Micelles micellar dendrimer

Chemistry of micelles is an important area of dendrimer research. A micellar structure depends on numerous factors, such as temperature, concentration, and mainly the molecular framework of the given amphiphiles. Revolutionary research in micellar chemistry is exhibited in the work of Menger et al. [57] and by Shinkai et al. [58]. 

Recently, several articles have been published dealing with water soluble dendrimers, due to their outstanding structure and properties which are similar to natural micellar systems, e. g., liposomes. Scheme 17 shows such an example of a dendritic micelle containing a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups. The most important difference to natural micelles is that dendritic micelles are covalently bound structures and not dyn-... 

The structure and properties of water soluble dendrimers, such as 46, is, in itself, a very promising area of research due to their similarity with natural micellar systems. As can be seen from the two-dimensional representation of 46 the structure contains a hydrophobic inner core surrounded by a hydrophilic layer of carboxylate groups (Fig. 12). However these dendritic micelles differ from traditional micelles in that they are static, covalently bound structures instead of dynamic associations of individual molecules. A number of studies have exploited this unique feature of dendritic micelles in the design of novel recyclable solubilization and extraction systems that may find great application in the recovery of organic materials from aqueous solutions [84,86-88]. These studies have also shown that dendritic micelles can solubilize hydrophobic molecules in aqueous solution to the same, if not greater, extent than traditional SDS micelles. The advantages of these dendritic micelles are that they do not suffer from a critical micelle concentration and therefore display solvation ability at nanomolar... 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

Poly(3inido amine) (PAMAM) Dendrimers. These dendrimers have a hydro-phihc polyamide interior that is relatively open to solvent, in contrast to a micellar assembly. Yet, the picture of a unimolecular micelle also emerged for a generation 4.5 PAMAM dendrimers having sodium carboxylate terminal groups (Fig. 11.4). 

The process utilizing supramolecular organization involves pore expansion in silicas. A schematic view of such micelles built from the pure surfactant and those involving in addition n-alkane is shown in Figure 4.9. Another example of pore creation provides a cross-linking polymerization of monomers within the surfactant bilayer [30]. As a result vesicle-templated hollow spheres are created. Dendrimers like that shown in Figure 4.10 exhibit some similarity to micellar structures and can host smaller molecules inside themselves [2c]. Divers functionalized dendrimers that are thought to present numerous prospective applications will be presented in Section 7.6. 

For polymer chemists it is interesting to know how well-known linear polymers can be linked with dendritic architectures and what the supramolecular consequences of this approach might be. Combination of dendrimers with linear polymers in hybrid linear-dendritic block copolymers has been employed to achieve particular self-assembly effects. Block copolymers with a linear polyethylene oxide block and dendritic polybenzylether block form large micellar structures in solution that depend on the size (i.e., the generation) of the dendritic block [10]. Amphiphilic block copolymers have been prepared by the combination of a linear, apolar polystyrene chain with a polar, hydrophilic poly(propylene imine) dendrimer [11] as well as PEO with Boc-substituted poly-a, -L-lysine dendrimers, respectively [12]. Such block copolymers form large spherical and cylindrical micelles in solution and have been described as superamphi-philes and hydra-amphiphiles , respectively. 

PAMAM dendrimers, synthesized subsequently, are likely the most widely investigated and used dendritic polymers to date. The first dendrimers commercialized in that family were the Starburst systems. These species were developed in the mid-1980s by Tomalia [5], at about the same time when Newkome developed similar dendritic architectures named Arborols [6]. A major incentive for the development of these molecules was the creation of covalently bonded (unimolecular) micelles comparable to the well-known multi- or intermolecular micellar systems. 

However, it is not only from an architectural viewpoint that dendrimers can be contrasted to organized assemblies. but micellar stabilization and organization of species in uncharacteristic environments are also suggested. Thus, stmctural attributes of branched frameworks ean be employed for such applications as aqueous solubilization of inherently water-insoluble species as well as molecular ordering based on noneovalent interaetions, such as //-bonding and ionic associations. The advent and current status of several dendritic micelles are herein ehronicled. 

Palmer, C.P. Micelle polymers, polymer surfactants and dendrimers as xpseudostationary phases in micellar electroki-netic chromatography. J. Cliromatogr., A1997, 780, 75-92. 

An elegant experiment by Crooks showed that the hydrophobic modification of PAMAM dendrimers by noncovalent interactions could also result in macromolecules that behave like inverted micelles (118). The spontaneous assembly between the fatty acids and amino periphery of the PAMAM dendrimer was driven by ionic interactions (Fig. 32). These dendrimers were shown to be capable of extracting hydrophilic dyes such as methyl orange fi om water into toluene. Similarly, these dendrimers were also shown to be excellent molecular containers for catalytically active metal nanoparticles. The inverted micellar nature of various dendrimers have been used by Crooks and others for the preparation of a variety of nanoparticles (119-122). A related macromolecule, but an architecture with less of a control, is a hyperbranched polymer. Hydrophobically modified hyperbranched polymers have also been shown to be capable of acting as inverted micelles (123,124). 

Various other combinations of hydrophobic-hydrophilic blocks are of interest as micellar microcontainers in drug-delivery applications, such poly(DL lactide)-poly(N-vinyl-2 pyrro-lidone) [301], poly(lactide)-depsipeptide [302], poly(malic acid)-poly(malic ester) [303], dendrimer unimolecular micelles [304], etc. 

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