360.0 g of methyl ethyl ketone was introduced into a 2-liter three-mouthed flask equipped with an agitator, a thermometer, a circulation cooling tube, and a nitrogen gas inlet tube, and was heated to 75° C. To this was added a mixed liquid comprising 180 g of methyl methacrylate, 32.4 g of methoxyethyl acrylate, 126.0 g of benzyl methacrylate, 21.6 g of methacrylic acid, 72 g of methylethyl ketone, and 1.44 g of “V-601” (manufactured by Wako Pure Chemical Industries Co., Ltd.), this mixed liquid being added dropwise at a uniform rate so as to complete dropping over two hours. When the dropwise addition was completed, a solution comprising 0.72 g of “V-601” and 36.0 g of methylethyl ketone was added, agitation was carried out for 2 hours at 75° C. and a solution comprising 0.72 g of “V-601” and 36.0 g of methylethyl ketone was also added and agitation was carried out for two hours at 75° C. Thereupon, agitation was continued for a further two hours at a raised temperature of 85° C., to yield a resin solution of a copolymer of methyl methacrylate/methoxyethyl acrylate/benzyl methacrylate/methacrylic acid (=50/9/35/6 (weight ratio)).
