Abiotic Hydrolysis of Poly[(R)-3-hydroxybutyrate] in Acidic and Alkaline Media

Poly[(R)-3-hydroxybutyrate)], P(3HB), is the most common member of polyhydroxyalkanoates, the natural biopolyesters of intrinsic biodegradability and biocompatibility. Abiotic hydrolysis of P(3HB) was investigated in acid and base media by monitoring the formation of two monomer products, 3-hydroxybutyric acid (3HB) and crotonic acid (CA), from three types of P(3HB) samples, amorphous granules, irregular precipitates and solvent cast films. The soluble monomeric products were not detected in acid solutions (0.1 to 4 N H⁺), but measured as the major hydrolytic products in base solutions (0.1 to 4 N OH⁻). Unlike the protons as catalyst in both hydrolysis and esterification, hydroxyl anions were consumed during formation of carboxylate anions. The amorphous granules of P(3HB) were decomposed 80- to 100-fold faster than the precipitates and solvent cast films. The latter two had around 71% crystallinity. The hydrolysis of amorphous grannules exhibited a quasi 0^th-order reaction rate and the activation energy of saponification was 82.2 kJ/mol, silimar to those of the biotic hydrolysis of P(3HB) by enzymes and living cells.