Kinetics and mechanism of the monomeric products from abiotic hydrolysis of poly[(R)-3-hydroxybutyrate] under acidic and alkaline conditions

Abiotic hydrolysis of poly[(R)-3-hydroxybutyrate] (PHB) was investigated in acid and base media by monitoring the formation of two monomeric hydrolytic products, 3-hydroxybutyric acid (3HB) and crotonic acid (CA) from three types of PHB matrix, native amorphous granules, precipitates and solvent-cast films. Control experiments were also performed with five esters of 3-hydroxybutyric acid. The monomeric products were not released from the PHB specimens in acidic solutions (0.1-4 N H+), but were measured as the major hydrolytic products from alkaline hydrolysis (0.1-4 N OH−). The amorphous PHB granules decomposed 30 times faster than the crystallized PHB precipitates and solvent-cast films. Formation of both CA and 3HB followed the quasi-0th-order kinetics and their mass ratio (CA/3HB = 0.42-0.68) increased with temperature because of the different activation energies, 88.4 kg/mol for CA formation and 78.9 kJ/mol for 3HB formation. Kinetic analysis and hydrolysis results revealed that dehydration of the 3-hydroxyl group is not a favorable reaction, compared to ester bond hydrolysis, under the chosen experimental conditions. A transient six-membered ring structure involving two neighboring monomers is most likely needed for crotonate formation.