RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

51. CHLOROMETHYLATION OF COPOLYMERS OF 3-VINYL-9-ETHYLCARBAZOLE WITH DIVINYLBENZENE

V. P. LOPATINSKII, E. E. SIROTKINA, M. P. GROSHEVA

(Presented by the Scientific-Methodological Seminar of the Organic Departments and Problem Laboratories)

Chloromethylation is an important stage in obtaining highly basic anion-exchange resins, as an intermediate step preceding the amination of the copolymer. The more completely the chloromethylation can be carried out, the greater the number of mobile chlorine atoms in the copolymer that can be replaced by amino groups, which, in turn, will increase the exchange capacity of the finished ion exchanger.

As the chloromethylating agent, monochlorodimethyl ether or dichlorodimethyl ether is most often used.

In the present work, the chloromethylation reaction of cross-linked copolymers of 3-vinyl-9-ethylcarbazole with divinylbenzene was studied with the aim of selecting the most suitable catalyst and establishing the optimum chloromethylation regime.

It is known that the difficulties encountered in the synthesis of chloromethylated derivatives of low-molecular aromatic compounds become more complicated when moving to processes of chloromethylation of copolymers. In this connection, we first carried out a study of the chloromethylation of 9-ethylcarbazole itself, with the aim of identifying the most effective catalyst.

For the study, pure 9-ethylcarbazole was prepared with a melting point of 68°C [1].

Chloromethylation was carried out with the aid of monochlorodimethyl ether (MChDME), and as catalysts such compounds were used that are most widely employed in the chloromethylation of copolymers of styrene with divinylbenzene, namely: SnCl₄; SnCl₂; ZnCl₂; AlCl₃.

The reaction was conducted in a round-bottom flask equipped with a reflux condenser, stirrer, and thermometer. 9-Ethylcarbazole was placed in the reaction flask, covered with monochlorodimethyl ether containing the catalyst suspended in it, and kept at the specified temperature. After the reaction was complete, the chloromethylated product was filtered off, washed first with methyl alcohol and then with a continuous stream of distilled water until traces of chlorine disappeared from the filtrate. The washed product was dried in air. The quantitative content of chlorine in the granules was determined by the Schöniger method [2]. The results of the experiments are presented in Table 1.

In all experiments, after the catalyst suspended in monochlorodimethyl ether was added to 9-ethylcarbazole, the latter hardened and turned into an insoluble compound. This is apparently caused by a secondary reaction of the chloromethyl groups and leads to the formation of a cross-linked polymer. A similar phenomenon has been observed by other authors [3]. As a result of such intermolecular cross-linking, the percentage of chlorine in the chloromethylated product decreases. As can be seen from Table 1, a noticeable cross-linking process is observed already at 0°C.

Table 1

Results of the chloromethylation of 9-ethylcarbazole
(molar ratio 9-ethylcarbazole : MKhDME = 1 : 1.25)

When aluminum chloride was used as the catalyst, the chlorine content in the chloromethylated product initially reached 3.21%; then, when the reaction duration was increased to 4.5 hours, it rose slightly, and with further chloromethylation the chlorine content decreased, probably due to the secondary reaction indicated above.

An increase in the amount of catalyst and in the reaction temperature did not lead to an increase in the chlorine content in the product; on the contrary, the cross-linking process intensified. Similar results were also obtained with the other catalysts.

In order to suppress the side process of cross-linking, chloromethylation of 9-ethylcarbazole was carried out in the presence of a solvent.

However, as is evident from Table 2, the desired effect could not be achieved. The maximum chlorine content in the chloromethylated product was obtained when stannous chloride was used as the catalyst. This catalyst was tested in the chloromethylation of a copolymer of 3-vinyl-9-ethylcarbazole with divinylbenzene. Granular copolymers were prepared by the interaction of 3-vinyl-9-ethylcarbazole with divinylbenzene (10% crosslinking) in the presence of azodiisobutyric acid dinitrile (1% of the weight of the monomers), according to the previously developed procedure [4].

Table 2

Chloromethylation of 9-ethylcarbazole in the presence of dichloroethane

(molar ratio
9-ethylcarbazole : MCDME = 1 : 1.25;
molar ratio
9-ethylcarbazole : dichloroethane = 1 : 6;
reaction temperature 0°C;
reaction time 1.5 hours)

For chloromethylation, the copolymer was placed in a reaction flask and covered with half of the total amount of monochlorodimethyl ether. Swelling was carried out for 14–17 hours at room temperature, after which the remaining portion of monochlorodimethyl ether with the catalyst suspended in it was introduced. The chloromethylated copolymer was treated with methyl alcohol and distilled water, as in the case of chloromethylated 9-ethylcarbazole.

In the course of chloromethylation of the copolymer of 3-vinyl-9-ethylcarbazole with divinylbenzene, we studied the effect on the amount of chlorine introduced of the molar ratio of copolymer to monochlorodimethyl ether, the swelling time of the copolymers, and the duration and temperature of the reaction.

The chloromethylation process is influenced above all by the amount of monochlorodimethyl ether. As is evident from Table 3, with an increase in the amount of monochlorodimethyl ether the chlorine content in the chloromethylated copolymer increases up to a certain limit. In the present case this limiting ratio, copolymer : monochlorodimethyl ether, is 1 : 3. With a further increase in monochlorodimethyl ether, the chlorine content in the copolymer decreases. It is possible that this is due to the occurrence of secondary reactions of the chloromethyl groups, as was observed in the chloromethylation of 9-ethylcarbazole, similarly to what Jones observed in the chloromethylation of polystyrene [5].

Table 3

Effect of the amount of monochlorodimethyl ether on the chlorine content in the chloromethylated copolymer

(SnCl₂ — 0.01 mol/mol of 3-vinyl-9-ethylcarbazole;
temperature 20°C; reaction time 8 hours; copolymer swelling time 16 hours)

For further studies we chose a molar ratio of copolymer : monochlorodimethyl ether equal to 1 : 2, since at the corresp—

ratio of 1 : 2.5 and 1 : 3, the percentage chlorine content in the chloromethylated copolymer does increase, but the mechanical strength of the granules decreases.

The data presented in Table 4 show that, with an increase in the swelling time of the copolymer, the chlorine content in the chloromethylated copolymer increases up to a certain limit. A further increase in the swelling time does not increase the chlorine content in the copolymer.

Table 4

Effect of the swelling time of the copolymer on the chlorine content in the chloromethylated copolymer

\((\mathrm{SnCl}_2 — 0.01\ \text{mole/mole of 3-vinyl-9-ethylcarbazole};\)
temperature \(20^\circ\mathrm{C}\);
reaction time 8 hours;
molar ratio copolymer : MKhDME \(1 : 2)\)

Table 5

Dependence of the chlorine content in the chloromethylated copolymer on the reaction time

\((\text{molar ratio copolymer : MKhDME } 1 : 2;\)
\(\mathrm{SnCl}_2 — 0.01\ \text{mole/mole of 3-vinyl-9-ethylcarbazole};\)
reaction temperature \(20^\circ\mathrm{C}\);
swelling time 16 hours)

The data presented in Table 5 show that the optimum reaction time proved to be 7 hours. With a further increase in the reaction time, the crosslinking process probably increases, as a result of which the chlorine content in the finished product decreases.

Table 6

Dependence of the chlorine content in the chloromethylated copolymer on the reaction temperature
\((\text{molar ratio copolymer : MKhDME } 1 : 2;\ \mathrm{SnCl}_2 — 0.01\ \text{mole/mole of 3-vinyl-9-ethylcarbazole};\) reaction time 8 hours; swelling time 16 hours)

From the data given in Table 6 it is evident that the greatest amount of chlorine can be introduced into the copolymer at room temperature.

Thus, the chlorine content in the chloromethylated copolymer depends not only on the amount of monochlorodimethyl ether, but also on the swelling time of the copolymer, the reaction temperature, and its duration.

As a result of the study it was established that the chloromethylation reaction of granular copolymers of 3-vinyl-9-ethylcarbazole with divinylbenzene can be successfully carried out in 7—8 hours at \(20^\circ\) in the presence of 0.01 mole of \(\mathrm{SnCl}_2\) and 2 moles of monochlorodimethyl ether per 1 mole of 3-vinyl-9-ethylcarbazole.

Conclusions

1. The chloromethylation of 9-ethylcarbazole was investigated.

2. The chloromethylation of a copolymer of 3-vinyl-9-ethylcarbazole with divinylbenzene in the presence of the catalyst SnCl₂ was studied. The principal optimal reaction conditions were established.

References

1. V. P. Lopatinskii, E. E. Sirotkina, L. I. Misailova. Izv. TPI, 112, 39, 1963.

2. W. Schöniger, Mikrochim. Akta, 123, 1955.

3. I. P. Losev, E. B. Trostyanskaya. Chemistry of Synthetic Polymers, Moscow, 1964.

4. V. P. Lopatinskii, E. E. Sirotkina, M. P. Grosheva. Izv. TPI (in press).

5. T. Jones. Ind. Eng. Chem., 44, 2686, 1952.