Counterion Effect of Amine Salts on Ring-Opening Polymerization of 1,3-Benzoxazines

The effect of nucleophilicity of counterions of amine HX salts as catalysts on the ring-opening polymerization (ROP) of 1,3-benzoxazines is investigated. The significant reduction observed for the ROP temperature is related to the nucleophilicity of counterions and found to show a general trend as I⁻ > Br⁻ > Cl⁻, as studied by differential scanning calorimetry and Fourier transform infrared spectroscopy. Moreover, the latent character of the amine salts is also demonstrated by tracking the benzoxazine–amine salt mixtures using ¹H NMR spectroscopy. The spectral analysis gives evidences for the dormant nature of the amine HX salts toward benzoxazine at room temperature in solvent. Besides, thermal stabilities of the cured benzoxazine monomers with and without catalysts are investigated by thermogravimetric analysis.