Polyrotaxane crown ether

Gibson et al. [109] and Sjen et al. [110] reported pseudo-polyrotaxanes and polyrotaxanes consisting of crown ethers with various polymers. The resulting polyrotaxanes were nonstoichiometric. Their properties - including solubility and glass transition temperatures - were different from those of the starting polymers. 

On the other hand, to constrain the cyclic, the blocking group (BG) has to be bulkier than the cavity of the cyclic molecule. Harrison found that a trityl group can only block cyclics up to 28-membered whereas the tris(p-t-butylphenyl)-methyl moiety can effectively constrain 42-membered cyclics [3, 13, 14]. These results have been widely applied as a guide in the preparation of polyrotaxanes e.g., monofunctional [18, 19] and difimctional [19—23] BG based on tri- and tetra-arylmethane derivatives were successfully prepared and used as end groups and in-chain units, respectively, to constrain crown ethers in a variety of polyrotaxanes. 

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. 

Swager and coworkers also applied the self-assembly process in side-chain systems [131, 132]. The bisphenylene crown ether was incorporated into a conjugated backbone, polyphenyleneacetylene 87. This polymer complexes with paraquat 88 to give a novel polyrotaxane structure (89). With a polythiophene backbone, a similar polyrotaxane was synthesized by the same approach. 

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. 

In early work no such NMR chemical shift changes relative to those of the parent components were observed for polypseudorotaxanes with aliphatic backbones and aliphatic crown ethers as the cyclic species [108, 109]. Model studies were performed with 18-crown-6 (18C6), which is so small that it cannot be threaded. The recovery of intact 18C6 under conditions identical with those for the syntheses of the polyrotaxanes ruled out the possibility of side reactions. The effective removal of the small crown ether by precipitation into a solvent which was poor for backbone but good for the cyclic demonstrated the effectiveness of the purification procedure. In addition, reaching a constant min value after multiple precipitations and the absence of the peak for free crown ether in GPC traces indicated that the larger crown ethers detected by NMR in the purified polymeric products were indeed threaded rather than simply mixed. 

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. 

Thus different phase behaviors of polyrotaxanes induced different thermal transitions. One-phase or two-phase materials can be obtained simply by proper choice of the components. The easy introduction of highly flexible cyclic components such as crown ethers with low T% surely expands the applications of otherwise brittle polymers into the low temperature range and also improves elasticity. The plasticizing effect of the crown ether is different from that of a normal plasticizer, because the cyclic is permanently connected to the backbone and no migration can occur. 

Therefore, die polarity and solubility of polymer can be modified deliberately by varying the nature of the components. High aqueous solubilities of polyamides and polyurethane threaded with crown ethers or CD are intriguing, because this observation implies potential applications of the polyrotaxane concept in coatings, adhesives, and water-borne processing. The observation of the emulsification of... 

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. 

Most of the reported polyrotaxanes are based on CD and crown ethers. Only a few polyrotaxanes are from other macrocycles, e.g. phenanthroline-based cyclics and bisparaquat cyclophane. Most CD-based polyrotaxanes were prepared by threading CD on to preformed polymers because CD are only soluble in polar solvents or water and not compatible with typical polymerization conditions. On the other hand, aliphatic crown ethers are soluble in water and most organic solvents. Therefore, they have broadened the scope of polyrotaxanes in terms of both polymerization conditions and types of backbones. They have often been threaded onto polymeric backbones by using them as solvents during polymerizations. 

Polyrotaxane networks 163 have been developed as prototype recyclable cross-linked polymers <2004AGE966>. These systems, which comprise a poly(crown ether)polyurethane and a bisammonium disulfide salt, are cross-linked via mechanical bonds ([3]rotaxane-like substructures) and are capable of undergoing reversible assembly and disassembly as they rely on thiol-disulfide reactions. Macroscopically, cross-linking determines gelation while de-cross-linking enables recovery of the starting materials. 

T. Takata, H. Kawasaki, N. Kihara, Y. Furusho, Synthesis of Side-Chain Polyrotaxane by Radical Polymerizations of Pseudorotaxane Monomers Consisting of Crown Ether Wheel and Acrylate Axle Bearing Bulky End-Cap and Ammonium Group , Macromolecules, 34, 5449 (2001)... 

A fascinating self-assembly process is based on the spontaneous threading of cyclodextrins or crown ethers onto polyether polyamine" or polyurethane chains (Figure 5.30). The formed polyrotaxanes possess several cyclodextrin units which are trapped after appropriate capping of the ends. Polymerization of the threaded cyclodextrins and removal of the central thread then leads to molecular cyclodextrane tubules with an inner hydrophobic tunnel and a diameter of about 5 A. 

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

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