Transition metal complex-cocatalyst systems

ETHYLENE/STYRENE COPOLYMERIZATION USING TRANSITION METAL COMPLEX-COCATALYST SYSTEMS... 

The wide diversity of cocatalysts and transition metal complexes suggests that the oxidation state of the transition metal is not a critical parameter. More important seems the availability of vacant coordination sites. In agreement with this, in the case of heterogeneous systems also,... 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

It is well known that in conventional catalyst systems a chemical interaction between the catalyst and the metal-alkyl takes place, which essentially leads to a variation of the transition metal oxidation state. This is likewise true with MgCl2 catalysts however, in this case there are many more possible reactions, given the greater complexity of the system. Thus, besides modifying the Ti valence, the metal-alkyl may interact with the Lewis base incorporated in the catalyst. The Lewis base added to the cocatalyst can, in turn, interact both with the support and with the TiCl4, as can the byproducts originating from the reaction between Al-alkyl and Lewis base. The situation appears to be quite complex. However, detailed knowledge about these processes is absolutely necessary for any attempt to rationalize the polymerization behavior of these catalytic systems. 

It is not always easy to deduce the mechanism of a polymerization. In general, no reliable conclusions can be drawn solely from the type of initiator used. Ziegler catalysts, for example, consist of a compound of a transition metal (e.g., TiCU) and a compound of an element from the first through third groups (e.g., AIR3) (for a more detailed discussion, see Chapter 19). They usually induce polyinsertions. The phenyl titanium triisopropoxide/aluminum triisopropoxide system, however, initiates a free radical polymerization of styrene. BF3, together with cocatalysts (see Chapter 18), generally initiates cationic polymerizations, but not in diazomethane, in which the polymerization is started free radically via boron alkyls. The mode of action of the initiators thus depends on the medium as well as on the monomer. Iodine in the form of iodine iodide, I I induces the cationic polymerization of vinyl ether, but in the form of certain complexes DI I (with D = benzene, dioxane, certain monomers), it leads to an anionic polymerization of 1-oxa-4,5-dithiacycloheptane. 

In addition, silver-catalyzed asymmetric aza-Diels-Alder reactions provide a useful route to optically active nitrogen-heterocyclic compounds such as piperidines or pyrid-azines. Substituted dihydrobenzofurans can also be enantioselectively prepared through silver-promoted allylation of aldehydes. Other types of silver-mediated cyclizations can also be used in the synthesis of tetrahydrofnrans, tetrahydropyrans, 1,2-dioxetanes, 1,2-dioxolanes, medium-sized lactones, dihydroisoqninolines, and so on. Silver salts can also be used as cocatalysts with other transition metals. Unique activity was observed for these silver-based systems in several cases. Conseqnently, the use of silver can enrich several available heterocyclization methods, and fnrther developments in the application of chiral silver complexes will hopefnlly appear in the near future. 

Interestingly, these hydrocarboxylation reactions also occur to some extent in metal-free systems, but the reaction efficiency can be improved significantly by the use of metal catalysts or promoters [18]. Among the variety of different transition metal catalysts, multicopper(II) compounds were usually the most active ones [18, 20], leading to product yields that are circa two to five times superior to those in the metal-free systems. The water-soluble tefracopper(II) complex [Cu4(/x4-0)(/u,3-tea)4 ( u,3-BOH)4][BF4]2 (6) was formerly used as a model catalyst in the hydrocarboxylations of C2-Q alkanes [18, 31]. Since then, the reactions have been optimized further [19-21] and extended to other alkanes and multicopper catalysts, namely including the dimer 2 [22], the trimer 5 [13], the tetramer 7 [14], and the polymers 11 [12], 12 [12], 13 [14], and 15 [15] (Table 3.1). Interestingly, in contrast to alkane oxidation, the hydrocarboxylation reactions do not require an acid cocatalyst. 

As shown by Tait under these conditions every zirconium atom forms an active complex and produces about 20000 polymer chains per hour. At temperatures above 50 °C, the zirconium catalyst is more active than the hafnium or titanium system the latter is decomposed by such temperatures. Transition metal compounds containing some halogene show a higher activity than systems that are totally free of halogen. Of the cocatalysts, methylalumoxane is much more effective than the ethylaluminoxane or isobutylalumoxane. 

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