[C:1]([O:4][CH:5]=[CH2:6])(=[O:3])[CH3:2].[CH2:7]([S:10]([O-:13])(=[O:12])=[O:11])[CH:8]=[CH2:9].[Na+:14].CC(N=NC(C#N)(C)C)(C#N)C>CO>[C:1]([O:4][CH:5]=[CH2:6])(=[O:3])[CH3:2].[CH2:7]([S:10]([O-:13])(=[O:12])=[O:11])[CH:8]=[CH2:9].[Na+:14]A glass polymerization vessel equipped with a reflux condenser, a dropping funnel, a thermometer and a stirrer was charged with 430 parts of vinyl acetate, 215 parts of methanol, 30 parts of sodium allylsulfonate and 0.20% by mole of azobisisobutylonitrile per mole of the vinyl acetate. After carrying out the polymerization at a temperature of 60° to 65° C. for 8 hours, the residual vinyl acetate was removed by a conventional method to give vinyl acetate-sodium allylsulfonate copolymer containing 2.5% by mole of sodium allylsulfonate unit. Then the copolymer was hydrolyzed by a conventional method to give a modified PVA of which degree of hydrolysis in vinyl acetate unit was 98.5% by mole.
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