Formation of Metal Nanoparticles in Modified Hyperbranched Polyglyerols and Use as Soluble Separable Catalysts

Introduction Well-defined structures of nanometer-dimensions are subject to intense research efforts. However, a rational control of structure formation on this length scale by convenient versatile means, and the control of functional properties and stability remain key challenges. Recent approaches to the controlled preparation of metal or semiconductor nanoparticles have utilized surfactants or the self-assembled nanostructures of block copolymer micelles. However, template structures self-assembled from a larger number of individual molecules are by their very nature dynamic in solution. Thus, their size and aggregation numbers are subject to fluctuations, and such structures can be shear-sensitive. Therefore, the concept of particle preparation in macromolecular compartments has been explored. Polymerized micelles, and very recently dendrimers have been employed. While (perfectly branched) dendrimers provide a well defined structure, their syntheses are usually tedious. The high selectivity and activity of homogeneous catalysts based on soluble transition metal complexes are opposed by a usually tedious seperation from the products, often involving thermal strain on the reactants. In most cases, the development of materials combining the advantages of classical homogeneous and heterogeneous catalysts has remained an elusive goal. For a number of synthetically useful transformations, soluble nanoparticles might bridge this gap. Different reactions of model substrates, catalyzed by metal particles prepared by some of the above routes have been reported recently. However, the fundamental issue of catalyst recycling has virtually not been addressed utilizing well defined systems. As a different approach, we have used readily accessible (statistically branched) hyperbranched polymers for the synthesis of stable metal nanoparticles. Only recently, a one-step synthesis allowing for a convenient access to hyperbranched polyglycerols of controlled, narrow molecular weight distributions was found. Partial esterification of the OH-groups with long chain fatty acids yields amphiphilic molecules (Scheme 1; Mw/Mn < 1.7). 6b