Amphiphiles micellar

Scheme 5.15. Convergent methodology has facilitated the construction of amphiphilic micellar dendrimers.

Surfactants Two other classes of host molecule, with, however, flexible cavities, are amphiphilic micellar and vesicular assemblies. In a simple view, these molecules bear a hydrophilic headgroup (e.g., ionic, zwitter ionic, or non-ionic) and a hydrophobic tail (carbon hydrogen chain). Depending on the solvent environment they organize in 3-dimensional surface-active assemblies. For example, in aqueous solutions their alignment... 

Here, the distinct domains of the resulting hybrid polymer are responsible for the self-assembly of the material. It should be noted that there are several other approaches to nanomaterials via ROMP, including the synthesis of dispersed latex nanoparticles, [29-34] hybrid nanoparticles via scaffolded initiation [35-39], and nanoparticles encapsulated in polymer matrices [40,41]. Amphiphilic micellar nanoparticles are by far the most prevalent systems in the literature relevant to a discussion of ROMP in nanoparticle synthesis, particularly those fully characterized in terms of particle formation and morphological characterization of the resulting polymer aggregates. Amphiphilic copolymers synthesized by ROMP that are not studied in this manner [42-45] or those nanoscale architectures involving only covalent interactions [46, 47] are not discussed here. 

Due to it s highly efficient and orthogonal nature, copper-catalyzed click chemistry is an attractive route to post-polymerization functionalization of ROMP copolymer backbones yielding amphiphilic micellar nanoparticles [103, 104]. However, it should be noted that post polymerization modification is necessary to yield azide or acetylene modified polymers, as azide and acetylene functionalities are not compatible with current ROMP catalysts. Ohe and coworkers [104] have synthesized amphiphihc triblock copolymers capable of assembly into discrete micellar nanoparticles by clicking an acetylene-modified hexaethylene... 

Advantage can be taken of the living polymer principle to prepare a wide range of highly controlled-structure polymers that are amphiphilic. The eopolymers of this type that have been studied most are block and graft copolymers of styrene and alkylene oxides. These polymers can form amphiphilic micellar aggregates (168) and polymers with unique phase-transfer activity (169). 

Likewise, Grieco, while working with amphiphile-like reactants, observed an enhanced preference for endo-adduct in aqueous solutions, which he attributed to orientational effects within the micelles that were presumed to be present in the reaction mixture ". Although under the conditions used by Grieco, the presence of aggregates cannot be excluded, other studies have clearly demonstrated that micelle formation is not the reason for the improved selectivities . Micellar a peg tes even tend to diminish the preference for endo adduct. ... 

FIG. 1 Self-assembled structures in amphiphilic systems micellar structures (a) and (b) exist in aqueous solution as well as in ternary oil/water/amphiphile mixtures. In the latter case, they are swollen by the oil on the hydrophobic (tail) side. Monolayers (c) separate water from oil domains in ternary systems. Lipids in water tend to form bilayers (d) rather than micelles, since their hydrophobic block (two chains) is so compact and bulky, compared to the head group, that they cannot easily pack into a sphere [4]. At small concentrations, bilayers often close up to form vesicles (e). Some surfactants also form cyhndrical (wormlike) micelles (not shown). 

The study on micellar models is still at the beginning. An amphiphilic ligand which can form micelles by itself has not yet been prepared. It is necessary to obtain complexes of higher stability in order to activate the hydroxyl group strong enough in the reactions of inactive esters or amides. Enantioselectivity must reach higher specifity. Nevertheless it seems to be clear that many features or some important clues have already been disclosed for further refinements of this micellar systems. More details about the present micellar reactions will be reported elsewhere in near future. 

The transformation of the hydrophobic periphery composed of bromo substituents into a hydrophilic wrapping of carboxylic acid functions was achieved by reacting 31 with (i) n-butyllithium and (ii) carbon dioxide. The polymer-analogous transformation provides water soluble, amphiphilic derivatives of 31 which constitute useful covalently bonded unimolecular models for micellar structures. 

Kim W, Thevenot J, Ibarboure E et al (2010) 5elf-assembly of thermally responsive amphiphilic diblock copolypeptides into spherical micellar nanoparticles. Angew Chem Int Ed 49 4257 260... 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

Ionic, polar, apolar, and amphiphilic molecules can coexist in the same liquid system, frequently coming in contact as a consequence of the micellar dynamics and of the large interfacial area between different domains (a typical value of the interfacial area is about 100 m /cm ). 

The different location of polar and amphiphilic molecules within water-containing reversed micelles is depicted in Figure 6. Polar solutes, by increasing the micellar core matter of spherical micelles, induce an increase in the micellar radius, while amphiphilic molecules, being preferentially solubihzed in the water/surfactant interface and consequently increasing the interfacial surface, lead to a decrease in the miceUar radius [49,136,137], These effects can easily be embodied in Eqs. (3) and (4), aUowing a quantitative evaluation of the mean micellar radius and number density of reversed miceUes in the presence of polar and amphiphilic solubilizates. Moreover it must be pointed out that, as a function of the specific distribution law of the solubihzate molecules and on a time scale shorter than that of the material exchange process, the system appears polydisperse and composed of empty and differently occupied reversed miceUes [136],... 

By small-angle neutron scattering experiments on water/AOT/hydrocarbon microemulsions containing various additives, the change of the radius of the miceUar core with the addition of small quantities of additives has been investigated. The results are consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water/surfactant interfacial region, decreasing the micellar radius, whereas toluene remains predominantly in the bulk hydrocarbon phase. The effect of n-alcohols on the stability of microemulsions has also been reported [119],... 

FIG. 6 Representation of spherical water-containing reversed micelles solubilizing a polar molecule (p) in the micellar core (A) or an amphiphilic molecule (a) in the palisade layer (B). 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

Epoxidation of cyclooctene and other alkenes with Oxone (KHSO5) was promoted effectively in an aqueous micellar solution of an amphiphilic ketone (3.3).52 The amphiphilic ketone can be easily derived from hepta(ethylene glycol) monodecyl ether. 

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]. 

The direct synthesis of poly(3-sulfopropyl methacrylate)-fr-PMMA, PSP-MA-fr-PMMA (Scheme 27) without the use of protecting chemistry, by sequential monomer addition and ATRP techniques was achieved [77]. A water/DMF 40/60 mixture was used to ensure the homogeneous polymerization of both monomers. CuCl/bipy was the catalytic system used, leading to quantitative conversion and narrow molecular weight distribution. In another approach the PSPMA macroinitiator was isolated by stopping the polymerization at a conversion of 83%. Then using a 40/60 water/DMF mixture MMA was polymerized to give the desired block copolymer. In this case no residual SPMA monomer was present before the polymerization of MMA. The micellar properties of these amphiphilic copolymers were examined. 

Oheme and co-workers investigated335 in an aqueous micellar system the asymmetric hydrogenation of a-amino acid precursors using optically active rhodium-phosphine complexes. Surfactants of different types significantly enhance both activity and enantioselectivity provided that the concentration of the surfactants is above the critical micelle concentration. The application of amphiphilized polymers and polymerized micelles as surfactants facilitates the phase separation after the reaction. Table 2 shows selected hydrogenation results with and without amphiphiles and with amphiphilized polymers for the reaction in Scheme 61.335... 

Recently, Porter et al. (1986b, 1988) have reported the synthesis of both meso- and ( )-forms of a series of two-chain carbonyl diacids made by joining two pentadecanoic acid units by a carbonyl group at the 3,3, 6,6, 9,9 and 12,12 positions, 3,5-didodecyl-4-oxoheptanedioic acid (C-15 3,3 ), 6,8-dinonyl-7-oxotridecanedioic acid (C-15 6,6 ), 9,11-dihexyl-10-oxononadecanedioic acid (C-15 9,9 ) and 12,14-dipropyl-13-oxopentacosanedioic acid (C-15 12,12 ), respectively. The diacids were used to probe further the question of stereochemical preference in two-chain amphiphiles. The method used for examining the diastereomeric preference was equilibration by base-catalyzed epimerization in homogeneous, bilayer and micellar media. This method allows for stereoselection based on hydrophobic/hydrophilic considerations rather than classic steric size effects. 

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