Abstract
A novel and successful application of ring-closing reactions of aminophenols has been proposed for the formation of a new type of 1,3-benzoxazine ionic derivatives. The optimization of the reaction and detailed computational studies have been reported for the estimation of heterocyclic ring stability and its further transformation, which is crucial in the polymerization process. The molecular structure of the obtained compounds has been fully characterized by applying X-ray analysis and spectroscopic methods. The novel benzoxazines undergo an intriguing thermal reaction leading to classical benzoxazines and chloroalkanes, which is the first step of transformation before polymerization. To gain more insights into the transformation behavior of ionic benzoxazine derivatives, the Fourier transform infrared (FT-IR) spectra of gaseous products were recorded in experiments with near simultaneous FT-IR/TGA measurements. The combination of thermogravimetry with FT-IR spectroscopy enables the quantitative and qualitative characterization of thermal transformation products and clarification of the reaction mechanism. The experimental data have been verified by applying DFT(B3LYP) and DFT(M062x) theoretical studies.
Highlights
Polybenzoxazines are perceived as both well-defined and still novel thermoset polymers
A new type of 1,3-benzoxazines have been synthesized by ringclosing reactions of aminophenols activated by NaH
The molecular structure of the newly obtained compounds have been fully characterized in solution by NMR spectroscopy and in the solid state by X-ray
Summary
Polybenzoxazines are perceived as both well-defined and still novel thermoset polymers. The appropriate tailor-made structural modification of benzoxazine monomers provides an opportunity to introduce new valuable properties into final polybenzoxazines in comparison to traditional phenolic resins.[1] The existing application of benzoxazines and polybenzoxazines in various fields is incontestable and still requires further attention They are already widely applied in the areas of coatings, adhesives, microelectronics, and aerospace technology.[2] The advantages and features worth underlining include low water absorption,[3,4] excellent dimensional stability, nearly zero volumetric shrinkage,[5,6] thermal stability, chemical resistance,[7,8] stable low dielectric properties, self-extinguishing/flame retardance, and good electronic properties.[9−12] it is important to note from a green chemistry point of view that the polymerization process proceeds without catalysts, byproducts, and harmful volatile formation,[13−15] and the monomers can be obtained from the renewable feedstock. The title compounds present a structural motif of an N-activated benzoxazine species postulated in the polymerization mechanism
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