): wherein R1 is a hydrogen atom or a methyl group, R2O is one species or a mixture of two or more species of an oxyalkylene group having 2 to 4 carbon atoms, wherein when R2O is a mixture of two or more species of oxyalkylene group having 2 to 4 carbon atoms, (R2O)m is a block or random copolymer, R3 is a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, and m is the average addition mol nu