Highly purified polylactic acid or a derivative thereof, a salt of the same, and purification method thereof

1. A method for purifying an alkali metal salt of a polylactic acid homopolymer, consisting of the following steps performed sequentially:

dissolving a polylactic acid homopolymer which has a number average molecular weight of 500-20,000 daltons into a water-miscible organic solvent which is acetone or acetonitrile;

adding an aqueous alkali metal salt solution selected from aqueous sodium bicarbonate and aqueous sodium carbonate to the solution of the polymer dissolved in the organic solvent, followed by mixing to produce an alkali metal salt of the polylactic acid, wherein the mixing is carried out by agitating the mixture at 40-100° C. for 10 minutes to 24 hours;

subjecting the mixture to phase separation by adding sodium chloride or potassium chloride to the mixture so that the mixture is separated into an organic solvent layer and an aqueous layer and to recover the organic solvent layer; and

removing the organic solvent from the organic solvent layer and recovering the purified alkali metal salt of the polylactic acid,

and wherein the aqueous alkali metal solution salt has a concentration of 0.05-0.1 g/mL.

2. A method for purifying an alkali metal salt of a polylactic acid homopolymer, consisting of the steps of:

dissolving a polylactic acid homopolymer which has a number average molecular weight of 500-20,000 daltons into a water-miscible organic solvent selected from acetone or acetonitrile;

adding an aqueous alkali metal salt solution selected from aqueous sodium bicarbonate and aqueous sodium carbonate to the solution of the polymer dissolved in the organic solvent, followed by mixing to produce an alkali metal salt of the polylactic acid;

subjecting the mixture to phase separation by adding sodium chloride or potassium chloride to the mixture so that the mixture is separated into an organic solvent layer and an aqueous layer and to recover the organic solvent layer;

removing the organic solvent from the organic solvent layer and recovering the purified alkali metal salt of the polylactic acid;

dissolving the recovered polymer into an anhydrous organic solvent, followed by filtering, to obtain a polymer-containing organic solvent; and

removing the anhydrous organic solvent from the polymer-containing organic solvent,

wherein the aqueous alkali metal salt solution has a concentration of 0.05-0.1 g/mL,

wherein the mixing is carried out by agitating the mixture at 40-100° C. for 10 minutes to 24 hours; and

wherein the steps are performed sequentially.

3. The method for purifying an alkali metal salt of a polylactic acid according to claim 2 , wherein the anhydrous organic solvent is anhydrous acetone or anhydrous acetonitrile.