TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Conditions: The thermal decomposition is performed in presence of organic solvent at 70-170 degrees C, preferably 70-150 degrees C.Preferred Compounds: The fluorine-containing 2-sulfo alkoxy propionic acid derivative of formula (I) is obtained by neutralizing fluorine-containing 2-fluoro sulfonyl alkoxy propionic acid fluoride derivative of formula (IV). The neutralization is performed using 4 equivalent or more of an alkali metal hydroxide with respect to 1 mole of compound of formula (IV).F-C(=O)-C(CF2X)F-O-(CF2C(Y1)F-O)n-(CFY2)m-SO2F (IV);X,Y1,Y2,m,n, = same as defined above.The derivative of formula (IV) is obtained by reacting cyclic addition product of formula (6), which is obtained by reacting sulfur trioxide with tetrafluoroethylene, with fluoride ion and hexafluoropropylene oxide.The alkyl vinyl ether of formula (II) undergoes chlorination, to obtain fluorine-containing chloro sulfonyl alkyl vinyl ether of formula (V), which subsequently undergoes fluorination, to obtain alkyl vinyl ether of formula (III).CF2=CF-O-(CF2C(Y)F-O)n-(CFY2)m-SO2Cl (V);Y1,Y2,m,n, = same as defined above.Preferred Method: The fluorine-containing 2-sulfo alkoxy propionic acid derivative is dried at 90 degrees C or less until the moisture content is 0.3-10 mass%, and subsequently dried at 100 degrees C or more until the moisture content is 0.2 mass% or less. Mechanical grinding is further performed, and temperature-fall of drying is carried out in order to obtain solid precipitate. Subsequently fluorine-containing sulfo alkyl vinyl ether of formula (II) is purified, and fluorinated, to obtain alkyl vinyl ether of formula (III).Preferred Process: The purification is performed by mixing organic solvent (B), and mineral salt to the aqueous solution containing fluorine-containing sulfo alkyl vinyl ether of formula (II), and organic solvent (A). The solvent (A) is removed and the obtained blend is further mixed with organic solvent (C), and the mineral salt is separated by filtration. When the mixture is mixed with water at 25 degrees C, it separates to two layers, and the solubility of the alkyl vinyl ether of formula (II) in the organic solvent is 10 mass% or less. The solvent (C) does not dissolve the mineral salt but dissolves the fluorine-containing sulfo alkyl vinyl ether. The blend is dried during the purification. The filtered mineral salt is further dried at 25-200 degrees C during purification.Preferred Solvent: The organic solvent has coordination ability to M1 ion or M2 ion. The organic solvent comprised 10-1000 mass parts with respect to 100 mass parts of fluorine-containing 2-sulfo alkoxy propionic acid derivatives, preferably 30-300 mass parts. The organic solvent consists of aprotic organic polar solvent chosen from ether solvent, sulfolane, hexamethyl phosphoric triamide, acetonitrile, dimethyl formamide, dimethyl sulfoxide, and/or tetramethyl urea. The ether solvent is glyme group solvent, diethyl ether, diisopropyl ether, tetrahydrofuran, dioxane, anisole, and/or crown ether. Glyme group solvent is dimethoxy ethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, tetraethylene glycol dimethyl ether, diethylene glycol monomethyl ether and/or diethylene glycol monoethyl ether. The aprotic organic polar solvent is preferably glyme group solvent, more preferably diethylene glycol dimethyl ether. The solvent (B) is halogen-containing hydrocarbon and/or ether, preferably chloroform, dichloromethane, dichloro pentafluoropropane Freon R225 (TM), or dichloro monofluoroethane Freon R141b(TM).Preferred Properties: The moisture content of diethylene glycol dimethyl ether is 250 ppm or less. The dielectric constant of solvent (B) is 4-10.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Salt: The mineral salt is sodium fluoride. Preferred Definitions:    n = 0;    X,Y2 = fluorine;    m = 2;    A1 = -OM1;    A2 = -OM3;    M1,M3 = sodium or potassium.