PLP-SEC Studies into the Propagation Rate Coefficient of Acrylamide Radical Polymerization in Aqueous Solution

Abstract

Acrylamide (AAm) is an important water-soluble monomer. Pulsed-laser polymerization in conjunction with size-exclusion chromatography (PLP-SEC) has been employed to measure the propagation rate coefficient, kp, in aqueous solution at monomer concentrations from 1 to 30 wt % and temperatures between 5 and 70 °C. Laser pulse repetition rate has been extensively varied, from 25 up to 500 Hz, to check for consistency of the measured kp values, which increase toward larger water content, e.g., by 50% upon lowering AAm concentration from 30 to 5 wt %. Variation of pH, up to pH 11, and addition of up to 1 mol L–1 NaCl to the polymerizing system do not significantly affect kp. The activation energy, EA(kp), of about 18 kJ mol–1 stays more or less constant within the monomer concentration range under investigation, which indicates that the changes of kp are primarily of entropic origin. The pre-exponential factor, A(kp), increases from 5.7 × 107 to 7.7 × 107 L mol–1 s–1 upon lowering the AAm concentration from 30 to 10 wt %. These high A(kp) numbers demonstrate the fluidizing action of water on the transition state structure for propagation and on the barrier toward internal rotation with AAm being lower than with acrylates.