The invention relates to a graft polymer obtainable for copolymerizing: at least one macromonomer of the formula (I) in which: X is a divalent branched or unbranched alkylene radical having from 1 to 12 carbon atoms; R1, R2, R3 are each independently a hydrogen atom or an alkyl group having from 1 to 4 carbon atoms; R4, R5 are each independently a hydrogen atom or a methyl group; R6 is hydrogen atom, an alkyl group having from 1 to 4 carbon atoms, a sulfate group, a phosphate group or a monoester of a dicarboxylic acid, and salts thereof; n is from 5 to 500; and at least one further monomer which has a polymerizable ethylenically unsaturated double bond.