Nitrobenzene, cationic polymerization

Sodium chloroaurate (NaAuCl4 2HaO) initiates polymerization of VCZ both thermally and photochemically. When the polymerization is conducted in nitrobenzene, photopolymerization under illumination of near ultraviolet to visible light proceeds with remarkable rapidity, whereas thermal polymerization starts after an induction period as shown in Fig. 8. This is a unique example of photoacceleration of cationic polymerization. Since the initiation mechanism is different from known cationic polymerization, the thermal system will be mentioned briefly before the discussion of the photochemical system. 

Aromatic nitro compounds inhibit radical polymerization. However, they do not seem to inhibit ionic polymerization [137]. On the contrary Rumanian authors reported [190] that nitrobenzene as welt as nitromethane and nitro ethane increase the rate of cationic polymerization of W-vinylcarbinol. 

Irradiation of a donor monomer-electron acceptor charge transfer complex, e.g. N-vinylcarbazole-sodium chloroaurate (2), -nitrobenzene (2) or -p-chloroanil (3) and -p opiolactone-uranyl nitrate (h), results in the initiation of cationic polymerization. 

The idea of two simultaneous homo-polymerizations was explored by Szwarc and his co-workers252. Pac and Plesch253 reported initiation of cationic polymerization of vinyl carbazole (VC) by tetranitromethane in nitrobenzene. They postulated the formation of a charge-transfer complex,... 

Table 7.7 Growth of polyoxymethylene crystals during cationic polymerization of 1,3,5-trioxane hi nitrobenzene at 35 °C I22>...

Simionescu et al. [104] used poly(ethylene oxide adipate) having tosylate groups at both ends as macroinitiators for the cationic polymerization of MeOZO to produce ABA-type block copolymers. Miyamoto et al. [105] further explored the concept and prepared block copolymers consisting of poly(propylene oxide) and poly(MeOZO) by using poly(propylene oxide)-p-nitrobenzene sulfonate as a... 

In the simplest case, cationic polymerizations are propagated by repeated addition of monomer molecules on to carbocations and onium ions. The growing macrocation is stabilized by positive charge delocalization. Examples are macrocations of isobutylene (I), nitrobenzene (II), vinyl methyl ether (III), acetaldehyde (IV), and tetrahydrofuran (V) ... 

Early studies on initiation of cationic polymerizations of styrene, isobutyl vinyl ether, and N-vinylcarbazole were carried out by dissolving AgQ04 in pure monomers or in nitrobenzene. Electric current was then passed through them at room temperature [81, 82]. Rubbery polymers formed as well as some copolymers, suggesting a cationic path of the polymerization. A mechanism was... 

Typical cationic polymerization solvents are methylene chloride, benzene, and nitrobenzene. 

Interesting effects were observed if cationic polymerization was carried out under an electric field [91,92]. Depending on the solvent, the degree of ion separation is decreased under the influence of the electrical field. However, if toluene is applied, the effect is small due to the low value of the solvent. In the range of intermediate values of (dichloroethane) the highest changes in rate are observed. In nitrobenzene, a solvent with high values, the ion separation is almost complete. Therefore, application of an... 

As far as cationic polymerization concerns, it was shown by Kricheldorf and Jenssen that EC did not react with a cationic-type catalyst such as methyl trifiate. No polymer or oligomer was found after 48 h of reaction carried out in nitrobenzene at 100 °C. 

The homopolymerization ofl consists of a room-temperature reaction of the monomer dissolved in nitrobenzene in the presence of anhydrous ferric chloride. Polymerizations were carried out under a stream of dry nitrogen. As depicted in Scheme 2, the homopolymerization of 1 to form 6FNE takes place by means of the Scholl reaction. The mechanism of the Scholl reaction was assumed to proceed through a radical-cation intermediate derived from the single-electron oxidation of the monomer and its subsequent electrophilic addition to the nucleophilic monomer. The reaction releases two hydrogens, both as protons, to form the... 

These early works have been reviewed by Fioshin (4) and well summarized by Bbeitenbach (5). Besides, Breitenbach has made a study of the polymerization mechanism using the copolymerization method and has shown that the reaction mechanism depends on the ions used in the electrolytic discharge and on the monomer present in the system. Cationic processes were also found to be initiated in a nitrobenzene solution of styrene by the anodic discharge of perchlorate and borotetrafluoride ions. The possibility that the three different mechanisms could occur simultaneously was demonstrated in the same system of acrylonitrile-styrene using a divided electrolytic cell. 

The cationic mechanism of the polymerization is supported by the reference experiments in various solvents. The polymerization is suppressed in basic solvents such as acetonitrile and dimethylformamide. Alkyl halides and nitrobenzene are preferable for the formation of PPS in this polymerization. 

Next to the styrene compounds, /V-vinylcarbazol has been most extensively studied by pulse radiolysis. When IV-vinylcarbazole is irradiated in aerated benzonitril, the cyclodimer of the Af-vinylcarbazole is formed the polymer is formed in both aerated and deaerated nitrobenzene. Tagawa et al. have proved that the radical cation of Af-vinylcarbazole plays an important role in both cyclodimerization and polymerization processes [39-43]. 

Fig. 7- Intermediate and product species formed in the dimerization and the polymerization of vinylcarbazole in nitrobenzene (N02) and be-nzonitrile (C/V) in the presence of oxygen (VCZ ) monomer cation radical (> cyclodimer (P) polymer [43]...

The similarity in the reactivities of free ions and corresponding ion pairs derived from the same cyclic monomer is more intriguing. Whereas ion pair reactivities are about 10 times smaller than corresponding free ion values in the anionic polymerization of vinyl monomers [39], and probably of the same relative proportions in cationic systems, the difference in cationic ring opening polymerizations is considerably less. For polymerization of THF in methylene chloride the factor is only 7, and for polymerization of 3,3-dimethylthietan, 40 in methylene chloride and 1 in nitrobenzene. Because the overall reactivity in cyclic monomer reactions is lower than for olefinic polymerizations, it might be expected that difference between free ion and ion pair reactivities, within one system, would also be less. However, this does not seem to be the whole answer. Plesch [44] has pointed out that in the polymerization of cyclic ethers and thietans (and presumably, therefore, other cyclic monomers)... 

The cationic copolymerization was carried out in air at 25°C in a nitrobenzene solution using stannic bromide as catalyst, while the anionic polymerization was studied in air with sodium dissolved in anunonia [186]. Although many monomers may be polymerized by quaternary ammonium salts, vinyl acetate does not produce any polymer on treatment with dimethylphenylbenzylammonium chloride [187]. 

In general, radical cations are less stable than their anionic counterparts, but relatively stable cations can be obtained from reactants in which those positions which carry the highest charge density in the radical are blocked with respect to either proton loss or nucleophilic attack. For example the 9,10-diphenylanthracene and rubrene radical cations are sufficiently stable to give reversible cyclic voltammo-grams. Radical cations appear to be more stable in nitrobenzene than in acetonitrile solutions. Coating of the anode with insoluble insulating polymeric ffims is a common hazard in anodic oxidation systems but it can be alleviated by the use of scraped electrodes and periodic polarity-reversal techniques. 

Other cationic systems studied include the polymerization of indene using tetrabutylammonium hexachloroantimonate in either dichloroethane or nitrobenzene. The antimonate was found to be more efficient and it was shown that polymerization occurred as a result of both direct oxidation of the monomer and by interaction with HSbCl, and HCIO4 produced electrochemically. Cerrai et al., investigating the polymerization of cyclohexyl vinyl ether, report that in dichloroethane with tetrabutylammonium perchlorate, only 0.05 mA was required to produce polymer by direct oxidation of the monomer. The same school, using cyclic voltammetry, rotating electrode voltammetry, and chromo-amperometry, report that the oxidation of anethole proceeds by an ECE mechanism with an intermediate chemical step of moderate velocity. 

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