Supramolecular Polymer Length

The average length n is critically dependent on the [M]/[A] ratio, a maximum average length being expected for a 1 1 ratio. For [M]/[A] 1, isolated A monomers predominate, while for [M]/[A] 1 small complexes of the type MAM predominate. This can be shown by measuring the steady state anisotropy of solutions of A and M in a 3 2 chloroform-methanol mixture as a function of [M]/[A], see Fig. 7.12. 

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In all cases, these aggregates had a uniform diameter of a few nanometers, that is, on the same length scale as the molecular dimensions. Hence, they appeared to be well-defined supramolecular polymers (rather than micellar or vesicular structures) with the propensity to higher-order structure formation. The exact natureofthelatterwasfoundtodependonthenumberofadditionalN—H G=C type hydrogen-bonding sites in the molecules end groups. 

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]...

SANS is a useful technique to derive some information on the structure and the shape of supramolecular polymers [ 189]. In particular, the length of rigid-rod supramolecular polymers can be determined if it is below about 50 nm [93, 166,190]. From the length of the rods, the molar mass can easily be deduced. 

By incorporating such motifs into handcuff -shaped monomers, it should be possible to combine some of the characteristics of supramolecular polymers - self-correcting, thermodynamically-controlled assembly - with the hallmark properties and stability of standard polymers with covalent backbones (Figure 8). Such monomers would, in the presence of a catalyst, spontaneously assemble to form mechanically linked polymers of precise length and architecture defined by the concentration at which they were prepared. Indeed, the various structures (linear, branched, hyperbranched etc.) would be intercon-... 

Fig. 6 Structures of the non-mesogenic bis-pyridyl (2) and bis-benzoic acid (3) monomers studied by Griffin et al. [67] and the proposed structure of the main chain liquid crystalline supramolecular polymer (2-3) highlighting the length of supramolecular mesogen...

Knoben W, Besseling NAM, Bouteiller L, Stuart AC (2005) Dynamics of reversible supramolecular polymers independent determination of the dependence of linear viscoelasticity on concentration and chain length by using chain stoppers. Phys Chem Chem Phys 7 (ll) 2390-2398... 

Chain stoppers, that is, monotopic monomers, have been widely used to reduce the chain length of supramolecular polymers and thus the viscosity of their solutions. More interestingly, chain stoppers can be used to block the concentration dependence of the length of the supramolecular... 

Figure 10 Schematic representation of the influence of concentration on the chain length of a supramolecular polymer without (a) or with (b) a chain stopper at a fixed stopper-to-monomer ratio. (Reproduced from Ref. 26. American Chemical Society, 2005.)...

Figure 10 FE-SEM of the filamentous precipitates of the supramolecular polymer of 14 and 15 formed from methylcy-clohexane (0.3 mM). The length of the bar across the ribbon in (c) is 100 nm. (Reproduced from Ref. 19. American Chemical Society, 2007.)...

All these most interesting features displayed by supramolecular polymers, however, raise important but difficult questions of characterization concerning composition, size, persistence length. 

Many applications of supramolecular polymers are based on the exceptionally strong dependence of their properties on environmental conditions such as temperature and solvent polarity. Because the bonds that keep the unimers of supramolecular polymers together are relatively weak, these parameters affect both the average length of the polymer chain as well as the dynamics of reversible chain breaking/recombination. This results in a viscoelastic response that may be dramatically stronger than in conventional polymers. 

Cgo is extremely hydrophobic thus, some of the initial attempts to capture Cgo molecules were carried out in water. Qo forms a 1 2 complex with a cyclodextrin molecule. Liu et al. synthesized water-soluble double-CD 16 (Figure 7.10). End-to-end intermolecular inclusion complexation of Cgo with double-CD 16 led to the formation of a supramolecular polymer in water. An STM image of the polymer displayed the regular linear arrangement of the Cgo nano-array. TEM observation of the supramolecular polymer confirmed the presence of a linear structure with a length in the range of 150 to 250 nm, suggesting that the polymer consists of 60 to 80 units of the minimum component. 

The polymer-like morphologies were observed by scanning electron microscopy (Figure 7.12a). The thicker entwined fibers had lengths exceeding 100 jm and widths between 250 and 500 nm. The addition of Cgo completely disrupted the network because Cgo molecules competitively occupied the cavities of the hosts, thus interfering with the complexation of 19. Consequently, further growth of the supramolecular polymers was not permitted in the solid state. 

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