Threading driving forces

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. 

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... 

Examples are the 1, l -dibenzyl-4, 4 -bipyridinium electron-acceptor dication threaded into the 1, 5-dinaphtho-38-crown-10 (Fig. 2a) [10], and the acyclic polyether containing a dioxybenzene electron-donor unit threaded into the electron-acceptor cyclobis(paraquat-p-phenylene) tetracationic cyclophane (Fig. 2b) [11]. Although in these cases a large contribution to the association driving force comes from the electron-donor/acceptor (charge-transfer, CT) interactions, hydrogen bonding can also play an important role, as clearly shown in the cases of pseudorotaxanes constituted by 4, 4 -bipyridinium [12a] or l,2-bis(pyridinium)ethane [12b] threads and crown ethers. 

A thread that runs throughout government reports and the more rational contemporary critiques of safety and society is that the perception people have about safety is critical. Both Lord Young and Professor Lofstedt agree on this, and Lofstedt (2011 87) specifically highlights the media as a driving force in the creation and perpetuation of the common misunderstandings around safety. 

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. 

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). 

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).

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