(175g) A Novel Approach for Preparing Heterogenized Homogeneous Catalysts in Mesoporous Polymer Supports
Homogeneous catalysts are widely used to catalyze chemical reactions in the liquid phase; however, the separation of the reactants and products from such a catalyst can be difficult and often limits their use in practice. For this reason there has been considerable interest in the heterogenization of homogeneous catalysts. The use of polymers for this purpose has been particularly attractive because of the availability of a wide range of polymeric structures and methods for catalyst incorporation. Our novel approach for heterogenizing homogeneous catalysts that involves the use of microphase separated block copolymers. Block copolymers are synthesized from a crosslinkable monomer (e.g., isoprene) that forms a supporting gel upon crosslinking, an organometallic monomer that is catalytically active (e.g., vinyl ferrocene), and a depolymerizable block (e.g., alpha-methyl styrene) that can be removed to form mass transfer pathways, or pores. When placed in a solvent, the polymer gel swells to facilitate transport of the reactants to the active centers and the transport of products from these centers. This is functionally similar to the synthesis of mesoporous siliceous materials created by an amphiphilic, structure-directing agent. However, in this system, placement of the catalyst site is also controlled by self-assembly. The performance of polymer-bound ferrocenium triflate catalysts for a number of reactions will be illustrated. It will also be demonstrated that in some cases the polymer-bound catalysts exhibit higher turnover frequencies (based on total Fe in the polymer) than for free ferrocenium triflate dissolved in the same solvent use to swell the gel.