Impact of Ionic Strength (Sodium Chloride Concentration) on Homopolymerization and Copolymerization Kinetics of Acrylamide and 2-Acrylamido-2-Methylpropane Sulfonic Acid

Anionic polyelectrolytes can be used for a variety of applications, including flocculation and enhanced oil recovery. While it is widely recognized that polyelectrolyte synthesis is impacted by the pre-polymer formulation and polymerization conditions, the specific relationships between these factors and the subsequent polymer properties are not well understood. Therefore, the current work intends to improve understanding of ionic strength (IS) effects during the copolymerization of 2-acrylamido-2-methylpropane sulfonic acid (AMPS) and acrylamide (AAm). The aims of the study are i) to use in situ H1 NMR to study copolymerization kinetics, and ii) to determine how increasing IS impacts copolymerization kinetics (and, by extension copolymer microstructure). It is found that altering IS prior to copolymerization has significant impacts on the reactivity ratios, and therefore impacts the microstructure through multiple mechanisms. Increasing IS causes a crowding effect, where the propagating chain develops a random coil conformation and causes steric hindrance of the large AMPS monomer, decreasing the likelihood of AMPS propagation. When the IS is increased further, the ionic shielding effect is more impactful, increasing the likelihood of AMPS propagation. Ultimately, this work will make it possible to manipulate IS to synthesize AMPS/AAm copolymers with specific desirable properties for target applications. Anionic copolymers are synthesized using free radical polymerization of 2-acrylamido-2-methylpropane sulfonic acid and acrylamide. Vial-scale polymerization and in situ 1H NMR are compared, and in situ 1H NMR is selected to study copolymerization kinetics. The effect of ionic strength on copolymerization kinetics is explored; varying ionic strength impacts the reactivity ratio estimates and resulting copolymer microstructure. image