Kinetics of the dispersion polymerization of acrylamide

Aqueous solutions of acrylamide were dispersed with non-ionic surfactants within isoparafinic hydrocarbons to particles of approx. 1 μm and polymerized in a batch reactor by water-soluble and oil-soluble azo-initiators at 42 to 57°C. The resulting conversion-time curves are S-shaped showing a strong gel effect. For maximum rate of polymerization, the following kinetic expressions were determined for the conditions investigated:

$\text{r}{}_{\mathrm{<mtext>max</mtext>}}\text{=kC}{}_{\mathrm{<mtext>I,o</mtext>}}{}^{0.5}\text{C}{}_{\mathrm{<mtext>M,o</mtext>}}$

for water-soluble initiators;

${}_{\mathrm{<mtext>max</mtext>}}\text{=kC}{}_{\mathrm{<mtext>I,o</mtext>}}\text{C}{}_{\mathrm{<mtext>E,o</mtext>}}{}^{\mathrm{?0.2}}$

For oil-soluble initiators, the overall rate constant k is a function of interface area and temperature. The interface area is dependent on the phase ratio, stirring speed and temperature. For constant interface areas, an activation energy of 26 kJ/mol was found. The overall activation energy of the polymerization is 88.2 kJ/mol, when temperature dependence of the interphase is not taken into account. Polymerization of acrylamide with oil-soluble initiators can be described at low conversions by a model which considers mass transfer of primary radicals, and to a lesser extent of initiator molecules, from the oil phase into the water phase as rate determining step and termination by primary radicals. The resulting molecular weights of the polymer are extremely high (10⁶g/mol) and depend on temperature, stirring speed and concentration of initiator, emulsifier and monomer.