The kinetics of acrylic acid (AA) solution polymerizations were characterized under various polymerization conditions using differential scanning calorimetry. The AA polymerization rate Rp decreased with increasing pH over all pH values due to the decreased reactivity of ionized AA relative to the unionized monomer. The rate behavior above pH 6 was suggestive of diffusion-controlled termination kinetics. The degree of ionization thus impacted not only the reactivity of species in solution but also the diffusive characteristics of propagating macroradicals. As the AA concentration was increased, the change in polymerization rate with pH was substantially less. The temperature dependence of Rp indicated that the overall activation energy for polymerization varied with temperature, due to diffusion-controlled termination kinetics. The polymerization rate was significantly higher when a multifunctional cross-linking agent was added, because of the gel effect, even while maintaining a constant double bond concentration.
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