Homogeneous catalyst cocatalyst

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 first kinetic model for propagation in homogeneous systems was proposed by Ewen [47], assuming that the propagation took place as shown in Fig. 9.18. This scheme, shown for Cp2Ti(IV) polymerization of propylene, is representative of the kinetics for dl of the polymerizations with Group IVB metallocenes. In the scheme, species 1 and 4 represent coordinatively unsaturated Ti(IV) complexes that are-formally 16-electron pseudo-tetrahedral species, species 2 represents the interacting catalyst/cocatalyst combination, while intermediate 3 is shown with the monomer coordinated... 

Catalysts and Kinetics. Hundreds of variants and combinations of catalysts, cocatalysts, catalyst pretreatments, and reaction conditions have been discovered and described, mostly in the patent literature (28). It is now generally agreed that most coordination polymerizations are heterogeneous, but that some are clearly homogenous. The basic characteristic that distinguishes all Ziegler/Natta-type stereoregular polymerization catalysts is that... 

Both homogeneous and heterogeneous catalysts are effective for this reaction. Homogeneous catalysts are usually 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 used in these catalysts are Mo, W, and Re, although other metals from groups 4-9 have also been used. Literally thousands of different... 

Homogeneous catalysts generally consist of (/) a well-characterised alkylidene (carbene) complex of a transition metal (Ru, Mo or W), or (ii) a combination of a transition metal halide or oxo-halide with an alkylating cocatalyst. 

Several investigations of supported catalysts for the polymerization of sPS have been reported (63-66). Generally, two approaches have been evaluated for preparing a supported catalyst system for sPS. Either a homogeneous catalyst, such as (CsHslTiCla, is attached to a solid support and activated by MAO or the MAO cocatalyst is attached to a support followed by reaction with the metallocene. Under certain conditions, high activity and S5mdiotacticity have been reported for these supported catalyst systems (67). 

The recent development of single-component "cationic" metallocene catalysts is an extraordinary new development which eliminates many of the problems associated with the methyalumoxane cocatalysts. A particular advantage of the cationic metallocene catalysts is that they are considerably more tolerant of functional groups than the conventional hetereogeneous catalysts or homogeneous catalysts based on methylaluminoxane. 

The most investigated homogeneous catalyst systems are based on bis(cyclopenta-dienyl)titanium(lV), bis(cyclopentadienyl)zirconium(rV), tetrabenzyltitanium, vanadium chloride, and trialkylaluminum or alkylaluminum halides as cocatalysts. Subsequent research on these and other systems with various alkyl groups has been conducted by Patat and Sinn [4], Shilov [5], Hemici-Olive and Olive [6], Reichert and Schoetter [7], and Fink et al. [8]. 

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