Friedel-Crafts halide

Further, while conventional Friedel-Crafts halides produce high molecular weight polyisobutylenes or polyisobutylene copolymers (e.g., butyl rubbers, HR) only at relatively low ( —100 °C) temperatures, alkylaluminum-based initiator systems produce high molecular weight materials at much higher ( —40 °C) temperatures. 

Since initiation with conventional Friedel-Crafts halides cannot be controlled, the fine-tuning of reactions becomes extremely cumbersome. In contrast, by the use of alkylaluminum compounds elementary events (initiation, termination, transfer) become controllable and thus molecular engineering becomes possible. Indeed, by elucidating the mechanism of initiation etc., a large variety of new materials, i.e., block3, graft4-6 bigraft7 copolymers, have been synthesized and some of their physical-chemical properties determined. 

With Friedel-Crafts halides (usually A1C13 and BF3) it is necessary to use equimolar or excess catalyst when alcohols are the alkylating agents. With primary alcohols usually 2 mol of catalyst per mole of alcohol must be used. Complexing the alcohol, as well as binding the water formed in the reaction explains the experimental findings. With secondary and tertiary alcohols, lesser amounts of catalyst are needed. The reactivity of different alcohols follows the order methyl < primary < secondary < tertiary, allyl, benzyl. The ease of carbocation formation according to Eq. (5.51) is most probably responsible for this reactivity order ... 

Acidic clay catalysts can also be used in alkylation with alcohols 98 The main advantages of these catalysts are the reduced amount necessary to carry out alkylation compared with conventional Friedel-Crafts halides, possible regeneration, and good yields. Natural montmorillonite (K10 clay) doped with transition metal cations was shown to be an effective catalyst 200... 

Isomerization catalysts were developed along two paths—by Friedel-Crafts halide systems or by dual site heterogeneous catalysts, originating with the commercial introduction of platinum-aluminas for catalytic reforming in the 1940,s. The Friedel-Crafts systems (aluminum chloride-hydrocarbon complexes) were used exclusively during the early stages of... 

Cationic polymerization is initiated by acidic substances. Bronsted acids and particularly Friedel-Crafts halides (viz., their complexes), which are adds in the Lewis sense, are excellent and widelyused initiators for electrophilic polymerizations. The number of cationoid substances which have been described to possess electrophilic polymerization activity is slowly increasing. These materials will be briefly surveyed here. 

Sigwalt (144) polymerized anethole with various Friedel-Crafts halides under various experimental conditions. Low molecular weight soluble materials were obtained in toluene whereas insoluble products... 

Roberts and Day (172) investigated the polymerization of a- and /S-pinenes with Friedel-Crafts halides as catalysts in toluene at 40—45° C. Under comparable conditions the / isomer polymerized much more vigorously than the a, and it gave higher conversions and higher softening products. 

When a-pinene is reacted with Friedel-Crafts halides, large amounts of dimers are formed containing unisomerized monomer units with one double bond per molecule. The authors believe that hydrogen transfer occurs during dimerization and suggest the following structure... 

Cationic Polymerization. Cationic polymerization is initiated by the transfer of a cation from the catalyst to the monomer. It allows a wider choice of monomers with double bonds, including carbonyls, cyclic ethers, and lactones. The ion may be within a carbonium or an oxonium ion. Friedel-Crafts halides, like AlCls or A CoHsJCL, are strong Lewis acids and initiate the polymerization directly. Weak Lewis acids need a... 

The scope of this reaction is eloquently given in a patent (72) describing the copolymerization of polar with nonpolar monomers in the presence of a Friedel-Crafts and a free radical initiator. In the process a polar monomer which contains strongly negative groups and that responds to free radical initiation but not Friedel-Crafts type of polymerization is complexed with a Friedel-Crafts halide, and then the complexed monomer is copolymerized, under anhydrous conditions, in the presence of a free radical initiator with a nonpolar monomer that responds to Friedel-Crafts but not free radical polymerization. ... 

Although the Friedel-Crafts halides which are listed include the typical halides of aluminum, boron, iron, titanium, and zinc, as well as others, all examples except one utilize zinc chloride as the complexing agent. 

In contrast to these alkylaluminum catalyst systems, the well-known Friedel-Crafts halides—e.g., A1C13, AlEtCl2, etc.—rapidly polymerize isobutylene to high molecular weight products in the absence of added cocatalyst. It is usually presumed, but remains to be proved, that traces of moisture or HC1 provide the necessary cocatalyst with the latter agents. 

Before general acceptance of the co-catalysis mechanism for cationic polymerizations initiated by Friedel-Crafts halides, zwitterions were regarded as possible intermediates4-. However, Horner seems to have been first to correctly identify a polymerization in which zwitterions are formed. Horner, Jurgeleit and Klupfel7) studied the polymerization of acrylonitrile by triethylphosphine and reported their findings in 1955. 

Unequivocal demonstration of the formation of macrozwitterions is confined to anionic polymerization. Ironically zwitterions were first postulated as intermediates in cationic vinyl polymerizations initiated by Friedel-Crafts halides4. Friedel-Crafts halides, probably the most widely used cationic initiators, are molecules, not ions. However, formation of an anion with a metal-carbon bond seems to be energetically unfavourable and initiation is thought to occur by self ionization or involve a co-catalyst. 

If there is no doubt that many Friedel-Crafts halides can undergo selfdissociation in certain solvents (particularly halogenated hydrocarbons), and that this ionc enic reaction is not due to basic impurities in the system, the attainment of the equilibrium relating ions to undissociated molecules often requires a long time This... 

Because olefins are soft bases and most Friedel-Crafts halides are hard acids, the primary interaction between these two types of compounds must be regarded as a weak one, the outcome of which is nerally limited to the equilibrium formation of the relatively feeble rr-complex. Only with the softer of the strong Lewis acids would one expect this interaction to proceed further and give direct Hunter-Yohe initiation titanium tetrachloride seems to comply with such a requirement, as suggested by the experimental evidence discussed in Sect. IV-B4-b). 

This hypothesis is in line with Pepper s proposition (23) that cationic polymerizations can be initiated by carbocations generated fi-om alkyl halides upon complexation with Friedel-Crafts halides. 

The traditional technique for sulphonylation with sulphonyl chlorides uses at least one mole of Friedel-Crafts halide per mole of sulphonyl chloride and it was realised that this would lead to difficulties in isolating polymers free from inorganic contaminants. However, the preparation of monomeric sulphones using catalytic quantities of FeCl had been reported in a war-time German patent [12] and we applied this procedure to reactions (1). Polymers of high molecular weight (reduced viscosity, RV, up to 1.2 for 1% solutions of polymer in dimethyl formamide at 25 C) were obtained... 

In cationic polymerization typical catalysts are represented by strong electron acceptors including Lewis acids, Friedel-Crafts halides, Brdnsted acids and stable carbenium-ion salts. However, many of them are not sufficient to initiate the polymerization and, therefore, require small amounts of a co-catalyst, usually a proton donor such as alcohols. 

According to Pepper, initiators generally employed for cationic polymerization are classified into (i) protonic adds, (ii) Friedel-Crafts halides, (iii) carbonium (carbo-cationic) salts, and (iv) catiogenic substances. Organometallic compounds, e.g., R AIC1 (R = alkyl m -+ n = 3), increasingly used in recent times, may also be used. ... 

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