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

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

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

Nanospheres with Polymerization Ability Coated by Polyrotaxane

β-Cyclodextrin (β-CD)-based nanosphere 1 initiated the oligomerization of δ-valerolactone (δ-VL) on the surface of 1 to give oligo(δ-VL)-tethered β-CD nanosphere 2 in bulk. Atomic force microscopy indicated that the molecular size of 2 is twice that of 1. The addition of α-CD to 2 leads to the formation of poly-pseudo-rotaxane on the surface of 2 to give a nanosphere with poly-pseudo-rotaxane (α-CD⊃2). 2D-NOESY NMR experiments showed correlation peaks between the inner protons of α-CD and the oligo(δ-VL) chains in an aqueous solution, indicating that the oligo(δ-VL) chains are included in the α-CD cavity. α-CD⊃2 has a core of β-CDs with poly-pseudo-rotaxanes on the surface. It should be noted that 2 did not show polymerization ability for δ-VL, but after the formation of poly-pseudo-rotaxanes, oligo(δ-VL) of α-CD⊃2 repropagated upon the addition of δ-VL. α-CD⊃2 is significantly larger than nanospheres 1 and 2. Additionally, postpolymerization increases the size of α-CD⊃2. These behaviors are reminiscent of the function of a spherical virus, which forms an ordered spherical structure and releases RNA chains from the capsid surface

Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by Molecular Machines

We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly­(ethylene glycol) (PEG) cross-linked by a [c2]­daisy chain, which is a double-threaded [2]­rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/vis spectroscopy results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene unit in the [c2]­daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest that αCD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that photoisomerization allows αCD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of αCD with cis-stilbene is quite low. Thus, the sliding motion of the αCD unit shrinks the [c2]­daisy chain, leading to the contraction of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier weight compared to the dry gel)

Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by Molecular Machines

We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly­(ethylene glycol) (PEG) cross-linked by a [c2]­daisy chain, which is a double-threaded [2]­rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/vis spectroscopy results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene unit in the [c2]­daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest that αCD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that photoisomerization allows αCD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of αCD with cis-stilbene is quite low. Thus, the sliding motion of the αCD unit shrinks the [c2]­daisy chain, leading to the contraction of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier weight compared to the dry gel)

Solvent-Free Photoresponsive Artificial Muscles Rapidly Driven by Molecular Machines

We prepared photoresponsive actuators as both hydrogels and dry gels consisting of 4-arm poly­(ethylene glycol) (PEG) cross-linked by a [c2]­daisy chain, which is a double-threaded [2]­rotaxane dimer with α-cyclodextrin (αCD) and stilbene. The obtained gels showed fast and large deformation triggered by UV irradiation in both wet and dry states. The UV/vis spectroscopy results, NMR measurements and tensile tests on the gels revealed that the actuation is driven by photoisomerization of the stilbene unit in the [c2]­daisy chain. The responsiveness of these gels depends on the molecular weight of the 4-arm PEG. These results suggest that αCD recognizes trans-stilbene prior to UV irradiation to maintain the length of the PEG chain in the polymer network and that photoisomerization allows αCD to leave the cis-stilbene moiety and move onto the PEG chain because the association constant of αCD with cis-stilbene is quite low. Thus, the sliding motion of the αCD unit shrinks the [c2]­daisy chain, leading to the contraction of the gels. In both wet and dry states, these actuations are repeatable through reversible photoisomerization of the stilbene moiety using different wavelengths of UV-light irradiation and can be used to perform bending and lifting actions (for 15 times heavier weight compared to the dry gel)

A Photoresponsive Polymeric Actuator Topologically Cross-Linked by Movable Units Based on a [2]Rotaxane

A photoresponsive polymeric material, in which [2]­rotaxane units with α-cyclodextrin threading onto an azobenzene derivative is used as a topological cross-link for the main chains of the polymeric material in aqueous media, was achieved. [2]­Rotaxane structures were found to act as movable links in the polymer network, and the mechanical properties of the material were enhanced to show a rupture strain of 2800%. The materials were reversibly deformed by irradiation with UV or visible light in aqueous media, which caused photoisomerization of the azobenzene moiety and changed the structure of the [2]­rotaxane linker, leading to deformation of the polymer network. Surprisingly, the dry materials, which had been uniaxially extended in air, showed a faster response than the hydrogel. The orientation of the polymeric network in the materials enables the efficient response. This dry material (5.6 mg) performed 5.6 μJ of mechanical work within 10 s, which is approximately 50 times higher than that achieved in our previous work