The present invention relates to a process for the preparation of a
peroxyester which can be used in particular in polymerizations, such as
the polymerization of vinyl chloride, or in the thermosetting of
polyester resins.This process comprises the reaction of a
hydroxyhydroperoxide salt with an acid halide or an acid anhydride under
the following conditions: the molar ratio R.sub.b of the
hydroxyhydroperoxide corresponding to the hydroxyhydroperoxide salt to
the acid halide is between 0.5 and 1.5; or alternatively the molar ratio
R.sub.b' of the hydroxyhydroperoxide corresponding to the
hydroxyhydroperoxide salt to the acid anhydride is between 1.0 and 3.0;
the hydroxyhydroperoxide salt was prepared beforehand by reaction of the
corresponding hydroxyhydroperoxide with a base according to a molar ratio
R.sub.a of the base to the hydroxyhydroperoxide of between 0.5 and 1.5;
and this process is carried out in aqueous media. The invention also
relates to an aqueous emulsion comprising the peroxyester obtained and to
a polymerization process employing a radical initiator comprising this
peroxyester.