Graft copolymer B, built up from: 15 to 45 wt.-%, preferably 20 to 40 wt.-%, based on graft copolymer B, of a non-cross-linked graft substrate polymer B1 having a glass transition temperature T G below 25°C (DSC, heating rate: 5K/min), consisting of polymerized units derived from monomers B11, B12 and optionally B13: from 95 to 99.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one C 1 -C 10 -alkyl acrylate; from 0.5 to 5 wt.-%, based on the total weight of B11, B12 and B13, of at least one monomer (= (meth)acrylate) of the formula H 2 C=CR 1 -CO-O-R 2 -X, wherein R 1 = H or CH 3, R 2 = C 1 -C 4 -alkanediyl, X =NH 2, OH or and from 0 to 4.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one other copolymerizable, monoethylenically unsaturated mon omer; and 55 to 85 wt.-%, preferably 60 to 80 wt.-%, based on graft copolymer B, of at least one polymer B2 having a glass transition temperature T G above 25°C, grafted (in the form of branches) from the graft substrate polymer, where polymer B2 consists of polymerized units derived from monomers B21 and optional comonomers B22: from 65 to 100 wt.-% of at least one vinylaromatic monomer and/or of a C 1 - C 8 -alkyl-(meth)acrylate, preferably of at least one vinylaromatic mon omer or its mixture with at least one C 1 -C 8 -alkyl-(meth)acrylate; and from 0 to 35 wt.-% of at least one other monofunctional comonomer; where components B1 and B2 give 100 wt.-% in total, and wherein • the graft substrate polymer B1 has a gel content (non-soluble fraction in toluene) below 5 wt.-%, preferably below 3 wt.-%, based on the total amount of B1; • the polymerized units derived from monomer B12 are chemically modified by moieties of the formula H 2 C=CR 1 -CO- (R 1 = H or CH 3) which are covalently bound to the functional groups X; and • the double bond of the moiety of formula is the active site from which polymerized units derived from monomer B21 and optionally comonomer B22 are grafted. 15 to 45 wt.-%, preferably 20 to 40 wt.-%, based on graft copolymer B, of a non-cross-linked graft substrate polymer B1 having a glass transition temperature T G below 25°C (DSC, heating rate: 5K/min), consisting of polymerized units derived from monomers B11, B12 and optionally B13: from 95 to 99.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one C 1 -C 10 -alkyl acrylate; from 0.5 to 5 wt.-%, based on the total weight of B11, B12 and B13, of at least one monomer (= (meth)acrylate) of the formula H 2 C=CR 1 -CO-O-R 2 -X, wherein R 1 = H or CH 3, R 2 = C 1 -C 4 -alkanediyl, X =NH 2, OH or and from 0 to 4.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one other copolymerizable, monoethylenically unsaturated mon omer; and from 95 to 99.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one C 1 -C 10 -alkyl acrylate; from 0.5 to 5 wt.-%, based on the total weight of B11, B12 and B13, of at least one monomer (= (meth)acrylate) of the formula H 2 C=CR 1 -CO-O-R 2 -X, wherein R 1 = H or CH 3, R 2 = C 1 -C 4 -alkanediyl, X =NH 2, OH or and from 0 to 4.5 wt.-%, based on the total weight of B11, B12 and B13, of at least one other copolymerizable, monoethylenically unsaturated mon omer; and 55 to 85 wt.-%, preferably 60 to 80 wt.-%, based on graft copolymer B, of at least one polymer B2 having a glass transition temperature T G above 25°C, grafted (in the form of branches) from the graft substrate polymer, where polymer B2 consists of polymerized units derived from monomers B21 and optional comonomers B22: from 65 to 100 wt.-% of at least one vinylaromatic monomer and/or of a C 1 - C 8 -alkyl-(meth)acrylate, preferably of at least one vinylaromatic mon omer or its mixture with at least one C 1 -C 8 -alkyl-(meth)acrylate; and from 0 to 35 wt.-% of at least one other monofunctional comonomer; where components B1 and B2 give 100 wt.-% in total, and wherein • the graft substrate polymer B1 has a gel content (non-soluble fraction in toluene) below 5 wt.-%, preferably below 3 wt.-%, based on the total amount of B1; • the polymerized units derived from monomer B12 are chemically modified by moieties of the formula H 2 C=CR 1 -CO- (R 1 = H or CH 3) which are covalently bound to the functional groups X; and • the double bond of the moiety of formula is the active site from which polymerized units derived from monomer B21 and optionally comonomer B22 are grafted. from 65 to 100 wt.-% of at least one vinylaromatic monomer and/or of a C 1 - C 8 -alkyl-(meth)acrylate, preferably of at least one vinylaromatic mon omer or its mixture with at least one C 1 -C 8 -alkyl-(meth)acrylate; and from 0 to 35 wt.-% of at least one other monofunctional comonomer; • the graft substrate polymer B1 has a gel content (non-soluble fraction in toluene) below 5 wt.-%, preferably below 3 wt.-%, based on the total amount of B1; • the polymerized units derived from monomer B12 are chemically modified by moieties of the formula H 2 C=CR 1 -CO- (R 1 = H or CH 3) which are covalently bound to the functional groups X; and • the double bond of the moiety of formula is the active site from which polymerized units derived from monomer B21 and optionally comonomer B22 are grafted.