[C:1]([O:4][CH:5]=[CH2:6])(=[O:3])[CH3:2].[C:7]([NH:11][C:12](=[O:15])[CH:13]=[CH2:14])([CH3:10])([CH3:9])[CH3:8].C(OOC(C)(C)C)(=O)C(C)(C)C.C(NC(=O)C=C)CCC>CO.O>[C:1]([O:4][CH:5]=[CH2:6])(=[O:3])[CH3:2].[C:7]([NH:11][C:12](=[O:15])[CH:13]=[CH2:14])([CH3:10])([CH3:9])[CH3:8]A 2-liter separable flask equipped with a stirring device, a condenser, a thermometer and three metering pumps was used as a polymerization reactor, and charged with 200 ml of methanol, 25.8 g (0.3 mole) of vinyl acetate, and 10.7 g (0.08 mole) of N-t-butyl acrylamide in an atmosphere of nitrogen. With stirring, the temperature of the reaction system was raised to 50° C. When the temperature of the reaction system reached 50° C., 3.48 g (0.02 mole) of t-butyl peroxypivalate was added, and the copolymerization was started. The copolymerization was continued while continuously feeding 172.2 g (2.0 moles) of vinyl acetate from a first metering pump for 2 hours at a rate of 1 mole/hr, a solution of 366.3 g (2.88 moles) of N-t-butyl acrylamide in 800 ml of methanol from a second metering pump for 2 hours and 20 minutes so that the rate of N-butyl acrylamide fed was 1 mole/hr, and a solution of 6.97 g (0.04 mole) of t-butyl peroxypivalate in 200 ml of methanol from a third metering pump for 2.5 hours so that the feed rate of the t-butyl peroxypivalate became 0.01 mole/hr. The copolymerization proceeded while maintaining a homogeneous system. After the copolymerization for 2.5 hours, the polymerization reaction mixture was poured into a large amount of water. The resulting copolymer precipitated and came afloat on the surface of the water. The copolymer was collected, washed thoroughly with water, pulverized by a mixer into a powder, collected by filtration, and dried at 40° C. in vacuo. The amount of the vinyl acetate/N-t-butyl acrylamide copolymer so obtained was 465 g. NMR spectroscopy in deuterochloroform showed that the N-t-butyl acrylamide unit content of the copolymer was 51 mole%.
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