Polymerization oxidative cationic

Benzene can be oxidative-cationically polymerized to brown or black products with iron chloride, aluminum trichloride/copper dichloride, etc. at mild temperatures. The insoluble material can be compressed at high pressures into mold inserts. 

Poly(/ -phenylene) has been synthesized by various procedures, such as oxidative cationic polymerization of benzene with Lewis acid-oxidant systems, electrolytic polymerization of benzene, coupling of aromatic halogen compounds, and condensation of aromatic nuclei with organometallic reagents, such as aryl Grignard reagents or aryllithium compounds [56]. 

The polymerization of benzene with aluminum chloride-cupric chloride has been suggested to proceed by oxidative cationic polymerization [57]. However, a cation radical initiation followed by a cationic propagation mechanism has generally been accepted [62]. A unique mechanism of propagation by coordination of benzene cation radical with benzene nuclei has been presented [63],... 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

Antimonic acid has been used as an ion-exchange material for a number of cations in acidic solution. Most interesting is the selective retention of Na" in 12 Af HQ, the retention being 99.9% (24). At lower acidities other cations are retained, even K". Many oxidation and polymerization catalysts are listed as containing Sb203. 

Several mechanisms for the polymerization of vinyl ether and epoxies have been suggested [20,22,23,25,27,28,33-35]. On irradiation with gamma rays or electrons, pure epoxies polymerize via a cationic mechanism [35]. However, this cationic polymerization is inhibited by just traces of moisture, as shown below for cyclohexene oxide in reaction 5. 

Cationic polymerization of ethylene oxide is accompanied by depolymerization and oligomerization. It has been reported that ethylene oxide polymerized cation-ically with the living dication of tetrahydrofuran and a surface active material was obtained290. ... 

Based on the synthesis of polyphosphazenes and of diblock copolyphosp-hazenes by the living cationic polymerization of phosphoranimines [237,241], the triblock poly(phosphazene-ethylene oxide) copolymer XVIII was synthesized by Allcock [223]. 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

Quite often in the ring-opening polymerization, the polymer is only the kinetic product and later is transformed to thermodynamically stable cycles. The cationic polymerization of ethylene oxide leads to a mixture of poly(ethylene oxide) and 1,4-dioxane. In the presence of a cationic initiator poly(ethylene oxide) can be almost quantitatively transformed to this cyclic dimer. On the other hand, anionic polymerization is not accompanied by cyclization due to the lower affinity of the alkoxide anion towards linear ethers only strained (and more electrophilic) monomers can react with the anion. 

In many polymerizations cyclic material is produced by a concurrent backbiting reaction as linear polymer is formed. For example dioxan Is formed In the cationic polymerization of ethylene oxide to polyoxyethylene, and polyoxyethylene can be degraded to... 

The lanthanide oxide cations [LnO]+ and the bare lanthanide ions Ln+ react differently with butadiene (162). Some bare Ln+ ions (La, Ce, Pr, Gd) activate butadiene but their oxide cations are inert toward butadiene. The lanthanides with weak M-O bonds, EuO and YbO, react by oxygen transfer to the butadiene. The oxide cations of Dy, Ho, Er, and Tm activate butadiene, whereas the bare metals of these lanthanides are unreactive with butadiene. The [HoO]+ ion has been studied in detail and is able to polymerize butadiene the mechanism of this reaction has been discussed. 

On the other hand, in cyclic ethers (alkene oxides, oxetans, tetrahydrofuran) and formals the reaction site is a carbon-oxygen bond, the oxygen atom is the most basic point, and, hence, cationic polymerization is possible. The same considerations apply to the polymerization of lactones Cherdron, Ohse and Korte showed that with very pure monomers polyesters of high molecular weight could be obtained with various cationic catalysts and syncatalysts, and proposed a very reasonable mechanism involving acyl fission of the ring [89]. 

Scheme 77.8 Strategies to prepare cationic polymerization catalysts (a) using oxide supports that have high Bransted and Lewis acidity (b) the addition of a co-catalyst to a neutral supported species (c) modification of the surface with Lewis acid co-catalysts prior to the grafting of the organometallic species [91, 96, 98].

Commercial polymers of formaldehyde are also produced using cationic polymerization. The polymer is produced by ring opening of trioxane. Since the polyacetal, POM, is not thermally stable, the hydroxyl groups are esterified (capped) by acetic anhydride (structure 5.22). These polymers are also called poly(methylene oxides). The commercial polymer is a... 

Many other cyclic ethers have been polymerized using cationic polymerization. Ethylene oxide (also called oxirane) polymerizes forming poly(ethylene oxide) (PEO) (structure 5.24) in the presence of acids such as sulfuric acid, producing a wide range of chain-sized polymers sold under various trade names including Carbowax and Poly ox. PEO is also used in cosmetics and pharmaceuticals (as water-soluble pill coatings and capsules). 

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