Yasuhito Koyama

Size‐Complementary Rotaxane Cross‐Linking for the Stabilization and Degradation of a Supramolecular Network

As cross-linked polymers are widely used fundamental materials, their chemical recycling becomes a quite important issue. A promising strategies is to utilize reversible crosslinking based on dynamic covalent chemistry (DCC). 3] For example, Chen et al. reported the efficient thermal reversibility of cross-linking by Diels–Alder reaction. On the other hand, supramolecular gels formed by intermolecular interactions, such as hydrogen bonds, can be efficiently de-crosslinked by specific stimuli without destruction of covalent bonds. 3d, 5–7] However, supramolecular gels are naturally stable only under limited conditions that are capable of keeping the intermolecular interactions strong. A polyrotaxane network (PRN) is a supramolecular gel stabilized by not only intermolecular interactions but also mechanical restriction. We previously reported a reversibly cross-linkable PRN based on DCC, consisting of poly(crown ether) backbone and bis(ammonium) cross-linker possessing a disulfide bond in its center (Figure 1a). The work gave a new impulse for the chemical recycling of cross-linked polymers, but it suffered from the disadvantage that it is a sluggish reaction. Therefore, we have developed a novel approach that enables the efficient de-cross-linking of PRNs without any cleavage of covalent bonds. Recently, we have found a novel procedure in which the axle component as a cross-linker has an end group the size of which is complementary to the macrocycle cavity placed on the trunk polymer (Figure 1b). The end groups provide an energy barrier to slow the dissociation, thereby kinetically stabilizing the rotaxane skeleton. Thus, the PRN stabilizes the network structure under normal conditions, but it can be de-cross-linked when certain conditions, such as those that accelerate the dissociation of the rotaxane skeletons, are satisfied. Because the decross-linking can be achieved without breaking the covalent bonds, the PRN is selectively degraded so as to not damage the trunk polymer. Furthermore, the stability and de-crosslinking capability of PRNs can be adjusted by the size of the end groups of the axle components. Herein we describe the concept of novel de-cross-linkable network polymers that utilize the size-complementary effect of the rotaxane crosslinks. First, we investigated a model for the size-complementary effect using crown ether/ammonium salt [2]rotaxanes 1 (Scheme 1). According to previous reports, roxaxanes 1 with suitable end groups (R = cyclohexyl, tBu, 4-tBuC6H4) were sufficiently stable to maintain its threaded structure owing to both the bulky end groups and hydrogen bonds between dibenzo [24]crown-8 ether (DB24C8) and the ammonium group. However, 1 dissociated into two parts, axle and wheel, when the hydrogen bonds were disturbed by a stimulus, in accordance with reported results (R = 4tBuC6H4). [12] Details are summarized in Table 1. As we envisioned, the dissociation rate depends on both the size of the end group and the kind of the external stimulus, as discussed below. An investigation into the dissociation behavior of 1 in dimethylsulfoxide (DMSO), a polar solvent that disturbed the hydrogen bonds, showed the decomposition of only 1a among three derivatives. The dissociation rate of 1a to DB24C8 and 2a obeyed first-order kinetics. The half-life (t1/2) was estiFigure 1. Strategy for de-cross-linking a PRN using a) a reversible cleavage of disulfide bond and b) characteristics of a rotaxane crosslink consisting of size-complementary components.

Rational control of a polyacetylene helix by a pendant rotaxane switch

Polyacetylene bearing a pendant rotaxane moiety with an optically active wheel component was synthesized to realize reversible structural control of its helical structure by position control of the wheel component. Polyacetylene formed a one-handed helical structure only when the optically active wheel component moved close to the main chain.

C−C Bond-Forming Click Synthesis of Rotaxanes Exploiting Nitrile N-Oxide

A click end-capping reaction exploiting nitrile N-oxide to rotaxane was described with emphasis of productivity of the protocol via stable C-C bond formation. Establishment of a pH-driven molecular shuttling system was also demonstrated by practical neutralization of the sec-ammonium group of the rotaxane axle with potassium hydroxide.

A Rational Design for the Directed Helicity Change of Polyacetylene Using Dynamic Rotaxane Mobility by Means of Through‐Space Chirality Transfer

Directed helicity control of a polyacetylene dynamic helix was achieved by hybridization with a rotaxane skeleton placed on the side chain. Rotaxane-tethering phenylacetylene monomers were synthesized in good yields by the ester end-capping of pseudorotaxanes that consisted of optically active crown ethers and sec-ammonium salts with an ethynyl benzoic acid. The monomers were polymerized with [{RhCl(nbd)}(2)] (nbd=norbornadiene) to give the corresponding polyacetylenes in high yields. Polymers with optically active wheel components that are far from the main chain show no Cotton effect, thereby indicating the formation of racemic helices. Our proposal that N-acylative neutralization of the sec-ammonium moieties of the side-chain rotaxane moieties enables asymmetric induction of a one-handed helix as the wheel components approach the main chain is strongly supported by observation of the Cotton effect around the main-chain absorption region. A polyacetylene with a side-chain rotaxane that has a shorter axle component shows a Cotton effect despite the ammonium structure of the side-chain rotaxane moiety, thereby suggesting the importance of proximity between the wheel and the main chain for the formation of a one-handed helix. Through-space chirality induction in the present systems proved to be as powerful as through-bond chirality induction for formation of a one-handed helix, as demonstrated in an experiment using non-rotaxane-based polyacetylene that had an optically active binaphthyl group. The present protocol for controlling the helical structure of polyacetylene therefore provides the basis for the rational design of one-handed helical polyacetylenes.

Catalyst- and Solvent-Free Click Synthesis of Cyclodextrin-Based Polyrotaxanes Exploiting a Nitrile N-Oxide

A catalyst- and solvent-free synthesis of cyclodextrin-based polyrotaxanes exploiting a stable nitrile N-oxide as an end-capping agent was achieved. The C-C bond-forming end-capping reaction of an allyl-terminated pseudopolyrotaxane with the nitrile N-oxide proceeded smoothly by solid-state grinding in a mortar to afford a polyrotaxane.

Fluorescence Control of Boron Enaminoketonate Using a Rotaxane Shuttle

The effect of rotaxane shuttling on the fluorescence properties of a fluorophore was investigated by exploiting fluorophore-tethered [2]rotaxanes. A fluorescent boron enaminoketonate (BEK) moiety was introduced in a rotaxane via transformation of an isoxazole unit generated as a result of an end-capping reaction using a nitrile N-oxide. The rotaxane exhibited a red shift of the fluorescence maximum along with a remarkable enhancement of the fluorescence quantum yield through wheel translation to the fluorophore.

Intramolecular 1,3-Dipolar Cycloaddition of Nitrile N-Oxide Accompanied by Dearomatization

Intramolecular 1,3-dipolar cycloaddition of 2-phenoxybenzonitrile N-oxides to benzene rings, accompanied by dearomatization, formed the corresponding isoxazolines in high yields. The X-ray single-crystal structure analysis revealed that the reaction formed the cis-adduct as a single isomer. The substituents on the benzene rings markedly affected the reaction rate, yield, and structure of the final product.

Selective synthesis of a [3]rotaxane consisting of size-complementary components and its stepwise deslippage.

An α-cyclodextrin-based size-complementary [3]rotaxane with an alkylene axle was selectively synthesized in one pot via an end-capping reaction with 2-bromophenyl isocyanate in water. Thermal degradation of the [3]rotaxane product yielded not only the original components but also the [2]rotaxane. Thermodynamic studies suggested a stepwise deslippage process.

Thermoresponsive Shuttling of Rotaxane Containing Trichloroacetate Ion

A thermoresponsive rotaxane shuttling system was developed with a trichloroacetate counteranion of an ammonium/crown ether-type rotaxane. Chemoselective thermal decomposition of the ammonium trichloroacetate moiety on the rotaxane yielded the corresponding nonionic rotaxane accompanied by a positional change of the crown ether on the axle. The rotaxane skeleton facilitated effective dissociation of the acid, markedly lowering the thermal decomposition temperature.

Versatile supramolecular cross-linker: a rotaxane cross-linker that directly endows vinyl polymers with movable cross-links.

A supramolecular cross-linked cross-linker, capable of introducing rotaxane cross-links to vinyl polymers, has been developed for the rational synthesis of polyrotaxane networks. The experimental results reveal that the combination of an oligocyclodextrin (OCD) and a terminal bulky group-tethering macromonomer (TBM) forms a polymer-network structure having polymerizable moieties through supramolecular cross-linking. Radical polymerization of a variety of typical vinyl monomers in the presence of the vinylic supramolecular cross-linker (VSC) afforded the corresponding vinyl polymers cross-linked through the rotaxane cross-links (RCP) as transparent stable films in high yields under both photoinitiated and thermal polymerization conditions. A poly(N,N-dimethylacrylamide)-based hydrogel synthesized by using VSC, RCPDMAAm , displayed a unique mechanical property. The small-angle X-ray scattering (SAXS) results, indicating patterns characteristic of a polyrotaxane network, clearly suggested the presence and role of the rotaxane cross-links. The confirmation of the introduction of rotaxane-cross-links into vinyl polymers strongly reveals the significant usefulness of VSC.