Polyrotaxanes poly

Fujita, H Ooya, T. and Yui, N. (1999) Thermally induced localization of cydodextrins in a polyrotaxane consisting of (3-cydodextrins and poly (ethylene glycol)-poly(propylene glycol) triblock copolymer. Macromolecules, 32, 2534-2541. 

The interest in macromolecular systems containing defined topological bonds, such as polyrotaxanes 7, multicatenanes 8, polycatenanes 9, poly[2]catenanes 10, and polymeric catenanes 11, is dual. First, these macromolecules represent daunting synthetic and characterization challenges which deserve attention in their own... 

Scheme 3. Pictorial representation of polyrotaxane 7, multicatenane 8, polycate-nane 9, poly[2]catenane 10, and polymeric catenane 11.

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... 

Wenz and Keller reported a-CD-based polyrotaxanes 50 and 51 with poly(iminoundecamethylene) and poly(iminotrimethylene iminodecamethylene) as backbones, respectively. These polyrotaxanes were prepared by mixing an acidic solution of the corresponding polymer with CD (Method 2, Figure 6) [88]. Polyrotaxane 50 was also transformed into 52 by attaching BG onto the NH sites. The threading process was monitored by proton NMR spectra and viscosity changes. The purified product had min values from 0.10 to 0.67, depending on the back-... 

By careful design, two CDs were also incorporated per pendant group (57) with poly(methyl methacrylate) backbones [102]. In polyrotaxane 58, pendant branched chains were threaded with CDs (one per branch) [97,101]. The chemical composition of the branched arm was very similar to that in 56. 

A side-chain polyrotaxane of Type 10 was also obtained by the reaction of de-protonated poly(benzimidazole) with a long-chain bromide bearing a BG at one end in the presence of / -CD [103], Because the CD was threaded onto the side chain before its reaction with backbone to form a hemirotaxane, the preparation is essentially Method 4. 

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. 

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. 

Gibson and coworkers utilized the expected complexation between crown ethers and acrylonitrile for the preparation of poly(acrylonitrile-crown ether rotax-ane)s 94 [137]. Relative to that with the polystyrene backbone, the enhanced threading supported the intermediacy of the expected complex. The reaction intermediates, the cations 95 and 96 in the preparation of poly(phenylene vinylene) (PPV) also provided a source for interaction with crown ethers [70], The solution polymerization of precursor 95 in the presence of crown ethers followed by transformation of 96 produced polyrotaxanes 97. 

CD are very rigid molecules and polyrotaxanes derived from them are expected to be more rigid than the starting backbone. Poly(methyl methacrylate side chain rotaxane) 56 had a Tg 20°C higher than the backbone itself. The same observation was also seen in side chain poly(ether ketone) and poly(ether sulfone) systems [96-102]. 

Paraquat, an ionic molecule, is more rigid than polymeric crown ethers. In addition, die crown ether has to adopt a restricted conformation for complexation to occur. The total increase of the rigidity for derived polyrotaxanes 84 depended on the min value [118, 119]. The higher the value of min, the higher Tg of the poly-rotaxane. Paraquat is a crystalline material but the polyrotaxanes are amorphous. 

The aqueous solubility of CD also enables their potential application as poly-rotaxane-based drug carriers. Yui and coworkers incorporated CD onto PEO chains in polypseudorotaxanes and polyrotaxanes [92-94], The releasing kinetics of CD from the polymer chain were studied. The release was governed by the inclusion complexation equilibrium. Biodegradation to cleave the BG units was shown to cause the release of the CD from the polyrotaxanes. 

Gibson and coworkers also found that the melt viscosity of a polymer is also altered by the formation of a polyrotaxane [19], Poly(ester rotaxane) 60 containing 42C14 as the cyclic component had a melt viscosity equivalent to that of the parent polyester with 2.5-fold higher molecular weight. This result clearly indicates that there is less chain entanglement in the polyrotaxane than in the backbone polymer by itself. 

Wilkes and coworkers studied polyrotaxanes derived from self-assembly of a polyurethane bearing paraquat moieties and BPP34C10 [130b]. The polyurethanes contained soft (poly(tetramethylene oxide)) and hard (paraquat ionene) segments. Interestingly, dynamic mechanical analysis indicated that polyrotaxanes had higher rubbery plateau moduli than the corresponding backbones. Thermal analysis revealed that the stability was enhanced by the formation of the polyrotaxanes. 

Beckham and coworkers studied the dynamic mechanical properties of poly(urethane-crown ether rotaxane)s [138]. No difference was observed between the backbone and polyrotaxane, probably because of the low min value (0.02). However, 13C solid-state NMR detected die presence of the crown ether as a mobile structure at room temperature. The same observation was seen in polyrotaxanes with ether sulfone and ether ketone backbones (77-80) [114]. Although no detailed properties were reported, the detection of the liquid-like crown ether provided very important information in terms of mechanical properties, because these properties are the result of molecular response to external forces. For example, mobile crown ethers can play the role of plasticizers and thus improve impact strength. 

Yamaguchi, I., Osakada, K., Yamamoto, T., Polyrotaxane containing a blocking group in every structural unit of the polymer chain. Direct synthesis of poly(alkylenebenzimidazole) rotaxane from Ru complex-catalyzed reaction of 1,12-dodecanediol and 3,3-diaminobenzidine in the presence of cyclodextrin. J. Am. Chem. Soc. 1996, 118, 1811-1812. 

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