Effect of Alkyl Chain Length of Acylated α-Cyclodextrin-Threaded Polyrotaxanes on Thermoresponsive Phase Transition Behavior

The temperature-dependent phase transition behavior of acetylated polyrotaxanes (PRXs) consisting of acylated alpha-cyclodextrins threaded onto a poly(ethylene glycol) chain in an aqueous solution is investigated. Acetyl (Ac), propionyl (Pr), and butyryl (Bu) group-modified PRXs with different degrees of substitution are synthesized to elucidate the effect of the alkyl chain length of the acyl groups on temperature responsivity. Ac-PRXs, Pr-PRXs, and Bu-PRXs are dissolved in water and exhibit temperature-dependent transmittance changes accompanied by coacervate formation. However, the temperature responsivity of acylated PRXs is seen in limited degree of substitution range (approximate to 10%). The maximum degree of substitution to show temperature responsivity of acylated PRXs decreases with increasing alkyl chain length of the acyl groups. Transmittance measurements of acylated PRXs in the presence of urea reveal that the alkyl chain length of the acyl groups determines the predominant force involved in the temperature-induced phase transition. During the temperature-induced phase transition of Pr-PRXs and Bu-PRXs, hydrophobic interactions are predominant; however, hydrogen bond formation may occur in Ac-PRXs. These fundamental findings related to acylated PRXs will facilitate the application of PRXs as thermoresponsive supramolecular materials.

Automated summary

The temperature-dependent phase transition behavior of acetylated polyrotaxanes (PRXs) consisting of acylated alpha-cyclodextrins threaded onto a poly(ethylene glycol) chain in an aqueous solution is investigated. Acetyl (Ac), propionyl (Pr), and butyryl (Bu) group-modified PRXs with different degrees of substitution are synthesized to elucidate the effect of the alkyl chain length of the acyl groups on temperature responsivity. The findings suggest that the alkyl chain length of the acyl groups determines the predominant force involved in the temperature-induced phase transition of acylated PRXs, with hydrophobic interactions being predominant for Pr-PRXs and Bu-PRXs, while hydrogen bond formation may occur in Ac-PRXs.