literature these bead polymers preferred for the purposes of the present invention and prepared in process step a have a gel structure and are prepared by processes as described in de a 19 852 667 the conversion of the copolymers to the adsorber takes place in stage b by haloalkylation followed by post crosslinking of the haloalkyl groups with aromatic rings in the polymer skeleton for the haloalkylation use is usually made of suitable reagents preferably chloromethyl methyl ethers these reagents particularly chloromethyl methyl ether may be used in unpurified form in which case the chloromethyl methyl ether may contain methylal and methanol for example as ancillary components an excess of the chloromethyl methyl ether is used and it acts not only as reactant but also as solvent and swelling agent it is therefore generally not necessary to use another solvent the chloromethylation reaction is catalyzed by adding a lewis acid examples of suitable catalysts for the purposes of the invention are iron iii chloride zinc chloride tin iv chloride and aluminum chloride the reaction temperature during the haloalkylation may be from 40 to 80 u00b0 c if the procedure is carried out at atmospheric pressure a particularly advantageous temperature range is from 50 to 60 u00b0 c during.