Formation of Cyclodextrin-Based Polypseudorotaxanes: Solvent Effect and Kinetic Study

Aqueous solutions of β- or γ-cyclodextrin were found to form polypseudorotaxanes upon threading of linear hydrophobic polymers such as poly(propylene glycol) bis-2-aminopropyl ether, H 3 CCH(NH 2 )CH 2 (OCH 2 CH(CH 3 )) n NH 2 (PPG-Am 2; MW ≈ 2000; n av ≈ 33), and pluronic 105, HO(CH 2 CH 2 O) 34 (CH 2 CH(CH 3 )O) 61 (CH 2 CH 2 O) 34 H (PLU; MW ≈ 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.