Driving Forces for Threading

A strong attractive force between the cyclic and the linear species is necessary to achieve high yield syntheses of rotaxanes and high min values for polyrotaxanes [6-8,12]. Different types of driving forces have been explored. Because the results from rotaxanes often provide the basis for polyrotaxanes, this section will briefly summarize various driving forces used in rotaxane syntheses so that we can understand polyrotaxane systems more completely. For specific details regarding rotaxane syntheses, interested readers are referred to those publications cited here and in other reviews [6-9]. 

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On the basis of results from polyrotaxanes 63, 64, and 65, the detailed threading and dethieading mechanisms were inferred [12, 17, 20, 22, 23, 114]. To shorten this discussion, polyrotaxane 63 of Type 6 will be used as an example (Figure 11). The initial driving force for threading is the hydrogen bonding between... 

They found that some cyclophanes 82 exchanged rapidly between the polymer chain ends and solution relative to the NMR time scale. However, some of the cyclics 82 were trapped in the middle of polymer chain on the NMR time scale and were thus excluded from the exchange process. These two papers demonstrated that this type of self-assembly provides a very strong driving force for threading. 

The driving force for building these multi-component structures is the formation of a bis-chelate complex. In fact, Cu(I) and 2,9-diphenyl-l,10-phenanthroline (dpp) derivatives form very stable bis-ligand pseudo-tetrah al complexes whereas monochelates of the type CuCdpp)" " are much less stable.[4] This has been applied in the simplest case ((a) of Figure 1) with unambiguous and quantitative formation of the threaded product.[2] If the string contains several coordination sites, the situation may become more complex and less predictable. 

Fig. 6.12. Normalized driving force on threading dislocation versus mismatch strain Cm and normalized film thickness h /b for the cubic material system represented in Figure 6.6. The surface has been truncated at G = 0 so that negative values are not shown. The kink in the surface, at the boundary between the flat and rising portions, is identical to the critical thickness condition shown in Figure 6.7 for To = 56. Adapted from Freund (1990).

Freund, L. B. (1990), The driving force for glide of a threading dislocaiton in a... 

A supramolecular C o dimer was built by the recognition between two independent fnllerene derivatives, one functionalized with an ammonium salt and the other with a crown ether. The contribution of N+-H O and C-H " O hydrogen bonds and ion-dipole interaction is the driving force for this assembled supramolecular system, where the Cgo-dibenzylammonium adduct was threaded through the cavity of a crown ether Cgo derivative (Figure 2.13). 

Figure 31. PRISM plus linearized R-MPY closure predictions" for the normalized inverse peak scattering intensity of the/ = 4 symmetric Gaussian thread diblock copolymer model. Results (top to bottom) for iV = 20, 200, 2000, and 20,000 are shown at fixed melt density and a Yukawa tail potential range parameter of a = 0.5, Here, the bare driving force for microphase separation, 1". varied by changing temperature. The inset shows the apparent exponent that describes the scaling relation between peak scattering intensity and /V as a function of inverse temperature (as extracted from the three largest chain lengths).

One of driving forces for the processes (a) and (b) is hydrogen bonding between CD molecules [12]. The threading process (a) and the final process (d) cause an increase in turbidity although the other processes are assumed not to increase the absorbance. 

Jorge Ibanez first conceived the idea for this book. Zvi Szafran (New England College, USA) induced us into making this a full textbook and not simply a laboratory manual. Margarita Hernandez was the architect and Jorge Ibanez the main driving force behind the project—they weaved the threads from the different chapters into an orderly whole. In addition, Carmen Doria endowed this book with her expertise... 

A classic example of the formation of a macrocycle by a neutral template is that of the versatile host compound and component of molecular machines, the so-called blue box, or cyclobis paraquat-para-phenylene. Reaction of the horseshoe precursor with dibromo-para-xylene leads to the formation of a tricationic intermediate that is capable of binding the template molecule (Scheme 3), which closes the macrocycle to form the tetracationic cyclophane. The jT-ir interactions of the charge-transfer variety (the complex of the product and template is colored, whereas the components are not) assisted by the charge on ihe product are a major driving force in the process, as revealed in X-ray structures of complexes. It should be noted that the interaction is of the jr-n type assisted by the complementary positive charge on the bipyridinium residues and r-electron-rich nature of the template. This supramolecu-lar synthon can be used for other cyclophanes, catenanes, and rotaxanes (see Self-Assembly of Macromolecular Threaded Systems, Self-Assembled Links Catenanes, and Rotaxanes—Self-Assembled Links, Self-Processes). 

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