Free-radical polymerization of 2-hydroxyethyl methacrylate (HEMA) supported by a high electric field

Abstract

In macromolecular science, tuning basic polymer parameters like molecular weight (M_n) or molecular weight distribution (dispersity, Đ) is an active research topic. Many prominent synthetic protocols concerning the chemical modification of a polymerization mixture (adding additional reagents) and equipment modification have been adopted for this purpose. On the other hand, less attention has been paid to studying the impact of external stimuli such as pressure, light, and spatial restrictions on the properties of resulting polymers. Here, we present a robust synthetic protocol in which a high electric field (an external factor) supports the thermally-induced free-radical polymerization (FRP) of 2-hydroxyethyl methacrylate (HEMA). The reactions were conducted with 0.1 wt% of 2′-azobisisobutyronitrile (AIBN) in the presence of high dc (direct current) electric fields with various magnitudes ranging from 0 kV cm⁻¹ to 140 kV cm⁻¹. By combining dielectric spectroscopy, nuclear magnetic resonance spectroscopy, exclusion chromatography, and differential scanning calorimetry, we have explored the effect of an external electric field on the progress of polymerization (via dielectric spectroscopy) and product characteristics. We found that HEMA FRP supported by a high dc voltage results in charged macromolecules (polyelectrolytes) with high conductivity ∼ 10⁻¹⁰ S cm⁻¹ at a glass transition temperature, markedly reduced molecular weight of M_n ≈41–58 kg mol⁻¹ (E > 14 kV cm⁻¹), and low/moderate Đ ∼ 1.1–1.3, which is an unexpected finding for non-controlled free radical polymerization. In contrast, the polymer produced in the absence of an electric field was characterized by M_n ≈ 10³ kg mol⁻¹ and much higher Đ = 1.73. Therefore, we found that the electric field can be another efficient external factor such as a spatial restriction or compression that allows for fine-tuning of polymer properties.