Rotaxanes polyrotaxanes

In this review, I describe our efforts to construct interlocked structures such as rotaxanes, polyrotaxanes and molecular necklaces incorporating cucurbituril as a molecular bead by utilizing the principles of self-assembly and coordination chemistry. A key to the success of this synthesis is the high affinity of cucurbituril toward alkyl diammonium ions, which allows formation of a stable pseudorotaxane... 

Later, gel formation was observed by Garcia et al. during the preparation of polyamides containing the benzo-18-crown-6 unit, and was ascribed to the formation of rotaxane/polyrotaxane and catenane/ polycatenane structures [156]. However, as the ring size of the benzo-18-crown-6 was considered too small to be threaded, it was thought uidikely that gel formation would occur due to catenane formation via threading of the crown ether moieties. 

Scheme 1. Schematic representations of a [2]rotaxane, a [2]pseudorotaxane, and one-, two-and three-dimensional polyrotaxanes...

Further studies by the same authors have led to the formation of [2]rotaxanes, [3]rotaxanes and pseudo-polyrotaxanes [85-87]. In all these interlocked species, in spite of the presence of aromatic rings in the axle and wheel, tt-ti interactions do not seem to play a role in the templating process. This highlights once again the importance of C-H---0 hydrogen bonding in the assembly of interlocked species. 

Polymers are generally not discussed in this book. However, a polymer with rotaxane structures involving cyclodextrins in side chains 408 [36], pseudorotaxane superstructures [37], doubly twisted polyrotaxane [38] as well as infinite polyrotaxane network (Figure 8.2.5) [39] can be mentioned here. 

Different types of polyrotaxanes, depending on how the cyclic and the linear units are connected, have been conceived [6-8, 12], According to the location of the rotaxane unit, polyrotaxanes can be defined as main-chain systems, Types 4, 5, 6, 7, and 8 (rows one and two in Table 1), and side-chain systems, Types 9, 10, 11, and 12 (rows three and four in Table 1). In main-chain polyrotaxanes the rotaxane unit is part of the main chain. In side-chain polyrotaxanes, the rotaxane moiety is located in the side chain as a pendant group. Polyrotaxanes can also be classified as polypseudorotaxanes and true polyrotaxanes, depending on their thermal stability toward dethreading. Polypseudorotaxanes are those without BG (column one in Table 1), in which the rotaxane components can be disassociated from each other by external forces. True polyrotaxanes are those with BG at the chain ends or as in-chain units (column two in Table 1), in which the rotaxane units are thermally stable unless one or more covalent bonds is/are broken. 

Therefore, polyrotaxanes can be simply defined as polymeric materials containing rotaxane units. They are different from conventional linear homopolymers because they always consist of two components, a cyclic species mechanically attached to a linear species. They also differ from polymer blends as the individual species are interlocked together and from block copolymers since the two components are noncovalendy connected. Thus new phase behavior, mechanical properties, molecular shapes and sizes, and different solution properties are expected for polyrotaxanes. Their ultimate properties depend on the chemical compositions of the two components, their interaction and compatibility. This review is designed to summarize the syntheses of these novel polymers and their properties. 

In addition to the types of structure and chemical compositions, the properties of a polyrotaxane are determined by the amount of cyclic incorporated. To define such quantities, the min value was introduced [7, 12], Min, file threading efficiency, was defined for systems of Types 4-6, 9, and 10 as the average number of cyclic molecules per repeat unit [7, 12]. However, this definition seems a little awkward for polyrotaxanes of Types 7, 8, 11, and 12, because in these polyrotaxanes the linear component penetrates through the polymeric cyclic instead of the cyclic being threaded on to the linear species. To fit all the types in Table 1, we redefine min as the proportion of rotaxane repeat units in the polymer. 

A strong attractive force between the cyclic and the linear species is necessary to achieve high yield syntheses of rotaxanes and high min values for polyrotaxanes [6-8,12]. Different types of driving forces have been explored. Because the results from rotaxanes often provide the basis for polyrotaxanes, this section will briefly summarize various driving forces used in rotaxane syntheses so that we can understand polyrotaxane systems more completely. For specific details regarding rotaxane syntheses, interested readers are referred to those publications cited here and in other reviews [6-9]. 

Similar to those for rotaxanes, different approaches have been employed for poly-rotaxane syntheses these will be summarized in the next section. The most important parameter in polyrotaxanes, the min value, is often employed as a measure of the effectiveness of the preparation method. Because this value mainly depends on the strength of the attractive force between the cyclic and the backbone, this section is again divided into subsections according to the types of driving forces rather than the types of polyrotaxanes. 

The same types of polyrotaxanes were also prepared by a different method, Method 2 (Figure 6). In this method, a preformed polymer is used and the cyclic is threaded onto the polymer in a melt or in solution. A solution of 28 and polystyrene in THF under reflux afforded a polyrotaxane with an min value of 5.0X1CT4, much lower than those via Method 1 [69]. Threading 28 on to poly (butylene sebacate) afforded poly(ester rotaxane) 33 of Type 4 [70]. Although a laige excess of cyclic was used, 33 only had a min value of 0.0017. This value is 100-fold lower than that for the corresponding polyrotaxane prepared by Method 1 [19]. A possible reason is that the concentration of chain ends is very low and the random coiled-chain conformation of a polymer disfavors threading. 

In 1979, Maciejewski et al. also explored Method 3 for the preparation of main-chain poly(vinylidene chloride-/ -CD rotaxane) 35 [74, 75]. Radiation polymerization and AIBN-initiated solution polymerization of the complex of vinyli-dene chloride with 21 gave products with min = 0.34 and 0.49, respectively. However, the polyrotaxane via Method 1 had a much lower min (0.087) with much lower CD/monomer feed ratio than for those via the polymerization of the complex (1 1 ratio). Therefore, the reported min values are not comparable, so the difference between the two methods in terms of threading efficiencies cannot be distinguished. Although the authors did not see any threading via Method 2 for the same polyrotaxanes, Ogino and coworkers prepared a true CD-based polyrotaxane of Type 5 using metal complexes as stoppers [76]. It was also found that... 

More recently, Harada et al. applied the complexation process to side-chain systems via Method 6 (Figure 10), in which the guest sites were introduced as pendant groups and thereafter the CD was threaded onto them [104, 105]. Different types of hydrocarbon chain as pendant groups were studied for their compatibility with different CDs. As the cyclic was not blocked, the products can be viewed as side-chain poly(pseudo rotaxane)s of Type 9. Probably because of the rapid exchange process between threaded and unthreaded forms, the isolation of the solid-state polyrotaxane was not reported. 

Incorporating different amounts of 1,10-decanediol into the above system, copoly (ester rotaxane)s 64 were also obtained [22]. The m/n values for these polyrotaxanes were related to the amount of BG applied the m/n value linearly in-... 

Similar to that in copoly(ester rotaxane)s 64, min for these poly(urethane rotax-ane)s increased with increasing BG, i.e. higher x values [116,117]. However, compared with the copoly(ester rotaxane), the dethreading occurred to lesser extent in these polyrotaxanes this is attributed to the fact that the NH groups retard dethreading by hydrogen bonding with the threaded crown ether as in structure 67. A linear relationship between min values and x was revealed. 

Poly(ether sulfone) and poly(ether ketone) rotaxanes 77, 78, 79, and 80 were reported by Xie and Gong via solution polymerization in a mixture of toluene and DMF in the presence of metal ions (K+ or Na+) and 30C10 [114, 123]. The min values depended on the reaction conditions and the amount of BG applied [114, 123]. Polyrotaxanes 77 and 78 were difficult to purify because these polymers formed emulsions in water or methanol. Because of different preparation conditions between those with or without BG, the absolute m/n values are not comparable and thus the effect of the BG on threading remains unknown. However, considering that a polar solvent, i.e., DMF, was used for polymerization, these m/n values are still significant. 

Whereas the structures of low molar-mass rotaxanes can be directly proved by NMR chemical shift changes, mass spectrometry, and X-ray crystallography [6-8], the formation of polyrotaxanes is much more difficult to prove because of their highly complicated structures. 

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