Intramolecular Hydrogen Bonding in Benzoxazines: When Structural Design Becomes Functional

Abstract

The future evolution of benzoxazines and polybenzoxazines as advanced molecular, structural, functional, engineering, and newly commercial materials depends to a great extent on a deeper and more fundamental understanding at the molecular level. In this contribution, the field of benzoxazines is briefly introduced along with a more detailed review of ortho-amide-functional benzoxazines, which are the main subjects of this article. Provided in this article are the detailed and solid scientific evidences of intramolecular five-membered-ring hydrogen bonding, which is supposed to be responsible for the unique and characteristic features exhibited by this ever-growing family of ortho-functionalized benzoxazines. One-dimensional (1D) (1)H NMR spectroscopy was used to study various concentrations of benzoxazines in various solvents with different hydrogen-bonding capability and at various temperatures to investigate in detail the nature of hydrogen bonding in both ortho-amide-functionalized benzoxazine and its para counterpart. These materials were further investigated by two-dimensional (2D) (1)H-(1)H nuclear Overhauser effect spectroscopy (NOESY) to verify and support the conclusions derived during the 1D (1)H NMR experiments. Only highly purified single-crystal benzoxazine samples have been used for this study to avoid additional interactions caused by any impurities.

Cerium Salts: An Efficient Curing Catalyst for Benzoxazine Based Coatings

“…Based on similar benzoxazine derivatives reported in the literature [43][44][45], they are ascribed to the trisubstituted benzene ring stretching, asymmetric stretching vibration of C-O-C, symmetric stretching vibration of C-O-C, and out-of-plane C-H, respectively. In the 1 H-NMR spectrum of 4EP-pPDA, the characteristic protons appear at around 4.51 (Ar-CH 2 -N) and 5.24 (O-CH 2 -N) ppm, confirming the formation of the 4EP-pPDA molecular structure [45][46][47]. Furthermore, the integration ratios of the peaks agree well with the proposed chemical structure.…”

Hydrogen-bonding characteristics and unique ring-opening polymerization behavior ofOrtho-methylol functional benzoxazine

“…The intramolecular hydrogen bonding hypothesis can explain the experiment results and let propose polymerization mechanisms. However, the intramolecular hydrogen‐bonding system in benzoxazines has rarely been studied and the intramolecular hydrogen‐bonding in hydroxyl functional benzoxazines still has not been experimentally demonstrated to exist . Moreover, it is known that impurities that exist during the synthesis of benzoxazine monomer acts as an efficient initiator and/or catalyst.…”

Developing Polybenzoxazine Composites Based on Various Carbon Structures

“…In addition, the synthesis process can be carried out even without solvent, depending on the melting points of the reagents, and moreover, the ring‐opening polymerization process does not require the introduction of any hardeners or catalysts as for the epoxy resins. PBZ resins show excellent dimensional stability, near‐zero volumetric shrinkage upon polymerization, exhibit byproduct free polymerization, low water absorption, high thermo‐oxidative stability, high char yields, high T g , reduced flammability, and good mechanical and dielectric properties. Despite all of these superb properties, the PBZ resins exhibit few important drawbacks that hinder the industrial production.…”