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.

Kinetic Studies of Elementary Reactions in SET-LRP / SARA ATRP

“…: the rates of comproportionation (k comp ) and disproportionation (k disp ) (in SET-LRP, disproportionation dominates; in SARA ATRP the equilibrium favors comproportionation); the rates of activation by copper(0) (k a0 ) and copper(I) (k a1 ) (in SET-LRP, all activation occurs by reaction with copper(0)); and the rate of bimolecular termination (zero or negligible in SET-LRP). The measurement of the rates of these elementary reactions has been a significant focus of our (19)(20)(21) and other groups' (22)(23)(24)(25) work in this field.…”

“…Thus performing polymerization in the presence of copper(0) in toluene (19,26,27) is a useful model for SET-LRP or SARA ATRP reactions in solvents that favor disproportionation or comproportionation, as equilibration between copper species can be neglected, leading to the simplified mechanism shown in Scheme 3. It is immediately apparent from Scheme 3 that the reaction involves net consumption of copper(0) and generation of dead polymer -indeed it could be summarized as:…”

“…As this occurs, the rates of activation by copper(I) and deactivation by copper(II) will also increase, causing the ratio of copper(I) to copper(II) to approach the value given by the ATRP equilibrium (19) (note that the concentration of copper(II) will always be slightly below the ATRP equilibrium value as a result of the continuing generation of copper(I) from copper(0)):…”

“…Full experimental details for simultaneous measurement of comproportionation and activation rate constants may be found in reference (19). Data for the measurement of chain length dependence on activation and termination rate constants were obtained from Levere et al, reference (12).…”

Copper‐mediated controlled radical polymerization in continuous flow processes: Synergy between polymer reaction engineering and innovative chemistry

“…Matyjaszewski has proposed that SET‐LRP cannot occur via an OSET process, but rather SET‐LRP should be thought of as a form of ARGET‐ATRP, with the copper(0) metal reducing copper(II) deactivator into copper(I) species, whereas at the same time acting as a supplemental activator (SARA‐ATRP). Other research groups have shown that in nonpolar solvents, copper(0) can act as an activator through reactions with the alkyl halide initiator to generate a secondary radical source in an analogous system to ICAR‐ATRP …”