Structural features in epoxy networks from N, N-diglycidyl epoxies and amines: 2. Ether ring formation and polymer structure in the reactions of N, N-diglycidylaniline with aniline and substituted anilines

Abstract

Intramolecular ether cyclization occurs much more readily than previously recognized in the cure of N, N-diglycidyl epoxies with aromatic amines. Even at a modest reaction temperature of 120°C, the polymer from N, N-diglycidylaniline (DGA) and aniline was shown by 13C n.m.r. spectroscopy to contain morpholinyl and perhydro-1,4-oxazepinyl end groups which accounted for 6 and 4%, respectively, of consumed epoxy groups, whereas intramolecular amine cyclization to give the simple perhydro-1,5-diazocine isomers accounted for 19%. ortho-Methyl substituents in the amine had little steric influence on the extent of ether cyclization, but electronegative substituents had a very marked effect such that it was increased to 28% with 2,4,6-trichloroaniline and 26% with 3,5-bis(trifluoromethyl)aniline. The polymer from DGA and aniline was linear and had a completely random tacticity: variation of reaction temperature and reactant ratio led to only modest improvements in molecular weight (maximum value about 3000) because of the relative ease of chain termination by ether cylization. Two new thermal reactions of the polymer are proposed to explain n.m.r. spectral and g.p.c. data, a cleavage reaction to give secondary amine and tetrahydroquinolyl end groups, and water elimination from adjacent hydroxyl groups to form an in-chain morpholine ring. The reactions were surface- and temperature-dependent. Polymer degradation also occurred slowly in tetrahydrofuran solution at room temperature.