We prepared acrylamide monomers with permethylated cyclodextrins (PM-CDAAmMe) or peracetylated cyclodextrins (PAc-CDAAmMe). PM-CDAAmMe and PAc-CDAAmMe are soluble in various hydrophobic liquid acrylate monomers, and they can form inclusion complexes with guest monomers such as adamantane or fluoroalkyl groups tethered to a vinyl residue. The bulk polymerization of the liquid acrylate monomers with the PM-CDAAmMe or PAc-CDAAmMe monomers and the guest monomers gave highly flexible and tough elastomers. Tensile tests on the obtained supramolecular elastomers showed fracture strains of over 800% and fracture energies that were 12 times larger than those of covalently cross-linked conventional elastomers, indicating that the host–guest cross-linking made the supramolecular elastomers quite tough. During the deformation process, the applied stress is dispersed into the supramolecular elastomers by dissociation and recombination of the reversible host–guest complex. Moreover, these host–guest complexes also allow the adhesion of fractured pieces of the supramolecular elastomers without adhesives. The mechanical strength of the fractured elastomer was restored to ∼99% of its initial strength within 4 h. The self-healing properties can be attributed to the reversible cross-linking by the host–guest interactions.

Highly flexible and tough elastomers were obtained from the bulk copolymerization of a peracetylated cyclodextrin (CD) monomer and small alkyl acrylate main chain monomers without a guest monomer. The main chains penetrated the cavity of the CD units, and the CD units on the polymer chain acted as movable cross-linking points in the obtained elastomer. In contrast, the copolymerization using a bulky main chain monomer with bulky side groups gave linear polymers. The CD units with the bulky main chain polymer cannot serve as movable cross-linking points. Introducing movable cross-linking into poly­(ethyl acrylate) resulted in a higher fracture energy comparable to that of conventional rubbers because of the stress-dispersion properties related to the sliding motion of the movable cross-linking points. The movable cross-linkers disperse applied external stresses more effectively than an elastomer with reversible cross-linking at a high Young’s modulus (150 MPa). Movable cross-linking can be introduced to enhance the fracture energy of polymeric materials.

Self-healing materials have attracted attention due to their ability to regain their structure and function after damage. In recent years, significant progress has been made in achieving various functions through supramolecular chemistry. This review describes an overview of the strategies used to prepare self-healing and self-restoring materials utilizing reversible and movable crosslinks. Reversible crosslinks, consisting of noncovalent bonds, can reversibly undergo repeated cleavage and reformation. Therefore, self-healing can be achieved by effectively regenerating reversible crosslinks between polymeric chains. Reversible crosslinks exploit many kinds of dynamic covalent bonds and noncovalent bonds, such as hydrogen bonds, metal coordination bonds, ionic interactions, π–π stacking, van der Waals forces, and hydrophobic interactions. Movable crosslinks exhibit self-restoring properties. Self-restoring materials can regain their original shape and mechanical properties after a cycle of loading and unloading external stress. Movable crosslinks consist of polymer chains that penetrate macrocyclic units and have self-restoring properties due to their sliding motion along the polymeric chains. In addition, multiple reversible cross-links produce synergistic effects to simultaneously achieve high toughness and effective self-healing. We believe that self-healing and self-restoring materials will play a substantial role in realizing a sustainable society.

Water plays important roles in various functions on the surface of polymers and peptides. In this work, host–guest hydrogels with different weight percentages of water were prepared via copolymerization of an acrylamide-modified cyclodextrin host monomer, an acrylamide-modified adamantane guest monomer, and acrylamide. The host–guest hydrogels showed high toughness and readhesion properties, in which the reversible host–guest cross-linking between the poly­(acrylamide) chains is important. The mechanical properties of the host–guest hydrogels depend on their water contents. The host–guest hydrogels showed maximum mechanical strength at different water contents. The maximum mechanical strength also varied depending on the type of poly­(acrylamide) in the backbone and the amounts of host and guest cross-linking units. The readhesion strength of the host–guest hydrogels varied with water content, indicating that the water in the host–guest hydrogels affects their mechanical strength and readhesion behavior. Tensile tests, contact angle measurements, and differential scanning calorimetry (DSC) revealed that water exists in three states (nonfreezing bound water, intermediate water, and free water) in the hydrogels, and the nonfreezing bound water and the intermediate water affect the mechanical properties, such as toughness and readhesion force. These results indicate that the host–guest interactions, which affect the mechanical properties of the host–guest hydrogels, depend on the nonfreezing bound water and the intermediate water hydrating the host–guest polymer network.

Bulk copolymerization of alkyl acrylates and cyclodextrin (CD) host monomers produced a single movable cross-network (SC). The CD units acted as movable crosslinking points in the obtained SC elastomer. Introducing movable crosslinks into a poly(ethyl acrylate/butyl acrylate) copolymer resulted in good toughness (Gf) and stress dispersion. Here, to improve the Young’s modulus (E) and Gf of movable cross-network elastomers, the bulk copolymerization of liquid alkyl acrylate monomer swelling in SC gave another type of movable cross-network elastomer with penetrating polymers (SCPs). Moreover, the bulk copolymerization of alkyl acrylate and the CD monomer in the presence of SC resulted in dual cross-network (DC) elastomers. The Gf of the DC elastomer with a suitable weight % (wt%) of the secondary movable cross-network polymer was higher than those of the SCP or SC elastomers. The combination of suitable hydrophobicity and glass transition of the secondary network was important for improving Gf. Small-angle X-ray scattering (SAXS) indicated that the DC elastomers exhibited heterogeneity at the nanoscale. The DC elastomers showed a significantly broader relaxation time distribution than the SC and SCP elastomers. Thus, the nanoscale heterogeneity and broader relaxation time distribution were important to increase Gf. This method to fabricate SCP and DC elastomers with penetrating polymers would be applicable to improve the Gf of conventional polymeric materials.