PURPOSE: To obtain titled polyester suitable for dispersants, etc., either by adding an acid sulfite to an unsaturated polyhydric alcohol followed by polycondensation with a polybasic acid, or by adding an acid sulfite to a polyester from an unsaturated polyhydric alcohol and polybasic acid.

CONSTITUTION: (A) An unsaturated polyhydric alcohol such as 2-butene-1, 4-diol and (B) sodium bisulfite are made to react, for example in the presence of an iron oxide by heating to 70°C to prepare a sulfonated polyhydric alcohol, which is then subjected to polycondensation with a polybasic acid such as oxalic acid to obtain the objective polyester. Alternatively, a polyester formed from said unsaturated polyhydric alcohol and polybasic acid is dissolved in, e.g., benzene followed by incorporating, in the presence of an iron oxide, 10W200mol% based on the unsaturated group, of sodium bisulfite in the system and carrying out a reaction at a temperature between 60°C and the boiling point to obtain the objective polyester.

USE: For pastes for use in fibers, paper processing agent, adhesives, coatings, water-soluble films, etc.