Oligomerization, cationic

The organophilic part of commercial dispersants is obtained by cationic oligomerization of isobutylene. 

When concentrated sulphuric acid alone was used as the initiator, the polymerization was found to follow a different path. It is well known that Bronsted acids can function as cationic/pseudocationic initiators in the oligomerization of olifins [174]. If the counter ion has a higher nucleophilicity as it forms cation-conjugate pairs, which collapse rapidly, polymerization will not take place. As the counter ion in the case of sulphuric acid is not very strong compared to the cation, oligomerization can take place, but may not be to a very high molecular weight. This, however, depends on the nature of the... 

The two stereoisomers of4-bromo-6,8-dioxabicyclo[3.2.1]-octan-7-one (47a and 47b) showed different behavior in their cationic oligomerizations at — 40 or 0 °C [50] The axial isomer 47 a showed a tendency to cyclodimerize, particularly at higher temperature, whereas the equatorial isomer 47 e was much less reactive and gave only a small amount of a cyclic dimer along with other oligomers. Reductive debromination using tri- -butylstannane converted the cyclic dimer 48 of 47 a to the cyclic dimer 43 of 41, indicating that the former also consisted of a pair of the enantiomeric monomeric units. 

In the presence of a cationic initiator such as trifluoromethanesulfonic acid and boron trifluoride etherate, 58, gave a mixture of dimers and higher oligomers. [69] On the basis of structural analyses of the dimers prepared from the racemic and optically active monomers, it was concluded that the cationic oligomerization involved protonation onto the amide nitrogen of 58 followed by selective scission of the C(5)—N(6) bond, while the C(5)—0(8) bond remained intact. 

Several years ago dimerization was essentially achieved (and is still currently performed) by means of acidic catalysts, sometimes as liquids but mainly as solids. However, in spite of its economic interest owing to its low price and low sensitivity to impurities, cationic oligomerization is limited in scope, the main drawbacks being its poor selectivity and low activity toward linear olefins. Organometallics of highly electropositive metals (aluminum, potassium) afford better selectivities but their specificity and their poor activity restrict their use to some specialized syntheses, e. g., dimerization of propene into 2-methyl-1-pen-tene (Al(/-Pr)3) or 4-methyl-1-pentene (K). Coordination catalysts offer a broader spectrum of activity (which is often the opposite of that observed in cationic reactions) and more diversified selectivities their practical use can be expected to grow. 

The experiment using BF3 Et2O did give a low molecular weight oil upon evaporation of the solvent and residual monomer, which was indicative of a cationic oligomerization of norbornene. 

Not only fluoride ions add readily to perfluoroolefms, but also carbanions themselves. Treatment of perfluoroolefms with catalytic amounts of CsF yields sometimes complex mixtures of various oligomers (the negative counterpart of cationic oligomerization Scheme 2.114). 

There is some controversy (ref. 10) about the extent to which ethylene participates in the quasi-equilibrium, since ethylene is the least reactive olefin in cationic oligomerization and the least readily obtained by a carbenium-ion cracking mechanism. 

Fig. 42. Dependence of critical micelles concentration CMC of cationic oligomeric surfactants on the number of surfactant arms ( degree of oligomerization ), as determined by surface tension measurements and conductimetry. (Data from [422])...

Recently Heitz et at. 20) have used cationic oligomerization of THF in the presence of acetic anhydride to calculate K, and K. They have assumed that the distribution of the oligomers carrying acetoxy end groups, determined by gpc, corresponds to the distribution of the living oligomers. The GPC chromatogram is shown in Fig. 2.1. 

Cationic Oligomerization of Oxiranes Leading to Linear Oligomers. Activated Monomer Mechanism... 

This mechanism is similar to that which seems to operate in the cationic oligomerization of oxiranes in the presence of compounds containing hydroxyl groups ( activated monomer polymerization ), discussed in detail in Chapter 4. 

Binary blends prepared by mixing iPP with a fully saturated ahcyclic hydrocarbon resin (obtained by cationic oligomerization of indene, a-methyl styrene, and vinyl toluene, followed by hydrogenation) showed good miscibihty in the melt state (12). 

For this purpose, a new catalyst was discovered (83), (diphenylcar-bene)pentacarbonyltungsten (molecule 4), and its effectiveness is notable for two reasons (a) because it supports the ideas about the mechanism of the olefin metathesis reaction, and (b) because the catalyst does not contain any metal halide or other Lewis acid and does not induce cationic oligomerizations even of sensitive olefins. 

The Ni-catalyzed oligomerization of olefins in ionic liquids requires a careful choice of the ionic liquid s acidity. In basic melts (Table 5.3-10, entry (a)) no dimerization activity is observed. Here, the basic chloride ions prevent the formation of free coordination sites on the nickel catalyst. In acidic chloroaluminate melts, an oligomerization reaction takes place even in the absence of a nickel catalyst (entry (b)). However, no dimers, but a mix of different oligomers, are formed by cationic oligomerization. Superacidic protons and the reactivity ofthe acidic anions [Al2Cl7] and [Al3dio] may accoimt for this reactivity. 

Oligomerization. NAFION has been used In the cationic oligomerization of styrene. Oligomers range from dimer to hexamers... 

One of the directions of piperylene application is cationic oligomerization for production of liquid rubber - the base of synthetic drying oil of SKOP brand. Use of piperylene hydrocarbon fraction as feed stock for synthetic drying oil production allows solving the problem of isoprene production by-product utilization and reducing consumption of plant oils in paintwork and other industry branches [206,241, 242]. 

As in industry including butyl rubber chlorination and piperylene cationic oligomerization we often have to use liquid flows with different physical characteristics (density, viscosity) it is necessary to study the influence of these parameters on conditions of reaction plan front formation - quasi-plug-flow mode, and consequently on the effectiveness of external heat removal. 

Electrophilic catalysts that are used for cationic piperylene oligomerization posses various activity (see 1.4.2). This determines principle possibility of improvement of industrial production of synthetic drying oil at stage of piperylene cationic oligomerization by selection of catalytic systems with corresponding activity and selectivity. 

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