Patent ID: 7691921

Claim:
A method for producing a copolymer for a cement admixture, which comprises a step of polymerizing monomer components comprising at least one unsaturated polyalkylene glycol ether-based monomer (A) represented by the following formula (1): wherein R 1 , R 2 and R 3 independently represent a hydrogen atom or a methyl group; R 4 represents a hydrogen atom or a hydrocarbon group with carbon atoms of 1 to 20; R a represents an alkylene group with carbon atoms of 2 to 18, provided that if m is 2 or more, each of oxyalkylene groups represented by R a O may be the same or different from each other; m represents an average addition mole number of oxyalkylene groups represented by R a O and is in the range of 1 to 300; and X represents a bivalent alkylene group with carbon atoms of 1 to 5, or when a group represented by R 1 R 3 C═CR 2 is a vinyl group, X represents a bond; and at least one unsaturated organic acid-based monomer (B) represented by the following formula (2): wherein R 7 , R 8 and R 9 independently represent a hydrogen atom, a methyl group or —(CH 2 ) z COOM 2 , wherein —(CH 2 ) z COOM 2 may form an anhydride together with —COOM 1 or another —(CH 2 ) z COOM 2 , in which case M 1 or M 2 is not present, and z is in the range of 0 to 2; and M 1 and M 2 independently represent a hydrogen atom, a metal atom, an ammonium group or an organic amine group; wherein the unsaturated polyalkylene glycol ether-based monomer (A) is added into a reactor in advance and the unsaturated organic acid-based monomer (B) is added thereto continuously or stepwise with a decreasing or increasing addition rate, and when the addition rate of the unsaturated organic acid-based monomer (B) is changed continuously, the ratio of the maximum addition rate (V MAX ) and the minimum addition rate (V MIN ) is not less than 1.2 times, and, when the addition rate of the unsaturated organic acid-based monomer (B) is changed stepwise, the ratio of the addition rates of the unsaturated organic acid-based monomer (B) before and after each change is not less than 1.2 times.