Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm band in infrared spectroscopy (IR). Historically, this band was assigned to the C-H out-of-plane bending of the benzene to which the oxazine ring is attached. This study shows that this band is a mixture of the O-C stretching of the oxazine ring and the phenolic ring vibrational modes. Vibrational frequencies of 3-phenyl-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-a) and 3-(tert-butyl)-3,4-dihydro-2H-benzo[e][1,3]oxazine (PH-t) are compared with isotope-exchanged and all-substituted compounds. Deuterated benzoxazine monomers, N-isotope exchanged benzoxazine monomers, and all-substituted benzoxazine monomers without aromatic C-H groups are synthesized and studied meticulously. The various isotopic-exchanges involved deuteration around the benzene ring of phenol, selective deuteration of each CH in the O-CH-N (2) and N-CH-Ar (4) positions on the oxazine ring, or simultaneous deuteration of both positions. The chemical structures were confirmed by H nuclear magnetic resonance spectroscopy (H NMR). The IR and Raman spectra of each compound are compared. Further analysis of N isotope-exchanged PH-a indicates the influence of the nitrogen isotope on the band position, both experimentally and theoretically. This finding is important for polymerization studies of benzoxazines that utilize vibrational spectroscopy.

The first benzoxazine incorporating cyanate ester group in its structure has been synthesized. The polymerization process investigated by differential scanning calometry (DSC) exhibits two clearly separated exotherm maxima, corresponding to cyanate ester trimerization and benzoxazine, respectively. The nature of each exotherm is studied by Fourier transform infrared spectroscopy (FT-IR). The activation energies for cyanate ester trimerization and benzoxazine ring-opening polymerization are determined by both Kissinger's and Ozawa's methods. Furthermore, both exothermic temperatures observed are lower than those of the published values of benzoxazine and dicyanate ester blends. Thermal properties such as glass transition temperature and char yield are studied by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). Those properties are found to be outstanding compared to the reported properties of general benzoxazine resins and polymerized blends of benzoxazine and dicyanate ester, offering a polymeric matrix for composite satisfying many requirements for such a material.

The pure monofunctional benzoxazines substituted by either electron donating or withdrawing groups are synthesized to verify the electronic effect on the polymerization behaviors without any complicated factors of the impurities.