Ring threading

For catenane formation from two separate rings, the most reasonable explanation involves dissociation of a ring, threading another ring on the thread, and reconnection of the ends of the thread (Scheme 2 a). However, careful mechanistic consideration of the rapid interconversion suggested a Mobius strip mechanism which involves molecular topology reminiscent of the well-known Mobius strip. 

The ring topology is the potential to form unique polymer structures. Like linear polymers, cyclic polymers not only can be branched or cross-linked, but also can form non-covalently linked structures based on their loop topology. These are referred to as topological polymers, including rotaxane, catenane, threaded rings, and rings threaded by network chains. Recently, much attention has been paid to how their particular properties not only differ from linear polymers, but also how they differ from a component of an interlocked polymer system, such as polycatenanes and polyrotaxanes. 

The simplest rotaxane morphology, as defined in the Introduction and represented in Figure 2.1, is noted as [2]-rotaxane to express that it is made up with two components a ring and a dumbbell.13 More generally, an [n]-rotaxane contains n — 1 rings threaded onto a dumbbell component. Figure 2.4 shows several relatively simple rotaxane morphologies that have been realized. 

Table 9 Control of the orientations of a-CD rings threaded onto asymmetric bola amphiphiles...

The overwhelming interest on the developments of CD inclusion polymers arose since the discovery of a-CD inclusion complexes with PEO of different molecular weights [16] and the subsequent synthesis of the polyrotaxanes composed of multiple a-CD rings threaded and entrapped on a polymer chain (Fig. 2) [17,18], So far, a large number of reports have been published on the CD-based polypseudorotaxanes and polyrotaxanes [12,18 19] and their applications in biomaterials [50-58],... 

Fig. 2 The synthesis of polyrotaxane composed of multiple a-CD rings threaded and entrapped on a PEO chain [17]...

Recently a polyrotaxane composed of multiple methylated a-CD rings threaded on a high molecular weight PEO chain and end-capped by bulky adamantyl groups was synthesized (Fig. 17) and investigated in terms of its thermo-reversible sol-gel transition and hydrogel formation [87],... 

A real rotaxane has recently been described which consists of a porphyrin-appended ring threaded by a string whose two ends are attached to a porphyrin. In this way, if the porphyrins are of different types (donor D and acceptor A), both are maintained in the same molecule by mechanical (or topological, for a catenane) bonds only. If... 

Early in 1994, Kaifer, Stoddart, and coworkers reported the first bistable molecular shuttle (Figure 70) based on a [2]rotaxane driven by chemical and electrochemical stimuli. The [2]rotaxane comprised a CBPQ U+ ring threaded by an axle consisting of two binding sites, benzidine and biphenol units. Under redox reactions and the addition of acid or base, the CBPQT + ring moves back and forth along the axle. This molecnlar shuttle can be switched by two different mechanisms and is a good candidate for the construction of complex molecular machines. 

The hypothesis is also supported by the number of a-CD rings threaded on a PEO chain and capped by bulky groups at the two ends [44], In the case of PEO-PHB-PEO block copolymer (PEO Mn = 5,000), the PEO segments covered by a-CD rings should be shorter than 2,000 from each end because of the steric hindrance effect, and the partial inclusion complexation of a-CD with PEO block with hydrophobic interaction of PHB blocks resulted in a strong network and a novel supramolecular hydrogel (Fig. 7c) [17]. 

To conclude, the nomenclature of macromolecular topological compounds should be briefly mentioned. In fact, it does not exist. The use of the Schill s nomenclature is difficult, since the name of polymeric catenanes and rotaxanes, where the chain consists of many interlocked rings or rings threaded on a long macromolecule, becomes cumbersome and hard to nronounce. Therefore, trivial names are often used. 

RCM has allowed the synthesis of a eatenane consisting of a large ring threaded through both eyelic units as in a handcuff-like structure. The synthesis is based on the model shown in Scheme 20 [50]. 

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