Oxazine Ring-Substituted 4th Generation Benzoxazine Monomers & Polymers: Stereoelectronic Effect of Phenyl Substituents on Thermal Properties

Oxazine ring-substituted benzoxazine monomers inherit a smart architecture to cater intriguing properties offered by this upcoming latest generation of polybenzoxazines. It is therefore encouraging to study the regio-effect of oxazine ring substitution on ROP (ring-opening polymerization) temperature, thermal stability of the monomers and polymers to guide future designing of structures. Herein, we report a systematic analysis on the thermal properties with variation in the number and position of substituent (2- and/or 4-) in phenyl-substituted benzoxazines. Successful syntheses of monomers and structural characterization were confirmed using 1D NMR (nuclear magnetic resonance spectroscopy), HRMS (high-resolution mass spectrometry), and XRD (X-ray diffraction). Polymerization behavior and thermal stability was studied by DSC (differential scanning calorimetry), FTIR (Fourier transform infrared) spectroscopy, and TGA (thermogravimetry analysis). In-depth structural analysis by 2D NMR [1H – 1H COSY (correlation spectroscopy), 1H – 13C HSQC (heteronuclear single quantum correlation), and HMBC (heteronuclear multiple bond correlation)] experiments, crystal data, TGA–GC–MS (thermogravimetry analysis-gas chromatography–mass spectrometry), and DFT (density functional theory) calculation provided mechanistic insights of the effect of substitution on thermal behavior of monomers. It is found that besides the molecular mass of monomers and intermediates, the stereo-electronic effect of substituents at the 2- and/or 4-positions governs both the evaporation of monomer/cleaved intermediates and ring-opening reaction. Activation energy (Ea) of polymerization depends upon the position and degree of phenyl substituent(s). Among the studied varieties, the 2,4-substituted benzoxazine monomer showed the lowest ROP temperature with the least mass-loss during polymerization, the lowest Ea value of polymerization. Additionally, the resultant 2,4-substituted polybenzoxazine showed the highest thermal stability and char yield. An astute choice of selective positioning of groups in the oxazine ring can be a guiding principle for the structural designing of monomers to advocate a wide variety of applications.