As associative thickener obtainable by free radical polymerization, the preparation thereof and the use thereof in paper coating slips are described. The associative thickener is formed from (a) acid monomers selected from ethylenically unsaturated C3- to C8-carboxylic acids, (b) associative monomers of the general formula H2C═CR1—COO-(EO)n—(PO)m—R2, in which R1 is hydrogen or methyl, n is a number of at least two, m is a number from zero to 50, EO is an ethylene oxide group, PO is a propylene oxide group and R2 is a C8-C30-alkyl group or a C8-C30-alkaryl group, and (c) nonionic, copolymerizable monomers differing from a) and b), the reaction product having been reacted, after the polymerization, with initiators forming nonionic radicals.