Ultra-High-Molecular-Weight, Narrow-Polydispersity Polyacrylamides Synthesized Using Photoiniferter Polymerization to Generate High-Performance Flocculants

Ultra-high-molecular-weight, water-soluble polyelectrolytes are commonly employed as flocculants for solid-liquid separation via colloidal destabilization, enabling the rapid and efficient removal of particulate matter from wastewater streams. A drive toward more sustainable and less polluting industrial practices, coupled with the desire to reduce freshwater usage and improve closed-loop systems, demands the development of flocculants with ever-higher dewatering dose performance. Herein, the use of trithiocarbonate-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization under either blue LED (λ_max = 470 nm) or UV (λ_max = 365 nm) irradiation, known as photoiniferter polymerization, was successfully utilized to generate ultra-high-molecular-weight (M_n > 1,000,000 g mol^-1) polyelectrolyte copolymer flocculants with narrow molecular weight distributions (M_w/M_n < 1.2). Cationic and anionic polyelectrolyte flocculants were synthesized containing various monomer compositions of acrylamide (AM), dimethylacrylamide (DMA), 3-(acryloyloxyethyll)trimethylammonium chloride (DMAEAq), 3-(acrylamidopropyl)trimethylammonium chloride (APTAC), sodium acrylate (NaAA), and sodium 2-(acrylamido)-2-methylpropylsulfonate (NaATBS) with high monomer conversion using simple experimental apparatus. The narrow molecular weight distribution cationic polyelectrolytes showed improved flocculation efficiency in the clarification of kaolin suspensions of up to 50% in comparison to a broad polydispersity (M_w/M_n > 5.0) commercial benchmark with an equivalent number average molecular weight. The improved performance of the narrow-polydispersity copolymers is attributed to the reduction in the content of the lower-molecular-weight polymer chains, which impart lower flocculation performance.