Cylindrical polymerized assemblies

Figure 7.10 Tapered macromonomers and their cylindrical polymerized assemblies...

Noncovalent self-assembly of complementary pairs of homoditopic building blocks (AA/BB-type), such as bis(crown ether) 125 and diammonium salt 126, has afforded well-defined supramolecular oligomeric/polymeric assemblies <2003JA3522>. In dilute solutions, entropy favored the formation of the cyclic dimer, whereas high equimolar concentration (>0.5 M) of the two components led almost exclusively to linear species aggregation, as revealed by H NMR analysis and viscosity measurements. Closely related linear poly[3]pseudorotaxane supramolecular arrays have also been prepared from cylindrical bis(crown ether) 127 and bisparaquat derivative 128 <2005CC1696>. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

Fig. 6a,b. Height micrographs obtained by tapping mode SFM from monolayers of a spherical b cylindrical molecules adsorbed on mica [84]. Top self-assembly of monodendron side groups into spherical and cylindrical supramolecular dendrimers. The process depends on the degree of polymerization of the polystyrene main chain... 

TMV consists of a cylindrical coat of 2,130 identical protein subunits enclosing a long RNA molecule of 6,400 nucleotides. In 1955, it was shown that the coat protein subunits and the RNA could be dissociated but would, under appropriate conditions, spontaneously self-assemble to reform fully active virus particles. This process is multistage, the critical intermediate being a 34-unit two-layered protein disc which, upon binding the RNA, is converted to a helical structure with 16.33 protein subunits per turn (Fig. 5-2). In the absence of the RNA, the protein may be polymerized into helical tubes of indefinite length. The presence of the RNA aids the polymerization process and results in a virus particle with a fixed length of 300 nm. 

Figure 1 Schematic representation of the structure and polymerization of microtubules. Microtubules are cylindrical polymers (diameter, 24 nm), with a distinct molecular polarity conferred by the orientation of tubulin subunits. They exist in a dynamic equilibrium, and their assembly and disassembly depends on the reversible addition and removal of tubulin, which is a heterodimer made up of an alpha and a beta subunit, at the ends of the microtubules.

In the case of ordered mesoporous oxides, the templating relies on supramolecular arrays micellar systems formed by surfactants or block copolymers. Surfactants consist of a hydrophihc part, for example, ionic, nonionic, zwitterionic or polymeric groups, often called the head, and a hydrophobic part, the tail, for example, alkyl or polymeric chains. This amphiphiUc character enables surfactant molecules to associate in supramolecular micellar arrays. Single amphiphile molecules tend to associate into aggregates in aqueous solution due to hydrophobic effects. Above a given critical concentration of amphiphiles, called the critical micelle concentration (CMC), formation of an assembly, such as a spherical micelle, is favored. These micellar nanometric aggregates may be structured with different shapes (spherical or cylindrical micelles, layered structures, etc. Fig. 9.8 Reference 70). The formation of micelles. 

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