Benzene Friedel—Craft polymerization

Method 5. Friedel-Crafts Polymerizations The aluminum chloride-catalyzed condensation of methylene chloride with benzene yields several products (79). 

Catalysts used in the polymerization of C-5 diolefins and olefins, and monovinyl aromatic monomers, foUow closely with the systems used in the synthesis of aHphatic resins. Typical catalyst systems are AlCl, AIBr., AlCl —HCl—o-xylene complexes and sludges obtained from the Friedel-Crafts alkylation of benzene. Boron trifluoride and its complexes, as weU as TiCl and SnCl, have been found to result in lower yields and higher oligomer content in C-5 and aromatic modified C-5 polymerizations. 

Propiophenone. Propiophenone [93-55-0] (ethyl phenyl ketone) is a colorless Hquid with a flowery odor. It can be prepared by the Friedel-Crafts reaction of benzene and propionyl chloride in the presence of aluminum chloride (346), or by the catalytic reaction of benzoic acid and propionic acid in the presence of water (347). Propiophenone is commercially available (348), and is sold in Japan at 2700 Y/kg (349). It is used in the production of ephedrine, as a fragrance enhancer, and as a polymerization sensitizer. 

Cumene is an important intermediate in the manufacture of phenol and acetone. The feed materials are benzene and propylene. This is a Friedel-Crafts alkylation reaction catalyzed by solid phosphoric acid at 175-225 °C and 400-600 psi. The yield is 97% based on benzene and 92% on propylene. Excess benzene stops the reaction at the monoalkylated stage and prevents the polymerization of propylene. The benzene propylene ratio is 8-10 1. 

The range of preparatively useful electrophilic substitution reactions is often limited by the acid sensitivity of the substrates. Whereas thiophene can be successfully sulfonated in 95% sulfuric acid at room temperature, such strongly acidic conditions cannot be used for the sulfonation of furan or pyrrole. Attempts to nitrate thiophene, furan or pyrrole under conditions used to nitrate benzene and its derivatives invariably result in failure. In the case of sulfonation and nitration milder reagents can be employed, i.e. the pyridine-sulfur trioxide complex and acetyl nitrate, respectively. Attempts to carry out the Friedel-Crafts alkylation of furan are often unsuccessful because the catalysts required cause polymerization. 

Polyhalide salts are prepared in situ in the bromination of less reactive aromatic substrates. Thus, tetra- -butylammonium bromide(TBAB) functions as a catalyst in the bromination of benzene, toluene and even benzyl bromides and a-bromoethylbenzene537 (both tend to polymerize in presence of normal Friedel-Crafts catalysts). Tetra-ethylammonium chloride in CH2Cl2/methanol catalyzes the bromination of anilines538. Improved selectivity is found in the chlorination of toluene catalyzed by onium chloride salts alone or combined with SnCl4539. 

Owing to its powerful Lewis acidity, BF3 is an effective reagent in organic synthesis, for example, promoting the conversion of alcohols and acids to esters, the polymerization of olefins and olefin oxides, and acylations and alkylations (in a manner similar to Friedel-Crafts processes). Mechanistic studies of some reactions of the latter type, such as the ethylation of benzene by QH5F, have shown that the BF3 functions as a scavenger for HF via the formation of HBF4 and thus participates stoichiometrically rather than catalytically. 

The idea is to add a latent Friedel-Crafts catalyst to the polymerization feed that will be inert during the polymerization. Then at >200 °C it becomes activated and converts the residual styrene monomer to phenethyl carbocations. These carbocations can either initiate cationic polymerization of the styrene or can alkylate the benzene rings of PS to become attached. With this approach PS itself is the scavenger for residual styrene monomer in PS (Scheme 4.3). 

A low purity (40-80%) divinylbenzene and chloromethylstyrene were used in the presence of a Friedel-Crafts catalyst (AICI3) for the synthesis of a polymeric phenolic antioxidant from p-cresol and l,3-bis(l-hydroxy-l-methylethyl)benzene in the presence of p-toluene sulfonic acid [115]. The obtained product was either used directly as AO for thermosetting resins or was treated consecutively with a-methylstyrene [116] or /ert-butylalcohol [117]. Other polymeric phenolic AO were obtained by reaction of phenol with p-men thane-1,8-diol, l-p-menthen-8-ol or limonene [118] or of p-cresol with 3- or 4-chloromethylstyrene in the presence of BF3-etherate or anhydrous AICI3 [119] the product thus obtained was finally aralkylated by a-methylstyrene. Thermostabilizer and/or LS for PUR was obtained, e.g. 98. 

In the field of ionic polymerization, we should mention the investigators87,114) who calculated the relative chain transfer constants for the polymerization of styrene in benzene solutions and its mixture with 1,2-dichloroethane on Friedel-Crafts catalysts. 

Styrene is obtained almost exclusively from the catalytic dehydrogenation of ethyl benzene (600°C, metal oxide). Ethyl benzene is obtained by a Friedel-Crafts reaction of benzene with ethylene. The separation of the styrene from the tetrafunctional, and therefore cross-linkable, divinyl benzene is important. In order to prevent premature polymerization, sulfur or dinitrophenols are added before distillation and t-butyl catechol is added before storing. 

An additional difficulty in the Blanc reaction is the tendency for activated aromatic rings to undergo polychloromethylation under the typically harsh reaction conditions. For example, in the chloromethylation of benzene, the product benzylchloride (4) is often accompanied by small amounts of / -xylylene dichloride 10, as well as a small amoxmt of diphenylmethane, the product resulting from Friedel-Crafts alkylation of benzene with benzylchloride (4). With more activated ring systems, such as phenols, the reaction is increasingly difficult to control, resulting often in the formation of polymeric materials. ... 

The self-condensation of sulfonoimidoyl chlorides such as A/ -tosyl (Ts) derivative of the benzene- and 4-phenoxybenzene-sulfonimidoyl chloride (142) utilizing a catalytic (AICI3 or FeCla) Friedel-Crafts reaction has been studied as a way to produce these polymers. It has been foimd that the reaction of the former compound yielded no polymeric product, whereas that of (142) produced exclusively methanol-insoluble polymer (143) of Mn = 14,500 with polydispersity = 1.45 in 80% yield (317). 

The Friedel-Crafts reaction was also employed to prepare a monomer. The reaction of benzene with 2,5-dichlorothiophene produced 2-chloro-4-phenylthio-phene, followed by chlorination with thionyl chloride to yield 2,5-dichloro-3-phenylthiophene [15]. This monomer was polymerized by dechlorinative coupling... 

Nafion-H is an efficent catalyst for Diels-Alder reactions (Table 3.39). The reactions of anthracene with maleic anhydride, dimethyl maleate, and dimethyl fuma late were carried out at 333 — 353 K in the presence of Nafion-H catalyst in either chloroform or benzene solvent. It should be noted that the reaction of dienophiles with very reactive dienes such as isoprene and 2,3-dimethylbutadiene can be carried out at room temperature to give the adduct in hi( yields. In usual systems, highly reactive dienophiles undergo polymerization during the desired reactions. In Diels-Alder reactions catalyzed by Friedel-Crafts Lewis acid catalysts, excess amounts of Lewis acid halides are required because of the formation of the complex between the halide and carbonyl oxygen atoms. Here again, Nafion-H catalysts allow easy and clean separation of products and the catalysts are not destroyed upon work up. 

Of the two pinene monomers (Fig. Ij, k), which can be isomerized into each other (cf. Scheme 3), the a-isomer exhibits an endocychc double bond and is thus the less reactive (and also less frequently used) in polymerization reactions. However, the polymerization of a-pinene was reported as early as 1937, using AICI3 as catalyst in hydrocarbon (i.e., benzene, toluene, xylene, or hexane) solution at <15°C, yielding 75%. Polymerization in the presence of aromatics, with AICI3 as Friedel-Crafts catalyst, takes place without the interaction of aromatic and terpene. However, structures and MWs of the polymers formed were not given [80]. A later comparative study shows that the polymerization of a-pinene produces 35% or less solid polymer with MWs of 0.6-0.7 kg/mol, depending on the catalyst used (p-pinene yield up to 96%, MW = 0.8-3.1 kg/mol). The molecular structure of the oligo(a-pinene) was, however, not provided [66]. 

Suspension polymerization was applied to prepare polynor-bomene aosslinked beads suitable for use as supports in organic synthesis. The monomers used included norbor-nene, norbom-2-ene-5-methanol, and aosslinking agents including bis(norbom-2-ene-5-methoxy)alkanes, di(norbom-2-ene-5-methyl)ether, and l,3-di(norbom-2-ene-5-methoxy) benzene. The initial resins, which were unsaturated, were subsequently modified using hydrogenation, hydrofluorination, chlorination, or bromination to yield saturated resins with varying properties. They were reported to be superior to more traditional styrene-divinylbenzene resins due to reduced interference in electrophilic aromatic substitution reactions (e.g., Friedel-Crafts acylation and nitration). 

Friedel-Crafts alkylation of various aromatic rings such as benzene, toluene or phenol lead to a variety of diaromatic products (equation 46). Nitration of this telechelic followed by reduction leads to a polymer with terminal amine groups. The phenol-terminated telechelics can be further derivatized as shown in Scheme 47. 57.458 Recently, tertiary esters and ethers were used in conjunction with BCI3 to provide the first example of a living carbocationic polymerization of isobutylene. Scheme 48 shows the polymerization mechanism suggested for initiation with tertiary esters. The resulting polymers are telechelics containing one or two tertiary chlorine chain ends. 

It has also been shown that living polydioxolane can be reacted directly onto polystyrene, whereby grafting takes place (Scheme 26). This is a kind of Friedel-Crafts reaction, resulting from the attack of the active site onto the benzene nucleus of a monomer unit. In this case, however, polydioxolane (PDXL) may remain ungrafted. The absence of any proton-donating impurity in the medium is required. The dioxolane polymerization has to be initiated by oxocarbenium salts and not by systems containing protons. 

---