([i] = 0.30) had two endothermic peaks at around 52 and 62 degrees C. The degree of X-ray crystallinity of solvent-cast P(3HB) films decreased from 62 to 40% as the [i] value was decreased from 1.00 to 0.68. The X-ray crystallinities of atactic ([i] = 0.46) and syndiotactic ([i] = 0.30) P(3HB) films were 0 and 26%, respectively. Enzymatic degradations of P(3HB) films with different stereoregularities were carried out for 3 h at 37 degrees C in 0.1 M potassium phosphate buffer (pH 7.4) in the presence of PHB depolymerases purified from Pseudomonas pickettii and Alcaligenes faecalis. The rates of enzymatic degradation of P(3HB) films ranging in [i] value from 0.68 to 0.92 were higher than that of bacterial P[(R)-3HB] film ([i] = 1.00). The highest rate of enzymatic hydrolysis was observed at the [i] value of 0.76. Little erosion of P(3HB) films was observed for the atactic and syndiotactic samples. The water-soluble products liberated during the enzymatic degradation of P(3HB) films were characterized by HPLC analysis. The bacterial P[(R)-3HB] film gave a mixture of monomer and dimer of 3-hydroxybutyric acid, but the steroirregular P(3HB) samples of [i] = 0.92-0.68 produced a mixture of monomer, dimer, trimer, and tetramer. In addition, the stereochemical effect on the rate of P(3HB) spherulite growth rate was studied.