ABSTRACT:The aqueous precipitation copolymerization of acrylonitrile (AN) and vinyl acetate (VAc) with NaClO 3 /NaHSO 3 redox initiation system is carried out continuously in a 10L pilot-plant reactor. The effects of pH value on ionization equilibrium of NaHSO 3 , polymerization rate, particle morphology, particle size and its distribution, molecular weight and its distribution have been investigated. It has been found that effective concentration of reductant H 2 SO 3 increases with the decrease in pH value. On the other hand, deceasing pH value intensifies the coalescence among particles and then reduces diffusion rate of monomer and radicals into particles. Experimental data shows that the increase of pH value results in higher conversion. It indicates that pH value's effect on particle stability prevails its influence on effective concentration of the reductant. This conclusion is verified by the molecular weight, particle size, and particle morphology.