Ethene Oligomerization Reactions

If the polymerization reaction occurs in the presence of steric constraints, the activation energy associated with the monomer insertion grows with the progress of the reaction, in the same way as discussed for the oligomerization reactions catalyzed by Bronsted acid sites (Section LB and Fig. 2a), because the available space is progressively reduced. Under such conditions, the formation of polymeric species is limited, and small oligomeric species can become observable. An example of this situation is ethene polymerization on Cr/silicalite, in which the transition metal center is grafted to the internal surface of a cavity (Section VI.C.l). 

Some metal oxide catalysts are activated by thermal reduction with hydrogen or carbon monoxide. For example, the catalytic activity of molybdenum oxide and tungsten oxide for the metathesis reaction of olefins is very much enhanced by their slight reduction (1). The catalytic activity for butene isomerization and ethene oligomerization appears on niobium oxide by its... 

Refs. [473,837,886,921]. Kustov et al. [922] published an interesting DRIFT spectroscopic study on ethene oligomerization, including a discussion of the role of Lewis- and Bronsted-acid sites and ethoxy groups as intermediates of this reaction. 

For the Mo/H-Beta zeolites, the formation of the Al2(Mo04)3 phase and the decrease in the concentration of Brpnsted acid sites explains the low catalytic activity of Mo/H-Beta in metathesis of ethylene and 2-butylene to propylene [149]. A promoting effect of Mg was revealed in the Mo/H-Beta-Al203 catalyst for cross-metathesis of ethene and butene-2 to propene [150]. The stability is improved at the Mg content of l-2wt.% due to the elimination of weak acid sites and suppression of the side olefin oligomerization reaction. 

The first biphasic process which has been commercialized uses Ziegler s Aufbaureaktion ("growing reaction") of ethene yielding oligomeric alkenes (Equation 5.7, [65]). 

Ni(sacsac)P(nBu3)Cl (sacsac = pentane-2,4-dithionate) was activated by AlEt2Cl to form a catalytically active species for the oligomerization of ethene and propene. This study is noteworthy in that it uses in situ UV-VIS spectroscopy to monitor the course of the polymerization. In this reaction, the aluminum reagent serves both to activate the transition metal and to scavenge any moisture present. 

The hydrovinylation reaction, the codimerization of ethene and styrene (Scheme 2), provides easy access to chiral building blocks from inexpensive hydrocarbon feedstocks, which can be used further for the preparation of fine chemicals. Key problems in this reaction include the selectivity of the reaction and the stability of the catalyst. The main side reactions are oligomerization and isomerization of the product to internal achiral alkenes. The latter reaction can be suppressed by... 

Espinoza, R. L., Korf, C. J., Nicolaides, C. P. and Snel, R. Catalytic oligomerization of ethene over nickel-exchanged amorphous silica-alumina effect of the reaction conditions and modeling of the reaction. Appl. Catal., 1987, 29, 175-184. 

Pd,122-124,1 8 133 134 pt 123,124,128 an(j q,127 Metal complexes of iminophosphines have been used in various catalytic reactions, such as the coupling of organostannanes with aryl halides,135 oligomerization of ethene,122 Heck reactions,136 and allylic alkylation reactions.123... 

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