The present invention relates to a process for preparing a waterborne dispersion comprising amphiphilic block copolymer comprising at least blocks [A] and [B], and polymer P comprising ethylenically unsaturated monomer(s) different from monomer(s) (i) (monomer(s) (iii)), whereby the amount of block copolymer is higher than 0.5 and lower than 50 wt. %, based on the total weight of monomers used to prepare the block copolymer and polymer P, wherein the process comprises at least the following steps: (I) subjecting at least ethylenically unsaturated monomer(s) (i) bearing acid-functional groups to a free-radical polymerization process in an aqueous medium in the presence of a free radical initiator and a cobalt chelate complex to obtain block [A], (II) subjecting at least ethylenically unsaturated monomer(s) (ii) different from monomer(s) (i) to an emulsion polymerization process in aqueous medium in the presence of block [A] and a free radical initiator, whereby the amount of ethylenically unsaturated monomer(s) (ii) in block [B] is at least 70 wt. %, relative to the total weight amount of monomers used to prepare block [B] and whereby the ethylenically unsaturated monomer(s) (ii) is (are) selected from the group consisting of methacrylic acid esters, dialkyl esters of itaconic acid, methacrylonitrile, α-methyl styrene and any mixture thereof, (III) subjecting at least ethylenically unsaturated monomer(s) (iii) different from monomer(s) (i) to an emulsion polymerization process in aqueous medium at a pH in the range of from 5 to 10 in the presence of the amphiphilic block copolymer to obtain the block copolymer-polymer P, and the process further comprises deactivating the cobalt chelate complex, which remains from step (I), prior to and/or during step (II).