The FT-IR and H-1 NMR spectra of model dimers, having different molecular sizes and pK(a), are investigated in order to understand the differences in the hydrogen-bonded network structures of polybenzoxazines. The correlation between the -(OHN)-N-... intramolecular hydrogen-bonding interaction and benzoxazine functional groups in the asymmetric dimers is investigated by H-1 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.