A water redispersible polymer powder with superior hydrophobicizing properties is a copolymer of vinyl acetate, a vinyl ester of an .alpha.-branched monocarboxylic acid, an alkyl(meth)acrylate, and a vinyl ester of a long chain unbranched monocarboxylic acid. The polymer is particularly useful as a hydrophobicizing agent for mineral-based chemical building products such as renders, mortars, etc.