condenser was charged with 159 0 g of ion exchanged water the reaction device was nitrogen purged under stirring and the solution temperature was controlled to 29 u00b0 c next an aqueous solution of 444 8 g of an unsaturated polyalkylene glycol ether hbve 50 which was formed by adding 50 mol on average of ethylene oxide eo to 4 hydroxybutyl vinyl ether dissolved in 211 2 g of ion exchanged water and an aqueous solution of 85 2 g of acrylic acid and 43 10 g of ion exchanged water mixed with each other were dropwise added to the reaction vessel over 1 5 hours simultaneously an aqueous solution of 1 21 g of l ascorbic acid and 10 2 g of 3 mercaptopropionic acid dissolved in 36 0 g of ion exchanged water and 9 35 g of a 10 aqueous solution of hydrogen peroxide were dropwise added to the reaction vessel over 1 75 hours after the dropwise addition the contents were stirred for 0 5 hours and then the polymerization reaction was completed the solution temperature was controlled between 27 u00b0 c and 30 u00b0 c during the reaction thereafter the reaction solution was neutralized to ph 7 using a 30 aqueous solution of sodium hydroxide thereby providing an aqueous solution of a polycarboxylic acid copolymer 15 having a weight average molecular weight mw of 9700 an aqueous solution of a comparative copolymer 1 shown in table 1 was produced in the same manner as in production.