The hydrogen-bonded network structure for polybenzoxazines is investigated by Fourier Transform Infrared Spectroscopy (FT-IR) with model dimer systems. Comparing the FT-IR spectra of the polybenzoxazines and model dimers, it is shown that the simpler structures of asymmetric dimers well simulate the hydrogen-bonded network structure between polymer chains while the structures of symmetric dimers reflect the hydrogen bonding scheme related to the end-groups of polymer chains. It is confirmed that the amine functional group in the Mannich bridge is greatly responsible for the distribution of hydrogen bonding species. Bisphenol A/methylamine-based polymer (BA-m) mainly consists of −OH···N intramolecular hydrogen bonding while bisphenol A/aniline-based polymer (BA-a) has a large amount of intermolecular hydrogen bonding and relatively weak hydrogen bonding groups in the polymer network structure. The possible network structure, in the sense of hydrogen bonding, for BA-m and BA-a polymers is proposed and a generalized explanation for the structure−property relationships in polybenzoxazines is also discussed.

A combination of molecular modeling, DFT calculations, and advanced solid-state NMR experiments is used to elucidate the supramolecular structure of a series of benzoxazine oligomers. Intramolecular hydrogen bonds are characterized and identified as the driving forces for ring-shape and helical conformations of trimeric and tetrameric units. In fast MAS (1)H NMR spectra, the resonances of the protons forming the hydrogen bonds can be assigned and used for validating and refining the structure by means of DFT-based geometry optimizations and (1)H chemical-shift calculations. Also supporting these proposed structures are homonuclear (1)H[bond](1)H double-quantum NMR spectra, which identify the local proton-proton proximities in each material. Additionally, quantitative (15)N[bond](1)H distance measurements obtained by analysis of dipolar spinning sideband patterns confirm the optimized geometry of the tetramer. These results clearly support the predicted helical geometry of the benzoxazine polymer. This geometry, in which the N...H...O and O...H...O hydrogen bonds are protected on the inside of the helix, can account for many of the exemplary chemical properties of the polybenzoxazine materials. The combination of advanced experimental solid-state NMR spectroscopy with computational geometry optimizations, total energy, and NMR spectra calculations is a powerful tool for structural analysis. Its results provide significantly more confidence than the individual measurements or calculations alone, in particular, because the microscopic structure of many disordered systems cannot be elucidated by means of conventional methods due to lack of long-range order.

The FT-IR and 1 H NMR spectra of model dimers, having different molecular sizes and pKa, are investigated in order to understand the differences in the hydrogen-bonded network structures of polybenzoxazines. The correlation between the -OH‚‚‚N intramolecular hydrogen-bonding interaction and benzoxazine functional groups in the asymmetric dimers is investigated by 1 H NMR spectra. While the FT-IR spectra of the model dimers indicate that the nature of hydrogen bonding is closely related to the basicity of the amine constituent, the spectra of the corresponding polymers suggest the existence of different hydrogen-bonding interactions. The existence of phenolic linkage formation and the stability of the Mannich base structure during polymerization are investigated by a dimerization reaction. It is demonstrated that benzoxazines based upon extremely bulky amines do not develop desirable properties due to the extensive degradation process.

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