Pathways for the Use of Carbazole in the Synthesis of High-Molecular-Weight Compounds¹

V. P. Lopatinskii

(Presented by Prof. L. P. Kulev, Doctor of Chemical Sciences)

Carbazole— a nitrogen-containing heterocyclic compound isolated from the anthracene fraction of coal tar—first found technical application in 1910, when it began to be used in Germany for the manufacture of fast Hydron dyes. Subsequently, the areas of technical application of carbazole were somewhat expanded. The most important stages in the development of industrial syntheses based on carbazole should be considered the production of azo dyes for ice dyeing, of certain insecticides (nitrocarbazoles), and, finally, of polymers and copolymers of 9-vinylcarbazole.

Industrial organic synthesis based on carbazole initially developed only in Germany. In subsequent years (from 1942), the production of polymers and copolymers of 9-vinylcarbazole has also been carried out in the United States. The question of the industrial use of carbazole as a raw material for products of organic synthesis has repeatedly been raised in the Soviet Union. Academician M. I. Il’inskii, Prof. N. A. Nikol’skii, Prof. I. Ya. Postovskii, and many other researchers made a major contribution to the development of methods for isolating carbazole from crude anthracene and to the ways of using it; however, to this day carbazole is not isolated on a large industrial scale and is not used in the organic-synthesis industry.

Meanwhile, the potential resources of carbazole in the products of the coke-chemical industry are sufficiently large for a large-tonnage industrial organic synthesis to be developed on its basis.

At the present scale of coking in the Soviet Union, the chemical industry can count on the use of approximately 50,000 tons of carbazole per year. Hence it becomes clear that the few directions of technical application of carbazole listed above will not be able to ensure consumption of all carbazole resources; consequently, it is necessary to expand research into ways of rational use of carbazole in the organic-synthesis industry.

Such a task was set by us several years ago in the Laboratory of Technology of Basic Organic Synthesis.

In solving this problem we proceeded above all from the necessity of finding ways to use carbazole in the synthesis of large-tonnage products, the production of which could consume significant quantities of carbazole. Therefore, we first of all turned our attention to the synthesis of unsaturated carbazole derivatives which, in homopolymerization and copolymerization with other monomers, could lead to the preparation of high-molecular-weight substances. Along with this, the synthesis, on the basis of carbazole, of starting materials for polycondensation products was of interest.

Finally, the advisability of developing syntheses of high polymers based on carbazole was also determined by the fact that from one unsaturated derivative of carbazole, namely 9-vinylcarbazole, very valuable polymers and copolymers had already been obtained.

Table 1

Properties of vinylcarbazoles

At present, a small number of vinyl derivatives of carbazole and its substituted compounds are known (Table 1). Of these, 9-vinylcarbazole, discovered in 1924 by the Englishmen Clemo and Perkin, Jr. [1], has been subjected to the greatest study. The properties of other vinyl derivatives of carbazole have been little studied. Clemo and Perkin [1] obtained 9-vinylcarbazole by dehydrochlorination of 9-(β-chloroethyl)carbazole, which in turn is obtained by the reaction between carbazole and the β-chloroethyl ester of p-toluenesulfonic acid. The yield of 9-β-chloroethylcarbazole reached 40%; the yield in the second stage was still lower. Thus, this route for the synthesis of 9-vinylcarbazole had no practical significance. Industrial production of this monomer began only after the fundamentals of the process of vinylation with acetylene at high temperatures and pressures had been studied. In 1935 Reppe [2] published a description of the synthesis of 9-vinylcarbazole by vinylation with acetylene; in subsequent years other methods for the synthesis of 9-vinylcarbazole were also proposed [6—17]. However, these methods differ from Reppe’s method in greater complexity and significantly lower yields of the final product. Therefore, in recent years only the Reppe method has undergone technical development. As a result of a careful study of the influence of conditions on the yield of 9-vinylcarbazole and the rate of the process, it proved possible to simplify the synthesis of 9-vinylcarbazole somewhat. In the initial variants [3, 18, 19, 20, 21, 22] the industrial synthesis of 9-vinylcarbazole was carried out in an autoclave under a pressure of 20—25 atm and at a temperature of 160—180°C in the presence of solvents—high-boiling alcohols (for example, ethylene glycol), aromatic or hydroaromatic hydrocarbons, most often in the presence of cyclohexane, hexahydroto-

toluene, hexahydroxylene, and decalin. KOH [2], KOH with the addition of NH₃ or heterocyclic nitrogen-containing bases [23], and, finally, a mixture of KOH and ZnO [24] were proposed as catalysts.

In the latter case, the duration of vinylation was reduced from 3 days to 1.5 hours, and the yield of 9-vinylcarbazole reached 90%. Subsequently [18], an industrial process used in Germany was described; the reaction began at 120° and ended at 280° over 4–5 hours on a catalyst consisting of a mixture of KOH and ZnO; the yield of the technical product was 80–85%, and of the pure product, 68–75%; the acetylene pressure was maintained within 10–20 atm [19, 21, 22].

In the Soviet Union, M. M. Koton and co-workers [16] showed that vinylation can be successfully carried out in a benzine medium (b.p. 120–160°) at an initial acetylene pressure of 7 atm (during the reaction it rises to 10–11 atm) and a temperature of 160–180°. Under these conditions the process was completed in 2 hours, and the yield of the technical product was 80–90%.

In other works, changes in the conditions were achieved by the use of other solvents (cyclohexane [25], vinyl ethyl ether [26], ligroin [27]) or other catalysts (a mixture of NaNH₂ and CaC₂ [27], NaOH [28], a mixture of ZnO and potassium carbazolide [34]).

However, the results of all investigations confirm that the vinylation of carbazole proceeds under comparatively severe conditions (pressure 7–25 atm and temperature 120–280°), that the process is sensitive to moisture, requires the use of very pure carbazole, and is carried out batchwise.

Meanwhile, 9-vinylcarbazole is of interest because, upon its polymerization and especially upon its copolymerization with certain monomers, synthetic resins with very valuable properties are obtained. Monovinylcarbazole itself has recently been used as an impregnating material for capacitors, which after polymerization acquires high heat resistance (up to 150–170°C). Polyvinylcarbazole, produced in Germany under the trade names Luvican M-100, M-125, M-150, and finally M-170 (in the USA—Polectron), is a thermoplastic resin amenable to pressing and injection molding, softening only at about 200°; it is extremely chemically resistant and exhibits excellent dielectric properties (the dielectric loss tangent reaches 0.0008 at a frequency of \(3 \cdot 10^9\) cycles [30, 33].

Many copolymers of 9-vinylcarbazole have been obtained, for example, with styrene, a mixture of styrene and acrylonitrile, p-fluorostyrene, 3,4-difluorostyrene, vinyl acetate, isobutylene, methyl methacrylate, vinylidene, simple vinyl ethers, vinyl chloride, and diene hydrocarbons [29].

The copolymer of 9-vinylcarbazole with styrene has the best properties; it is distinguished by the same dielectric properties as styrene, but has a considerably higher heat resistance (120–140° instead of 80°).

All these circumstances prompted us to test other methods for obtaining 9-vinylcarbazole, of which we settled on the reaction of vinylation of carbazole with vinyl acetate and on synthesis via 9-(β-oxyethyl)carbazole. In the first case, it was of interest to vinylate carbazole under such conditions as would bring about cleavage of vinyl acetate with liberation of a free vinyl group, through the addition of which the vinylation of carbazole would be accomplished. Thus, the following reaction was expected (see p. 13).

The results of the investigations confirmed our supposition regarding the possibility of vinylating carbazole by this route. The best results were obtained when the reaction was carried out in the presence of salts of divalent-

mercuric acetate in an acidic medium. In this case, however, not only was quantitative conversion of carbazole into 9-vinylcarbazole achieved, but also simultaneous conversion of the latter into polymer, since under the reaction conditions the monomer is unstable and rapidly polymerizes [35, 36].

\[ \ce{carbazole + CH3COOCH=CH2 ->[\ce{Hg^{2+}},\ \text{acid}] 9-vinylcarbazole + CH3COOH} \]

The last method for obtaining poly-9-vinylcarbazole differs from the previously proposed methods in that polyvinylcarbazole is formed directly from carbazole in a single stage, while 9-vinylcarbazole is here only an intermediate product. Thus, the process of obtaining polyvinylcarbazole is shortened by one stage, and moreover by a very inconvenient one because of the toxicity of the monomer. The reaction is carried out with equimolar amounts of carbazole and vinyl ether in a solution of aliphatic ketones, in the temperature range from −50 to +50°C and in the presence of catalysts containing divalent mercury. Addition of acid accelerates the polymerization. Quantitative conversion of carbazole into poly-9-vinylcarbazole is attained, depending on the temperature, in 2–24 hours. Among the positive aspects of the process developed is also the fact that the reaction is carried out under mild conditions (at low temperatures and atmospheric pressure), and the duration of the process is short. The process of vinylation of carbazole with vinyl acetate is considerably simpler than the Reppe process, is readily regulated and reproduced, and therefore is of practical interest.

At the same time, we studied another route for the synthesis of 9-vinylcarbazole through 9-(β-hydroxyethyl)carbazole. The most important stage of this process is the preparation of 9-(β-hydroxyethyl)carbazole, which had previously been accomplished by two routes: by the interaction of sodium or potassium carbazole with ethylene oxide in xylene for 5 hours at 50°C with a yield of 92% of theory [10, 18, 37], and by interaction of sodium carbazole with ethylene chlorohydrin upon boiling in acetone [15] or methyl ethyl ketone [14] for 30 hours with a yield of 40% of theory. 9-Vinylcarbazole was obtained from 9-(β-hydroxyethyl)carbazole by heating with NaOH in vacuo [11] and by dehydration with KOH [10, 18, 37], as well as by pyrolysis of 9-(β-acetoxyethyl)carbazole at 575–600°C in the absence of catalysts in a nitrogen atmosphere [13, 15], or at 540°C in the presence of a copper–boron–aluminum catalyst and a residual pressure of 3–20 mm, with a yield of 23% [18, 37, 14]. In studying this route for the synthesis of 9-vinylcarbazole, we found that quantitative conversion of carbazole into 9-(β-hydroxyethyl)carbazole is achieved if its interaction with ethylene oxide is carried out under such conditions that intermediate formation of potassium or sodium carbazole is observed, i.e., in alkaline acetone or methyl ethyl ketone. For example, in the presence of KOH and methyl ethyl ketone, with an equimolar ratio of carbazole and ethylene oxide, a 97–98% yield of 9-(β-hydroxyethyl)carbazole is obtained at 50°C in 50 minutes [38]. The same results are achieved not only when the process is carried out in the liquid phase, but also when gaseous ethylene oxide is passed through a suspension of carbazole in aliphatic ketones in the presence of alkalis. 9-Vinylcarbazole was obtained by us through 9-(β-iodoethyl)carbazole with a yield in the last stage of 91.6% of theory [38]. The simplicity of the indicated processes and the high yields at all stages make the synthesis of 9-vinylcarbazole through 9-(β-hydroxyethyl)carbazole also practically promising. It should be noted that

Under analogous conditions we synthesized, in quantitative yield, 9-(β-oxyethyl)carbazole, from which high-molecular-weight products can likewise be obtained.

In addition to developing and improving methods for the synthesis of 9-vinylcarbazole and its polymers, we began a study of methods for the synthesis of vinylcarbazoles containing vinyl groups in the benzene rings of the carbazole molecule. Such monomers are of still greater practical interest than 9-vinylcarbazole, since they can yield still more heat-resistant polymers. In this direction only one investigation had been carried out (Kawai et al. [5]), which showed that certain 9-alkyl and 9-aryl derivatives of 3,6-divinylcarbazole (Table 1) can, upon polymerization, give polymers with good dielectric properties and high heat resistance (for example, the polymer of 9-ethyl-3,6-divinylcarbazole had a softening temperature of 410°C). In the course of studying methods for the synthesis of 9-alkyl-3-vinylcarbazoles and 9-alkyl-3,6-divinylcarbazoles, we worked out and considerably improved methods for synthesizing a number of intermediate products. The preparation of 9-alkylcarbazoles (the first stage of the synthesis) by the reaction of carbazole with alkyl bromides or iodides was brought to a quantitative yield [31, 32]. Acetylation of 9-alkylcarbazole with acetyl chloride with addition of acetyl bromide (the second stage of the synthesis) can, depending on the conditions, lead to an 80% yield of 9-ethyl-3-acetylcarbazole or to an 80% yield of 9-ethyl-3,6-diacetylcarbazole [31, 32]. The latter result is significantly better than the results achieved previously [39, 40], where 9-ethyl-3,6-diacetylcarbazole is obtained in 10% yield. Reduction of acetylcarbazoles by means of aluminum isopropylate and subsequent dehydration lead to the preparation of the corresponding 3-vinyl-9-alkylcarbazoles and 3,6-divinyl-9-alkylcarbazoles. These vinylcarbazoles are low-melting, colorless substances that are readily polymerized into transparent colorless polymers with heat resistance above 200°C.

Interesting prospects, in our view, are offered by the route for the synthesis of 3-vinyl- and 3,6-divinylcarbazoles through the corresponding β-oxyethyl derivatives of carbazole, the process for preparing which is now being developed by us.

Finally, of considerable interest for the practical use of carbazole may be syntheses, on its basis, of starting substances for polycondensation products. Such substances should include, first of all, certain bifunctional derivatives of carbazole, for example, diamino- and dioxycarbazoles, and, on the other hand, the development of syntheses of silicon-containing derivatives of carbazole is highly promising. It should be noted that very little has yet been done in this direction. Even the syntheses of amino derivatives of carbazole and its substituted compounds have been poorly studied. As regards silicon-containing derivatives of carbazole, the literature contains only a reference to a patent [41], in which the preparation of dicarbazolesilanediol is described.

Thus, the development of methods for the synthesis of monomers and high-molecular-weight substances based on carbazole is still at an initial stage. These methods require serious and comprehensive study, which will make it possible to accelerate the solution of the problem of the practical use of carbazole as a raw material in the chemical industry.

LITERATURE

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2. G. W. Johnson, English pat. 470077 (1937); C. A. 32, 591, 1938; English pat. 460116 (1937); C. A. 32, 591, 1938.

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19. E. Keyßner, W. Wolff, German pat. 642424 (1937); Friedländer, XXIII, 273, 1937.

20. E. Keyßner, German pat. 642939 (1937); Friedländer, XXIII, 274, 1937.

21. H. Beller, R. Christ, F. Wuerth, English pat. 641437 (1950); C. A. 45, 8044, 1951; Amer. pat. 2472085 (1949); C. A. 43, 7036, 1949.

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29. I. P. Losev, E. B. Trostyanskaya. Chemistry of Synthetic Polymers, GKhI, 1960.

30. H. Davidge, J. Appl. Chem., 9, 241, 1959.

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33. G. S. Kolesnikov. Synthesis of Vinyl Derivatives of Aromatic and Heterocyclic Compounds, Academy of Sciences of the USSR, 1960.

34. V. P. Lopatinskii, E. E. Sirotkina, I. P. Zherebtsov. Proceedings of the Tomsk Polytechnic Institute, 112, 1963.

35. N. Buu-Hoi, R. Royer, Rec. trav. chim., 66, 533, 1947; C. A. 42, 5014, 1948.

36. N. Buu-Hoi, R. Royer, J. Org. Chim., 15, 123, 1950.

37. M. Kadonaga, Jap. pat. 1381 (53); C. A. 48, 12178, 1954.

1. Communication 7 in the series “Investigations in the Chemistry of Carbazole Derivatives.” ↩