Kinetic Modeling of Radical Homo-, Co- and Terpolymerization in Alcohol/Aqueous Solutions

Abstract

Multi-functional polymers used for personal care products can be modified post-production or be used “as-is” to simplify formulations, while providing the same properties as a mixture of polymers. Typical compositions consist of acrylic acid (AA) copolymerized with non-functional monomers such as methyl acrylate (MA) and N-tert-butylacrylamide (t-BuAAm) synthesized by radical polymerization in alcohol/water solutions. However, solvents capable of forming or disrupting hydrogen bonds cause the polymerization kinetics of acrylate and acrylamide monomers to deviate from those observed in bulk or non-polar solvents. Though a good foundation of literature has been developed for the solvent-based kinetics of hydrophobic acrylates in organic solvents and hydrophilic acrylates and acrylamides in water, there is limited knowledge as to how the kinetics evolve as the solvent transitions from organic to aqueous. In this work, the impact of hydrogen bonding solvents on the kinetics of homo-, co- and terpolymer systems is systematically studied using in-situ NMR batch experiments to investigate how reaction temperature, initial monomer content, comonomer composition and ethanol/water ratio in the solvent mixture affect the rate of monomer conversion, copolymer composition drift and final polymer molar masses (MMs). Furthermore, the results from a pulsed laser polymerization investigation are used to develop correlations that capture the variation in homo- and copolymerization propagation rate coefficients with reaction conditions. The investigations are combined to guide the development of a terpolymerization model that includes the formation and consumption of midchain radicals and introduces solvent dependencies for all relevant kinetic mechanisms. The model successfully represents monomer conversion profiles and polymer MMs, branching levels, and copolymer compositions under both batch and industrially-relevant starved-feed semi-batch operating conditions over a broad range of temperatures, monomer contents, solvent compositions and initiator levels. While implemented for MA/AA/t-BuAAm, the model structure can be used to support the development of other acrylate/acrylamide systems as part of the continued efforts to utilize “green” solvents to reduce the environmental impacts of industrial polymerization processes.