Alkenes, oligomerization/polymerization

In conclusion, also in the case of alkene oligomerization/polymerization on metal-containing catalysts, the rate of surface reactions and hence, the concentrations of the surface species can be modified by tuning temperature and pressure... 

Boratabenzene complexes of Group 3 and Group 6 metals serve as effective catalysts for the oligomerization/polymerization of ethylene. For example, [(C5H5B-Ph)2ScPh]2, pretreated with H2, oligomerizes ethylene to furnish 1-alkenes.17a In the case of a Cr(III)-boratabenzene complex, ethylene is polymerized to afford... 

Along with enabling technological advances in alkene polymerizations, the application of new ancillary ligands or cocatalysts has become significant in the field of alkene oligomerizations to obtain a-olefms. This research exploits a new phase in the development of known catalyst systems containing Gr, Ni, Pd, Ti metal centers, as well as Fe and Co as newcomers. 

Fast side reactions under the conditions of acid-catalyzed alkylation are oligomerization (polymerization) and conjunct polymerization. The latter involves polymerization, isomerization, cyclization, and hydrogen transfer to yield cyclic polyalkapolyenes. To suppress these side reactions, relatively high (10 1) alkane alkene ratios are usually applied in commercial alkylations. 

The alkene elimination product usually dissociates readily but remains coordinated in some cases, as in a compound of the type (21-XXII).106 The /3-H elimination is the predominant termination step in catalytic alkene oligomerizations and polymerizations. 

A number of stable carbenium ions can be generated in superacid media most NMR data on chemical shifts and coupling constants of carbenium ions have been reported in superacid media [7-10]. However, superacids cannot be used for polymerization studies because excess superacid not only stabilizes the ionic species, but also protonates any alkene which would otherwise be available for oligomerization/polymerization. 

The oligomerization/polymerization of alkenes on catalysts containing transition metal ions can be discussed on the basis of a scheme very similar to that reported in Fig. 2, where the active center is represented by an isolated transition metal on the catalyst surface (F ). The only difference with respect to the previous case concerns the height of the energy barriers associated with the insertion process, which is definitely lower (A isteric constraints, the scheme of Fig. 2a can be modified to give that as shown in Fig. 3. 

Methylaluminoxane (MAO), sometimes referred to as polymethylaluminoxane (PMAO), currently gamers the most industrial interest of aU the alkylaluminoxanes owing to its activity as a cocatalyst for metallocene-catalyzed alkene polymerization (see Oligomerization Polymerization by... 

Metallacyclopentanes are of considerable importance as intermediates in catalytic alkene dimerization (see Oligomerization Polymerization by Homogeneous Catalysis). 

An analogous scheme can be advanced to rationalize the stereospecificity with which alkyl migration proceeds in metal alkyl-transition metal-catalyzed alkene oligomerization and polymerization reactions such as Ziegler-Natta polymerization and alkene metathesis. " ... 

Since cationic titanium complexes have been proposed as intermediates in alkene polymerization by soluble Cp2TiCl2/AlR3-based Ziegler-Natta catalysts see Ziegler-Natta Catalysts), there has been considerable interest in the synthesis and reactivity of cationic titanium complexes (see Oligomerization Polymerization by Homogeneous Catalysis). 

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