Ring-opening mechanism of epoxides with alcohol and tertiary amines

Abstract

The ring-opening mechanism of epoxides in the presence of tertiary amines was studied in this work. Gas chromatography (GC) and size-exclusion chromatography (SEC) were used to measure the monomer conversion and molecular weights of the polymers. Nuclear magnetic resonance (NMR) spectroscopy was used to analyze the polymer structures. It has been found that only in the system where tertiary amines and hydroxyl-containing compounds coexist can the ring-opening reaction occur, indicating that there is an interaction between tertiary amines and the hydroxyl groups, which is confirmed by the NMR analysis of a mixture of triethylamine (TEA) and ethanol (EtOH), and the polymerization of TEA and glycidol (GL). As for normal epoxides, such as propylene oxide, polymers contained tertiary amine moieties before and after dialysis, suggesting that the tertiary amine moieties were covalently linked with the polymer backbone but were not present as the counter ions, and this is the first NMR proof confirming that tertiary amine initiates the ring-opening reaction of epoxides. In sharp contrast, a reaction at huge excesses of EtOH and TEA was carried out at [GL]₀/[EtOH]₀/[TEA]₀ = 1/50/50, and a model compound with only one GL unit was obtained. The NMR analysis of this model compound confirms that the alkoxy anion from ethanol rather than TEA attacks and opens the epoxy ring of GL. The presence and absence of the tertiary amine moiety in the polymers from GL/tertiary amine before and after dialysis further support the reaction mechanism that an alkoxy anion initiates the polymerization, and the positively charged tertiary amine units are present only as counter ions and are removed by dialysis. These demonstrate that the ring-opening mechanisms catalyzed by tertiary amine for epoxides will be different from each other depending on their structures.