Particle size control in the soap-free emulsion polymerization of styrene by an oil-soluble initiator with a weakly acidic water-soluble initiator

•Polystyrene particles were able to be synthesized by oil- and water-soluble initiators.•V-501 worked as radical initiators and weak acid to promote the particle growth.•Particle size was enabled to be controlled by the concentration of V-501.•Number concentration of particle was maintained by V-501.

The soap-free emulsion polymerization of styrene by an oil-soluble initiator, 2,2′-azobis(2-methylpropionitrile) (AIBN), with a water-soluble initiator, 4,4′-azobis(4-cyanovaleric acid) (V-501), was conducted in water to control the size of the synthesized polymeric particles. The particle size and surface potential were monitored as concentration of V-501 used for the polymerization was varied. When the V-501 concentration was increased, the particle size increased because V-501 was a weak acid and reduced the surface potential, thereby promoting particle growth through coagulation. Additionally, V-501 can also function as a radical initiator, increasing the efficiency of the polymerization reaction and, thus, the number concentration of the particles.

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