| Cyclodextrins (CDs) with hydrophilic exterior and hydrophobic cavity tend to accommodate hydrophobic molecules, or a great variety of polymers. The concept of molecular machine has received much attention in various fields of science in recent years. ICs based on host-guest interactions between cyclodextrin and long chain polymers are of critical importance for preparing molecular machines. Inoue et al. reported single threading and dethreading machine based on PEG-substituted ?-CD. Tian and co-workers developed several examples of molecular shuttling containing a-CD, working in the form of simulated rotaxane. Liao et al. developed photo-responsive pseudopolyrotaxane hydrogel based on a-CD and PEG.The inclusion process of single-stranded inclusion between CD and PEG has been well studied. y-CD is able to form double-stranded IC with PEG theoretically due to its larger internal diameter (7.5 A) than a-CD (4.7 A). Harada et al. successfully prepared double-stranded IC based on terminal modified PEG, while it was found that y-CD formed trace IC with non-modified PEG, and the amount was so small that obtained IC could not be characterized. On the one hand, the terminals greatly affect inclusion process, some terminals even prohibit the inclusion process. On the other hand, modification increases the complexity of the preparation process. In the present work, we introduce a simple method to prepare tunable ICs between ?-CD and non-modified PEG by choosing suitable y-CD concentrations and PEG molecular weights.Solid ICs with double-stranded structure and controlled amount between y-CD and PEG were successfully prepared and systematically characterized by XRD, DSC and 1H NMR. The IC amount, structure and property are tunable by the temperature and PEG's molecular weight. It is of great significance to increase our understanding on the formation of multi-stranded ICs between CDs and polymers and to lay the foundation for the preparation of more sophisticated molecular machines as well. |
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