Homogeneous cocatalysts

A final example of homogeneous catalysis is the use of metallocene catalyst systems in chain growth polymerization processes. The metallocene, which consists of a metal ion sandtviched between two unsaturated ring systems, is activated by a cocatalyst. The activated catalyst complexes with the monomer thereby reducing the reaction s energy of activation. This increases the rate of the reaction by up to three orders of magnitude. 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Both heterogeneous and homogeneous disproportionation catalysts are known. All contain a transition metal component with derivatives of Mo, W, and Re being the most important. Heterogeneous catalysts are generally metal oxides deposited on a support such as silica or alumina (1, 4). Homogeneous catalysts in general require a non-transition metal derivative as cocatalyst (2, 3). 

The traditional cocatalyst, diethylaluminumchloride or triethylaluminum, shows only a pure polymerization activity and was used as a homogeneous system to understand the polymerization, which is simpler in soluble than in heterogeneous systems. Kinetic studies and applications of various methods have helped to define the nature of the active centers, to explain aging effects, to establish the mechanism of the interaction with olefins, and to obtain quantitative evidence of some elementary steps [12,13]. 

Both homogeneous and heterogeneons catalysts are effective for this reaction. Homogeneons catalysts are nsnally composed of a transition metal compound combined with a main group metal alkyl cocatalyst, or they consist solely of a well-defined transition metal carbene complex. The most common transition metals nsed in these catalysts are Mo, W, and Re, although other metals from groups 4-9 have also been used. Literally thousands of different... 

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