Reaction Kinetics and Mechanistic Shift in Non/Catalytic Esterification of PEGs: Microwave-Assisted vs. Conventional Esterification
Abstract
Esterification reactions play a crucial role in both industrial and biological systems, essential for the production of pharmaceuticals, polymers, and specialty chemicals. The esterification of polyethylene glycol (PEG) with chloroacetic acid (CAA) is important for surfactants, drug delivery systems, etc. This study investigates the esterification kinetics in a microwave-assisted, non-catalytic system, employing an alternative kinetic parameter estimation approach compared to conventional methods. Instead of separately analyzing temperature-dependent reaction rates, an optimization algorithm was used to simultaneously evaluate data from different temperatures. This novel method enhances the accuracy and reliability of kinetic parameters, while the absence of a catalyst significantly influenced the reaction mechanism, emphasizing the role of high-dielectric constant molecules. The esterification kinetics between CAA and PEGs were studied under both conventional and microwave-assisted conditions. Optimized kinetic data revealed that increasing PEG chain length resulted in decreased reaction rates, which was attributed to increased viscosity and steric hindrance effects. In the microwave method, lower rate constants were associated with the shorter thermal exposure of the reaction. Additionally, the collision frequency factor decreased with an increase in microwave power, due to the lower dielectric constants of PEG-based compounds. A significant change in reaction order was also observed under non-catalytic conditions; under microwave irradiation, PEG200 exhibited a reaction order of 1.5, while PEG400 showed second-order dependence. These findings offer new insights into the kinetic behavior of non-catalytic reactions and highlight the influence of molecular properties on reaction pathways. Ester formation was confirmed by FTIR and NMR analyses, providing evidence of successful esterification.
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