The mechanism of acid curing of epoxy resins catalyzed with tertiary amines was investigated by using model systems composed of phenylglycidyl ether and benzoic acid or acetic acid anhydrides in the presence of benzyldimethylamine. The reaction was studied by NMR spectrometry, liquid chromatography, and ozone absorption. The main findings are that (1) the tert‐amine is bound chemically and irreversibly during the reaction under the formation of a quaternary ammonium salt magnified image and (2) 1‐phenyloxypropanediol‐2,3‐dibenzoate or diacetate is the main reaction product. The suggested reaction mechanism involves initiation in which the tertiary amine reacts with the epoxy group, giving rise to a zwitterion that contains a quaternary nitrogen atom and an alkoxide anion; the latter immediately reacts with the anhydride and quaternary salt is formed. In a later stage the carboxy anion of the quaternary salt reacts first with the epoxy group, then with the anhydride. By this reaction diester is formed and the carboxy anion is regenerated.

The molar masses of linear polyesters formed in the polymerization of stoichiometric mixtures of 1,7-heptanedicarboxylic acid (4) with 2,2-bis[4-(2,3-epoxypropoxy)phenyl]propane (2) (the diglycidyl ether of Bisphenol A), and of 1,2-cyclohexanedicarboxylic anhydride (3) with 2,3-epoxypropyl phenyl ether (l), catalyzed with tertiary amines, and the effect of the concentration of the tertiary amine catalysts were compared. In the reaction between anhydride 3 and epoxide 1 the molar mass of polyester, determined by GPC, was found to decrease with increasing concentration of the tertiary amine, whereas in the reaction of the acid it is independent of the catalyst content. The molar mass distribution is much narrower in the reaction of the anhydride. These results correspond to the mechanisms determined earlier: a stepwise mechanism in the reaction between epoxide and acid, and an "initiation" mechanism in the case of anhydride. In the polymerization of the dicarboxylic acid 4 with the diepoxide 2 good agreement was observed with the theoretical dependences, calculated by using the theory of branching processes.

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