Rapid Epoxy Polymerization Induced by Aliphatic Cyclic Tertiary Amines: The Roles of Nucleophilicity and Leaving Ability

Epoxy resins exhibit variations in their polymerization mechanisms, curing temperatures, curing times, and mechanical properties of the cured products depending on the curing agent used. Elucidating the mechanism of the reaction between the curing agent and epoxy groups is important. However, the polymerization mechanism of epoxy groups induced by tertiary amines remains unclear, particularly regarding the relationship between the properties of the amines and their polymerization ability based on their nucleophilicity because the number of tertiary amines investigated is limited. Herein, we demonstrate that nucleophilicity and leaving ability of the base, rather than basicity, are the critical factors controlling reactivity in the epoxy polymerization when using tertiary amines. Quinuclidine, which combines these important properties, can polymerize epoxy monomers significantly faster than the conventionally used N,N-dimethylbenzylamine. This study establishes criteria for the selection of amines for the curing of epoxy resins using tertiary amines and provides guidelines for developing new curing agents based on molecular design.