Synthesis of Schiff base–containing benzoxazine derivatives

Abstract

The excellent chemical and physical properties of benzoxazine resins and the functionality of Schiff bases were combined in one compound's structure, creating newly designed benzoxazine derivatives that can form complexes with metals. The new type of benzoxazine monomer was synthesized via the ring-closure reaction of formaldehyde, aniline, and three newly designed Schiff bases. The presence of the Schiff base in the molecular structure of these novel benzoxazine monomers enables them to trap metals as the functional compounds, like Cu, from a solution. Thermally initiated polymerization occurs at a lower temperature by the formation of intermolecular and intramolecular hydrogen bonds between imine, oxazine, Schiff base hydroxyl groups, and the newly generated hydroxyl groups. The thermal behavior of the bisbenzoxazine monomers was investigated with Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and differential scanning calorimetry, and then they were cured at 120, 180, and 230 C. According to the magnetic susceptibility, FTIR, scanning electron microscopy with energy dispersive X-ray spectroscopy, TGA, and microwave plasma atomic emission spectroscopy results, it is shown that Cu(II) complexes of the compounds were also succesfully synthesized, and they proved to be successful in catching metal. This is due to the functionality of Schiff bases forming the metal complexation in the compounds. The poly(bisbenzoxazine)s also showed high limiting oxygen index (31-37), low ring-opening temperature (150-190 C), high char yield (35%-49%), and excellent thermal stability, due to the highly crosslinked nature of the polymers.

Melamine as cross‐linking agent for mono‐cyclic benzoxazine with aldehyde groups: higher crosslinking density and thermal properties

“…In the 1 H NMR spectra (Figure 1b), the proton peaks of ArCH 2 N and OCH 2 N on the oxazine ring appear at 4.78 and 5.59 ppm, manifesting the formation of benzoxazine 50 . The peak of the proton Ar‐H on the benzene ring appears at 6.5–8.0 ppm, and the peak at 9.82 ppm is the absorption peak of the hydrogen proton on the CHO.…”

“…In the 1 H NMR spectra (Figure 1b), the proton peaks of Ar CH 2 N and O CH 2 N on the oxazine ring appear at 4.78 and 5.59 ppm, manifesting the formation of benzoxazine. 50 The peak of the proton Ar-H on the benzene ring appears at 6.5-8.0 ppm, and the peak at 9.82 ppm is the absorption peak of the hydrogen proton on the CHO. With the introduction of MA and the proceeding of prepolymerization reaction, the intensity of the methylene Ar CH 2 N and The new peak at 4.25 ppm is attributed to the MA amino hydrogen proton, and near 7.9 ppm is the newly generated Schiff-base hydrogen proton, the intensity of which increased with increasing MA content, indicating that more Schiff-base structures were generated.…”

“…It is expected that the introduction of melamine, which has an active amino and triazine structure, can enhance the thermal stability and crosslink density of polymers 46 . Also, it was found that the introduction of Schiff‐base structure could enhance the crosslink density and stability of the resin 47–52 . As the report by Xu, 49 two diepoxides containing a two‐benzene‐ring‐conjugated Schiff base when cured with an aromatic diamine result in high‐performance thermosets.…”

“…46 Also, it was found that the introduction of Schiff-base structure could enhance the crosslink density and stability of the resin. [47][48][49][50][51][52] As the report by Xu, 49 two diepoxides containing a two-benzene-ring-conjugated Schiff base when cured with an aromatic diamine result in high-performance thermosets. However, the exchange reaction of Schiff bases makes the composition of the reaction products complicated to obtain high purity products.…”