A Molecular Necklace: Threading β-Cyclodextrins onto Polymers Derived from Bile Acids.

A molecular necklace of polypseudorotaxanes was prepared by threading β-cyclodextrins (β-CD) onto biodegradable and thermoresponsive polyurethanes derived from bile acids. These polyurethanes were synthesized via a simple step condensation of bile acid-based dicarbonate with poly(ethylene glycol)-diamine. The β-CD rings slide onto the poly(ethylene glycol) segments and selectively recognize the bile acid units of the polyurethane chains, whereas the poly(ethylene glycol) segments remain crystalline with a lower crystallinity. This bio-compound-derived molecular necklace can be visualized by scanning tunneling microscopy. The polypseudorotaxanes show thermosensitivity in water and the phase transition temperature may be fine-tuned by varying the molar ratios of β-CD to the bile acid units. Such an interesting necklace model of polypseudorotaxane constructed from natural compounds may lead to the further exploration of their applications, such as as an enzyme model, due to their biological nature.