Polymerization conjunct

When ethylene is reacted at 573 K in the presence of water in static conditions, oligomerization and conjunct polymerization give rise to paraffinic, olefinic and aromatic products (8). Nevertheless, the distribution of the aliphatics and aromatics is quite different from that of the steam-cracking products. In the former a great variety of products is formed they include propane, n-butane, isobutane and isopentane as aliphatics, and toluene, xylenes and ethylbenzene as aromatics (Figure 6B). 

The list of termination and transfer reactions given here is certainly not exhaustive. Furthermore, it must be remembered that hydrocarbons under the influence of metal halides may undergo alkylation and isomerization reactions, and conjunct polymerizations [24], especially if the temperature is much above 0°. It is therefore of the greatest importance that in future the simplest systems should be selected for investigation, and that the nature of the reaction products be examined with particular care. 

Depending on the reaction conditions, alkenes may undergo either of two types of catalytic polymerization. The products of the first type, which may be termed true polymerization, consist of alkenes having molecular weights which are integral multiples of the monomer alkene. The second type, conjunct polymerization, yields a complex mixture of alkanes, alkenes, alkadienes, cycloalkanes, cycloalkenes, cycloalkadienes, and, in some cases, aromatic hydrocarbons the products do not necessarily have a number of carbon atoms corresponding to an integral multiple of the monomer. 

Sulfuric acid finds commercial application in the polymerization of isobutylene to diisobutylene (a mixture of the two 2,4,4-trimethylpen-tenes) and the copolymerization of isobutylene with the n-butylenes to obtain a more complex mixture of octylenes. Dilute (65-70 %) acid is used at 20-35° for the polymerization of only the isobutylene. The copolymerization occurs in the so-called hot acid polymerization process in which the dilute sulfuric acid is employed at about 80-90°. If more concentrated sulfuric acid is used, particularly acid of concentration above about 90%, conjunct polymerization occurs even at —35°. 

With the other metal halides and the other alkenes either true or conjunct polymerization may predominate depending largely on the particular catalyst and conditions. 

By proper choice of catalyst and conditions, all alkenes can be made to undergo what may be termed conjunct polymerization (Ipatieff and Pines, 70), that is, polymerization accompanied by the formation of saturated hydrocarbons. Indeed, with some catalysts, for example with aluminum chloride, true polymerization of the olefin takes place only under special conditions. With sulfuric acid, the type of polymerization depends on the concentration of the acid conjunct polymerization... 

The polymerization of ethylene in the presence of aluminum chloride is fundamentally changed by the presence of metallic aluminum (Hall and Nash, 72). The product which was obtained at a reaction temperature of 100-200° under superatmospheric pressure was a mobile fuming liquid which was shown to contain diethyl aluminum chloride, a liquid spontaneously inflammable in air. Less conjunct polymerization occurred, the lower-boiling product consisting of olefins mixed with only minor amount of paraffins. 

Propene also undergoes conjunct polymerization in the presence of dilute phosphoric acid at high temperatures and pressures (Monroe and Gilliland, 58). When propene was treated with 10-30% phosphoric acid at 260-305° and at 170-410 atmospheres pressure, the only operating variable which appreciably affected the composition of the polymer was the extent to which the feed was polymerized. At constant percentage reaction of the feed under these conditions, the temperature, pressure, and acid catalyst concentration had no effect on the product composition. At low conversions, the polymer consisted of nearly pure dimer at 50% polymerization, two-thirds of the total was dimer and even when the feed was almost completely polymerized, the dimer fraction amounted to 35-40 % of the total polymer. The dimer and trimer fractions obtained at temperatures of 305° or lower using a acid concentrations below 30% contained about 25% paraffins and little or no naphthenes or aromatic hydrocarbons. 

The late Herman Pines (98) described the process of conjunct polymerization by which olefins reacted in sulfuric acid to give a complex mixture of... 

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. 

Conjunct polymerization involves hydride abstraction from an olefin or oligomer by a carbocation. For example, during the oligomerization of isobutylene by dissolving it in 96% sulfuric acid, octene products may react with carbocation 1 [Eq. (13.4)] before they eliminate a proton to form trimethylpentenes 11... 

The activity of acid-type catalysts for polymerization of olefins to higher boiling product has long been known, as reviewed by Oblad et al. (1). Depending on reaction conditions, the product can be predominantly olefinic, or a mixture of olefins, paraffins, naphthenes and aromatics ("conjunct" polymerization). 

The products are 95% olefins, with the remainder mostly paraffins and some naphthenes in the higher carbon numbers. At higher temperature conjunct polymerization occurs (involving olefin cyclization to naphthene, followed by hydrogen transfer from naphthenes to olefin). The reaction proceeds essentially to completion at 730°F to give the mixture of paraffins and aromatics shown in Table II. 

Conjunct polymerization should be kept to a minimum, as it represents a loss of olefin and acid. Retardation of conjunct polymerization is favored by a low temperature, as in alkylation, especially in that portion of the absorber with the strongest... 

Three-c(H)rdinate carbenium ion and five-coordinate carbonium ion intermediates satisfactorily account for many of the acid-catalyzed reactions of hydrocarbons at high temperatures. Yannoni et al. have characterized the structure and dynamics of several carbenium ions trapped in (noncatalytic) solids at low temperatures [32,94,95), but lifetimes of such ions on active surfaces at higher temperatures would preclude NMR observation in all but special cases. Maciel observed triphenyl carbenium ion on alumina 196). The alkyl-substituted cyclopentenyl ions discussed earlier are also special ions they are commonly observed products in conjunct polymerization reactions of olefins in acidic solutions. The five member ring cannot easily rearrange to an aromatic structure, and ions like I and II are apparently too hindered to be captured by the framework to form alkoxy species. 

The discovery of conjunct polymerization, whereby hydrogen transfer occurs between molecules of olefins, leading to the formation of saturated hydrocarbons and polyenes, became the forerunner of the discovery of alkylation and isomerization of paraffins. During World War II these processes were used for the manufacture of high octane aviation gasoline to be used in pursuit planes, and presently products from the alkylation reaction assumed new importance as a major component of unleaded gasoline (Figure 4). 

But prior to writing his dissertation he published his first paper (1 ). It was a paper on the dehydration of butanol Herman Pines earliest discovery in acid catalysis was what he and Professor Ipatieff called conjunct polymerization (2). When low molecular weight olefins, for example, a butylene, are dissolved... 

Although the above reactions are in effect dehydrogenation reactions, the cracking catalyst as such should not be considered to be a dehydrogenation-hydrogenation catalyst of the usual type, such as nickel, cobalt, copper-chromite, and others. Indeed it is difficult to visualize the occurrence of the reported true hydrogenation reaction (Parravano, Hammel, and Taylor, 3) without the presence of a small amount of a true hydrogenation catalyst impurity or the participation of other reactions, such as conjunct polymerization. 

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