Benzoxazines with tolyl, p-hydroxyphenyl or p-carboxyphenyl linkage and the structure–property relationship of resulting thermosets

Three benzoxazines (7–9) with a tolyl, p-hydroxyphenyl or p-carboxyphenyl structure, respectively, were successfully synthesized by a three-pot or two-pot procedure. In the three-pot approach, the first step is the condensation of 2-hydroxybenzaldehyde with p-toluidine, 4-aminophenol and 4-aminobenzoic acid, respectively, forming intermediates (1–3) with an imine linkage. The second step is the addition of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) on the imine linkage, resulting in intermediates (4–6) with a secondary amine linkage. The last step is the ring closure condensation of (4–6), leading to benzoxazines (7–9). In the simplified two-pot approach, the first two steps-the condensation and imine reduction were carried out in one reactor, so the yield of (4–6) was increased. The structures of monomers (1–9) were characterized and confirmed by 1D and 2D NMR spectra. The synthesized benzoxazines were thermally self-cured or copolymerized with a commercial benzoxazine, bis(3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazin-6-yl)methane (F-a). IR analysis was utilized to monitor the ring-opening reaction of (7–9) and to propose the structures of P(7–9). The microstructure and the structure–property relationship of the resulting homopolymers and copolymers are studied.