Synthese von Block-Copolymer stabilisierten Palladiumnanopartikeln für die Katalyse von C C Kupplungsreaktionen in Wasser

Die vorliegende Arbeit beschäftigt sich mit der Verwendung von Nanopartikeln in der organischen Katalyse. Nanopartikel haben gegenüber klassischen heterogenen Katalysatoren vor allem den Vorteil der größeren Oberfläche und Reaktivität, wodurch Katalysatoren mit höherer Effizienz zugänglich sind. Die...

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Bibliographic Details
Main Author: Luy, Christoph
Contributors: Greiner, Andreas (Prof. Dr.) (Thesis advisor)
Format: Doctoral Thesis
Published: Philipps-Universität Marburg 2013
Online Access:PDF Full Text
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This work is concerned with the use of nanoparticles in organic synthesis. In comparison with classical heterogeneous catalysts they show higher surface area and reactivity, which leads to catalysts with higher efficiency. Most catalysts based on nanoparticles require the use of organic solvents to carry out catalysis. To use water instead of organic solvents generates some major advantages. Water is nontoxic, non-flammable, shows highly effective heat capacity and is available cheaply. The aim of this work was to produce water dispersible palladium nanoparticles for organic catalysis. The use of amphiphilic block-copolymers leads to incorporation of the particles into micellar structures. This leads to a higher stability of the particles in water. Besides the amphiphilic character leads to a more homogeneous dispersion of the mostly hydrophobic educts in water. The nanoparticles were stabilised with coordinating polymers. In this case block-copolymers were required which consists of a hydrophilic and a hydrophobic polymer-block as well as a coordinating agent. In the first established system a Polyethyleneglycol-polystyrene-poly-4-vinylpyridine triblock-copolymer was produced. In this case the Poly-4-vinylpyridine block acts as the coordinating agent. The innovation of this system is the synthesis and use of a triblock-copolymer which encloses the particles into an amphiphilic double shell and leads to water soluble particles. As polymerisation method Atom Transfer Radical Polymerisation (ATRP) was used. As starting material Polyethyleneglycol with one hydroxyl end-group and a small molecular weight distribution was used. Through esterification with α Bromoisobutyrate an ATRP-macroinitiator was received which was used for the ATRP of Styrene. The resulting diblock-copolymer was used as initiator for the ATRP of 4-Vinylpyridine. The produced triblock-copolymer was used to stabilise palladium nanoparticles which were produced by reduction of palladium salt with Superhydride®. The resulting particles were water dispersable and showed catalytic activity in nitrophenole reduction. However one disadvantage of this system is that the available block-length of the Polyethyleneglycol is limited to commercial available polymers. In addition the coordinating agent is quite undefined. The Poly-4-vinylpyridine block underlies a molecular weight distribution so the polymer chains have a different number of monomer units which can coordinate to the particle surface. Particularly when only a short polymer block is aimed it results in a broad molecular weight distribution and part of the chains can be unfuctionalised. For this reasons a second system was established which allows better control on the coordinating agent. As coordinating agent 2,2’-Bipyridine was used which is known as a good ligand for palladium surfaces and was used before to stabilise nanoparticles. Additionally it is relatively easy to insert 2,2’-Bipyridine as polymer end-group just by adding it to the reaction mixture after anionic polymerisation. By this it was possible to produce 2,2’ Bipyridyl endfuntionalised Polystyrene in a one pot synthesis. To receive an amphihilic diblock-copolymer it was necessary to functionalise the second end of the Polystyrene block with a group which can be used as initiator for an additional polymerisation. This functionality is determined by the choice of the anionic initiator. 3-tert-Butyldimethylsilyloxy-1-propyllithium was used before as initiator for the synthesis of α Hydroxy¬polystyrene. By using this initiator it was possible to insert a protected hydroxyl group. The use of a protecting group was necessary to mask the hydroxyl group during lithiation and anionic polymerisation. This initiator was not commercially available so it was produced in a two step reaction. After the polymerisation of Styrene the protecting group was cleaved and the resulting α Hydroxy ω bipyridylpolystyrene was esterificated with α Bromoisobutyrylbromide to receive the bromo end-functionalised polymer. This polymer was used to initiate the ATRP of Poly(oligo(ethyleneglycol)methacrylate). The resulting diblock-copolymer showed good water dispersibility and cryo-TEM measurements revealed the micellar structure of the aqueous dispersion. In situ reduction of palladium salt in polymer solution led to palladium nanoparticles with sizes below 10 nm. The synthesised particles showed good water dispersibility and cryo-TEM pictures showed the metal particles inside of block-copolymer micelles. These particles showed good catalytic activity in nitrophenole reduction with hydrazine. To show the usability of the particles in the catalysis of carbon-carbon coupling reactions a Heck reaction was carried out in water. It could be shown that the desired reaction could be catalysed successfully and that under the given reaction conditions the nanoparticles showed higher catalytic activity than the heterogeneous standard catalyst palladium on charcoal.