An aqueous multistage emulsion polymer formed by the free radical polymerization in at least two stages of, in each stage, at least one ethylenically unsaturated nonionic acrylic monomer, the polymer in two of the stages having glass transition temperatures (Tg) differing by at least 10 ° C.; the polymerization, in at least one stage, being effected in the presence of 0.01-1.0%%, by weight based on the dry weight of the stage polymer, t-alkyl hydroperoxide, t-alkyl peroxide, or t-alkyl perester wherein the t-alkyl group includes at least 5 Carbon atoms, preferably t-amyl hydroperoxide, is provided. Also provided is a method for preparing the aqueous multistage emulsion polymer, an aqueous coating composition including the aqueous multistage emulsion polymer and a method for treating a substrate with the aqueous coating composition.