Method for producing a spherical ion exchange resin

A method for producing a spherical crosslinked copolymer, which comprises polymerizing a monofunctional addition-polymerizable monomer and a polyfunctional addition-polymerizable monomer by suspension polymerization in an aqueous medium in the presence of a radical polymerization initiator of the following formula (a) or (b) which has a decomposition temperature for a half-life period of 10 hours within a range of from 60 to 120 DEG C and which is capable of forming at least four radical initiating points per molecule by the decomposition: wherein R1 is a linear or branched C1-10 alkyl group, R2 is a n-substituted C4-15 hydrocarbon group, and n is an integer of at least 2, wherein the plurality of R1 per molecule may be the same or different; (R4-O-O &rparstr& mR5 (b> wherein R4 is a linear or branched C1-10 alkyl group, R5 is a m-substituted C4-25 hydrocarbon group, and m is an integer of at least 2, wherein the plurality of R4 per molecule may be the same or different.