Ring-Sliding Gel

Ring-like molecules with large enough cavities such as crown-ethers, cyclodextrins can form inclusion complexes with guest molecules of suitable sizes. They can also be threaded on to linear molecules, blocked by bulky end groups, and slide on it in response to environmental stimuli. This supramolecular structure is known as rotaxane, or polyrotaxane if more than one rings are threaded in one linear molecules of enough length, e.g. a polymer chain.

Polyrotaxanes formed between cyclodextrins and polyethylene glycol (PEG) have been familiar since the first synthesis by A. Harada (Nature 1992, 356, 325-327. DOI: 10.1038/356325a0). The preparation and properties of this supramolecular structures were also early investigated. Although some interesting properties such as a gel-sol transition at elevated temperature (Polym. J. 1994, 26, 1019-1026) was reported, and the possible mechanism was postulated as the sliding behavior of the cyclodextrins along the PEG chains. This explanation, although easy to understand, was not fully tested or applied in the following years^†, until a recent communication by K. Ito and co-workers (Chem. Commun. 2006, 4102-4103. DOI: 10.1039/b607373e). They simply controlled the number of cyclodextrins threaded to each PEG chain so that there were free spaces for sliding, and as well end-capped the chain with bulky end-groups to prevent de-threading of the cyclodextrin at elevated temperature, where the cyclodextrins can only closely pack with each other at the chain end to form a block and crystallize into crosslinking points. Gelation thus occurs.

I am currently also working on possible designs between cyclodextrins with PEG-containing block copolymers which may show similar gelation behaviors.