Origin of Initial Uncontrolled Polymerization and Its Suppression in the Copper(0)-Mediated Living Radical Polymerization of Methyl Acrylate in a Nonpolar Solvent

Abstract

We report the controlled polymerization of methyl acrylate in the presence of elemental copper at room temperature in a nonpolar solvent. As in single electron transfer (SET) living radical polymerization with nonactivated Cu(0), uncontrolled polymerization is observed early in the reaction. In the absence of marked disproportionation of copper(I) in our system, we propose that copper(0) is oxidized to copper(I) while activating the initiator, which triggers uncontrolled polymerization. Copper(I) then reacts with the alkyl halide initiator to generate active species and copper(II), leading to the establishment of the equilibrium between copper(I) and (II). Delaying the addition of monomer until the initial equilibrium between copper(I) and (II) is established, or addition of a small amount of TEMPO, as radical scavenger for the initial uncontrolled propagating radical species, prevent the early loss of control in polymerization. The system provides high chain end fidelity, and the polymers generated can be chain extended with high efficiency. This system displays many similarities to ICAR, with the reaction between RBr and Cu playing the role of the radical initiator. Use of copper(0) also has the advantage that only a very small amount of catalyst is utilized to mediate polymerization, and the copper catalyst is easily removed by filtration.