Formation of initiating cation

Cationic polymerizations are presently overshadowed by other methods of polymer synthesis. The main reason is the extraordinary complexity of the chemical processes in cationic polymerizations, and the consequent difficulty in controlling technological problems. Some monomers cannot, however, be polymerized in any other way. This is a sufficient reason for studying the generation and reactions of carbocations (and also carboxonium and oxonium ions), 

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Several hypotheses have been proposed for explaining the mechanism of the formation of initiating cations by means of Friedel-Crafts catalysts . 

Polymerizations initiated via addition-fragmentation reactions can also be classified as an initiation process involving radicalic species. The principle of this class of reactions consists in the reaction of a photolytically formed radical with an allyl-onium salt generating a radical cation intermediate. These reactive species undergo a fragmentation giving rise to the formation of initiating cations. 

In the above expressions, [HM. .. (AR) 1 represents the total concentration of all the propagating ion pairs K is the equilibrium constant for formation of initiating cations. When a steady state exists, the rates of initiation and termination are equal to each other ... 

Radiation-Induced Polymerization. In 1956 it was discovered that D can be polymerized in the soHd state by y-irradiation (145). Since that time a number of papers have reported radiation-induced polymerization of D and D in the soHd state (146,147). The first successhil polymerization of cychc siloxanes in the Hquid state (148) and later work (149) showed that the polymerization of cycHc siloxanes induced by y-irradiation has a cationic nature. The polymerization is initiated by a cleavage of Si—C bond and formation of silylenium cation. 

The dienaldehyde 117 cyclizes to a mixture of the octalins 120 and 121 on treatment with concentrated orthophosphoric acid. It was suggested that the reaction is initiated by formation of the cation 118, which undergoes ring-closure to the bicyclic cation 119. Proton loss in two alternative ways leads to the products (equation 62)72. 

Figure 6. Electrophilic polymerization of thiophene. Initial 71-bonding, formation of radical cation, electrophilic addition.

In an effort to explore the factors that govern anodic C-C bond formation, Swenton and coworkers have also been exploring the intramolecular coupling of phenols and olefins (Scheme 28) [44]. In these reactions, initial oxidation of the phenol followed by loss of a proton and a second oxidation led to the formation of a cationic intermediate (26). This intermediate was trapped by the olefin to form a second cation that was in turn trapped by methanol to form the final product 28. When R2 was equal to methyl (25b) or phenyl (25c) the reaction led to a good yield of the cyclized product. Reactions where the R2 was equal to a hydrogen (25a and 25d) were not so successful. The cyclizations were compatible with the incorporation of the olefin into a third ring (25e). 

The expressions (Eqs. 5-34 and 5-42) for Rp in cationic polymerization point out one very significant difference between cationic and radical polymerizations. Radical polymerizations show a -order dependence of Rp on while cationic polymerizations show a first-order depenence of Rp on R,. The difference is a consequence of their different modes of termination. Termination is second-order in the propagating species in radical polymerization but only first-order in cationic polymerization. The one exception to this generalization is certain cationic polymerizations initiated by ionizing radiation (Secs. 5-2a-6, 3-4d). Initiation consists of the formation of radical-cations from monomer followed by dimerization to dicarbo-cations (Eq. 5-11). An alternate proposal is reaction of the radical-cation with monomer to form a monocarbocation species (Eq. 5-12). In either case, the carbocation centers propagate by successive additions of monomer with radical propagation not favored at low temperatures in superpure and dry sytems. 

Radical anions are produced in a number of ways from suitable reducing agents. Common methods of generation of radical anions using LFP involve photoinduced electron transfer (PET) by irradiation of donor-acceptor charge transfer complexes (equation 28) or by photoexcitation of a sensitizer substrate (S) in the presence of a suitable donor/acceptor partner (equations 29 and 30). Both techniques result in the formation of a cation radical/radical anion pair. Often the difficulty of overlapping absorption spectra of the cation radical and radical anion hinders detection of the radical anion by optical methods. Another complication in these methods is the efficient back electron transfer in the geminate cation radical/radical anion pair initially formed on ET, which often results in low yields of the free ions. In addition, direct irradiation of a substrate of interest often results in efficient photochemical processes from the excited state (S ) that compete with PET. 

Squalene is an important biological precursor of many triterpenoids, one of which is cholesterol. The first step in the conversion of squalene to lanosterol is epoxidation of the 2,3-douhle bond of squalene. Acid-catalysed ring opening of the epoxide initiates a series of cyclizations, resulting in the formation of protesterol cation. Elimination of a C-9 proton leads to the 1,2-hydride and 1,2-methyl shifts, resulting in the formation of lanosterol, which in turn converted to cholesterol by enzymes in a series of 19 steps. 

Our radiolysis studies also indicate that phosphonates react quite slowly with the superoxide anion radical. Although our studies do not support the formation of radical cations as an initial oxidation step, we cannot rule out the possibility that radical cations are not involved in the oxidation of the C—P bond, as previously proposed [44], It is also possible that more electron-rich organphosphorus compounds or organophosphorus compounds in the adsorbed state may exhibit different redox and hydroxyl radical chemistries than what is observed under pulse radiolysis employing homogeneous conditions. 

According to the studies of monomers in the organic glass matrices mentioned so far, the ion radicals formed from solute monomers relate their radiation-induced ionic polymerization to the primary effect of ionizing radiations on matter. It is concievable that the initiating species in the anionic polymerization (caxbanions) are formed by the addition of the monomer molecules to the anion radicals which result from electron transfer from the matrices to the solute monomer. The formation of the cation radicals is necessary also to initiate the cationic polymerization. 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

One of the commonest reactions shown by benzene derivatives is the electrophilic substitution of hydrogen by some other group. The mechanisms of these reactions are relatively well-understood and a vast range of electrophiles have been shown to be effective. The reaction involves the initial formation of a cationic intermediate which then rearomatises with the loss of a proton to give the substitution product (Fig. 8-1). 

There is very little known about the neutral vanadate species, V04H3, because it is, at best, only a minor component in aqueous solution [35], The initial protonation of V04H2 at about pH 3 is accompanied by a second protonation that cannot be separated from the first. The result is the formation of a cationic species. Thermodynamic and spectroscopic evidence [33] suggests that formation of this compound is accompanied by incorporation of water to form the octahedral derivative, V02(H20)4+, commonly referred to as V02+. Theoretical calculations also support the assignment of tetrahedral coordination to the monoanion and octahedral geometry to the cationic form of vanadate [36],... 

The formation of cation-radicals by carbazoles is well documented. In 1968, it was observed that oxidation of carbazoles with lead tetraacetate in acid conditions led to the formation of the cation-radical of the corresponding 3,3 -dicarbazole. The radicals are persistent and uninfluenced by air or water.466 This work was subsequently pursued further, and similar oxidations were effected by other typically single-electron oxidants.467 Indications from electrochemical work that the simple cation-radicals are the reactive intermediates appeared about the same time as the initial observation.468 This work, too, has been followed-up in depth. Electrochemical... 

The LB films of TTF [73,74] and BEDT-TTF [75-81] derivatives were investigated and secondary treatments, such as iodine or ICI doping, rendered the films conductive. The structure of the LB film of C18TET-TTF doped with iodine is investigated by UV/visible and IR spectroscopies [76— 79]. The initial stage of the chemical reaction involves formation of the cation radical of the donor, which is converted spontaneously into a dimeric form. At this step the film is insulating. Gradual evaporation of I2 from the film takes place afterward and the conductivity of the film reaches a maximum of about 0.1 S/cm. (See the footnote in Section II.A. 1.)... 

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