Persulfate-Initiated Polymerization of Acrylamide at High Monomer Concentration

Acrylamide was polymerized in inverse-microsuspension using potassium persulfate as the initiator. Experiments were performed between 40 and 60°C with initial monomer concentrations of 25-50 wt % of the aqueous phase. The rate of polymerization was determined to be proportional to the monomer concentration to the 1.34 power, with 95% confidence limits of ±0.12. The rate order, therefore, does not significantly deviate from 1 25, a dependence reported previously at low and moderate monomer levels This suggests the rate exponent is invariant to the acrylamide concentration up to its solubility limit in water. Limiting conversions have also been observed and have been attributed to the depletion of the persulfate initiator. A reciprocal dependence between the limiting conversion and the initial acrylamide concentration implies that a secondary, monomer-enhanced decomposition reaction is occurring. A hybrid complex-cage mechanism, in which initiator-monomer association is a necessary precursor to enhanced decomposition, will be shown to give good quantitative predictions of the polymerization rate, monomer and initiator consumption, and molecular weight. Further, it overcomes the thermodynamic inconsistencies characteristic of unmodified cage or complex theories.