Process for preparing an aqueous polymer dispersion

A process for preparing an aqueous polymer dispersion D(P), the process comprising

• (i) providing a first aqueous mixture M(1) comprising water and ethylenically unsaturated monomers, wherein at least x weight-% of said ethylenically unsaturated monomers consist of one or more monomers according to formula (I)
• wherein R1 is H or CH3, X is CO or CH2, A1 is O or NH, A2 is selected from the group consisting of CH2, C(CH3)2 and CH(OH) and R2 is selected from the group consisting of an ester group, an amido group, a urea group, an alkylamine group, a carbonyl group, an alkyl group and an alkoxy group, wherein from 95 to 100 weight-% of M(1) consist of water and said ethylenically unsaturated monomers;
• (ii) providing a second aqueous mixture M(2) comprising water and at least two different ethylenically unsaturated monomers, wherein from 55 to 98 weight-% of said unsaturated monomers consist of one or more of vinyl aromatic monomers, C1-C20-alkyl esters of methacrylic acid and C1-C20-alkyl esters of acrylic acid, and wherein y weight-% of said ethylenically unsaturated monomers consist of the one or more monomers according to formula (I) with 0≤y<x, and wherein from 95 to 100 weight-% of M(2) consist of water and said at least two different ethylenically unsaturated monomers;
• (iii) introducing M(2) into a polymerization vessel and subjecting the ethylenically unsaturated monomers comprised in M(2) to co-polymerization for a first polymerization period Δt(1), obtaining an intermediate mixture M(i);
• (iv) introducing, after Δt(1), M(1) into the polymerization vessel comprising M(i), admixing M(1) and M(i) obtaining a polymerization mixture M(P), and subjecting the ethylenically unsaturated monomers to co-polymerization, obtaining, after a second polymerization period Δt(2), the aqueous polymer dispersion D(P), wherein at least 95 weight-% of the polymers comprised in D(P) after Δt(2) are based on the ethylenically unsaturated monomers comprised in M(1) and M(2).