INTERACTION OF AMINES WITH VINYL ETHERS

V. VINYLATION OF INDOLE WITH VINYL ACETATE AND SYNTHESIS OF POLY-N-VINYLINDOLE

V. P. Lopatinskii, E. E. Sirotkina, L. F. Kovaleva

(Presented by the scientific seminar of the Faculty of Chemical Technology)

The present work is part of a study in the field of the synthesis of polymers of N-vinylamines by the vinylation of secondary amines with complex vinyl ethers. We have found that the interaction of indole with vinyl acetate in a solution of aliphatic ketones in the presence of a catalyst—mercury(II) sulfate—proceeds analogously to the reaction of carbazole with vinyl acetate described by us earlier [1] and leads to the preparation of poly-N-vinylindole in a single stage.

The literature describes the monomer—N-vinylindole—which was obtained by vinylation of indole with acetylene [2, 3, 4], but there is only a reference to its polymer [4]. Under the conditions studied by us, N-vinylindole is apparently formed as an intermediate product, but it polymerizes rapidly, as a result of which its polymer is the final product.

As a result of the investigation, we established that quantitative conversion of indole into poly-N-vinylindole is achieved if equimolecular amounts of indole and vinyl acetate are used in the reaction, acetone or methyl ethyl ketone is used as the solvent, and mercury(II) sulfate is used as the catalyst. The optimum amounts of catalyst are 2–4% of the weight of indole. A change in temperature from −10°C to 30°C leads to an increase in the yield of polymer and of its high-molecular-weight fraction, which is accompanied by an increase in the softening temperature (Table 1).

Increasing the reaction temperature above 25°C leads to darkening of the polymer as a result of oxidation.

Under optimum conditions, quantitative conversion of indole into poly-N-vinylindole is achieved at 20°C in 4 hours. Since acetic acid is liberated in this process (formed during the decomposition of vinyl acetate), in all experiments the completeness of conversion was monitored simultaneously by the content of acetic acid in the reaction products. For the process to proceed successfully, it is necessary that the catalyst be obtained in the reaction medium (from mercury acetate and sulfuric acid). Mercury acetate was prepared by a known method [5].

Poly-N-vinylindole was isolated from the reaction mixture by adding water or methanol. Since the low-molecular-weight products

the reaction are soluble in methanol; in this way the high-molecular-weight fraction of the polymer was separated, the molecular weight of which was determined cryoscopically. Fractions with molecular weights of 1500—1650 are amorphous white powders with softening temperatures of 175—190° (in a capillary). Poly-N-vinylindole dissolves well in aromatic hydrocarbons, chlorobenzene, pyridine, dioxane, and chloroform; it is poorly soluble in aliphatic alcohols (methyl and ethyl) and insoluble in water and acetic acid. The structure of the polymer as poly-N-vinylindole is proved by the fact that it does not give a nitroso derivative.

Table 1

Effect of temperature on the reaction of indole with vinyl acetate
(with a duration of 4 hours and 2% catalyst)

Experimental Part

The starting indole was of “pure” grade and was then recrystallized twice from benzene to a melting point of 52.5°C. Vinyl acetate was freshly distilled, with b.p. 73°C.

Into a three-necked flask equipped with a thermometer, stirrer, dropping funnel, and placed in a thermostat, 0.49 g of mercury acetate \(((\mathrm{CH}_3\mathrm{COO})_2\mathrm{Hg})\) and 30 ml of acetone are charged, and, with stirring, 0.75 ml of sulfuric acid (sp. gr. 1.84) is added. As a result of the exchange reaction, the catalyst is formed—freshly prepared mercury(II) sulfate. Then 23.4 g of indole is introduced into the flask and 18.4 ml of vinyl acetate is added dropwise with continuous stirring. The reaction is carried out at a constant temperature of \(20 \pm 1^\circ\mathrm{C}\) for 4 hours. Toward the end of the reaction, a sample is taken for the content of acetic acid. After completion of the reaction, the catalyst is filtered off from the reaction mixture. The filtrate is poured into 300 ml of methanol, in which the polymer precipitates. The polymer is filtered off, washed with water to a neutral reaction, and dried.

The yield of the polymer fraction with molecular weight 1630 reaches 73.0% of theoretical. The softening temperature of the polymer is 175—190°C.

Found, %: N 11.80; 11.85. \((\mathrm{C}_{10}\mathrm{H}_9\mathrm{N})_n\). Calculated, %: N 11.9.

Amount of acetic acid formed in the reaction of indole with vinyl acetate, excluding the acid formed from mercury acetate: found 12.05 g; calculated 12.13 g.

Conclusions

1. The interaction of indole with vinyl acetate in the presence of mercury(II) sulfate, leading to the preparation of the polymer N-vinylindole, has been investigated.

1. It has been shown that, under optimal conditions, the quantitative conversion of indole into poly-\(N\)-vinylindole can be achieved, and the yield of the fraction with a softening temperature of \(175—190^\circ\mathrm{C}\) reaches 73%.

References

1. V. P. Lopatinskii, E. E. Sirotkina. Izvestiya Tomskogo politekhnicheskogo instituta, 111, 44, 1961.
2. Chemistry of Acetylene. Izdatinlit, 1954.
3. W. Reppe, E. Keyßner, German Pat. 618120, 1935; Friedländer, 22, 332.
4. C. E. Schildknecht, Vinyl and Related Polymers, N.-Y., 1952.
5. Yu. V. Koryakin, I. I. Angelov. Pure Chemical Reagents, Goskhimizdat, 1955.