Thermoresponsive behavior of polyrotaxanes based on α-cyclodextrins threaded on poly(2-alkyl-2-oxazoline)-poly(ethylene glycol)-poly(2-alkyl-2-oxazoline) triblock copolymers

Abstract

Polyrotaxanes with poly-2-alkyl-2-oxazoline blocking moieties have been successfully synthesized. The structure of the synthesized samples was confirmed by NMR spectroscopy, and the molar masses were measured by light scattering and GPC. The density of cyclodextrin rings on polyethylene glycol did not depend on the structure of poly-2-alkyl-2-oxazoline chains and ranged from 2 to 5. A high specific optical rotation of the prepared polyrotaxanes is found which is caused by the formation of hydrogen bonds between neighboring macrocycles. The behavior of water-salt solutions of synthesized polyrotaxanes depended on the size of the poly-2-alkyl-2-oxazoline side groups. Only polymers with poly-2-isopropyl-2-oxazoline and poly-2-n-propyl-2-oxazoline blocking moieties were thermoresponsive in aqueous solutions, and the phase separation temperature decreased at passage from a polymer with poly-2-isopropyl-2-oxazoline to a sample with poly-2-n-propyl-2-oxazoline. Aqueous solutions of polyrotaxanes with smaller side groups were molecularly dispersed at all temperatures, and the polymer with poly-2-butyl-2-oxazoline blocking chains did not dissolve in water.