Polysiloxanes as supports for catalysts

Introduction Immobilization (heterogenization) of catalysts aims to obtain catalysts which combine the advantages of homogeneous catalysts (high activity, selectivity) and heterogeneous ones (stability, ease of separation from the reaction medium and the possibility of re-use in subsequent reaction cycles). One of the most promising directions of research in this field is the use of polymers as catalyst carriers [1]. The main factor that causes the polymer matrix is an interesting alternative to the inorganic carriers is the ability to control the structure of the support (the size and stiffness of the molecules, the density of the groups anchoring catalyst). This makes possible to control its solubility in the reaction medium, the chemical and thermal resistance as well as the capacity, and consequently, catalytic activity and selectivity of the obtained catalysts. Particularly interesting group of supports are soluble polymers. By using such catalysts, organic reactions can be carried out in a homogeneous manner and thus may have similar catalytic activity and stereoselectivity as the homogeneous parent system. When the reaction is completed, the catalyst can be separated by either solvent or heat precipitation, membrane filtration, centrifugation, or size-exclusion chromatography. The use of soluble polymers for classical and combinatorial synthesis, and catalysis has recently been reviewed [2÷6]. Recyclable catalysts allow for significant savings in expensive metal complex or enzyme. Immobilized catalyst is easier to distribute and control its concentration, for example, in combinatorial reactions. High density of ligands on the surface of the support makes possible the synthesis of multifunctional catalysts in which more than one active agent is bound to the carrier. Supported analogs of toxic, explosive, or odorous reagents are safer and more convenient to handle than the corresponding soluble chemicals. Furthermore, supported catalysts allow the use of a wider range of solvents, they can also be more resistant towards side reactions deactivating the catalyst, such as reduction, autooxidation, or hydrolysis. It is possible to stabilize highly reactive coordinatively unsaturated compounds, which cannot exist as separate species in solution. Despite these advantages, polymeric supports are not yet used on a large scale in industrial processes. The main reason for this is insufficient stability of immobilized catalysts and loss of activity due to leaching of metal and/or ligand. In addition, the immobilized systems often exhibit lower catalytic activity than homogeneous catalysts due to poor accessibility of the active sites, steric effects of the matrix, and a carrier-solvent incompatibility or due to inhomogeneities resulting from the formation of the different bonds between the carrier and complex. Immobilization of chiral catalysts often results in lower activities and enantioselectivities as compared to those observed for their homogeneous counterparts [5]. Synthesis of the polymeric carrier involves the copolymerization of monomers functionalized with a suitable ligand or the grafting of ligands on a pre-formed polymer support. Organic polymers, such as polystyrene, polyethylene, poly(ethylene oxide), poly(vinylpyridine), and others, have usually been used as supports for catalysts [7]. Immobilization of catalysts on polymeric carriers usually involves covalent binding of the metal with a functional group on the polymer, as a result of ligand exchange between a soluble metal complex and the polymer. Methods for the immobilization of biocatalysts (enzymes) on polymers are more complex and comprise a number of techniques ranging from physical adsorption through a covalent bond formation to the trapping and encapsulation in the polymer network [8, 9]. Potential benefits of the biocatalyst immobilization include mainly the higher resistance to temperature, pH variation and poisoning/contamination of the catalyst. The ease of separation from the reaction mixture and the possibility of reuse are also of great importance. Moreover, the enhanced activity of the enzyme is often observed, due to a better availability of catalytic sites resulting from a conformational change imposed by the formation of a covalent bond to polymer. However, the process of immobilization of enzymes also has its disadvantages, such as the additional cost of the carrier and additional reagents and increased mass transfer resistance (due to diffusion limitations). It should be emphasized that there is no universal method for the immobilization of any enzyme or for all uses of the particular enzyme. Selection of the optimal procedure, carrier, or even a suitable enzyme is strongly dependent on the chosen process. The immobilization method must be adjusted individually for each system. Immobilization of the enzyme can thus be too expensive and time-consuming, especially, when the enzyme itself is inexpensive.