Process for producing an aqueous polymer dispersion

A process for producing an aqueous polymer dispersion of polymerized ethylenically unsaturated monomers M, where the monomers M comprise

• a) at least one monomer M0 which is selected from compounds of formula 1,
• wherein X is Cl, Br or OS(O)2R3,
• Y is H, C1-C10-alkyl, Cl, Br, OS(O)2R4, or C(═O)OR5,
• R1 is H or C1-C10-alkyl,
• R2 is H or C1-C10-alkyl,
• R3 is C1-C10-alkyl,
• R4 is C1-C10-alkyl, and
• R5 is C1-C20-alkyl;
• b) at least one monomer M1 which is selected from nonionic monoethylenically unsaturated monomers having a solubility in deionized water of at most 60 g/L at 25° C. and 1 bar, and being different from monomers M0,
• c) at least one monomer M2 which is selected from ethylenically unsaturated monomers having a carboxylic acid group,
• d) optionally one or more ethylenically unsaturated monomers M3 that are different from monomers M0, M1, and M2,
• where the process comprises
• i) a first polymerization step of an aqueous radical emulsion polymerization of a first monomer composition M.a to prepare a polymer, wherein the first monomer composition M.a comprises:
• at least 0.1% by weight, based on the total weight of the monomer composition M.a, of at least one monomer M0, and
• at least 1% by weight, based on the total weight of the monomer composition M.a, of at least one monomer M2,
• ii) neutralizing at least 50% of the carboxylic acid groups derived from monomers M2 of the polymer of step i) by addition of a base, and subsequently
• iii) at least one further polymerization step of an aqueous radical emulsion polymerization of a second monomer composition M.b in the presence of the polymer of step ii), where the monomers M comprise
• a. 5 to 95 wt %, based on the total amount of the monomers M, of the monomer compositions M.a., and
• b. 5 to 95 wt. %, based on the total amount of the monomers M, of the monomer composition M.b.