Inverse copolymer micelles

For the amphiphilic block copolymer in the non-polar selective solvent, the unpolar blocks form the corona, which provides solubilization and stabilization, while the polar or hydrophilic and functionalized blocks form the core, which is able to dissolve metal compounds due to coordination, followed by the nucleation and growth of metal particles upon reduction. Also the internal structure of block-copolymer micelles, as given by the size of core and corona and the density profile in each domain, has been carefully characterized by static and dynamic light scattering [146] and by small angle neutron scattering using contrast variation techniques [147], The micellar corona has many of the characteristics of a spherical polymer brush. 

This allows a quantitative understanding of the corona density profile enabling one to improve or adjust the steric stabilization of polymeric or inorganic colloids in a number of applications. 

In most cases block copolymers form spherical micelles in dilute solution. In only a few studies was the formation of non-spherical aggregates reported. For example, cylindrical or worm-like micelles were observed for polystyrene-polybutadiene-polystyrene (PSt-PB-PSt) triblock copolymers in ethylacetate [148], PSt-PI (polyisoprene) in N,N-dimethylformamide (DMF), or PEO-PPO-PEO triblock copolymers in aqueous solutions [149]. Conditions for the formation of non-spherical micelles currently seem to be clear only for ionic block copolymers. Due to enormous interfacial tension these systems are in a thermodynamic state close to the super-strong segregation limit (SSSL) [150]. Under these conditions, a sequence of shape transitions from spherical - cylindrical - lamellar is possible. Such transitions can be induced by increasing the ionic strength of the solution or by increasing the relative length of the core block. 

The important step involves the solubilization of inorganic compounds into the micellar core. As a guideline for optimum precursor materials and micellar core blocks, one can use Pearsons hard / soft acid /base (HSAB) concept [151], which has been generalized to include metals and semiconductors [152]. The general strategy is to start from weakly coordinated metals, e.g. Pd(OAc)2 or Pd(C104)2 which are complexes of a soft acid (the transition metal ions) and a hard base (acetates, perchlorates, etc.). The formation of more stable complex of a soft acid with a softer base, e.g. polyvinylpyridine, to assemble the micellar core, is the driving force for solubilization. The polymer complex should not be too stable since over-stabilization could prevent the formation of the desired colloid in the subsequent chemical reaction. 

Supported Au nanoclusters synthesized from diblock copolymer (PSt-b-P2VP) micelles can be reliably prepared with well-controlled sizes and dispersions [158]. 

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In 1997, Antonietti et al. reported on catalytically active palladium nanoparticles prepared by reduction of palladium(II) compounds in inverse block copolymer micelles, namely polystyrene-ib-poly(4-vinylpyridine) (PS-b-P4VP). Activated aryl bromides were coupled reproducibly in Heck reactions [18]. Small partide sizes were a prerequisite for high conversions, as indicated by qualitative TEM investigations. Very high total turnovers were reported (0.0012 mol% palladium, 68% conversion in five days, corresponding to 56 000 TO) (Table 1). Catalyst activity was found to be dependent on the structure of the block copolymer employed, which was attributed to a better accessibility of the metal particles in smaller micelles with a high surfacer area and thinner polystyrene layer. 

As an extension of the perspective of micelle formation by amphiphihc block copolymers the following part will focus on two other types of polymers. The micellar structures that will discussed are (i) micelles and inverse micelles based on a hyperbranched polymers and (ii) polysoaps, that are copolymers composed of hy-drophihc and amphiphihc or hydrophobic monomers. Whereas the first class of polymers is stiU very new and only few examples exist of the synthesis and appH-cation of such stracture in catalysis, the synthesis and aggregation characteristics of polysoaps has already been intensively discussed in the hterature. 

Riess demonstrated recently that poly(styrene-b-oxirane) copolymers could act as non-ionic surfactants and lead to water/ toluene microemulsions (29, 30). Using isopropanol as cosurfactant, both 0/W and W/0 microemulsions are obtained (3l). This is a very important conclusion, since PO based diblock copolymers give rise only to 0/W microemulsions under the same experimental conditions (8, 31,). In this respect the "branched structure" of the PO hydrophilic component could favor a decrease in the packing density of the inverse micelle forming molecular and explain the different behavior of the linear and star-shaped PS/PO block copolymers in the W/0 microemulsification process. 

The phase behaviour established for concentrated aqueous solutions of PEO-PPO-PEO copolymers has its counterpart in PEO/PBO copolymer solutions. A phase diagram for PE058PB0i7PE0M based on tube inversion experiments is shown in Fig. 4.14 (Luo et al. 1992). The hard gel is isotropic under the polarizing microscope and can be characterized as a cubic phase formed from spherical micelles of a similar size to those in the dilute micellar solution. 

The aggregation behavior of AB silicone surfactants in nonpolar oils including several hydrocarbon oils has been reported by Rodriguez [46]. They found that inverse micelles were formed in all oils, adjacent to the inverse cubic phase formed by the neat copolymers and by concentrated mixtures of copolymer and oil. The CMC depended strongly on the length of the pEO chain but only weakly on the pDMS chain. Inverse hexagonal phase was also observed. 

A method based on fluorescence quenching that did not depend on the nature of the transition was used to determine the micelle size of the hexyl copolymer (24). The basic idea underlying this method is that, in a solution containing luminescent probe and quencher molecules, both solubilized in an excess of micelles, the quenching will be inversely related to the number of micelles, because the more micelles there are, the smaller is the chance of both a probe and a quencher molecule inhabiting the same micelle (25-27). The hexyl copolymer used in our study had a degree of polymerization of 1700. The fluorescent probe was tris(2,2 -bipyridine)ruthenium(II) ion [Ru(bpy)3 ], the quencher was 9-methylanthracene (9-MeA), and the solvent was an aqueous 0.1 M LiCl solution. The fluorescence experiments were supplemented with solubilization experiments from these, the distribution of the 9-MeA between the polymer molecules and the solvent molecules, as well as the extent to which the polymer was in micellar form, could be simultaneously determined. The results indicated that the micelles inside the domain of a macromolecule encompassed approximately 24 repeat units, and that this micelle size was independent of the polymer concentration, of the probe concentration, and the extent to which the polymer was micellized. 

Assuming that the model system is a valid model for a MHAP prepolymerization solution, these results for the model system can be applied to the MHAP. The model system results suggest that the micelle solution copolymerization process can be a means of producing multiblock copolymers in which hydrophobic and hydrophilic blocks alternate. At constant monomer concentration, the number of blocks in the copolymer molecule is inversely related to the number of monomers in the block. Thus, factors that increase the sequence length of a block decrease the number of blocks. The monomer concentrations and the polymer MW are also factors governing the sequence length and number of blocks in the copolymer. Polymerization conditions (e.g., surfactant type and concentration) can be used to control the block size to some extent. 

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