Phosphorus-containing epoxy curing agents via imine linkage

Two new phenol-type curing agents, (3) and (4), were synthesized by the condensation of 4 (or 2)-hydroxybenzaldehyde and 4-aminophenol, followed by the addition of DOPO to the resulting imine linkage. Two reaction mechanisms, conjugate-addition and carbocation mechanisms, were proposed. The fact that the ratio of the configuration of the two diastereomers depends on the reaction solvent indicates that the carbocation mechanism is the more reasonable one. (3) and (4) served as curing agents for epoxy resins, and the resulting epoxy thermosets exhibited good flame retardancy, and moderate T-g and thermal stability. The nitrogen-phosphorus synergistic effect explains the high flame retardancy.