Catalyst-free synthesis, characterization and photoluminescence of main-chain luminescent polybenzoxazines

Two novel luminescent main-chain polybenzoxazine polymers, (Poly1)_main and (Poly2)^x_main, were synthesized and characterized to explore their structural, thermal, morphological and photophysical properties. Polymer (Poly1)_main was obtained via a Mannich condensation reaction without a catalyst, followed by thermal polymerization to produce the crosslinked polymer (Poly2)^x_main. Structural analyses using Fourier transform infrared spectroscopy and X-ray diffraction confirmed the successful formation of the polymers, with (Poly2)^x_main exhibiting a higher degree of crosslinking and partial ordering in an otherwise amorphous structure. Scanning electron microscopy imaging revealed that thermal polymerization significantly altered the morphology, transforming the porous structure of (Poly1)_main into a denser, layered morphology in (Poly2)^x_main. Thermogravimetric analysis and differential scanning calorimetry highlighted the improved thermal stability of (Poly2)^x_main due to extensive crosslinking. Photophysical studies showed that (Poly1)_main in solution exhibited yellow-green luminescence with a broad emission maximum at 522 nm and CIE coordinates (0.39, 0.48). In contrast, the powders of both polymers displayed sharp red luminescence with an emission peak at 658 nm and CIE coordinates (0.72, 0.27), attributed to molecular packing effects and exciton coupling in the solid state. These results underscore the interplay among structural, morphological and photophysical properties, highlighting the potential of these polymers in optoelectronics, sensing and luminescent materials. © 2025 Society of Chemical Industry.