ting units of the formula (II): a polymer (III) having repeating units of the formula (III): wherein R1 is a hydrogen atom or a methyl group, R2 is a C1-8 alkylene group, R3 is a C1-18 alkyl group, and X1 is a halogen atom; and a polymer (IV) having repeating units of the formula (IV): wherein R4 is a hydrogen atom or a methyl group, R5 is a C1-8 alkylene group, R6 is a C1-18 alkyl group, and X2 i