Acid curing of epoxy resins. A comparison between the polymerization of diepoxide‐diacid and monoepoxide‐cyclic anhydride systems

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 ( 1 ), 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.