Supramolecular Polymers in Solution

Ringsdorf and co-workers have shown that triphenylenes can form alternating donor—acceptor supramolecular polymers in solution by doping them with equimolar amounts of electron acceptors.128129 Supramolecular polymers formed in this manner allow for electron transfer perpendicular to the molecular planes upon excitation of the chromophores, i.e., unidirectional charge-transport through the column. 130 The formation of these donor—acceptor pairs is favored in apolar solvents. In more polar solvents the triphenylenes alone do not polymerize and consequently donor—acceptor polymers with low DP are formed. 

Many reports on the formation of supramolecular polymer in solution have been mentioned in this chapter. In particular, the interest in cydodextrin-based supra-... 

In view of the extensive literature, this chapter does not aim to be comprehensive, but uses selected notable examples from the field to demonstrate the concepts and applications of these fascinating species. For example, there are numerous examples of linear supramolecular polymers in solution [32], solvated nanotubes [33], vesicles [34] and gel networks [35-38] that rely on donor-acceptor interactions however, this review focuses on unsolvated, cross-linked supramolecular polymers and their bulk properties. 

Calix[5]arene-based supramolecular polymers have also been developed. Paris and Cohen et al. used a heteroditopic calix[5]arene bearing an amino group on the lower rim. -phe amino group was protonated readily by contact with acidic solution. The ammonium terminus was encapsulated iteratively within the calix[5]arene cavity to produce the calix[5]arene-based supramolecular polymers. DLS experiments supported the formation of sizable supramolecular polymers in solution. SEM provided clear evidence for the formation of fibrillar networks of the supramolecular polymers. 

The book is divided into three sections. The first section provides a general overview of the fundamentals of supramolecular polymers. In Chapter 1, Thibault and Rotello provide a brief introduction to these systems and in Chapter 2, Azagarsamy, Krishnamoorthy, and Tha5mmanavan describe the rapidly emerging area of amphiphilicity in polymer and dendrimers self-assembly. Interactions at interfaces are sometimes similar but often quite different than those in solution, a topic covered by Loveless, Kersey, and Craig in Chapter 3. 

In addition to spectroscopic techniques used to identify supramolecular behaviour in solution it is possible to observe molecular aggregation in solution and undertake analyses to give the mass distribution of the species present. Although not widely used in supramolecular chemistry, the technique of osmometry allows researchers to determine the masses of assemblies in solution and relate those to the masses of the single molecules to give aggregation numbers. It is of particular value in polymer science where it is used to determine the average mass, and mass distribution, of... 

Fig. 10 Transition temperature (T ) between the thin filaments and tubes for supramolecular polymer 3 solutions in aromatic solvents, versus length (L) and width (W) of the solvent molecules. The length (L), width (W) and thickness (Th) are defined as the respective dimensions of the smallest right-angled parallelepiped containing the molecule, such that L>W>Th. Reprinted with permission from [42]...

Triphenylenes provided with nonionic di(ethylene oxide) side-chains (25f)132 134 or with ionic alkyl chains (25g)135 form supramolecular polymers in water.136 The arene—arene interactions of the aromatic cores allow for the formation of columnar micelles . At low concentrations the columns are relatively short, and the solutions are isotropic. At higher concentrations the longer columns interact and lyotropic mesophases are formed.133 Computer simulations showed that in the isotropic solution the polymerization of the discotics is driven by solute-solute attraction and follows the theory of isodesmic linear aggregation the association constants for dimerization, trimerization, and etc., are equal and the DP of the column thus can easily be tuned by concentration and temperature.137 138 At higher concentrations the sizes of the columns are influenced by their neighbors, the columns align, and the DP rises rapidly. 

The molecular weight reached saturation at about 18000 Da. 3-CiNH-a-CD was found to form supramolecular polymers in aqueous solutions (Fig. 3.19). 

As mentioned above, 3-p- BocCiNH-a-CD forms helical supramolecular polymers in aqueous solutions. In contrast, 6-p- BocCiNH-) -CD was synthesized and found to form an intramolecular complex using NMR measurements. When adamanta-necarboxylic acid (1-AdCA) was added as a competitive guest to an aqueous solution of 6-p- BocCiNH- -CD, NMR spectra showed that the Boc-cinnamamide part in its own fi-CD cavity was expelled into water. The 6-p- BocCiNH- -CD underwent a conformational change to form an inclusion complex with AdCx. Moreover, with an addition of an excess amount of a-CD, the cinnamoyl part was found to be included in a-CD to form a hetero dimer (Fig. 3.27). 

We further characterized this copolymer mixture by using an Ubbelohde viscometer to measure the solution viscosity of a mixture of D20-PMMA and T12-PMMA in tetrahydrofuran (THF). The formation of supramolecular polymers in the D20-PMMA/T12-PMMA blends provided a higher solution viscosity relative to that of pure PMMA as shown in Figure 2.15 in addition, the viscosity increased upon increasing the concentrations of the copolymers. This supramolecular polymer also could be observed macroscopically—in the form of a gel—from a 1 1 mixture of D20-PMMA/T12-PMMA in THF at a concentration of 30 g/dL at the same concentration, a solution of PMMA, which lacked any specific interpolymer hydrogen-bonding interactions, flowed freely (see the inset to Figure 2.15). 

Figure 9 Chemical structures of Cs-symmetrical bipyridine-based discotics and schematic representation of their propeller-like conformation assembling into linear supramolecular polymers in dilute isotropic solution. Reproduced from Palmans, A. R. A. Vekemans, J. A. J. M. Havinga, E. E. etal. Angew. Chem. Int. Ed 1997, 36, 2648-2651. ...

It was not until 1997, when the UPy unit was discovered, that it became possible to assemble supramolecular polymers in dilute solution This work highlighted... 

D. Liquid Crystals from Cyclic, Hydrogen-Bonded Aggregates Hydrogen-Bonded, Supramolecular Polymers in Isotropic Solution... 

By using the click reaction, Stoddart and co-workers connected a viologen unit to the pillar[5]arene moiety to obtain a mono-functionalized pillar[5]arene (7.13 Scheme 7.1). As the concentration increased in the range of 0.1-100 mM in dichloromethane, the assemblies of 7.13 changed from self-inclusion complexes to linear supramolecular polymers. In addition, gels could be formed after a sealed solution of 7.13 at concentrations above 25 mM in dichloromethane had been left to stand for 12 h. 

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