Synthesis and Thermal Properties of Difunctional Benzoxazines with Attached Oxazine Ring at the <i>Para</i>-, <i>Meta</i>-, and <i>Ortho</i>-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 <i>para</i>-, <i>meta</i>-, and <i>ortho</i>-position. The ROP was examined by DSC analysis, which revealed a reduction in curing temperature in the order of <i>meta</i> (225 °C) < <i>ortho</i> (239 °C) < <i>para</i> (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 <i>meta</i>-position. The differences in their positioning also changed their ROP mechanism, an unusual intramolecular polymerization was observed in <i>meta</i>, while in <i>ortho</i> and <i>para</i> the polymerization proceeded in a regular manner. A curing mechanism responsible for lower curing temperature and faster ROP in <i>meta</i> 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 <i>T</i>_g and in the order of <i>para</i> (291 °C) > <i>meta</i> (270 °C) > <i>ortho</i> (266 °C). Even though <i>meta</i>-PBZ2 displayed an earlier degradation with <i>T</i>_d10 of 358 °C as compared to <i>para</i> and <i>ortho</i> (<i>T</i>_d10: 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 “<i>meta</i>” in the backbone is a beneficial strategy to have a low curing benzoxazine without sacrificing the thermal stability of resulting polybenzoxazine