Benzoxazine: Synthetic Modification, Mechanism, and Microstructure of Its Resulting Thermoset

A main chain-type polybenzoxazine precursor, P(BF-bapp)-1, with high molecular weight (MW) was prepared through a strategy of A-A and B-B polycondensation, avoiding by-reaction that occurred in Mannich condensation. To discuss the effect of the MW of benzoxazine on the properties and microstructure of the resulting thermoset, anthother three structurally similar benzoxazines with difference MW were syntheized in this work. To discuss the mechanism of ring-opening polymerization (ROP) of benzoxazine, three polybenzoxazine precursors were prepared. Among the polybenzoxazine precursors, free ortho positions to the O and N of oxazine are available for PBz-0M. The ortho positions to the O of oxazine are blocked by methyl group for PBz-2M. The ortho positions to the O and N of oxazine are blocked by methyl group for PBz-6M. According to DSC, IR, and thermal analysis, we found that the ROP of PBz-2M can be carried out through free ortho positions to the N of oxazine. A reaction mechanism was proposed to explain the polymerization in this work. Blends of cyanate ester and benzoxazine have been independently studied by several researchers, some of them suggest that the rapid trimerization of cyanate ester in the blend is related to the ring-opened structure of benzoxazine. Recently, we unexpectedly observe that gelation occurred in solution of P-oda/BACY blend after 24 h at room temperature. However, the possibility of ring-opening polymerization for benzoxazine at room temperature is rare. Therefore, it is highly likely that the catalytic effect results from the benzoxazine itself. We tried to find the reason of gelation through model reaction in this work, and proposed a catalytic mechanism of benzoxazine for the trimerization of cyanate ester.