SET-LRP of 2-hydroxyethyl acrylate in protic and dipolar aprotic solvents

Abstract

Cu(0) wire-mediated single-electron transfer living radical polymerization (SET-LRP) of 2-hydroxyethyl acrylate (HEA) was performed in protic solvents, MeOH, binary mixtures of MeOH and EtOH with H₂O, and H₂O, and in the dipolar aprotic solvent, DMSO. The tertiary alkyl halide initiator, ethyl 2-bromoisobutyrate (EBiB), and the tris[2-(dimethylamino)ethyl]amine (Me₆-TREN) ligand mediated rapid SET-LRP of HEA providing poly(HEA) (PHEA) with narrow M_w/M_n. When SET-LRP of HEA was performed at high H₂O content in MeOH, and in H₂O, gel formation was observed exclusively on the Cu(0) wire surface. This demonstrated the heterogeneous nature of the Cu(0)-mediated SET activation that promotes a strong adsorption of PHEA and slow diffusion of PHEA radicals generated from activation on the Cu(0) wire surface by the hydrophobic effect. High molecular weight PHEA was obtained at [M]₀/[I]₀ = 400 and 800 with M_w/M_n < 1.45 in MeOH + 40% H₂O in 50 min. This suggests significantly less termination and a much higher rate of SET-LRP of HEA at 25 °C than in the previously reported CuX-catalyzed polymerization of HEA in bulk (90% conversion, M_n = 30 000 after 14 h at 90 °C) or solution (87% conversion, M_n = 14 700 after 12 h at 90 °C) at elevated temperatures. When targeting higher [M]₀/[I]₀, M_w/M_n increases with conversion but first order kinetics was observed. This was attributed to the increased hydrophobic effect of PHEA at higher M_n that is accompanied by slow desorption of the polymer from the Cu(0) wire and a reduced exchange rate between dormant and active species.