Comparative study of inclusion complex formation between β-cyclodextrin (host) and aromatic diamines (guests) by mixing in hot water, co-precipitation, and solid-state grinding methods

Aromatic diamines are essential components of polyimide and many other thermosetting polymers. Recent attention has been growing on the threading of cyclodextrins (CDs) onto diamine monomers intended to improve the solubility in water and thermal stability of resultant polymers. The co-precipitation method is often used to isolate inclusion complexes (ICs) of aromatic diamines and other sparingly water-soluble aromatic guest molecules with β-CD. To find the viability of other methods, we studied IC formation between β-CD and some aromatic diamines by mixing in hot water, co-precipitation, and solid-state grinding. ICs formation in water was carried out by solid guest dispersion into the β-CD aqueous solution at 80 °C with high-speed magnetic stirring. In contrast, solid-state grinding was employed by adding a small amount of water to promote IC formation. Thus, ICs prepared by mixing in hot water and solid-state grinding methods were crystallized from water by cooling to 4 °C. Structures of the ICs in solution were confirmed by chemical shifts changes of cavity protons of β-CD in ¹H NMR and the cross-peaks between aromatic protons and cavity protons in ¹H-¹H ROESY NMR. Job’s plot and NMR titration experiments were used to determine the stoichiometric ratio of host and guest in solution.