Daisy-chain polymers

Figure 20.5 ADMET synthesis of daisy-chain polymers. (Reprinted with permission from Ref [48].)...

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [32,60-68]. Cyclic tri[2]rotaxanes (daisy chain necklace) containing cyclodextrins have been prepared from the mixture of 6-(4-aminocinnamoyl)-Q -CD and 2,4,6-trinitrobenzene sulfonic acid sodium salt [50,59] in an agueous solution (Fig. 21). If the molecule changes its conformation (or co-conformation), the ring may expand or shrink by external conditions (temperature, solvents, photochemically, elec-trochemically). These compounds are important because the cycle can be used as a chemical valve as seen in ion channels in biological membranes. 

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [45-53]. Harada et al. [31] treated 6-p-aminoCiO-a-CD (40 mM) with 2M excess 2,4,6-trinitrobenzenesulfonic acid sodium salt (TNBS) as bulky stoppers in aqueous solutions. The resulting precipitate was found to be mainly a cyclic trimer by H NMR and TOF mass spectra. After purification of the crude product, the 2D ROESY spectrum of the cyclic trimer shows cross-peaks between phenyl protons close to an amino group and secondary hydroxyl groups (0(2)H). A trinitrophenyl group is found at the secondary hydroxyl group side. A proposed structure of a cyclic trimer (cyclic daisy chain) is shown in Fig. 3.12. Kaneda et al. [38] reported the preparation of cyclic di[2]rota-xane fashion constructed tail-to-tail by azobenzene derivatives of permethylated a-CDs and showed its computer-generated supramolecular structures (Fig. 3.13). Easton et al. [39] also reported the preparation of cyclic di[2]rotaxane constructed by stilbene-appended a-CDs in tail-to-tail fashion (Fig. 3.14). Kaneda et al. [40]... 

An atypical rotaxane structure is that involving a thread covalently attached to the CyD ring. Such a system acts as both host and guest and it has to be specially designed so that the thread is induded spontaneously in a cavity other than the one bearing it intermolecular inclusion), whereas intramolecular indusion is prevented. The resulting assembly, named a daisy chain by J.F. Stoddart, is either an acyclic supramolecular polymer 8 or a cyclic supramolecular oligomer 9 [50] (Fig. 12.4). Acyclic cyclodextrin polymers were first observed in the solid state in... 

Fig. 12.4. Daisy-chain supramolecular polymer 8 and cyclic supramolecular dimer 9.

A [c2] daisy chain dimer polymer with extension and contraction properties was reported by Grubbs et al. via ring-closing olefin metathesis. This polymer is similar to natural fibers because it can experience both extension and contraction states. It is worthy of mention that the radius of the polymer was observed to increase by 48% after expansion by adding a base. These good mechanical properties make it an exceptional precursor for functional materials. 

Mechanically interlocked polymers ( daisy chains ) have attracted considerable interest, not just for their unique material properties but also for the synthetic challenge involve din their creation. Grubbs and coworkers [188] reportedthe synthesis of a bis-functionalized [c2] daisy-chain monomer, which was subsequently polymerized by ADMET. 

Thus, the formation of a cylinder-like supramolecular dimer is the key step in the construction of some micellar and vesicular assemblies. Several methods can be employed to make building blocks having cylinder-shaped structures. For example, cylinder-like supramolecular dimers can be initially formed from mono-functionalized pillararenes through supramolecular interactions such as ti-ti stacking. Functional groups can be used to modify pillararenes, and the pillararene derivatives obtained may form daisy-chain type dimers through the complexation. However, such dimers prefer to form further supramolecular polymers rather than spherical assemblies at certain concentrations. Therefore, it should be noted that the functional groups... 

Guidry, E.N., Li, J., Stoddart, J.F., and Grubbs, R.H. (2007) Bifunctional [C2]daisy-chains and their incorporation into mechanically interlocked polymers. Journal if the American Chemical Society, 129,8944—8945. 

---