The present invention relates to [1] a method of producing a polymer emulsion for water-resistant coating films with excellent water resistance, including emulsion polymerizing a radical polymerizable monomer (B) in the presence of a compound (A) represented by the following formula (1); and [2] a method of producing a polymer coating film, including a step 1 of emulsion polymerizing a radical polymerizable monomer (B) in the presence of a compound (A) represented by the following formula (1), to obtain a polymer emulsion having an average particle diameter of 30 nm or more and 300 nm or less, and a step 2 of applying the obtained polymer emulsion on a substrate and then drying: ##STR00001##
wherein R.sup.1 represents an alkyl group having 16 or more and 24 or less carbon atoms; m represents an average addition molar number of (CH.sub.2H.sub.2O) and is 0.5 or more and 10 or less; and M represents a cation or a hydrogen atom.

The method includes providing a high impact polystyrene (HIPS) reaction system, wherein the HIPS reaction system has a devolitalizer downstream of a reactor and injecting a retarding agent into the HIPS reaction system prior to the devolitalizer.