Supramolecular polymers inclusion complexes

Hydrogels formed by PEO block copolymers have previously been proposed as sustained release matrix [85,86], The a-CD-PEO hydrogel delivery system differs in that the gelation relies on the formation of a polymer inclusion complex induced by the PEO-threaded CDs. The properties of the supramolecular hydrogel can be fine-tuned with the composition, molecular weight and chemical structure of the polymer or copolymers. 

Summary The analysis of supramolecular structures containing polymers, and the discussion about the effect of polymeric materials with different chemical structures that form inclusion complexes is extensively studied. The effect of the inclusion complexes at the air-water interface is discussed in terms on the nature of the interaction. The entropic or enthalpic nature of the interaction is analyzed. The description of these inclusion complexes with different cyclodextrines with several polymers is an interesting way to understand some non-covalent interaction in these systems. The discussion about the generation and effect of supramolecular structures on molecular assembly and auto-organization processes is also presented in a single form. The use of block copolymers and dendronized polymers at interfaces is a new aspect to be taken into account from both basic and technological interest. The effect of the chemical structure on the self-assembled systems is discussed. 

The structure of 6-(tert-butylthio)-y0-CD was characterized by Tabushi et al. [21]. The compound of 6-O-(tert-butylthio)-y0-CD was prepared from the reaction of 6-O-(p-toluenesulfonyl)-y0-CD with tert-butylmercaptan and recrystallized in water. This is the first example of the determination of crystal structure of monosubstituted CD derivatives and the first evidence concerning the supramolecular polymer of an inclusion complex of a monosubstituted CD. The crystal structure of 6-O-(tert-butylthio)-y0-CD was arranged around the two-fold axis to yield a polymeric structure, in which the tert-butyl group is intermolecularly included in the cavity of CD (Fig. 3). 

Most of the supramolecular assemblies have been constructed from the solutions of each component. More recently, the authors found that CD formed inclusion complexes with PEG with high selectivity only by mixing powdered crystals of CDs and polymer samples without any solvents under ambient conditions (Scheme 2) [120]. When crystals of a-CD and PEG (Mw = 400) were mixed without solvent (a-CD ethylene glycol unit =1 2), the X-ray powder patterns changed a peak at 26 = 22° characteristic for the cage type decreased, and a peak at 26 = 20° characteristic for the channel type appeared... 

The advances in the studies on the inclusion complexes of CDs threading onto polymer chains have led to interesting development of supramolecular hydrogels with many different molecular and supramolecular structures. Both physical and chemical hydrogels of many different types were developed based on the CD-based polypseudorotaxanes and polyrotaxanes. 

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). 

Inclusion complexation has developed to becoming another widely exploited supramolecular interaction for the formation of supramolecular polymer networks, mostly in water [197, 198]. Several classes of macrocycles have been developed, including crown ethers [199, 200], porphyrins [201, 202], cyclophanes [203], catenanes [204], cavitands [205, 206], cryptophanes [207], calix[n]arenes [208], and carcerands [209]. Macrocyclic-based supramolecular gels can either be formed from low molecular weight precursors or from macromolecular building blocks. The following discussion focuses on the latter. 

Harada and coworkers developed a variety of supramolecular polymers based on the host-guest interaction of cyclodextrin (CD) derivatives with hydrophobic guests, such as the adamantyl and cinnamoyl groups [45,46]. Consequently, various structures were obtained by conjugating cinnamoyl and hydrocinnamoyl groups to a- and/3-CD. When the hydrocinnamoyl group was attached to -CD, the flexibility of the linker resulted in the formation of a self-inclusion complex - in other words. 

Figure J4.12 Supramolecular polymers obtained by Harada and coworkers, (a) Self-inclusion complex (cyclic monomer) formed as a result of the flexibility of the linker between the hydrophobic guest and yS-cyclodextrin (CD) [47] (b) Cyclic dimer and trimer formed when more rigid linkers were applied [48] (c) Supramolecular...

The self-assembly of supramolecular structures such as host-guest inclusion complexes has r ently bem the focus of a number of research efforts. This approach allows the d ign and building of nanoscale molecular device. It constitutes a conveni t route to realizing the polymerization of hydrophobic molecules (guests) such as thiophene derivatives in aqueous soluticm by using CDs (hosts) and to customizing the polymer ardiitectures 9-11). 

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