An Ultrastrong and Highly Stretchable Polyurethane Elastomer Enabled by a Zipper-Like Ring-Sliding Effect.

Elastomers with excellent mechanical properties are in substantial demand for various applications, but there is always a tradeoff between their mechanical strength and stretchability. For example, partially replacing strong covalent crosslinking by weak sacrificial bonds can enhance the stretchability but also usually decreases the mechanical strength. To surmount this inherent tradeoff, a supramolecular strategy of introducing a zipper-like sliding-ring mechanism in a hydrogen-bond-crosslinked polyurethane network is proposed. A very small amount (0.5 mol%) of an external additive (pseudo[2]rotaxane crosslinker) can dramatically increase both the mechanical strength and elongation of this polyurethane network by nearly one order of magnitude. Based on the investigation of the relationship between molecular structure and mechanical properties, this enhancement is attributable to a unique molecular-level zipper-like ring-sliding motion, which efficiently dissipates mechanical work in the solvent-free network. This research not only provides a distinct and general strategy for the construction of high-performance elastomers but also paves the way for the practical application of artificial molecular machines toward solvent-free polyurethane networks.

A supramolecular network derived by rotaxane tethering three ureido pyrimidinone groups.

A supramolecular network and its film were prepared via intermolecular hydrogen bonding of a [2]rotaxane with three ureido pyrimidinone groups. Adding less polar solvents such as CHCl3 to the rotaxane afforded a swollen rotaxane-cross-linked network. The properties of the film obtained by drying were evaluated to characterize it.

Towards a hexa-branched [7]rotaxane from a [3]rotaxane via a [2+2+2] alkyne cyclotrimerization process.

A facile synthesis methodology to obtain a crown ether-based hexa-branched [7]rotaxane, has been achieved by using Co-catalyzed [2+2+2] alkyne cyclotrimerization from a [3]rotaxane. The [2+2+2] alkyne cyclotrimerization process, which requires only 10 mol% of Co2(CO)8 as a catalyst and affords an efficient production of target rotaxanes, represents a new strategy for the construction of highly complicated dendritic rotaxanes.

Integrative Self-Sorting: One-Pot Synthesis of a Hetero[4]rotaxane from a Daisy-Chain-Containing Hetero[4]pseudorotaxane.

The structural complexity of mechanically interlocked molecules are very attractive to chemists owing to the challenges they present. In this article, novel mechanically interlocked molecules with a daisy-chain-containing hetero[4]rotaxane motif were efficiently synthesized. In addition, a novel integrative self-sorting strategy is demonstrated, involving an ABB-type (A for host, dibenzo-24-crown-8 (DB24C8), and B for guest, ammonium salt sites) monomer and a macrocycle host, benzo-21-crown-7 (B21C7), in which the assembled species in hydrogen-bonding-supported solvent only includes a novel daisy-chain-containing hetero[4]pseudorotaxane. The found self-sorting process involves the integrative recognition between B21C7 macrocycles and carefully designed components simultaneously containing two types of secondary ammonium ions and a host molecule, DB24C8 crown ether. The self-sorting strategy is integrative to undertake self-recognition behavior to form one single species of pseudorotaxane compared with the previous report. This self-sorting system can be used for the efficient one-pot synthesis of a daisy-chain-containing hetero[4]rotaxane in a good yield. The structure of hetero[4]rotaxane was confirmed by 1 H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry.

One-pot synthesis of hetero[6]rotaxane bearing three different kinds of macrocycle through a self-sorting process.

In this article, a six-component self-sorting process that involves three types of crown ether macrocycle and three types of cation guest molecule was carefully and thoroughly investigated. The six components include three kinds of crown ether, namely bis(p-phenylene-34-crown-10) (BPP34C10), dibenzo-24-crown-8 (DB24C8) and benzo-21-crown-7 (B21C7), and their corresponding cation guest molecules, namely a 4,4'-bipyridine dication (BPY2+) and dibenzylammonium (DBA) and benzylalkylammonium (BAA) ions, respectively. Based on this well-established highly selective six-component self-sorting process, a hetero[6]rotaxane bearing three different kinds of crown ether macrocycle was designed and successfully synthesized through a facile and efficient one-pot "click" stoppering strategy. Such work is proposed to be a significant advance in the construction of mechanically interlocked molecules with high structural complexity, as well as a good supplement in the areas of multi-component self-sorting and noncovalent self-assembly.

A Braided Hetero[2](3)rotaxane.

A novel braided hetero[2](3)rotaxane is demonstrated by integrating the braided structure and mechanically interlocked rotaxane, in which a heterotritopic linear tris(dialkylammonium) guest penetrates a heterotritopic tris(crown ether) host, resulting in the formation of braided pseudohetero[2](3)rotaxane with different crossing and threading points. Then a braided hetero[2](3)rotaxane is constructed through the "CuAAC" click reaction.

Triggering a [2]Rotaxane Molecular Shuttle by a Photochemical Bond-Cleavage Strategy.

The successful triggering of ring-shuttling motion between two stations in a [2]rotaxane is demonstrated by employing a photochemical bond-cleavage strategy. A photolabile bulk barrier is covalently introduced into two identical stations of the thread to prevent dynamic shuttling of the macrocycle, resulting in a "gated" state. Irradiation of UV light (λ = 365 nm) results in the complete removal of the bulk barrier and the balanced shuttling motion of the macrocycle, indicating an "open" state of the rotaxane. In addition, the process from the "open" rotaxane to the "gated" rotaxane was executed by a chemical-rebonding method.

One-pot synthesis of a [c2]daisy-chain-containing hetero[4]rotaxane via a self-sorting strategy.

The construction and efficient synthesis of hetero[n]rotaxanes with high structural complexity are always attractive challenges. Herein, we demonstrate a facile one-pot preparation of a hetero[4]rotaxane, by employing a self-sorting strategy, which contains an interpenetrated dibenzo-24-crown-8 (DB24C8) based [c2]daisy chain structure and is ended with a benzo-21-crown-7 (B21C7) based rotaxane at each side. The key to the design involved encoding the selective threading using a steric hindrance-related "language", where highly selective self-assemblies occurred in a three-component self-sorting process, which included the threading of a benzylalkylammonium into a B21C7 and interpenetrated dimerized formation of a DB24C8 based [c2]daisy chain simultaneously; the precise pre-assembled system resulted in the efficient synthesis of hetero[4]rotaxane with a high-level of structural complexity under the "CuAAC" reaction.

Two Stepwise Synthetic Routes toward a Hetero[4]rotaxane.

Heterorotaxanes have been emerging as an important class of mechanically interlocked molecules and have attracted much attention in recent years. Driven by the distinguishable host-guest interactions between crown ether macrocycles and ammonium with different sizes, a novel hetero[4]rotaxane was successfully prepared by employing the combination of copper-catalyzed "click" reaction and P(n-Bu)3-catalyzed esterification reaction as stoppering reactions. The hetero[4]rotaxane contains an interlocked species in which a dibenzo[24]crown-8 ring threaded by a dibenzylammonium-containing component with two benzo[21]crown-7 macrocycles at both ends to act as stoppers, and each of the two benzo[21]crown-7 rings is also threaded with a benzylalkylammonium unit to form the second interlocked species. The hetero[4]rotaxane was prepared through two different stepwise synthetic routes, and the complicated chemical structure of the hetero[4]rotaxane was well-characterized by (1)H NMR spectroscopy and high-resolution electrospray ionization (HR-ESI) mass spectrometry. The investigation shows that the construction of complicated topological heterorotaxane can be achieved via distinct approaches with high efficiencies, which may provide a foundation for the construction of more sophisticated heterorotaxane systems or functional supermolecules.