Tougher when stressed: Hydrogels are typically weak materials because they contain only a small fraction of polymeric material highly swollen with water. Strength can be increased by adding extra cross-linking or interpenetrating networks with some sacrificial bonds to enhance toughness; however, these properties deteriorate upon stretching and may be slow to recover after relaxation. Liu et al. developed polyethylene glycol hydrogels cross-linked with moderate fractions of polymers that form sliding rings. These allow the chain to orient in parallel when stretched, leading to rapid and reversible strain-induced crystallization and thus much tougher hydrogels. Science , aaz6694, this issue p. 1078

The present review article deals with recent novel studies on the preparation and application of polyrotaxanes comprised of cyclodextrins (CDs) and various linear polymers, especially poly(ethylene glycol) (PEG). First, a brief introduction of the historical background of the pioneering work on the preparation of an inclusion complex and polyrotaxane is provided. Subsequently, the authors have focused on the recently developed solvent systems for the polyrotaxane. These new solvents are interesting from two fundamental viewpoints: (1) from the perspective of the clarification of the hydrogen-bonding-based dissolution mechanism of polyrotaxanes; and (2) from the practical viewpoint of the preparation of modified polyrotaxanes or slide-ring gels containing ionic liquids. A wide variety of polyrotaxane derivatives, whose cyclodextrin moiety was modified to carry various functional groups, and their intriguing characteristics are introduced in this article. Finally, many instances of the application of the PEG–CD polyrotaxane to soft materials, such as gels, molecular tubes and multivalent ligand systems, are summarized.

We prepared a mechanically tough, highly transparent elastomer by using a polyrotaxane cross-linker acting as a molecular pulley.

A novel type of gel, called a topological gel, has been recently developed with a supramolecular architecture with topological characteristics. In the topological gel, polymer chains with bulky end groups are neither covalently crosslinked as in chemical gels nor attractively interacting as in physical gels but are topologically interlocked by figure‐eight shaped crosslinks. Hence, these crosslinks can pass along the polymer chains freely to equalize the tension of the threading polymer chains similarly to pulleys; this is called the pulley effect. This concept can be applied not only to gels but also to a wide variety of polymeric materials without solvents. Then, polymeric materials with movable crosslinks are referred to as slide‐ring materials (SRMs) in a wider sense. Here, we review the synthesis, structure, physical properties, and applications of topological gels and SRMs. In particular, slide‐ring elastomers show remarkable scratch‐proof properties for application to coating materials for automobiles, cell phones, mobile computers, golf clubs, and so on. © 2014 The Authors Journal of Applied Polymer Science Published by Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40509.