Oxygen inhibition and the influence of pH on the inverse emulsion polymerization of the acrylic monomers

The influence of the inhibition periods and the pH of the water phase on the polymerization rate, and consequently on determined rate orders, was observed by a kinetic study of the inverse emulsion polymerization of the water soluble acrylic monomers: acrylamide (AM) and acrylic acid (AA) salts. When the concentration of the inhibiting oxygen in emulsion was lowered by evacuation of the dispersion, the polymerization rate increased with decreasing inhibition period while the molecular weight of the resultant polymer decreased. These kinetic data indicate that ineffective decay of the initiator ammonium persulfate during the inhibition period probably occurs and the retardation of the polymerization rate is only the result of the lower concentration of the initiator at the beginning of the polymerization period. In a kinetic study of the inverse emulsion polymerization of AA, the strong influence of the polymerization rate of AA on the pH of the water phase was observed, This behavior is explained by the same reason as in the polymerization of BA in dilute solution, i.e., by repulsion of the negatively charged radicals and monomeric anions at lower pH and the formation of ion pairs on the ends of propagating radicals at higher pH. Because the inhibition period and the pH of the water phase influence the rate of polymerization of water soluble monomers, their values must be maintained constant in the determination of rate orders. (C) 2001 Elsevier Science Ltd. All rights reserved.