Main-chain polyrotaxanes

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. 

Figure 15. Preparation of branched or cross-linked polymer via the formation of main-chain polyrotaxanes 73.

A rotaxane composed of many ring parts is referred to as a poly[ ]rotaxane [6] these, in turn, include mostly main-chain and side-chain polyrotaxanes (Figure 36.1). In main-chain polyrotaxanes, the ring components are threaded along a polymeric axle, to form a polymer of [2]rotaxanes. A variety of polymers can be used as the long axle molecules, ranging from polymethylene chains to aromatic polymers. Among the different types of side-chain polyrotaxane can be included pseudo-polyrotaxanes and poly(2]rotaxanes. 

Figure 36.1 Main-chain polyrotaxane and side-chain polyrotaxane. Yellow = ring blue = axle red = stopper.

Many other main-chain polyrotaxanes have been reported, and polymers with grafted polyrotaxanes have been also prepared. 

A variety of polyrotaxane structures can be envisioned, as shown in Scheme 1 [1-3]. True rotaxanes polymers possess bulky groups at the ends of the linear species or along the backbone to prevent diffusive loss of the ring component. However, it has been shown that main chain polypseudo-rotaxanes dethread extremely slowly due to random coiling of the backbones [4,5], so that the distinction between pseu-dorotaxane and rotaxane polymers of the main chain type is somewhat blurred for these systems. Main chain polyrotaxanes are the most common type, although all of the other types shown in Fig. 43.1 have been synthesized. 

A main chain polyrotaxane, a mechanically interlocked structure that can be considered as a string of pearls, in which the strand is the polymer backbone and the pearls are the cychc species threaded onto the strand. The backbone polymer can be a polyester, polyamide, poly(ethylene oxide), or virtually any linear polymer. The cyclic species are typically crown ethers, cyclodextrins (CDs), cucurbimrils, and calix-arenes. Polyrotaxanes can be made by two different methods statistical threading or via the template approach (enthalpi-cally driven). The statistical approach utilizes le Chatelier s principle with an excess macrocycle during the polymerization. The template approach is driven by attractive interactions of the macrocyclic species with either the monomer... 

A different strategy to the synthesis of main-chain polyrotaxanes has recently been reported. Here the macrocyclic component for the polyrotaxane acts as a catalyst for the polymerization reaction. Cucurbituril is known to catalyze the 1,3-dipolar cycloadditions within its cavity. Therefore, this macrocycle was added to a reaction mixture containing diazide and dialkyne monomer units, which both have stopper units as the core of the monomer. As a consequence, the reaction between the alkyne and azide groups that occur within the macrocyclic cavity will also necessarily result in the formation of the rotaxane. This reaction is accelerated by a factor of 10 when bound within the cucurbituril cavity and therefore each new repeat unit will also contain exactly one macrocycle. GPC of the resulting polyrotaxane found that = 5.1 x 10, Mw = 9 x 10 with H-NMR estimating to be 13 x 10. ... 

One conceptually easy way to access polyrotaxane structures is to prepare a rotaxane monomer which has reactive groups on (or as) its stoppers and/or on its macrocyclic component and then subsequently carry out a conventional polymerization reaction. This potentially opens up access to a wide range of interlocked polymeric architectures from simple main-chain polyrotaxanes to molecular necklaces, the daisy-chain systems and even dendritic interlocked species. 

Scheme 14,4 Synthesis of [3]rotaxanes 6 and S, and of a perfect main-chain polyrotaxane...

Gong, C., Ji, Q., Subramaniam, C., and Gibson, H.W. (1998) Main-chain polyrotaxanes by threading crown ethers onto a preformed polyurethane preparation and properties. Macromolecules, 31,1814-1818. 

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... 

Fig. 13 Various types of main chain and side chain polyrotaxanes blocking group (shaded circle) hollow circle (cyclic component) ellipses (cyclics threaded by linear species) [from Gong and Gibson (reproduced with permission from ref. 58)].

Synthesis of Polyrotaxanes from Main Chain Pseudopoly rotaxanes... 

Scheme 1 illustrates the simplest structures of rotaxane, catenane, and knot besides polyrotaxane and polycatenane. From the fact that the main chain-type polyrotaxane at the left side is the only interlocked polymer synthesized so far among the three polymers shown at the bottom of the scheme, progress in synthesis of interlocked polymers appears to be sluggish judging from the level of activity in synthetic polymer chemistry in the world. 

More detailed general structures of the representative polyrotaxanes and polycatenanes are shown in Scheme 2. Polyrotaxanes can be categorized into two types one is the polyrotaxanes consisting of the main chains of covalent type as shown in the top four examples (A-D), while the other involves the polyrotaxanes of which monomer linking units are constructed by the rotaxane structure as shown in the following two structures (E, F). The essen-... 

Main Chain-Type Polyrotaxane, B Poly[2]rotaxane, E... 

Main Chain-Type Polyrotaxanes Bearing Crown Ethers as The Wheel Components... 

Main-chain Type Polyrotaxanes Having Cydodextrins as The wheel Components... 

Main-chain-type polytotaxane Side-chain-type polyrotaxane... 

E and F). The essential difference in the main-chain structure between the two types of polyrotaxanes causes large differences in their physical or mechanical properties. Polyrotaxanes A-D are further divided into main- (A and B) and side-chain type (C and D). Meanwhile, the synthesis of poly[2]rotaxane (E) and poly[3]rotaxane (F) has been very recently achieved. Genuine poiyrotaxane seems to be one of the polyrotaxanes like the latter topological polyrotaxanes, which may reflect their truly unique structures to their properties. 

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