Experimental and modeling study of acrylamide copolymerization with cationic monomers in aqueous medium

Aqueous free-radical copolymerization of acrylamide with the cationic monomer acryloyloxyethyltrimethyl ammonium chloride is investigated in this work by both experimental analysis and model development with emphasis on the copolymer composition. Solution polymerization experiments carried out by in-situ 1H-NMR technique at different values of monomer concentration, initial monomer mixture composition, and ionic strength provided accurate data of monomer composition as a function of conversion. The results revealed a remarkable dependence of the composition behavior upon monomer and electrolyte concentration, as it has been observed in similar polyelectrolyte systems. An advanced model of copolymer composition which takes into account the non-conventional features of this system is proposed and applied to estimate the corresponding reactivity ratios. On the one hand, modeling of aqueous free-radical copolymerization systems involving charged monomers is challenging due to the presence of electrostatic effects arising from the interaction between ionic moieties. The repulsive forces between equally-charged monomer units may result in the dependence of reaction rate coefficients and reactivity ratios upon the concentration of charges in the system due to electrostatic screening effects. Namely, the copolymerization behavior is likely to be affected by parameters such as the concentration of the charged monomer and, more generally, the ionic strength of the reaction medium. On the other hand, acrylate polymers are known to be subject to relevant secondary reactions which originate a non-negligible fraction of mid-chain radicals (MCRs) in the system, with consequences on the rate of monomer consumptions as well as copolymer composition. The developed model accounts for both previous aspects. In particular, the electrostatic effect has been simulated through a DLVO-based kinetic approach to express effective rate coefficients as a function of the ionic strength. On the other hand, the role of the secondary reactions has been included by defining reactivity ratios for backbiting and MCR propagation reactions. The proposed model of copolymer composition as a function of conversion explains the experimental dependencies upon monomer and electrolyte concentration for a wide range of experimental conditions. The proposed approach appears to be general enough to be successfully applied to this type of complex copolymerization systems