AbstractCopolymerization of unsaturated polyester with styrene in water‐in‐oil (W/O) type emulsion was carried out with the use of various basic compounds as emulsifiers. It was found that a stable, gellike W/O type emulsion of unsaturated polyester resin is formed only when pKa's of the bases are above 6 and their concentrations are higher than some critical value. In these conditions, water can be dispersed in emulsion up to 900 to the resin. By polymerization, the stable W/O type emulsion is transformed to a white solid copolymer which is dry to the touch and which contains 90–95 of initially added water. It was confirmed that the basic compounds react with the carboxylic group of the polyester to form at the water‐resin interface polyester salts, which act as true emulsifying agents. The stabilization mechanism of the emulsion at various concentrations of the polyester salt was investigated, mainly by microscopic observations, and an interpretation of the critical value of emulsifier concentration is p
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