Formation of cyclodextrin-based polypseudorotaxanes: Solvent effect and kinetic study

Aqueous solutions of beta- or gamma -cyclodextrin were found to form polypseudorotaxanes upon threading of linear hydrophobic polymers such as poly(propylene glycol) bis-2-aminopropyl ether, H3CCH(NH2)CH2(OCH2-CH(CH3))(n)NH2 (PPG-Am-2; MW approximate to 2000; n(av) approximate to 33), and pluronic 105, HO(CH2CH2O)(34)(CH2CH(CH3)O)(61)(CH2-CH2O)(34)H (PLU; MW approximate to 6500). The kinetics of the threading process was determined as a function of the temperature and solvent composition (water, heavy water, and urea). When the water solution of cyclodextrin was added to the polymer's dispersion, a thick solid precipitate was promptly formed, depending on the concentration and temperature. Turbidity measurements allowed us to determine the time necessary for a complete threading of the linear polymer (the "threading time"); this parameter depends on the solution temperature and on the solvent nature. A simple kinetic model provides the activation free energy of the process, and the number of cyclodextrin molecules threaded around a single linear chain. Enthalpy, entropy, and heat capacity changes are consistent with a process ruled by hydrophobic effects.