RESEARCH IN THE CHEMISTRY OF CARBAZOLE DERIVATIVES

60. STUDY OF THE CHLOROMETHYLATION PROCESS OF COPOLYMERS OF STYRENE WITH 9-VINYLCARBAZOLES

E. E. Sirotkina, V. P. Lopatinskii, S. I. Kudinova, A. F. Ogarev

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

Earlier, in studying the chloromethylation reaction of crosslinked copolymers of 9-vinylcarbazole with styrene and divinylbenzene in the presence of zinc chloride, it was found that, during the reaction, additional crosslinking occurs, leading to a decrease in the swelling capacity of the resin; and the greater the content of 9-vinylcarbazole units in the copolymer, the more highly crosslinked the copolymers obtained [1, 2]. Exploratory experiments on the chloromethylation of linear copolymers of 9-vinylcarbazole with styrene confirmed the previously observed crosslinking of three-component copolymers containing 9-vinylcarbazole, whereas polystyrene under chloromethylation in these conditions does not form crosslinked copolymers.

Thus, our preliminary studies showed that, in the chloromethylation of copolymers of 9-vinylcarbazole with styrene, crosslinking proceeds through the carbazole units. It was therefore of interest to study in more detail the chloromethylation process of these copolymers and to show the possibility of synthesizing crosslinked copolymers suitable for producing ion-exchange resins.

Cases of obtaining crosslinked copolymers during the chloromethylation of polystyrene have been described in the literature; however, the process is carried out under different conditions, and in this case it is not possible to obtain crosslinked copolymers of regular structure [3].

The initial copolymers for studying the chloromethylation process were obtained by bead polymerization of 9-vinylcarbazole with styrene according to a previously developed procedure [1]. However, the amount of initiator in this case was increased to 1%, and washing of the copolymer from unreacted monomers was carried out with boiling methyl or ethyl alcohol. As the studies showed, in obtaining linear copolymers the purity of the carbazole monomer plays a greater role than in the case of the synthesis of crosslinked copolymers; it is necessary to use a monomer with a purity of not less than 99%. Moreover, in this case the monomer must not contain even a slight impurity of anthracene, since the latter, being a strong inhibitor of the copolymerization reaction, does not allow solid copolymers to be obtained. The copolymers obtained are whitish granules, insoluble in aliphatic alcohols and hydrocarbons, but soluble in aromatic hydrocarbons.

The characteristics of the copolymers obtained are presented in Table 1. The introduction of chloromethyl groups into the copolymer was carried out with monochlorodimethyl ether (MCDME) in the presence of anhydrous zinc chloride. The degree of chloromethylation was monitored by analysis for total and active chlorine, and the degree of crosslinking by swelling in xylene.

Table 1

Characteristics of the copolymers obtained

The chloromethylation reaction was investigated as a function of the following factors: reaction temperature, the amount of monochlorodimethyl ether, and the amount of catalyst.

To study the chloromethylation process, a copolymer obtained by copolymerization of 20% 9-vinylcarbazole and 80% styrene was taken.

Experiments studying the swelling of the copolymer in monochlorodimethyl ether showed that, already during swelling at 20°C, crosslinking of the copolymer occurs; however, it partially passes into solution. If swelling is carried out in monochlorodimethyl ether containing catalyst, after only 10 minutes the granules swell completely while retaining their shape. As can be seen from the data in Table 2, the sufficient quantity of

Table 2

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

MCDME should be considered to be 10 parts by weight per 1 part by weight of copolymer, since a further increase in the amount of ether affects neither the swelling of the chloromethylated copolymer nor the number of chloromethyl groups introduced into the copolymer and is apparently determined by the degree of adsorption of the ether by the copolymer during swelling.

In studying the effect of temperature, it was found that already at 40°C chloromethylated crosslinked copolymers with stable-

swelling capacity (Fig. 1). However, the optimum reaction temperature should be considered to be 60°C, since under these conditions a significantly larger amount of chlorine is introduced, while the swelling remains at the same level.

From the data in Fig. 2 and Table 3 it is evident that the optimum amount of catalyst should be considered to be 50% of the weight of the copolymer, since in this case the greatest amount of introduced chlorine is achieved while the swelling remains constant.

Table 3

Effect of the amount of catalyst on the chlorine content in the chloromethylated copolymer

Note. Reaction temperature—60°; reaction duration—4 hours; MCME : copolymer 10 : 1.

The assumption that crosslinking of linear copolymers of 9-vinylcarbazole with styrene during chloromethylation occurs through the carbazole nuclei was confirmed in the chloromethylation of a copolymer containing 40% 9-vinylcarbazole units. As can be seen from the data in Table 4, in the chloromethylation of this copolymer, other conditions being equal, the chloromethylation products obtained are more highly crosslinked and have a lower chlorine content.

In the chloromethylation of a copolymer of 3-methyl-9-vinylcarbazole with styrene, a crosslinked chloromethylated copolymer is also obtained, as is evident from the data in Table 4.

Table 4

Characteristics of chloromethylated copolymers

Note. Reaction temperature—60°; amount of ZnCl₂—50%; reaction duration—4 hours.

Experimental Part

Chloromethylation was carried out in a round-bottom flask equipped with a stirrer, a reflux condenser, and placed in a thermostat. Zinc chloride and MCME were introduced into the flask. After dissolution of the cata-

Fig. 1. Effect of the reaction temperature on the content of total chlorine in the chloromethylated copolymer and on its swelling in xylene. Amount of ZnCl₂ — 30%, MHDE : copolymer 10 : 1, reaction time — 4 h.

Fig. 2. Effect of the amount of catalyst on the content of total chlorine in the chloromethylated copolymer; reaction temperature — 60°C, MHDE : copolymer 10 : 1.
Curve I — 50% ZnCl₂
Curve II — 30% ZnCl₂
Curve III — 10% ZnCl₂
Curve IV — 5% ZnCl₂

the copolymer was added to the catalyst, and the contents of the flask were stirred for 10 minutes. After this the temperature was raised to the specified value and maintained for a definite time. Samples were periodically taken from the reaction flask. After completion of the reactions, the copolymer was filtered off and washed with a fivefold amount of methanol or ethanol and a fivefold amount of a 5% hydrochloric acid solution. The chloromethylated copolymer was then washed with distilled water until a negative reaction for chloride ion was obtained. The product in the samples taken was treated in the same way. The air-dried copolymer was analyzed for the content of total and active chlorine, and its swelling in xylene was determined.

By aminating the chloromethylated copolymer with a 20% aqueous solution of trimethylamine at 45°C for 8 hours, an anion exchanger was obtained with an exchange capacity, with respect to hydrochloric acid, of 3.6–3.8 g-eq/g.

Conclusions

1. The process of chloromethylation of linear copolymers of 9-vinylcarbazole with styrene has been investigated, and it has been shown that the final reaction products are cross-linked chloromethylated copolymers suitable for the synthesis of ion-exchange resins.

2. By amination of the chloromethylated copolymer, an anion exchanger was obtained with an exchange capacity of up to 3.8 g-eq/g.

References

1. V. P. Lopatinskii, E. E. Sirotkina, S. I. Kudinova. Izv. TPI (in press).

2. V. P. Lopatinskii, E. E. Sirotkina, S. I. Kudinova. Izv. TPI (in press).

3. K. Paper, H. Paisley, M. Joung. J. Chem. Soc., 1953, 4097.