Ziegler catalysts, coupling

The mechanistic similarity between Ziegler-Natta polymerization of olefins and the alkene cyclization reactions described above suggested that early transition metal catalysts would be effective catalysts for the coupling of... 

A soluble titanium-based modified Ziegler-Natta catalyst [Ti(OR)4-Et3Al, R = n-Bu, isoPr] is employed in the reaction.42 Since similar catalysts may be used for the oligomerization and polymerization of ethylene, the nature and oxidation state of the metal and reaction conditions determine selectivity. Ti4+ was found to be responsible for high dimerization selectivity, whereas polymerization was shown to be catalyzed by Ti3+. According to a proposed mechanism,42,43 this catalyst effects the concerted coupling of two molecules of ethylene to form a metal-lacyclopentane intermediate that decomposes via an intramolecular p-hydrogen transfer ... 

Therefore, in such heterogeneous polymerizations, almost all industrial catalysts are supported, for example on silica, whereas the typical Ziegler s titanium catalysts are by definition supported on magnesium chloride. These catalysts are adsorbed at the surface or incorporated into the crystal structure of the support. Other catalysts, such as Phillips chromium catalysts, can be coupled at the support surface by a chemical bond. 

An ESI mass spectrometer coupled online to a microreactor was used to intercept the catalytically active cationic intermediates of the Ziegler-Natta polymerization of ethylene with the homogeneous catalyst system [Cp2Zr(Me)Cl]-MAO (MAO = methylaluminoxane). For the first time these intermediates were studied directly in the solution and their catalytic activity proved.60... 

As already discussed, most Ziegler-Natta and related coordination catalysts polymerise a-olefins by a 1,2-insertion mechanism, yielding highly regio- and stereoregular polymers. When some nickel-based coordination catalysts are used, however, isomerisation of the active species may accompany the propagation, and polymers containing 2, co-coupled monomeric units are formed [183,191],... 

Ziegler-Natta catalysts. The second one involves hydrocarbon coupling in the solid acid zeolite chabazite. 

In contrast to other catalysts, a Ziegler system containing nickel and triethylaluminum gives coupling products in the hydrosilylation of 1-alkynes64 (equation 17). 

Ziegler-Natta polymerization [241,242] is an important method of vinyl polymerization because it allows synthesis of polymers of specific tacticity. As reported by Santos and Metzger, Ziegler-Natta polymerization can be carried out in a flow microreactor system coupled directly to the electrospray ionization (ESI) source of a quadrapole time-of-flight (Q-TOF) mass spectrometer (Fig. 37) [243]. In the first micromixer (Ml), a catalyst (CP2Z1O2/MAO) and a mmiomer solution are mixed continuously to iiutiate the polymerization. The polymerization occurs in a microtube reactor. The solution thus obtained is introduced to the second micromixer (M2), where the polymerization is quenched by acetonitrile. The quenched solution is fed directly into the ESI source. The transient cationic species... 

---