(305h) Understanding the Role of H-Bonding in a Model Reaction for Polyester Glycolysis

Kinetic studies of polymer deconstruction reactions are challenging due to the poor solubility of many commodity polymers in common solvents and sequential reactions that lead to products of various chain lengths. To explore the kinetics of ester glycolysis without the complication of polyethylene terephthalate (PET) solubility and complex product distributions, we studied the catalytic transesterification of a model compound possessing ester moieties similar to those in PET. In particular, we explored the reaction kinetics for transesterification of 2-(benzoyloxy)ethyl benzoate with various alcohols (EG, propylene glycol (PG), 1,2-butylene glycol (BG), 1,3-propanediol, and 1-butanol) at 75 °C catalyzed by the strong base triazabicyclodecene (TBD). The measured turnover frequency and apparent activation energy were greatest for the reaction with EG and lowest for the reaction with 1-butanol. Moreover, the reaction was nearly zero order in benzoate during transesterification in excess glycol but nearly first order in benzoate when in excess 1-butanol. The rate and order of reaction in benzoate for transesterification with 1,3-propanediol were between that performed in EG and butanol. We also observed that dilution in acetonitrile increased the rate of transesterification with EG, suggesting an influence of solvent on the reaction energetics. Results from ab initio quantum chemical calculations reveal that the critical reaction intermediate formed in EG is stabilized by intramolecular hydrogen bonding, which accounts for the higher apparent activation energy and zero order kinetics. The presence of acetonitrile solvent apparently lowers the transition state energy associated with EG substitution during the reaction. These findings suggest potentially important roles of intramolecular H-bonding and/or solvent effects in the kinetics of PET deconstruction via transesterification.