Novel mono and di peroxyesters characterized by chloro-substitution of a tertiary carbon atom at the alpha position relative to the carbonyl group(s), the monoperoxyesters having no less than 9 carbon atoms and the diperoxyesters having no less than 16 carbon atoms, are utilized as polymerization initiators. Typical is the polymerization of styrene and the copolymerization of vinyl chloride and vinyl acetate.