Metallated block copolymer micelles

In a more general way, the loading of metal salts into preformed block copolymer micelles has become the most used route for the incorporation of precursors into block copolymer nanostructures because it allows precursor loading with tolerable loading times, it is quite versatile, and it is applicable to a wide variety of precursor/block copolymer/solvent systems. The accordingly synthesized polymer-coated metallic or semiconducting nanoparticles exhibit increased stability, which results in, e.g., protection against oxidation as illustrated by Antonietti et al. [108]. 

Application of amphiphilic block copolymers for nanoparticle formation has been developed by several research groups. R. Schrock et al. prepared nanoparticles in segregated block copolymers in the sohd state [39] A. Eisenberg et al. used ionomer block copolymers and prepared semiconductor particles (PdS, CdS) [40] M. Moller et al. studied gold colloidals in thin films of block copolymers [41]. M. Antonietti et al. studied noble metal nanoparticle stabilized in block copolymer micelles for the purpose of catalysis [36]. Initial studies were focused on the use of poly(styrene)-folock-poly(4-vinylpyridine) (PS-b-P4VP) copolymers prepared by anionic polymerization and its application for noble metal colloid formation and stabilization in solvents such as toluene, THF or cyclohexane (Fig. 6.4) [42]. 

Block copolymers self-assemble to form nanoscale organized structures in a selective solvent. The most common structures are spheres, with the insoluble core surrounded by a solvent-swollen corona. In some instances, disk- or worm-like micelles form, and are of particular interest, since the control of their association can lead to a broad range of new applications [1,2]. An important subset of block copolymer micelles are those which contain metal atoms, through covalent attachment or by complexa-tion [3], These structures are interesting because they take advantage of the intrinsic properties of their components, such as the mechanical properties of the polymer micelles and the optical and magnetic characteristics of the metal atoms. Moreover, the assembly permits the control of the uniformity in size and shape of the nanoparticles, and it stabilizes them. 

Nanoparticles consisting of noble metals have recently attracted much attention because such particles exhibit properties differing strongly from the properties of the bulk metal [1,2], Thus, such nanoparticles are interesting for their application as catalysts [3-5], sensors [6, 7], and in electronics. However, the metallic nanoparticles must be stabilized in solution to prevent aggregation. In principle, suitable carrier systems, such as microgels [8-11], dendrimers [12, 13], block copolymer micelles [14], and latex particles [15, 16], may be used as a nanoreactors in which the metal nanoparticles can be immobilized and used for the purpose at hand. 

Selvan and coworkers167168 utilized a block copolymer micelle of polystyrene-block-poly(2-vinylpyridine) in toluene exposed to tetrachloroauric acid that was selectively adsorbed by the micelle structure. On exposure of this solution to pyrrole monomer, doped PPy was obtained concurrently with the formation of metallic gold nanoparticles. The product formed consisted of a monodispersed (7-9 nm) gold core surrounded by a PPy shell. Dendritic nanoaggregate structures were also reported... 

The outstanding features of metal clusters prepared in block copolymer micelles [81] are their high catalytic activity combined with high stability. Such micellar catalyst systems can be recovered after reaction by precipitation or ultrafiltration. In many cases high selectivity and stability have been observed. Cyclohexadiene, for instance, is selectively hydrogenated by Pd colloids just to cyclo-octene [69]. High activity and stability of such catalyst particles have been reported for the Heck-reaction with unusually high turnover numbers of... 

This approach was first described in [64, 65]. When metal nanoparticles are located in the block copolymer micelle cores [64] or in microgels [65] and these polymeric systems are used as templates for silica casting, both pore size and... 

PS-b-P4VP-based Catalysts In the mid-1990s we developed nanoparticulate catalysts based on block copolymer micelles derived from the polystyrene-felocfe-poly-4-vinylpyridine (PS-b-P4VP) [11, 46]. In selective solvents (toluene and THE) these block copolymers form micelles with the P4VP cores, the latter serve as nanoreactors for metal nanopartide formation. The Pd nanopartides of 2.6 nm... 

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. 

Cores of Block Copolymer Micelles. In selective solvents (a good solvent only for one block), amphiphilic block copolymers form micelles, the characteristics of which (size and shape) depend on the chemical structure and molecular weight of each block and on the solvent type [27-29]. If the coreforming block contains functional groups, which are able to react with metal... 

Figure 4.8. Schematic image of metallation of the block copolymer micelle cores.

Unlike PEO- -P2VP, the PEO-fe-PB block copolymer micelles in water are very dense, so they successfully fulfill two roles They serve as nanoreactors for Pd, Pt, and Rh nanoparlicle formation and as metal-particle-containing templates for mesoporous silica casting [41]. Figure 4.10 presents PEO- -PB micelles (M = 13,400 = 19,200) filled with Pd nanoparticles and meso-... 

This chapter illustrates our approaches in the design of metal nanocomposites, both in bulk and in solution. The key advantages of soluble systems, especially if they retain solubility alter nanoparticle formation, are their potential use for raicroheterogeneous catalysis and magnetic liquids or formation of thin deposited or free-standing films (the latter were obtained with block copolymer micelles and some functional polysilsesquioxane colloids). These opportunities... 

Notably, gold is by no means the only material that can be incorporated into block copolymer micelles. Indeed, other metals such as palladium, silver, and cobalt have been successfully applied, and even nanoparticles of CdS and PbS have been... 

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