Highly Elastic Supramolecular Hydrogels Using Host–Guest Inclusion Complexes with Cyclodextrins

Supramolecular hydrogels, which are cross-linked via host–guest interactions, show high-performance physical properties, such as elasticity and toughness. Herein we prepare a supramolecular hydrogel without chemical cross-linker. The supramolecular hydrogel was prepared by polymerization of the inclusion complexes between β-cyclodextrin acrylamide and adamantane acrylamide monomers. The β-cyclodextrin–adamantane gel (βCD–Ad gel) shows a high stretching property (990%). The initial strain (0%) is restored in several minutes for a βCD–Ad gel stretched to 180% of the initial strain without altering the physical history. However, chemically cross-linked poly­(acrylamide) does not show the reversible stretching property. These results indicate that host–guest interaction inside the supramolecular hydrogel plays an important role in the shape recovery properties

One-Pot Synthesis of γ-Cyclodextrin Polyrotaxane: Trap of γ-Cyclodextrin by Photodimerization of Anthracene-Capped <i>p</i><i>seudo</i>-Polyrotaxane

One-Pot Synthesis of γ-Cyclodextrin Polyrotaxane:  Trap of γ-Cyclodextrin by Photodimerization of Anthracene-Capped pseudo-Polyrotaxan

Double-Threaded Dimer and Supramolecular Oligomer Formed by Stilbene Modified Cyclodextrin: Effect of Acyl Migration and Photostimuli

We observed changing supramolecular structures of stilbene-α-cyclodextrin (StiO-α-CD) by photoirradiation and migration. Stilbene derivatives show photoinduced isomerization under irradiation with λ =340 nm to give 2-cis-StiO-α-CD and with λ =254 nm to give 2-trans-StiO-α-CD. Photoisomerization of StiO-α-CD shows the photostationary state during 30 min. 2D NMR and diffusion coefficient studies revealed that 2-trans-StiO-α-CD forms a double-threaded dimer but 2-cis-StiO-α-CD changes to a supramolecular oligomer by photoirradiation. We found that the mutual migration of a stilbene group (StiO) on α-CD occurs under neutral conditions. The StiO group of α-CD (StiO-α-CD) moves between the C2 and C3 positions on the secondary hydroxyl group of StiO-α-CD (the wider rim of α-CD) to give 3-trans-StiO-α-CD. 3-trans-StiO-α-CD forms a supramolecular oligomer, whereas 3-cis-StiO-α-CD changes to a double-threaded dimer, indicating that 3-StiO-α-CDs gives the opposite results in the supramolecular structures of 2-StiO-α-CDs. The thermal isomerization (migration) is very slow. It takes about 300 h to reach the equilibrium state. Moreover, the migration rate constant (ktrans3→2) of the trans-StiO group from the C3 position to the C2 position of α-CD is faster than ktrans2→3 from the C2 position to the C3 position of α-CD. On the other hand, kcis2→3 of the cis-StiO group from the C2 position to the C3 position of α-CD is faster than kcis3→2 from the C3 position to the C2 position, meaning kcis2→3 > kcis3→2, which is the opposite result for ktrans3→2 > ktrans2→3. The formation of a stable double-threaded dimer would suppress the migration of the StiO group of StiO-α-CDs in aqueous solutions

Cyclodextrin-Initiated Polymerization of Cyclic Esters in Bulk: Formation of Polyester-Tethered Cyclodextrins

Cyclodextrins were found to initiate ring-opening polymerization of some cyclic esters selectively to give a polyester with a CD at the chain end

Highly Elastic Supramolecular Hydrogels Using Host–Guest Inclusion Complexes with Cyclodextrins

Supramolecular hydrogels, which are cross-linked via host–guest interactions, show high-performance physical properties, such as elasticity and toughness. Herein we prepare a supramolecular hydrogel without chemical cross-linker. The supramolecular hydrogel was prepared by polymerization of the inclusion complexes between β-cyclodextrin acrylamide and adamantane acrylamide monomers. The β-cyclodextrin–adamantane gel (βCD–Ad gel) shows a high stretching property (990%). The initial strain (0%) is restored in several minutes for a βCD–Ad gel stretched to 180% of the initial strain without altering the physical history. However, chemically cross-linked poly­(acrylamide) does not show the reversible stretching property. These results indicate that host–guest interaction inside the supramolecular hydrogel plays an important role in the shape recovery properties

Chiral Supramolecular Polymers Formed by Host−Guest Interactions

α-Cyclodextrin with a p-t-butoxyaminocinnamoylamino group in the 3-position (3-p-tBocCiNH-α-CD) has been found to form a supramolecular polymer in an aqueous solution. The degree of polymerization of the supramolecular polymer is higher than 15 at 20 mM, as proved by VPO (vapor pressure osmometry) measurements and turbo ion spray TOF MS measurements. The existence of substitution/substitution interactions between adjacent monomers of the supramolecular polymer have been confirmed by the observation of positive and negative Cotton bands in circular dichroism spectra. The mechanism for the induction of the chirality was confirmed using model compounds. The substituents were found to exist as a left-handed anti configuration in supramolecular polymers. The supramolecular polymer was found to take a helical structure. The structure of the supramolecular polymer was observed by STM measurements

An Artificial Molecular Chaperone: Poly-<i>pseudo</i>-rotaxane with an Extensible Axle

Poly-pseudo-rotaxanes CDs⊃1 (CDs; cyclodextrins, 1; poly(δ-valerolactone) having single β-CD at the end of the polymer chain) initiate polymerization of δ-valerolactone (δ-VL) in the solid state when CDs (α-CD, β-CD, and 2,6-di-O-methyl-β-CD) are threaded onto the polymer chain. 1 without threaded CDs did not show any polymerization ability for δ-VL. An adamantane molecule (Ad) inhibited the polymerization ability of CDs⊃1 for δ-VL, indicating that β-CD at the end of CDs⊃1 could not bind δ-VL because the β-CD cavity was occupied by Ad. It should be noted that the insertion reaction and the polymerization took place inside the β-CD cavity at the end of CDs⊃1 and that the formation of poly-pseudo-rotaxane is necessary for the initiation of δ-VL. The structures of β-CD⊃1 and 1 were characterized by powder X-ray diffraction measurements and solid-state NMR spectroscopies. The polymer chain of β-CD⊃1 was found to elongate in the solid state, whereas the polymer chain of 1 formed a random coil conformation. 1 was deactivated for the polymerization by blocking the active cavity of β-CD with the polymer chain. CDs threaded onto 1 are immune to the initiation of δ-VL directly but have an essential role to fold the polymer chain in a proper way as an artificial chaperone

Selection between Pinching-Type and Supramolecular Polymer-Type Complexes by α-Cyclodextrin−β-Cyclodextrin Hetero-Dimer and Hetero-Cinnamamide Guest Dimers

Novel supramolecular complexes have been prepared from an α-cyclodextrin−β-cyclodextrin hetero-dimer (α-CD−β-CD hetero-dimer) and hetero-cinnamamide guest dimers, G-t-Boc and G-NH2, having adamantyl groups in aqueous solutions. On addition of the competitive guest, the supramolecular structure formed by a mixture of the α-CD−β-CD hetero-dimer and G-t-Boc was found to be different from that of a mixture of the α-CD−β-CD hetero-dimer and G-NH2 by the 1H NMR spectroscopy, the ROESY NMR spectroscopy, and the circular dichroism spectroscopy. The size of the supramolecular complex from the mixture of the α-CD−β-CD hetero-dimer and G-NH2 is larger than that from the mixture of the α-CD−β-CD hetero-dimer and G-t-Boc, which was proved by the pulse field gradient spin−echo NMR and the atomic force microscopy. These results suggest that the mixture of the α-CD−β-CD hetero-dimer and G-t-Boc formed a pinching-type complex, and the mixture of the α-CD−β-CD hetero-dimer and G-NH2 formed a supramolecular polymer-type complex

Polymerization of Lactones Initiated by Cyclodextrins: Effects of Cyclodextrins on the Initiation and Propagation Reactions

Cyclodextrins (CDs) were found to initiate ring-opening polymerizations of lactones selectively to give polyesters in high yields, although lactones did not give any polymers under the same conditions in the absence of CD. The order of the polymer yield of β-butyrolactone (β-BL) with CDs is α-CD ≅ β-CD ≫ γ-CD > no CD. On the other hand, that of δ-valerolactone (δ-VL) is β-CD > γ-CD ≫ α-CD ≅ no CD. The yields of the polyesters depend on the cavity size of CDs and structures of lactones, indicating that the reaction took place via inclusion of lactones in the CD cavity. The β-CD−adamantane inclusion complex did not show any polymerization activity for δ-VL under the same conditions because adamantane is strongly included in the cavity of β-CD to inhibit formation of the inclusion complex between β-CD and δ-VL. The included lactones in the CD cavity are activated by the formation of hydrogen bonds between the hydroxyl group of CDs and the carbonyl oxygen of lactones in the initiation step, which was observed by FT−IR spectroscopy. The products were found to be a polymer chain attached to the C2-hydroxyl group of a single glucopyranose unit of CD via an ester bond. The lactones are activated by other remaining secondary hydroxyl groups to give the propagation step by way of insertions of monomers between CD and the polymer chain. The initiation and the propagation steps of the polymerization of lactones by CDs were observed by solid state 13C NMR techniques

An Artificial Molecular Chaperone: Poly-<i>pseudo</i>-rotaxane with an Extensible Axle

Poly-pseudo-rotaxanes CDs⊃1 (CDs; cyclodextrins, 1; poly(δ-valerolactone) having single β-CD at the end of the polymer chain) initiate polymerization of δ-valerolactone (δ-VL) in the solid state when CDs (α-CD, β-CD, and 2,6-di-O-methyl-β-CD) are threaded onto the polymer chain. 1 without threaded CDs did not show any polymerization ability for δ-VL. An adamantane molecule (Ad) inhibited the polymerization ability of CDs⊃1 for δ-VL, indicating that β-CD at the end of CDs⊃1 could not bind δ-VL because the β-CD cavity was occupied by Ad. It should be noted that the insertion reaction and the polymerization took place inside the β-CD cavity at the end of CDs⊃1 and that the formation of poly-pseudo-rotaxane is necessary for the initiation of δ-VL. The structures of β-CD⊃1 and 1 were characterized by powder X-ray diffraction measurements and solid-state NMR spectroscopies. The polymer chain of β-CD⊃1 was found to elongate in the solid state, whereas the polymer chain of 1 formed a random coil conformation. 1 was deactivated for the polymerization by blocking the active cavity of β-CD with the polymer chain. CDs threaded onto 1 are immune to the initiation of δ-VL directly but have an essential role to fold the polymer chain in a proper way as an artificial chaperone