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酸基含有ビニル系単量体及びアクリル酸エステル単量体を構成成分とする共重合体であって、該共重合体中に残存する重合開始剤の含有量が２０ｐｐｍ以下であり、かつ残存するアクリル酸エステルの含有量が１０００ｐｐｍ以下であるアルカリ可溶型ビニル系重合体。A copolymer comprising an acid group-containing vinyl monomer and an acrylate monomer as constituents, the content of the polymerization initiator remaining in the copolymer being 20 ppm or less and remaining alkali-soluble vinyl polymer content of acrylic acid ester le is 1000ppm or less. 反応系内のゲージ圧力を０．１ＭＰａ以上とすることによって得られる、請求項１記載のアルカリ可溶型ビニル系重合体。 The alkali-soluble vinyl polymer according to claim 1, obtained by setting the gauge pressure in the reaction system to 0.1 MPa or more. １０時間半減期温度が６０〜１２０℃の有機過酸化物系開始剤を使用することによって得られる、請求項１又は２記載のアルカリ可溶型ビニル系重合体。 The alkali-soluble vinyl polymer according to claim 1 or 2, which is obtained by using an organic peroxide initiator having a 10-hour half-life temperature of 60 to 120 ° C.