The structural transformation during polymerization of benzoxazine/FeCl3 and the effect on the thermal stability

The mixture of bisphenol A/aniline-based benzoxazine (BA-a) and FeCl3 was prepared at 40 °C and then was cured to get the polymer at elevated temperatures. The thermal stability of the modified polybenzoxazine was improved and its char yield at 800 °C increased by 11%. The reason for the improvement of the thermal stability and the structural transformation during polymerization was systematically investigated by Fourier transform infrared (FTIR) spectroscopy, 1H nuclear magnetic resonance spectroscopy (1H NMR), thermo-gravimetric analysis (TGA), pyrolysis-gas chromatography/mass spectrometry (Py/GC–MS), TGA-IR and element analysis (EA). The results show that BA-a can partially undergo the ring-opening polymerization at a low temperature in the presence of FeCl3 and generate some oligomers containing N, O-acetal-type bridge structures. At a high temperature, this structure can be transformed into the arylamine Mannich bridge structure which can delay degradation of the aniline moiety. Besides, the volatilization of fragmented Schiff base structures can be found during cure and more thermostable crosslinked structures are formed in polybenzoxazine. The entire polymerization mechanism of BA-a with FeCl3 was proposed.