Modified styrene-maleic acid copolymer and use thereof

A modified styrene-maleic acid copolymer, which is produced by a process comprising reacting a carboxylic acid salt in a styrene-maleic acid copolymer with a halogen compound containing one halogen and no polymerizable unsaturated groups, the halogen compound being selected from the group consisting of benzyl chloride and a benzyl chloride compound having a substituent bound to the benzene ring, except for nitrobenzyl chloride, wherein the styrene-maleic acid copolymer to be modified is represented by a formula,

• where A is a metal element, m is a number from 1 to 3, n is a number from 3 to 300, and both ends are hydrogen atoms, with the proviso that when A is a di- or more valent metal element, the metal element may form a salt with plural caboxy groups which are not limited to ones in a same molecule, and the modified styrene-maleic acid copolymer has at least one ester group formed by a reaction of at least one of carboxyl groups of the styrene-maleic acid copolymer and the group other than halogen of the halogen compound and wherein a modification rate of the carboxyl groups of the styrene-maleic acid copolymer is from 70% to 95%.