Seishi Ohashi

Simple and low energy consuming synthesis of cyanate ester functional naphthoxazines and their properties

Naphthoxazines functionalized with a cyanate ester group are synthesized in high yield under moderate conditions, including room temperature synthesis, compared with general benzoxazine synthesis. Additionally, this synthesis can shorten reaction pathways for another cyanate ester functional benzoxazine whose polymer exhibits higher thermal properties than general polybenzoxazines. Catalytic polymerization evaluated by differential scanning calorimetry (DSC) indicates multiple exotherm maxima, seemingly including cyanate ester trimerization and polymerization of naphthoxazine. The nature of each exotherm is studied by Fourier transform infrared spectroscopy (FT-IR). Furthermore, their observed exothermic temperatures are lower than those of the reported normal benzoxazines, dicyanate ester blends and cyanate ester functional benzoxazines. Thermal properties determined by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA), such as the char yield and glass transition temperature, are relatively high compared to those of ordinary polybenzoxazines, and polymerized blends of benzoxazines and dicyanate esters.

Oxazine Ring-Related Vibrational Modes of Benzoxazine Monomers Using Fully Aromatically Substituted, Deuterated, 15N Isotope Exchanged, and Oxazine-Ring-Substituted Compounds and Theoretical Calculations

Polymerization of benzoxazine resins is indicated by the disappearance of a 960-900 cm-1 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-C2 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, 15N-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 CH2 in the O-CH2-N (2) and N-CH2-Ar (4) positions on the oxazine ring, or simultaneous deuteration of both positions. The chemical structures were confirmed by 1H nuclear magnetic resonance spectroscopy (1H NMR). The IR and Raman spectra of each compound are compared. Further analysis of 15N 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.

Synthesis and ring-opening polymerization of 2-substituted 1,3-benzoxazine: the first observation of the polymerization of oxazine ring-substituted benzoxazines

2-Substituted 1,3-benzoxazines (two benzoxazine monomers used in this paper are abbreviated as PH-a-[2]ba and PH-pda-[2]ba) having a phenyl group as an oxazine ring substituent are synthesized with benzaldehyde through 2-hydroxy-N-phenylbenzylamine structures. The polymerization of these monomers is observed for the first time and confirmed using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FT-IR). The poly(PH-a-[2]ba) derived from 2-substituted 1,3-benzoxazines exhibits good thermal properties based on TGA analysis, despite showing slight inferiority to poly(PH-a), an unsubstituted counterpart of PH-a-[2]ba. Additionally, benzylideneaniline is extracted during the polymerization, indicating that this compound is the byproduct of the polymerization mechanism. Structural verification is achieved by synthesizing benzylideneaniline and comparing its 1H-NMR spectrum with the reaction byproduct.

Benzoxazines with enhanced thermal stability from phenolated organosolv lignin

Lignin-based benzoxazines are synthesized for the first time using organosolv lignin as the phenolic component and aniline or propargyl amine as the amine component through the Mannich condensation reaction. Acid-catalyzed phenolation of organosolv lignin is performed to increase the phenolic structure with the open ortho-position, which is a requirement for an oxazine ring formation. Two model compounds using o-cresol and p-cresol as the phenolic component and propargylamine as the amine component are also synthesized for comparison. The successful syntheses are verified by Fourier transform infrared spectroscopy (FT-IR); proton, carbon and phosphorus nuclear magnetic resonance spectroscopy (1H, 13C and 31P NMR); and elemental analysis. Further structural characterization of the precursor resins is performed using heteronuclear single quantum coherence (HSQC) NMR technique. The polymerization process is followed by both differential scanning calorimetry (DSC) and in situ isothermal FT-IR technique. The polymerization of the lignin-based benzoxazines proceeds faster than ordinary benzoxazine monomers due to the catalytic effect of the residual phenolic moieties in the lignin units. The majority of polymerization process takes place in less than 15 min at 180 °C for both lignin-based benzoxazines studied. The thermal stability of the polymers under study is evaluated by thermogravimetric analysis (TGA). The char yields of the polybenzoxazines derived from the lignin-based benzoxazines are close to 50%, which lead to LOI values considered self-extinguishing.

Quantitative studies on the p-substituent effect of the phenolic component on the polymerization of benzoxazines

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. The analytical data of each compound are collected using 1H-NMR, 13C-NMR, and differential scanning calorimetry (DSC). In order to quantify the electronic effect on the polymerization behavior, the Hammett substituent constant is utilized and plotted against resonances of 1H-NMR, 13C-NMR spectra and DSC exotherm maximum temperature. The use of the Hammett substituent constant is reexamined by calculating the natural charge on the phenolic moiety via ab initio calculation using the Gaussian program and correlated with the polymerization exotherm temperature. The activation energies obtained using the Kissinger and Ozawa methods are related to the electronic effect of the substituents on the phenolic part.

Development of Hydrogen-Rich Benzoxazine Resins with Low Polymerization Temperature for Space Radiation Shielding

A systematic study has been carried out to develop a material with significant protection properties from galactic cosmic radiation and solar energetic particles. The research focused on the development of hydrogen-rich benzoxazines, which are particularly effective for shielding against such radiation. Newly developed benzoxazine resin can be polymerized at 120 °C, which meets the low-temperature processing requirements for use with ultrahigh molecular weight polyethylene (UHMWPE) fiber, a hydrogen-rich composite reinforcement. This highly reactive benzoxazine resin also exhibits low viscosity and good shelf-life. The structure of the benzoxazine monomer is confirmed by proton nuclear magnetic resonance and Fourier transform infrared spectroscopy. Polymerization behavior and thermal properties are evaluated by differential scanning calorimetry and thermogravimetric analysis. Dynamic mechanical analysis is used to study chemorheological properties of the benzoxazine monomer, rheological properties of the cross-linked polybenzoxazine, and rheological properties of UHMWPE-reinforced polybenzoxazine composites. The theoretical radiation shielding capability of the composite is also evaluated using computer-based simulations.

Chapter 49 – Preparation of High Purity Samples, Effect of Purity on Properties, and FT-IR, Raman, 1H and 13C NMR, and DSC Data of Highly Purified Benzoxazine Monomers

Abstract Many kinds of benzoxazine resins are synthesized and thoroughly purified to obtain the single crystals. The benzoxazine monomers with different purification methods show huge difference in the DSC themograms. Herein, it is proven that the impurities in the monomers can enormously influence their polymerization behavior. The large single crystals of BA-a, which is commonly used in benzoxazine research, are displayed here, and FT-IR, Raman, 1H- and 13C-NMR, and DSC data of the single or multiple crystals of the monomers are also listed to indicate the high purity.