Electrostatic effects on the polymerization of vinyl acetate in three-component anionic microemulsions

Abstract

The polymerization of vinyl acetate (VA) in three-component microemulsions stabilized with Aerosol OT is examined as a function of pH. To investigate the role of electrostatic effects on the polymerization kinetics, two initiators (potassium peroxodisulfate and V-50) that decompose into negatively-charged and positively-charged free radicals, respectively, were used. The pH of the reacting medium influences the efficiency of potassium peroxodisulfate (KPS) to initiate the reaction but it has no effect on V-50 efficiency. At neutral pH, faster reaction rates and higher conversions (after 90 min) are achieved with KPS. At equal free radical fluxes and pH's, KPS gives faster reaction rates and larger conversions than V-50 because of the different electrostatic interactions of KPS and V-50 free radicals with the negatively-charged microemulsion droplets and the reacting particles. Final latices contain nanosize polymer particles with particle size of ca. 30 nm in reactions initiated with KPS and between 26 and 40 nm in polymerizations initiated with V-50. In all cases, linear poly(vinyl acetate) is obtained because chain transfer to monomer is the controlling mechanism of chain growth, even at high conversions. This appears to be due to the small particle size that allows the fast desorption rate of the monomeric free radicals from the particles.