Synthesis and Thermal Properties of Difunctional Benzoxazines with Attached Oxazine Ring at the Para-, Meta-, and Ortho-Position

To investigate the role of position of oxazine ring in the benzoxazine backbone on their ring-opening polymerization (ROP) and thermal stability of resulting polybenzoxazine, we have synthesized difunctional monomers solely containing benzoxazine moieties (BZ2) with attached oxazine ring at the para-, meta-, and ortho-position. The ROP was examined by DSC analysis, which revealed a reduction in curing temperature in the order of meta (225 °C) ortho (239 °C) para (251 °C). The differences in structural and geometrical parameters were investigated by NMR (1H, 13C, 1H–1H NOESY) and X-ray crystallography analysis. The electronic effects and the intramolecular interaction between oxazine ring and aromatic hydrogen were higher when the attached oxazine ring was at the meta-position. The differences in their positioning also changed their ROP mechanism, an unusual intramolecular polymerization was observed in meta, while in ortho and para the polymerization proceeded in a regular manner. A curing mechanism responsible for lower curing temperature and faster ROP in meta has been proposed, which involves an intramolecular electrophilic substitution of iminium ion, resulting in aza-cyclic rings along with classical phenolic Mannich bridges in the networked structure. The cured resin showed a high Tg and in the order of para (291 °C) > meta (270 °C) > ortho (266 °C). Even though meta-PBZ2 displayed an earlier degradation with Td10 of 358 °C as compared to para and ortho (Td10: 373 °C) due to aza-cyclic rings, the main backbone degradation was observed to be coinciding in all PBZ2s at 417 °C with a char yield of 57% at 600 °C. Thus, changing position of oxazine ring to “meta” in the backbone is a beneficial strategy to have a low curing benzoxazine without sacrificing the thermal stability of resulting polybenzoxazine.