Chloromethylation Reactions
Chloromethylation reactions of the aromatic rings of polystyrene and styrene copolymers are being carried out extensively. Chlorodimethyl ether (a carcinogen) is a good solvent for these polymers. It is, therefore, commonly employed as the reagent [181, 182]. Laboratory preparations can be carried out in mixtures of carbon disulfide and ether, using zinc chloride as the catalyst. A 9-h reaction at room temperature yields 10% substitution [183]. The chloromethylation process [184] occurs in two steps. Benzyl methyl ether forms as an intermediate. Cross-linking reaction between the aromatic nuclei and the formed CH2Cl group occurs as side reaction. There are strong indications that the chloromethylation takes place only at one position on the ring [185]. The same is true of Bromo methylation [185]. Stannic chloride is a very effective catalyst for this Friedel-Craft reaction [186]. Iodomethylation can also be carried out in the same manner with similar results [179]. When the reactions are carried out on cross-linked styrene copolymers with chlorodimethyl ether and stannic chloride catalyst, they are accompanied by strong morphological changes [187]. If these reactions are carried out with low levels of chloromethylating agents or catalysts, they occur more or less homogeneously. Larger levels of either of them, however, result not only in greater levels of chloromethylation, but also in higher degrees of secondary cross-linkings and in uneven distributions of the chloromethyl groups [188]. Another technique of chloromethylating polystyrene is to react it with methylal and thionyl chloride in the presence of zinc chloride [189]:
Chloromethyl-substituted polystyrenes can also be prepared from poly (p-methyl styrene) s by treating them with aqueous sodium hypochlorite in the presence of a phase transfer catalyst, like benzyl triethylammonium chloride [190]. The conversions of methyl to chloromethyl groups can be as high as 20%without any detectable morphological changes [187]. If these reactions are carried out with low levels of chloromethylating agents or catalysts, they occur more or less homogeneously. Larger levels of either of them, however, result not only in greater levels of chloromethylation, but also in higher degrees of secondary cross-linkings and in uneven distributions of the chloromethyl groups [188]. Another technique of chloromethylating polystyrene is to react it with methylal and thionyl chloride in the presence of zinc chloride [189]. Techniques for chloromethylating poly aryl ether sulfones, polyphenylene oxide, phenolic resins, and model compounds were described recently [191]. When the subsequent products are converted to quater nary amines, there is a decrease in the quaternization rate with increase in degree of substitution. This may be duetosteric effects imposed by restricted rotation of the polymeric chains [191]. This phenomenon was not observed in quaternization of poly (chloromethyl styrene). The chloromethylation reaction of a polysulfone with chloromethyl ether, catalyzed by stannic chloride, can be illustrated as follows:
Vinyl benzyl chloride monomer is available commercially. It is possible, therefore, to simply prepare the chloromethylated polystyrene or copolymers from the monomer without the chloromethylation reactions.
