Aqueous SET-LRP catalyzed with “in situ” generated Cu(0) demonstrates surface mediated activation and bimolecular termination

Abstract

The aqueous SET-LRP catalyzed with “in situ” generated Cu(0) of the two amphiphilic monomers 2-hydroxyethyl acrylate (HEA) and oligo(ethylene oxide) methyl ether acrylate (OEOMEA) was investigated at temperatures from −22 to +25 °C. The k^app_p values of both monomers are higher at 0 °C (4.61 min⁻¹ for OEOMEA and 2.60 min⁻¹ for HEA) than at 25 °C (1.60 min⁻¹ for OEOMEA and 1.12 min⁻¹ for HEA). These unexpected and unprecedented results are explained by the lower Cu(0) particle size obtained by the disproportionation of CuBr at 0 °C in H₂O. Poly(OEOMEA) obtained by aqueous SET-LRP at 0 °C with the unexpectedly high k^app_p = 4.61 min⁻¹ exhibits 88% chain-end functionality at 100% monomer conversion, while the theoretical value would have to be ∼0%. This high experimental chain-end functionality was explained by the slow desorption of the hydrophobic backbone containing the propagating radicals of these amphiphilic polymers from the surface of the catalyst due to their strong hydrophobic effect. Polymer radicals adsorbed on the surface of Cu(0) undergo monomer addition and reversible deactivation but do not undergo the bimolecular termination that requires desorption. This amplified adsorption–desorption process that mediates both the activation and the bimolecular termination explains the unexpectedly high chain-end functionality of the polymers synthesized by SET-LRP.