Itaconic acid is an unsaturated dibasic acid. Its polymerization with an appropriate cross-linking agent would be expected to produce a weakly acidic ion exchange resin having a high exchange capacity. In actual operation, however, there are technical difficulties involved in the polymerization of high melting point (168 degrees centigrade) itaconic acid. According to a process wherein itaconic acid is directly used as a monomer, it is difficult for the polymerization to progress and for resin having a satisfactory exchange capacity to be produced.
In addition, there is a process in which a resin having a bead-like cross-linked structure is obtained from polymerization of liquid itaconic acid diester and divinyl benzene, followed by hydrolysis of the same. In this case, however, the hydrolysis hardly progresses and the process has proved unsuccessful.
Another process in which itaconic anhydride is used as a monomer has been tried. However, the range of application of this process is narrow in view of the fact that copolymerization and hydrolysis progress satisfactorily only when diallyl itaconate, which is highly soluble mutually with itaconic anhydride, is used as the cross-linking agent. Moreover, the ring structure of itaconic anhydride is easily opened by water, thereby forming itaconic acid. Accordingly, it is not possible to obtain bead-like resin by employing water suspension polymerization.
Therefore, a process has been employed in which copolymerization is carried out between the beta-monoalkyl ester of itaconic acid and a cross-linking agent (with the itaconic acid monoester being dispersed as oil drops in water), followed by hydrolysis (U.S. Pat. No. 3,219,596). Satisfactory bead-like ion exchange resin has come to be obtained by this process.
Ion exchange resin obtained by a process using either itaconic anhydride or the beta-monoalkyl ester of itaconic acid as described above has a high exchange capacity and is different from other similar carboxylic acid type resins such as resins made from methacrylic acid as the monomer since it possesses a pair of carboxylic radicals working together. It is known to be particularly effective in adsorbing heavy metal ions, and is useful as a heavy metal capturing agent. In view of the fact that the beads of these resins have a compact structure of a firm gel-type nature, their ion exchange rates are low, which is a major shortcoming.
Under the circumstances, the development of a porous itaconic acid ion exchange resin which retains the aforementioned high exchange capacity and a high level of heavy metal capturing ability and whose ion exchange rate is high has been desired for a long time. However, it has been difficult to prepare the same since itaconic acid resins have all lacked adequate porosity.