ISSN 1560-0904, Polymer Science, Ser. B, 2006, Vol. 48, Nos. 1–2, pp. 5–10. © Pleiades Publishing, Inc., 2006. Original Russian Text © A.V. Orlov, S.Zh. Ozkan, G.N. Bondarenko, G.P. Karpacheva, 2006, published in Vysokomolekulyarnye Soedineniya, Ser. B, 2006, Vol. 48, No. 1, pp. 126–133.
Oxidative Polymerization of Diphenylamine: Synthesis and Structure of Polymers A. V. Orlov1, S. Zh. Ozkan, G. N. Bondarenko, and G. P. Karpacheva Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Leninskii pr. 29, Moscow, 119991 Russia Received May 19, 2005; Revised Manuscript Received August 16, 2005
Abstract—Three procedures for the chemical oxidative polymerization of diphenylamine, namely, in solutions of sulfuric acid, in an H2SO4–tert-butanol mixture, and via the interfacial process, are considered. It was shown that the highest molecular mass products are formed by the interfacial process. Oxidative hydrolysis and chain termination reactions predominate in a homogeneous medium. The effects of polymerization conditions, such as the concentration of reagents, their ratio, and the reaction temperature, on the yield and molecular-mass characteristics of polydiphenylamine were studied. The structure of reaction products was investigated by UV spectroscopy. It was demonstrated that, even when ammonium persulfate is in excess, the degree of oxidation of polydiphenylamine is rather small and chain propagation proceeds as a C–C rather than N–C addition as in the case of aniline. DOI: 10.1134/S1560090406010027
INTRODUCTION
In this paper, we are concerned with the oxidative polymerization of diphenylamine in concentrated sulfuric acid, in the sulfuric acid–tert-butanol mixture, and under interfacial conditions, where the monomer is soluble in organic solvent, while an oxidizer and an acid show solubility in water.
Even though polymers based on aniline and its derivatives were synthesized more than a century ago, they are attracting progressively growing attention of researchers owing to the ease of their synthesis, high stability, and combination of valuable electrophysical properties [1, 2]. The advancement of engineering aspects of polyaniline chemistry follows two directions: elaboration of new synthetic procedures and widening of the scope of monomers for oxidative polymerization. A number of achievements, for example, template [3, 4], intercalation [5, 6], and border-line [7, 8] polymerization, belong to the first direction, while the progress in the second direction is insignificant. The employment of various alkyl [9, 10], alkoxy [11, 12], halo [13, 14], sulfo [15, 16], and other [17–19] aniline derivatives primarily results in worsening of physicochemical characteristics of the resulting polymers. A great number of promising monomers turned out to be unsuitable for oxidative polymerization owing to their insolubility under synthesis conditions diphenylamine is among them. The electrochemical oxidation of this monomer is unusual from the viewpoint of both the polyaddition mechanism and the structure of products [20, 21]. This stimulated the authors of this study to search for new alternative procedures for the oxidative polymerization of diphenylamine. 1 E-mail:
EXPERIMENTAL Diphenylamine (Aldrich) was recrystallized two times from isopropyl alcohol. To this end, diphenylamine (80 g) was dissolved in alcohol (150 ml) at 60°C. The resulting solution was rapidly filtered and allowed to stand for 4 h at –2–0°C. The colorless lamellar crystals were separated from the mother solution and dried under dynamic vacuum conditions for 8 h at 20°C to a constant weight. The procedure was repeated, and the final drying was performed for no less than 20 h until a constant weight was achieved. The yield of diphenylamine after double recrystallization was 38 g (46% based on the initial weight); Tm = 55°C. The monomer was stored at 5°C in a dark container before use. Ammonium persulfate of the analytical grade was recrystallized as described in [22]. Hydrochloric acid of the reagent grade and sulfuric acid of the analytical grade, as well as methyl alcohol (Baker), isopropyl alcohol of the special purity grade, and tert-butyl alcohol of the analytical grade were used as received. Toluene of the analytical grade, was fractionated before use [23]. Aqueous solutions of reagents were prepared using distilled water.
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Table 1. Polymerization of diphenylamine in solution of sulfuric acid ([(NH4)2S2O8] : [diphenylamine] = 1.25 (mol/mol) and a temperature of –2–0°C) [Diphenylamine], mol/l
[H2SO4], mol/l
Reaction time, h
Yield, % with respect to theory
Mw × 10–3
Mw / Mn
0.1
17.7
144
6.3
2.1
3.7
0.1
9.0
48
16.1
4.1
2.2
0.1
5.0
4
40.3
6.7
1.4
0.01
5.0
8
13.2
3.5
2.9
0.05
5.0
4
58.4
4.9
2.0
0.2
5.0
4
32.3
5.3
2.3
The polymerization of diphenylamine in the solution of sulfuric acid or in the H2SO4–tert-butanol mixture was carried out as follows. To a solution of the monomer in sulfuric acid or a mixture of equal volumes of 2 M H2SO4 and the alcohol of the desired concentration that were temperature-controlled under constant stirring at –2–0°C, a solution of ammonium persulfate in the same solvent (1/4 of the total volume) was added dropwise. The reaction temperature was maintained at a level of –2–0°C. Polymerization was conducted under continuous intense stirring. When the process was completed, the mixture was precipitated into a 10-fold excess of ice water. The resulting product was filtered off and washed many times initially with methanol and then with water to remove the unreacted reagents. Polydiphenylamine was neutralized by a 3% solution of NH4OH for 1 day, filtered off, and washed many times with excess of distilled water to the neutral reaction and finally vacuum dried. Low-molecular-mass oligomers were extracted by methanol in a Soxhlet extractor for 1 day and dried to a constant weight. To perform the interfacial oxidative polymerization of diphenylamine, the desired amount of the monomer was dissolved in toluene, and ammonium persulfate and hydrochloric acid were dissolved in distilled water. Both solutions were cooled to –2–0°C and mixed at once in a temperature-controlled U-shaped flask. The process was performed under intense stirring at a temperature not higher than –2–0°C. When the reaction was completed, the mixture was precipitated in a 5-fold excess of isopropyl alcohol acidified by 0.1 M HCl, filtered off, and washed many times first with methanol and then with distilled water. The IR spectra of polydiphenylamine samples were recorded on a Specord M-82 spectrophotometer in the 4000–400 cm–1 range and processed using the SoftSpectra program. Samples were prepared as KBr pellets. The molecular mass of polymers was measured by GPC on a Milton Tory instrument equipped with PL gel columns having pore sizes of 100, 500, and 105 Å. A DMF–0.1 M LiBr mixture was used as an eluent. The
elution rate was 0.5 ml/min. Calibration was made relative to poly(ether sulfone). The ESCA study was performed with an XSAM800 two-chamber instrument (Kratos Analytical Ltd.) The characteristic line of MgKα (hν = 1253.6 eV) was used as exciting radiation. The instrument was calibrated using standard samples relative to lines of Au 4f (84.0 eV), Ag 3d (368.3 eV), Cu 2p (932.7 eV), and CuLMM (918.7 eV). RESULTS AND DISCUSSION Methods of Synthesis The chemical oxidative polymerization primarily involves dissolution of a monomer and an oxidizer in dilute aqueous solutions. If aniline is easily protonated to phenylammonium and gives rise to soluble salts in the majority of dilute solutions, then diphenylamine, being a much weaker base, forms a salt only with sulfuric acid monohydrate [24]. However, the use of concentrated sulfuric acid as a reaction medium provides an example of the most severe reaction conditions and the product arising from the oxidation of diphenylamine is a dark brown lowmolecular-mass compound that is fully soluble in acetone, DMF, and N-methylpyrrolidone. As the aggressive state of the reaction medium is reduced owing to dilution with H2SO4 to 9 mol/l, the rate of oxidation grows and the yield of polydiphenylamine increases from 6 to 16% (Table 1). The limiting concentration of sulfuric acid below which the monomer loses solubility amounts to 5 mol/l. Under these conditions, the yield of oxidation products is nearly 40%. In the H2SO4-doped state, a dark green powder insoluble in organic solvents is isolated; after neutralization by the ammonia solution, the color of the powder changes to gray-blue. A similar situation is observed in the oxidative polymerization of aniline [25]. As the concentration of the acid is increased (above 2 mol/l), the rate of reaction and the yield of polyaniline decrease significantly.
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The yield of reaction products plotted as a function of the concentration of diphenylamine shows a maximum in the range of 0.05 to 0.10 mol/l (Table 1). This is explained, on the one hand, by deceleration of oxidation in the low-concentration range and, on the other hand, by intensification of side processes and poor solubility of the monomer with an increase in concentration above 0.2 mol/l. The [oxidizer] : [monomer] molar ratio was maintained close to stoichiometric (1.25 : 1.0) since this ratio ensured the best results in sulfuric acid. Despite all efforts, the molecular mass of products arising from the oxidation of diphenylamine did not exceed (6–7) × 103, whereas in the case of polyaniline, this value achieved several hundred thousands [26, 27]. We managed to decrease the concentration of the acid to 2 mol/l, while preserving the solubility of the monomer and oxidizer using a 4 M H2SO4–tert-butanol mixture (equal volumes). This alcohol is characterized by a higher oxidation stability as compared to other alcohols. However, this trick was unsuccessful. Neither the rate of reaction nor the yield of polydiphenylamine changed significantly. However, the share of the lowmolecular-mass fraction extracted by methanol increased markedly from 5–7% in the case of H2SO4 and to 25–30% in the case of the H2SO4–tert-butanol mixture. The molecular mass of the products decreased to (4–5) × 103. An analysis of the effect of reaction conditions on the yield and molecular-mass characteristics of polydiphenylamine (Table 2) revealed that, as in the former case, the yield of the polymer increased with the monomer concentration, while the effect of the oxidizer concentration on the process was somewhat different than that in the polymerization of aniline. As the [oxidizer] : [monomer] molar ratio was increased above stoichiometric, the yield and molecular mass of polydiphenylamine did not decrease but, on the contrary, increased up to a 5-fold excess of (NH4)2S2O8 with respect to diphenylamine. Thus, it is suggested that higher molecular mass products will be synthesized if oxidative polymerization is performed in dilute aqueous solutions of the acid and a sufficiently high concentration of the oxidizer. It is necessary, on the one hand, to exclude the presence of a solvent inducing chain termination and, on the other hand, to minimize oxidative hydrolysis processes that proceed intensely under these conditions. We managed to solve this problem using interfacial oxidation in which an oxidizer with an acid and a monomer occur in two immiscible phases—aqueous and organic. In this case, the choice of a suitable organic solvent makes it possible to widen the scope of previously inaccessible monomers and the separation of the oxidizer and acid into the separate phase allows one to control their type and concentration, while prePOLYMER SCIENCE
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Table 2. Polymerization of diphenylamine in solution of sulfuric acid and tert-butanol (4 M H2SO4 : tert-butanol = 1 : 1 (vol/vol), a temperature of –2–0°C, and a reaction time of 4 h) [Diphenyl amine], mol/l
[Oxidizer] : Yield, % with [monomer], respect Mw × 10–3 Mw /Mn mol/mol to theory
0.05
1.25
38.6
3.6
3.9
0.1
1.25
45.7
4.9
3.8
0.2
1.25
45.4
5.5
3.5
0.1
1
30.9
4.7
2.6
0.1
1.5
46.4
5.1
4.4
0.1
5
20.1
4.6
4.3
serving low-stability oxidation products. Since oxidation proceeds only on the interphase, there is no need to gradually dose the reagents. In the case of diphenylamine, a system composed of the toluene solution of the monomer and the aqueous solution of (NH4)2S2O8 and HCl taken at a ratio of 1 : 1 (vol/vol) turned out to be the most suitable. Table 3 illustrates the effect of concentrations of the monomer, oxidizer, and acid on the yield and molecular-mass characteristics of polydiphenylamine. These data show that in terms of the yield and molecular mass of the polymer, the interfacial process should be performed at sufficiently high concentrations of the monomer (5 mol/l) and acid (2–2.5 mol/l) and at a stoichiometric or a somewhat higher [oxidizer] : [monomer] ratio (1.25–1.50). A variation in the molecular mass of polydiphenylamine with temperature follows the same pattern as in the case of polyaniline [28, 29]; namely, the molecular mass decreases with temperature. To synthesize higher molecular mass products, the temperature of the reaction mixture should be maintained at a level of −10 to +15°C. The data presented above indicate that the interfacial oxidative polymerization provides a way of increasing the molecular mass of polydiphenylamine by more than a factor of 4 as compared to the two previously described methods. Simultaneously, the degree of polydispersity decreases owing to the better solubility of the monomer and the intermediates and owing to the lessening of side reactions (hydrolysis and degradation of macromolecules). A short-term contact of the reaction products with the aqueous medium giving rise to hydrolysis is observed only at the moment when they fall into the reaction zone—on the interphase—where rapid oxidation followed by chain growth takes place and then the reaction products return to the organic phase where degradation processes are minimized. 2006
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Chemical Structure of Polydiphenylamine The structure of products prepared by three methods—polymerization in sulfuric acid, polymerization in the H2SO4–tert-butanol mixture, and the interfacial process—was studied by IR spectroscopy. The IR spectra of all the test samples (Fig. 1) exhibit a broad band at 3400 cm–1. This band is the superposition of several bands of different intensities. These are primarily bands related to the stretching vibrations νN−H in phenyleneamine (3392 cm–1) and quinoimine (3174 cm–1) moieties, a weak band corresponding to the stretching vibrations νë–H in a benzene ring (3024 cm–1), bands corresponding to the antisymmetric (3520 cm–1) and symmetric (3432 cm–1) stretching vibrations νN–H of end groups, and hydrogen bonds (3272 cm–1) formed by these groups. The smallest amount of amino end groups and, as a consequence, a single band at 3392 cm–1 are observed for the interfacial process (Fig. 1c). The spectra of the samples prepared in the H2SO4– tert-butanol mixture (Fig. 1b) show an additional triplet at 2900 cm–1 and a band at 1445 cm–1. These bands are assigned to the stretching and bending vibrations of C−H bonds in saturated hydrocarbons, respectively. In the IR range from 1600 to 1450 cm–1, two intense bands are observed at 1592 and 1504 cm–1. These bands characterize the stretching vibrations νë–ë in 4,4'-substituted quinoid and benzoid rings. The skeletal vibrations of C=N and C–N bonds in quinodiimine and phenyleneamine groups appear as a weak band at 1650 cm–1 and two bands at 1316 and 1232 cm–1, respectively. It should be noted that the degree of oxidation of the polydiphenylamine being
formed is extremely small compared to polyaniline, especially in the interfacial process. According to ESCA studies, the content of quinodiimine units in the polymer does not exceed 5%. The band near 1272 cm–1 corresponds to the stretching vibrations νë–ë between two rings. This fact, in combination with out-of-plane bending vibrations δë–H due to 4,4'-substituted rings (824 cm–1), provides evidence for the tail-to-tail mechanism of C–C addition in contrast to the C–N head-to-tail addition in polyaniline. Bands at 1176–1168 and 872 cm–1 are related to the in-plane and out-of-plane bending vibrations δë–H of aromatic rings. The occurrence of a weak band at 791 cm–1, indicating the existence of other substitutions along with para-substitution in addition to weak bands at 767 cm–1 (C–Cl bonds) or 1053 and 1044 cm–1 (S=O) bonds), suggests the occurrence of substitution—chlorination or sulfonation—via aromatic rings. An analysis of the IR spectra of the polymers (Fig. 1) and the monomer shows that the bands corresponding to a monosubstituted phenyl ring (δë–H = 750 and 695 cm–1) are preserved to some extent in the spectra of all the samples. If, in the IR spectrum of polydiphenylamine prepared by the interfacial process, end groups of this type predominate, then in the IR spectra of low-molecular-mass samples synthesized in sulfuric acid and in the H2SO4–tert-butanol mixture, end groups resulting from oxidative hydrolysis prevail (C=O (1700 cm–1) and NH2 (3520 and 3432 cm–1)). The content of quinodiimine groups (νë=N = 1650 cm–1) in these samples is also higher. It is pertinent to note that, in the range of 3600– 2600 cm–1, marked widening of IR bands and increased
Table 3. Interfacial polymerization of diphenylamine (toluene : water = 1 : 1 (vol/vol), a temperature of –2–0°C, and a reaction time of 4 h) [Diphenylamine], mol/l
[Oxidizer] : [monomer], mol/mol
[HCl] : [monomer], mol/mol
Yield, % with respect to theory
0.1
1.25
5
60.0
8.5
1.2
0.2
1.25
5
62.0
9.4
1.1
0.5
1.25
5
56.8
11.6
1.2
0.2
0.66
5
37.8
5.2
2.5
0.2
1.5
5
65.0
7.9
2.1
0.2
5
5
68.9
5.8
5.0
0.2
1.25
1
55.7
9.3
1.2
0.2
1.25
10
58.0
16.8
1.3
0.2*
1.25
5
56.4
12.2
1.1
0.5*
1.25
5
47.8
23.9
1.4
0.2**
1.25
5
60.9
5.8
1.9
Mw × 10–3 Mw /Mn
* At –13 to –10°C and **38–40°C. POLYMER SCIENCE
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Absorption
36
(a)
(a)
(b)
(b)
(c)
(c)
20
4 ν × 10–2, cm–1
36
Fig. 1. IR spectra of polydiphenylamine prepared by (a) polymerization in the solution of sulfuric acid, (b) polymerization in the H2SO4–tert-butanol mixture, and (c) interfacial polymerization. [Monomer] = 0.2 mol/l, [oxidizer] = 0.25 mol/l, and [H2SO4] = (a) 5 and (b) 2 mol/l. (c) [HCl] = 1 mol/l.
background absorption are observed in the IR spectra of reaction products prepared in sulfuric acid and in the H2SO4–tert-butanol mixture. The reason behind this finding, in addition to the above-mentioned increase in the amount of NH2 end groups and hydrogen bonds (−C=N···H–N– and –C = O···H–N–) being formed, is that the polymers under consideration undergo selfdoping involving bound sulfo groups. Interesting results were obtained in the study of the effect of the oxidizer concentration on the structure of polydiphenylamine (Fig. 2). It is seen that, for the interfacial process, as the [oxidizer] : [monomer] ratio is increased from 1.0 to 1.5 to 5.0 through, the degree of oxidation of the polymer also increases. The absorption band at 1650 cm–1 that corresponds to the quinoid structure of diphenylene units and that appears as a small shoulder at a ratio of 1 : 1 appreciably grows with an increase in the amount of the oxidizing agent. Simultaneously, the intensity of bands at 3600–3200 and 1700 cm–1 related to degradation processes (–NH2 and C=O end groups) also increases. POLYMER SCIENCE
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4 ν × 10–2, cm–1
Fig. 2. IR spectra of polydiphenylamine prepared by interfacial polymerization in relation to the concentration of (NH4)2S2O8 . [Monomer] = 0.2 mol/l; [HCl] = 1 mol/l; and [oxidizer] = (a) 0.2, (b) 0.3, and (b) 1.0 mol/l.
However, the molecular mass of reaction products also increases at least to [oxidizer] : [monomer] = 5 (Table 3). This is also evident from the ratio of intensities of bands assigned to monosubstituted (750 and 695 cm–1) and disubstituited (820 cm–1) phenyl rings. This implies that an increase in the concentration of the oxidizer, on the one hand, promotes a rise in the molecular mass of polydiphenylamine and, on the other hand, assists quite the reverse process—chain degradation via oxidative hydrolysis. CONCLUSIONS Methods of oxidative polymerization implemented in sulfuric acid and in the H2SO4–tert-butanol mixture and especially via the interfacial process allow one to markedly widen the scope of monomers useful for the chemical synthesis. The first two methods are suitable for rather stable amines and the related products. Interfacial polymerization is convenient to a greater extent for the synthesis 2006
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of polymers subjected to oxidative hydrolysis in the course of synthesis. This method not only enables one to select the most suitable solvent for a monomer but also to vary the type of the oxidizer and acid and their concentrations, while preserving low-stability oxidation products. The example is provided by the oxidative polymerization of diphenylamine. The study of oxidation of diphenylamine in various reaction media showed that, in order to prepare highmolecular-mass products, it is necessary, on the one hand, to maintain sufficiently high concentrations of the oxidizer and acid and, on the other hand, to minimize the related oxidative hydrolysis and, as a consequence, degradation of polymers. Oxidative hydrolysis is mostly typical of reactions proceeding under homogeneous conditions—in the solution of sulfuric acid or in the H2SO4–tert-butanol mixture. Furthermore, the latter reagent terminates growing polymer chains. Fragments of this alcohol were detected in the polydiphenylamine being formed. The interfacial process reduces the probability of oxidative hydrolysis owing to separation of reaction products and the oxidizer. This makes it possible to increase the molecular mass of polydiphenylamine by a factor of nearly 4 as compared to the homogeneous process. In addition to technological features, the polymerization of diphenylamine is characterized by the unusual mechanism of polyaddition. IR experiments showed that the structure of the resulting polymer results from C–C addition in para-positions of phenyl rings rather than N–C addition as in the case of polyaniline. Therefore, polydiphenylamine is an aromatic polyamine in which diphenylene units are separated by amino groups. One should also note that the products of polymerization of diphenylamine are characterized by an unusually low degree of oxidation compared to aniline even when the [oxidizer] : [monomer] molar ratio is higher than stoichiometric. Possibly, this is related to the structural strain arising in the course of oxidation of diphenylamine units to diquinodiimine ones and their lower thermodynamic and hydrolytic stabilities. REFERENCES 1. A. Malinauskas, Polymer 42, 3957 (2001). 2. A. G. McDiarmid, Synth. Met. 125, 11 (2002). 3. C. R. Martin, Chem. Mater. 8, 1739 (1996). 4. M. Delvaux, J. Duchet, P.-Y. Stavaux, et al., Synth. Met. 113, 275 (2000).
5. Q. Wu, Z. Xue, Z. Qi, and F. Wang, Polymer 41, 2029 (2000). 6. M. Kryszewski, Synth. Met. 109, 47 (2000). 7. A. V. Orlov, S. G. Kiseleva, G. P. Karpacheva, et al., J. Polym. Sci. 89, 1379 (2003). 8. A. V. Orlov, O. Yu. Yurchenko, S. G. Kiseleva, et al., Vysokomol. Soedin., Ser. A 43, 890 (2001) [Polymer Science, Ser. A 43, 572 (2001)]. 9. T.-C. Wen, C. Sivakumar, and A. Gopalan, Spectrochim. Acta, Part A 58, 167 (2002). 10. P. A. Kilmartin and G. A. Wright, Synth. Met. 104, 145 (1999). 11. L. H. C. Mattoso, L. G. Paterno, S. P. Campana, and O. N. Oliveria, Jr., Synth. Met. 84, 123 (1997). 12. L. H. C. Mattoso and L. O. S. Bulhoes, Synth. Met. 52, 171 (1992). 13. A. H. Kwon, J. A. Conklin, M. Makhinson, and R. B. Kaner, Synth. Met. 84, 95 (1997). 14. X.-H. Wang, L.-X. Wang, X.-B. Jing, and F.-S. Wang, Synth. Met. 69, 149 (1995). 15. X.-H. Wang, J. Li, L.-X. Wang, et al., Synth. Met. 69, 147 (1995). 16. E. V. Strounina, A. P. Leon, and G. G. Kane-Maguire, Synth. Met. 106, 129 (1999). 17. H. S. O. Chan, S. C. Ng, L. S. Leong, and K. L. Tan, Synth. Met. 68, 199 (1995). 18. F. Cataldo, Eur. Polym. J. 32, 43 (1996). 19. D. Goncalves, R. C. Faria, M. Yonashiro, and L. O. S. Bulhoes, J. Electroanal. Chem. 487, 90 (2000). 20. U. Hayat, P. N. Bartlett, G. H. Dodd, and J. Barker, J. Electroanal. Chem. 220, 287 (1987). 21. J. Guay and H. Dao Le, J. Electroanal. Chem. 274, 135 (1989). 22. Yu. V. Karyakin and I. I. Angelov, Pure Chemicals (Khimiya, Moscow, 1974) [in Russian]. 23. A. Weissberger, E. Proskauer, T. Riddick, and E. Toops, Organic Solvents. Physical Properties and Methods of Purification (Wiley, New York, 1955; Inostrannaya Literatura, Moscow, 1958). 24. B. D. Berezin and D. B. Berezin, Modern Organic Chemistry: A Manual for Institutes (Vysshaya Shkola, Moscow, 1999) [in Russian]. 25. F. Lux, Polymer 35, 2915 (1994). 26. P. N. Adams, P. J. Laughlin, A. P. Monkman, and A. M. Kenwright, Polymer 37, 3411 (1996). 27. P. N. Adams, P. J. Laughlin, and A. P. Monkman, Synth. Met. 76, 157 (1996). 28. J. Stejskal, A. Riede, D. Hlavata, et al., Synth. Met. 96, 55 (1998). 29. P. N. Adams and A. P. Monkman, Synth. Met. 87, 165 (1997).
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