Pyridinyl-containing benzoxazine: Unusual curing behaviors with epoxy resins

A pyridinyl-containing benzoxazine (6) was synthesized from the Mannich condensation of 4-phenyl-2,6-bis(4-aminophenyl) pyridine (2), phenol, and paraformaldehyde. For the purpose of properties comparison, a benzoxazine (7), which is structurally similar to (6) except for the pyridinyl group, was prepared. The solvent effect on the synthesis of (6) was discussed, and toluene/ethanol was found to provide (6) with the best purity and yield. The pyridinyl group provides solubility and acts as a catalyst for the ring opening of benzoxazine, as supported by the forward curing in the DSC thermograms. When curing with epoxy resins, a carbonyl absorption at 1670 cm−1 and 192 ppm was observed in the IR and 13C NMR spectra. It is proposed that the formation a cyclic amide structure is responsible for the absorption. A reaction mechanism including nucleophilic addition, Diels–Alder reaction, and rearrangement was proposed. The pyridinyl group acts as a crosslinking site, and results in thermosets with good thermal properties.