Into a mixed solvent of 32.4 mL of methanol and 3.6 mL water (methanol/water=9/1) were dissolved 4.9823 g (17.1 mmol) of sodium 11-methacrylamidoundecanoate (NaMAU), 186.5 mg (0.9 mmol) of 2-acrylamido-2-methylpropanesulfonic acid (AMPS), 39.6 mg (0.99 mmol) of sodium hydroxide, and 7.4 mg (0.045 mmol) of azobisisobutyronitrile. The solution was deaerated by bubbling argon for 30 minutes, the container was covered with a septum, and the polymerization was conducted by heating the solution at 60° C. for 12 hours. After the completion of the polymerization reaction, the reaction solution was dropwise added into a large excess of ether, and the resulting precipitate was collected by filtration under suction. This precipitate was dissolved in water, dialyzed against pure water for 1 week, and 2.78 g of random NaMAU/AMPS copolymer (95/5) was obtained by freeze-drying (yield: 53.71%).
