Ultrasonic Emulsion Copolymerization of Styrene and 2-Hydroxyethyl Methacrylate without an Exogenous Chemical Initiator

Due to the complementary combination of hydrophilic and hydrophobic properties, copolymers of styrene (St) and 2-hydroxyethyl methacrylate (HEMA) have been widely employed in coating applications. However, their emulsion polymerization remains challenging because of the strong hydrophilicity of HEMA, which exhibits poor compatibility with hydrophobic St. Ultrasound (US) energy generates acoustic cavitation, leading to the formation of radicals through water sonolysis and sodium dodecyl sulfate (SDS) decomposition. Providing a simpler and greener approach, US-assisted polymerization eliminates the need for chemical initiators while mitigating oxygen inhibition. In this study, an emulsion copolymer was synthesized with styrene (St) and 2-hydroxyethyl methacrylate (HEMA) in aqueous medium using SDS as a surfactant via ultrasonic initiation without using an exogenous chemical initiator. The effects of five key parameters on polymerization yield were studied by employing a one-factor-at-a-time (OFAT) approach to assess the direct effect of individual parameters on polymerization yield. The St-to-HEMA molar ratio, monomer (M)-to-water (W) ratio, and SDS-to-M ratio were varied within the ranges of 1:1-3:1, 5-15 wt%, and 5-15 wt%, respectively. Additionally, the US intensity and temperature were examined between 20-50 W and 30-70 °C, respectively. The optimal conditions were obtained as St/HEMA molar of 1.5, M/W ratio of 10 (w/w)%, and SDS/M ratio of 10 (w/w)%, US power of 34-37 W, and the reaction temperature of 50 °C. Under these conditions, the highest polymerization yield was achieved as 89.10.3%. The copolymer structure of spherical latex nanoparticles was confirmed via FTIR and ¹H-NMR, while the average diameter was measured as approximately 80 nm through SEM.