Kinetics and Mechanisms of Acrylamide Polymerization from Absolute, Online Monitoring of Polymerization Reaction

An automatic, continuous, online monitoring technique was used to follow the polymerization of acrylamide under a variety of temperature and initiator conditions, without chromatographic columns. The technique furnishes, as a function of time, the weight-average polyacrylamide mass Mw, the monomer conversion, reduced viscosity, and certain measures of polydispersity. After a complex initial phase following initiator addition, wherein impurities competed with monomer for free radicals, monomer conversion followed a first-order decay during most of the subsequent reaction. For fixed monomer concentration, at every point in conversion beyond very early points, Mw was proportional to the inverse square root of the initiator concentration. Furthermore, the monomer decay time also scales in the same way, and Mw vs conversion is linear during most of the conversion, with a negative slope. Hence, the overall reaction scheme falls within the quasi-steady state approximation (QSSA) of ideal polymerization kinetics. The rate constant for initiator decay, as well as the ratio of propagation rate constant squared to termination rate constant were determined. The activation energy for the potassium persulfate initiator decomposition was also determined. Deviations from the ideal kinetics at early and late conversion are rationalized by existing models. Using a technique for determining instantaneous polydispersity from the derivative of Mw, it was possible to follow the evolution of the polydispersity for the polyacrylamide reactions.