MECHANISM AND KINETICS OF THE PERSULFATE-INITIATED POLYMERIZATION OF ACRYLAMIDE

Acrylamide was polymerized at high monomer concentrations (25-50 wt %) at temperatures between 40 and 60-degrees-C with potassium persulfate as the initiator. The rate of polymerization was found to be proportional to the monomer concentration to the 5/4th power, a dependence extensively reported at low and moderate levels, suggesting that the rate order is invariant to the acrylamide concentration up to its solubility limit in water. Limiting conversions have also been observed and are reciprocally related to the initial monomer concentration. Both the high rate orders and limiting conversion are found to be manifestations of the same phenomena: the monomer-enhanced decomposition of potassium persulfate. A "hybrid cage-complex" mechanism, in which hydrogen bonding between the monomer and initiator lead to association, has been derived. This postulates that the monomer-initiator associate leads to donor-acceptor interactions between the amide and the persulfate. The decomposition of this charge-transfer complex leads to a secondary initiation reaction, which proceeds in competition with and often in preference to the thermal bond rupture of the peroxide. It will be shown to give good quantitative prediction of the polymerization rate order, monomer and initiator consumption, and molecular weight. Furthermore, the mechanism avoids the free-energy inconsistencies characteristic of prior theories and is generalizable to other nonionic and ionogenic acrylic water-soluble monomers in polar solvents.