Aluminum alkyls reaction with propylene

Cumene as a pure chemical intermediate is produced in modified Friedel-Crafts reaction processes that use acidic catalysts to alkylate benzene with propylene (see Alkylation Friedel-CRAFTSreactions). The majority of cumene is manufactured with a soHd phosphoric acid catalyst (7). The remainder is made with aluminum chloride catalyst (8). 

Diphenylamine can be alkylated exclusively in the ortho positions by reacting with an olefin in the presence of aluminum diphenylamide (7), which can be readily obtained by heating DPA with powdered aluminum, or more easily by treating sodium diphenylamide with AIQ. - Ethylene is more reactive than propylene, which in turn is more reactive than isobutylene. Eor a typical reaction, a small amount of the amide is generated in a DPA melt and ethylene is introduced under pressure (5 —30 MPa) at 200—400°C. The absorption of ethylene stops after about 30 min and 2,2 -diethyldiphenylamine is obtained in 95% yield. With propylene only a 25% yield of the 2,2 -diisopropyldiphenylamine is obtained. 

Reactions other than those of the nucleophilic reactivity of alkyl sulfates iavolve reactions with hydrocarbons, thermal degradation, sulfonation, halogenation of the alkyl groups, and reduction of the sulfate groups. Aromatic hydrocarbons, eg, benzene and naphthalene, react with alkyl sulfates when cataly2ed by aluminum chloride to give Fhedel-Crafts-type alkylation product mixtures (59). Isobutane is readily alkylated by a dipropyl sulfate mixture from the reaction of propylene ia propane with sulfuric acid (60). 

Another simple oligomerization is the dimerization of propylene. Because of the formation of a relatively less stable branched alkylaluminum intermediate, displacement reaction is more efficient than in the case of ethylene, resulting in almost exclusive formation of dimers. All possible C6 alkene isomers are formed with 2-methyl-1-pentene as the main product and only minor amounts of hexenes. Dimerization at lower temperature can be achieved with a number of transition-metal complexes, although selectivity to 2-methyl-1-pentene is lower. Nickel complexes, for example, when applied with aluminum alkyls and a Lewis acid (usually EtAlCl2), form catalysts that are active at slightly above room temperature. Selectivity can be affected by catalyst composition addition of phosphine ligands brings about an increase in the yield of 2,3-dimethylbutenes (mainly 2,3-dimethyl-1-butene). 

Organotitanium compounds have been intensively studied, initially mainly because of the discovery by Ziegler and Natta that ethylene and propylene can be polymerized by TiCl3-aluminum alkyl mixtures in hydrocarbons at 25°C and 1 atm pressure84 (Section 22-9). Organic compounds have been found to react with N2 and to act as catalysts in a number of other reactions. 

The availability of such data would be very useful for a direct experimental verification of those reaction models which provide for competitive adsorption of the monomer, the aluminum alkyl and the hydrogen on the catalyst surface86 87. The studies carried out to-date on the effect of temperature do not even permit to establish clearly, whether the decline in catalytic activity, observed in propylene polymerization above 60-70 °C with TiCl4. EB/MgCl2—AlEt3 type catalysts 45,69,98), is due to an irreversible deactivation of active centers or to some other phenomena. 

One first assumed that polymerization with Ziegler-Natta catalysts, such as aluminum-alkyls plus halides, works by a simple ionic mechanism. Since single aluminum alkyls normally cause anionic and titanium halides a cationic chain reaction (Chapter 8), the two components of the initiator should neutralize each other and only the excess one over the other should be active. If this were true, then either one of the components alone should be able to initiate the polymerization of ethylene or propylene, but this is not the case. A simple anionic or cationic mechanism can therefore not explain the polymerization with Ziegler-Natta catalysts. 

Olefin homologation. The methyl groups of 1 exchange with some aluminum alkyls and halides, but the methylene group is unreactive. However, 1 reacts with ethylene (toluene, 20", 18 hours) to form propylene in 327. yield. Reactions of this type are generally improved by addition of trimethylamine. This olefin homologation may involve the transient species a formed by CH2—Al dissociation (equation II). 

Although presently lacking industrial importance, alternating copolymers can be made from propylene and butadiene, also from propylene and isoprene. Copolymers of propylene and butadiene form with vanadium- or titanium-based catalysts combined with aluminum. alkyls. The catalysts have to be prepared at very low temperature (-70 C). Also, it was found that a presence of halogen atoms in the catalyst is essential.Carbonyl compounds, such as ketones, esters, and others, are very effective additives. A reaction mechanism based on alternating coordination of propylene and butadiene with the transition metal was proposed by Furukawa. ... 

A major advance in polymer science occurred in the 1950s when Ziegler and Natta both established that aluminum alkyls could polymerize ethylene under high pressure. They discovered that addition of transition metal compounds such as TiCL or VCI5 accelerated the reaction, so that ethylene could be polymerized at atmospheric pressure and room temperature (Eq. 13.23). Propylene could also be polymerized by these systems. While these very simple, and readily available, olefins can be polymerized under radical conditions with difficulty, such reactions were far from optimal. The discovery of Ziegler-Natta polymerization launched a huge effort in polyethylene and polypropylene science that continues to this day, with tens of millions of pounds of each being produced every year. 

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