Intramolecular Lactonization of Poly(α-hydroxyacrylic acid): Kinetics and Reaction Mechanism

Abstract

Poly(α-hydroxyacrylic acid), PHA, is one of the few polymers with biodegradable properties used in mechanical pulp bleaching to stabilize hydrogen peroxide. A new method for the in situ follow-up of the lactone ring formation of PHA has been developed. The results have further been applied to describe the reaction kinetics of the lactonization and hydrolysis reactions through parameter estimation. In addition, the reaction mechanism is elucidated by multivariate data analysis. Satisfactory identification and semiquantitative separation of the lactone ring as well as the acyclic (carboxyl and hydroxyl groups) forms have been established by 1H NMR in the pH range of 1–9. The lactonization reaction approaching equilibrium can be described by pseudo-first-order kinetics in the pH range of 1–6. The rate constants of the pseudo-first-order kinetic model have been estimated by nonlinear regression. Due to the very low rates of lactonization as well as the weak pH dependency of the reaction, an addition-elimination mechanism is proposed. Additionally, the presence of a transient reaction intermediate during lactonization reaction could be identified by subjecting the measurement data to multivariate data analysis (PCA, principal component analysis). A good correlation was found between the kinetic and the PCA models in terms of model validity.