To obtain a metal salt capable of extremely suppressing the formation of a cluster when dispersing a metal cation in an organic polymer material in a high concentration by forming the metal cation into a metal salt by locating the metal cation near a monobasic acid of a branched polymer by an electrostatic interaction.
This metal salt is represented by the formula Mn+(RA-)n {Mn+ is a n-valent metal cation; RA- is an anion of a monobasic acid; (n) is an integer of 1-4; R is a branched polymer residue; A is carboxylate or sulfonate; a monobasic acid of the formula RA-H satisfying two relative equations 500≤Mn(GPC)≤20,000 and 1.1≤Mw(GPC)/Mn(GPC)≤15 [Mn(GPC) is a number average molecular weight measured by a gel permeation chromatography method; Mw(GPC) is a weight average molecular weight measured by a gel permeation chromatography method]}. For example, the metal salt is obtained by mixing and heating a metal cation Mn salt of a carboxylic acid such as formic acid and acetic acid with a monobasic acid RA-H corresponding to the RA- to perform an anion exchange reaction.
