Cyclodextrins-Based Polyrotaxanes: From Functional Polymers to Applications in Electronics and Energy Storage Materials

The concept of polyrotaxane comes from the rotaxane structure in the supramolecular field. It is a mechanically interlocked supramolecular assembly composed of linear polymer chains and cyclic molecules. Over recent decades, the synthesis and application of polyrotaxanes have seen remarkable growth. Particularly, cyclodextrin-based polyrotaxanes have been extensively reported due to the low-price raw materials, good biocompatibility, and ease of modification. Hence, it is also one of the most promising mechanically interlocking supramolecules for wide industrialization in the future. Polyrotaxanes are widely introduced into materials such as elastomers, hydrogels, and engineering polymers to improve their mechanical properties or impart functionality to the materials. In these materials, polyrotaxane acts as a slidable cross-linker to dissipate energy through sliding or assist in dispersing stress concentration in the cross-linked network, thereby enhancing the toughness of the materials. Further, the unique sliding-ring effect of cyclodextrin-based polyrotaxanes has pioneered advancements in stretchable electronics and energy storage materials. This includes their innovative use in stretchable conductive composite and binders for anodes, addressing critical challenges in these fields. In this mini-review, our focus is to highlight the current progress and potential wider applications in the future, underlining their transformative impact across various domains of material science.

Preparation and Application of Polyrotaxane Cross-Linking Agent Based on Cyclodextrin in Gel Materials Field

The cross-linking point of a conventional chemical cross-linking agent is fixed. Therefore, gels that are prepared with a conventional cross-linking agent have poor deformability, strength, shear resistance, and further properties. Some researchers have prepared a new cross-linking agent using cyclodextrin (CD). In a polyrotaxane cross-linking agent, the cross-linking points can slide freely along the molecule chain. The special "slide ring" structure can provide better elongation, strength, and other properties to gels, which can effectively expand the application of the gel's materials. This paper summarizes the preparation methods and applications from different types of CD and compares the improvements of properties (swelling, viscoelastic properties, etc.). In addition, the current results of our group are presented, and some ideas are provided for the development of polyrotaxane cross-linking agents.

Synthesis, Characterization, and Potential Application of Cyclodextrin-Based Polyrotaxanes for Reinforced Atelocollagen Threads

Preparing strong and flexible atelocollagen-based materials for biomedical applications is still a challenging task. To address this challenge, this study describes the synthesis and characterization of water-soluble polyrotaxanes (PRs) with different coverage ratios and molecular weights of axle polymers, and their potential applications for PR-reinforced atelocollagen threads (PRATs). A novel method was established for the syntheses of PRs with relatively low coverage ratio at the sub-gram scale, in which the aldehyde groups were employed as crosslinking sites for preparing the PRATs via reductive amination. The aldehyde groups were successfully quantified by 1H nuclear magnetic resonance spectroscopy using 1,1-dimethylhydrazine as an aldehyde marker. Fourier-transform infrared and thermogravimetric analysis measurements supported the characterization of the PRs. Interestingly, tensile testing demonstrated that coverage ratio affected the mechanical properties of the PRATs more strongly than molecular weight. The insights obtained in this study would facilitate the development of soft materials based on atelocollagens and PRs.

Micromechanics and damage in slide-ring networks

We explore the mechanics and damage of slide-ring gels by developing a discrete model for the mechanics of chain-ring polymer systems that accounts for both crosslink motion and internal chain sliding. The proposed framework utilizes an extendable Langevin chain model to describe the constitutive behavior of polymer chains undergoing large deformation and includes a rupture criterion to innately capture damage. Similarly, crosslinked rings are described as large molecules that also store enthalpic energy during deformation and thus have their own rupture criterion. Using this formalism, we show that the realized mode of damage in a slide-ring unit is a function of the loading rate, distribution of segments, and inclusion ratio (number of rings per chain). After analyzing an ensemble of representative units under different loading conditions, we find that failure is driven by damage to crosslinked rings at slow loading rates, but polymer chain scission at fast loading rates. Our results indicate that increasing the strength of the crosslinked rings may improve the toughness of the material.

Chemorheological Monitoring of Cross-Linking in Slide-ring Gels Derived From α-cyclodextrin Polyrotaxanes

Despite hundreds of studies involving slide-ring gels derived from cyclodextrin (CD)-based polyrotaxanes (PRs), their covalent cross-linking kinetics are not well characterized. We employ chemorheology as a tool to measure the gelation kinetics of a model slide-ring organogel derived from α-cyclodextrin/poly (ethylene glycol) PRs cross-linked with hexamethylenediisocyanate (HMDI) in DMSO. The viscoelastic properties of the gels were monitored in situ by small-amplitude oscillatory shear (SAOS) rheology, enabling us to estimate the activation barrier and rate law for cross-linking while mapping experimental parameters to kinetics and mechanical properties. Gelation time, gel point, and final gel elasticity depend on cross-linker concentration, but polyrotaxane concentration only affects gelation time and elasticity (not gel point), while temperature only affects gelation time and gel point (not final elasticity). These measurements facilitate the rational design of slide-ring networks by simple parameter selection (temperature, cross-linker concentration, PR concentration, reaction time).

Metastable doubly threaded [3]rotaxanes with a large macrocycle

Ring size is a critically important parameter in many interlocked molecules as it directly impacts many of the unique molecular motions that they exhibit. Reported herein are studies using one of the largest macrocycles reported to date to synthesize doubly threaded [3]rotaxanes. A large ditopic 46 atom macrocycle containing two 2,6-bis(N-alkyl-benzimidazolyl)pyridine ligands has been used to synthesize several metastable doubly threaded [3]rotaxanes in high yield (65-75% isolated) via metal templating. Macrocycle and linear thread components were synthesized and self-assembled upon addition of iron(ii) ions to form the doubly threaded pseudo[3]rotaxanes that could be subsequently stoppered using azide-alkyne cycloaddition chemistry. Following demetallation with base, these doubly threaded [3]rotaxanes were fully characterized utilizing a variety of NMR spectroscopy, mass spectrometry, size-exclusion chromatography, and all-atom simulation techniques. Critical to the success of accessing a metastable [3]rotaxane with such a large macrocycle was the nature of the stopper group employed. By varying the size of the stopper group it was possible to access metastable [3]rotaxanes with stabilities in deuterated chloroform ranging from a half-life of <1 minute to ca. 6 months at room temperature potentially opening the door to interlocked materials with controllable degradation rates.

Direct enhancement of intercomponent interactions in polyrotaxane and its pronounced effects on glass state properties

Strong interactions between the host cyclodextrin and the threading guest polymer were introduced by selective modifications to the polymer of a polybutadine-based polyrotaxane. The changes in the intercomponent interactions influenced the mobility of the threading polymer that was confined in the glassy host framework, resulting in different mechanical properties.

Hydrogen-Bonded Structure of Water in the Loop of Anchored Polyrotaxane Chain Controlled by Anchoring Density

Hydrogen-bonded network of water surrounding polymers is expected to be one of the most relevant factors affecting biocompatibility, while the specific hydrogen-bonded structure of water responsible for biocompatibility is still under debate. Here we study the hydrogen-bonded structure of water in a loop-shaped poly(ethylene glycol) chain in a polyrotaxane using synchrotron soft X-ray emission spectroscopy. By changing the density of anchoring molecules, hydrogen-bonded structure of water confined in the poly(ethylene glycol) loop was identified. The XES profile of the confined water indicates the absence of the low energy lone-pair peak, probably because the limited space of the polymer loop entropically inhibits the formation of tetrahedrally coordinated water. The volume of the confined water can be changed by the anchoring density, which implies the ability to control the biocompatibility of loop-shaped polymers.

Supramolecular complex formation of polysulfide polymers and cyclodextrins

We report the first preparation of a supramolecular polysulfide polymer, which is a polyrotaxane containing sulfur-styrene copolymer and methylated α-cyclodextrins (TMαCDs) as linear and cyclic molecules, respectively (SPRx). Compared to the sulfur-styrene copolymer prepared by a copolymerization method typically used to synthesize polysulfide polymers, the environmental and thermal stabilities of SPRx are significantly improved because the polysulfide polymer is covered with TMαCD.

Anisotropic Amorphous X-ray Diffraction Attributed to the Orientation of Cyclodextrin

The beauty of cyclic molecules is reflected in their host-guest complexation reactions, as well as their unique X-ray diffraction patterns. Cyclodextrins, the longest known host molecules with rigid ring structures, show anisotropic X-ray diffraction characteristic of their single-molecule structure, rather than their intermolecular relationships. Amorphous derivatives of α-cyclodextrin exhibit broad and strong halo diffractions in the solid, melted, and dilute solution states. The diffraction angle corresponds to the intramolecular distance between neighboring glycosidic oxygen atoms located at the vertices of a regular hexagonal array. Because the hexagon is parallel to the aperture plane of the rigid cyclic molecule, the diffraction appears only in the direction parallel to this plane. The anisotropy was confirmed by stretching an amorphous thermoplastic polymer threaded through the inclusion cavities of a sequence of cyclodextrins. The resultant unique anisotropic X-ray diffraction suggests the possible use of rigid cyclic molecules as molecular orientation probes.

Analysis of High-Molecular-Weight Polyrotaxanes by MALDI-TOF-MS Using 3-Aminoquinoline-Based Ionic Liquid Matrix

Polyrotaxane (PR) is a necklace-like supramolecule composed of cyclic components, such as cyclodextrin (CD), and a threading polymer capped with bulky end groups. PR exhibits peculiar mechanical properties attributed to the intermolecular cross-links with CD. Various CD molecules threaded on a linear PEG chain are often modified with chemical groups to add specific physicochemical properties. In general, the stoichiometry between CD and the PEG chain is a significant parameter that defines the unique physical properties of CD-based polyrotaxane (CD-PR). To date, mass spectrometry (MS) has been applied to investigate the molecular distribution of CD-PR, modifications of CD, and the threaded ratio of CD. However, only molecular weights (MWs) up to several 10s of kDa can be subjected to such analysis, whereas the MW of CD-PR used as industrial materials is much greater. Herein, we applied two ionic liquid matrices composed of 3-aminoquinoline and a high mass detector to analyze PRs using MALDI-TOF-MS. High to very high MW PRs in the range of 90-700 kDa were successfully analyzed using this method. The threaded ratio of CD was estimated from a single MW of CD, PEG, and PR. The ratios obtained were consistent with that obtained using ¹H NMR. Furthermore, a single-stranded form of PR in γ-cyclodextrin threaded PR (γCD-PR) was clearly distinguished from a double-stranded form, which is only possible in γCD -PR because of its large host cavity.

Exploring and Exploiting the Symmetry-Breaking Effect of Cyclodextrins in Mechanomolecules

Cyclodextrins (CDs) are cone-shaped molecular rings that have been widely employed in supramolecular/host–guest chemistry because of their low cost, high biocompatibility, stability, wide availability in multiple sizes, and their promiscuity for binding a range of molecular guests in water. Consequently, CD-based host–guest complexes are often employed as templates for the synthesis of mechanically bonded molecules (mechanomolecules) such as catenanes, rotaxanes, and polyrotaxanes in particular. The conical shape and cyclodirectionality of the CD “bead” gives rise to a symmetry-breaking effect when it is threaded onto a molecular “string”; even symmetrical guests are rendered asymmetric by the presence of an encircling CD host. This review focuses on the stereochemical implications of this symmetry-breaking effect in mechanomolecules, including orientational isomerism, mechanically planar chirality, and topological chirality, as well as how they support applications in regioselective and stereoselective chemical synthesis, the design of molecular machine prototypes, and the development of advanced materials.

Mechanically interlocked materials. Rotaxanes and catenanes beyond the small molecule

An overview of the progress in mechanically interlocked materials is presented. In particular, we focus on polycatenanes, polyrotaxanes, metal–organic rotaxane frameworks (MORFs), and mechanically interlocked derivatives of carbon nanotubes (MINTs).

A molecular modeling study for miscibility of polyimide/polythene mixing systems with/without compatibilizer

Molecular models were established to predict the miscibility of polyimide/polythene mixing systems and the enhancing effects of compatibilizer addition of maleic anhydride grafted polythene (MAH-g-PE). Molecular dynamics simulations were applied to investigate radial distribution functions and Flory-Huggins parameters of the mixing systems. Results show that polyimide/polythene is miscible to a certain degree, and the miscibility gets better after adding MAH-g-PE. Dissipative particle dynamics (DPD) simulations display that micro-phase separation occurs in the polyimide/polythene mixing systems, however, effective interfaces appear between polyimide and polythene phases after adding MAH-g-PE. The results of molecular mechanics simulations indicate that the ability of mixing systems to resist stretch, compression and shear deformation increases after adding MAH-g-PE. This work offers a promising technique to predict miscibility properties for polyimide/polythene system prior to actual production and attempt to find a suitable compatibilizer for that system.

Structure and properties of tough polyampholyte hydrogels: effects of a methyl group in the cationic monomer

The very small steric bulk of methyl exhibits significant effects on the strength and distribution of ionic bonds in gels.