Method for producing fluorine-containing polymer and composition thereof

A method for producing a fluorine-containing polymer (A) comprising as a main component a structural unit represented by the following formula (A3):

• wherein R6 to R9 each independently represent fluorine, C1-C5 perfluoroalkyl, or C1-C5 perfluoroalkoxy,
• wherein the monomer comprises a monomer (M) corresponding to the structural unit contained as the main component in the fluorine-containing polymer (A);
• the polymerization reaction is performed in an aprotic solvent (B);
• the aprotic solvent (B) is a hydrofluoroether;
• the polymerization initiator is at least one member selected from the group consisting of
• a compound represented by formula (C1):
• wherein R31 and R32 are the same or different, and each is perfluoropropyl, perfluoroisopropyl, perfluoro-2-phenyl-2-propyl, perfluorobutyl, perfluoro-sec-butyl, perfluoro-tert-butyl, perfluoropentyl, perfluoroisopentyl, perfluoroneopentyl, perfluoro-2-methyl-2-pentyl, perfluoro-2,4,4-trimethyl-2-pentyl, perfluorohexyl, perfluoro-2-methylhexyl, perfluoro-2-ethylhexyl, perfluorocyclohexyl, perfluoro-4-methylcyclohexyl, perfluoro-4-ethylcyclohexyl, perfluoroheptyl, perfluoro-2-heptyl, perfluoro-3-heptyl, perfluorooctyl, perfluoro-2-methyl-2-octyl, perfluorononyl, perfluorodecyl, perfluorophenyl, perfluoro-2-methylphenyl, perfluoro-3-methylphenyl, or perfluoro-4-methylphenyl, in which at least one fluorine atom is replaced by hydrogen;
• a compound represented by formula (C2):
• wherein R33 and R34 are the same or different, and each is a group in which at least one fluorine atom of C3-C10 perfluoroalkyl optionally substituted with perfluorophenyl is replaced by hydrogen, or a group in which at least one fluorine atom of perfluorophenyl optionally substituted with linear or branched C1-C4 perfluoroalkyl is replaced by hydrogen;
• a compound represented by formula (C3):
• wherein R35 and R36 are the same or different, and each is a group in which at least one fluorine atom of C1-C10 perfluoroalkyl optionally substituted with perfluorophenyl is replaced by hydrogen, or a group in which at least one fluorine atom of perfluorophenyl optionally substituted with linear or branched C1-C4 perfluoroalkyl is replaced by hydrogen;
• and
• an inorganic peroxide;
• and
• the polymer (A) is dissolved in the aprotic solvent (B), and the amount of the polymer (A) dissolved in the aprotic solvent (B) is 20 mass % or more based on the total mass of the polymer (A) and the aprotic solvent (B).