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

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, k(p), in aqueous solution at monomer concentrations from 1 to 30 wt % and temperatures between 5 and 70 degrees C. Laser pulse repetition rate has been extensively varied, from 25 up to 500 Hz, to check for consistency of the measured k(p) 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 k(p). The activation energy, E-A(k(p)), of about 18 kJ mol(-1) stays more or less constant within the monomer concentration range under investigation, which indicates that the changes of k(p) are primarily of entropic origin. The pre-exponential factor, A(k(p)), increases from 5.7 X 10(7) to 7.7 X 10(7) L mol(-1) s(-1) upon lowering the AAm concentration from 30 to 10 wt %. These high A(k(p)) numbers demonstrate the fluidizing action of water,on the transition state toward internal rotation with AAm being lower than with acrylates.