Mechanism of Carbon−Halogen Bond Reductive Cleavage in Activated Alkyl Halide Initiators Relevant to Living Radical Polymerization: Theoretical and Experimental Study

Abstract

The mechanism of reductive cleavage of model alkyl halides (methyl 2-bromoisobutyrate, methyl 2-bromopropionate, and 1-bromo-1-chloroethane), used as initiators in living radical polymerization (LRP), has been investigated in acetonitrile using both experimental and computational methods. Both theoretical and experimental investigations have revealed that dissociative electron transfer to these alkyl halides proceeds exclusively via a concerted rather than stepwise manner. The reductive cleavage of all three alkyl halides requires a substantial activation barrier stemming mainly from the breaking C-X bond. The activation step during single electron transfer LRP (SET-LRP) was originally proposed to proceed via formation and decomposition of RX(•-) through an outer sphere electron transfer (OSET) process (Guliashvili, T.; Percec, V. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 1607). These radical anion intermediates were proposed to decompose via heterolytic rather than homolytic C-X bond dissociation. Here it is presented that injection of one electron into RX produces only a weakly associated charge-induced donor-acceptor type radical anion complex without any significant covalent σ type bond character between carbon-centered radical and associated anion leaving group. Therefore, neither homolytic nor heterolytic bond dissociation applies to the reductive cleavage of C-X in these alkyl halides inasmuch as a true radical anion does not form in the process. In addition, the whole mechanism of SET-LRP has to be revisited since it is based on presumed OSET involving intermediate RX(•-), which is shown here to be nonexistent.

Surface Initiated Polymerizations via e-ATRP in Pure Water

“…Also omitting the Cu(II) from solution did not provide any polymer layer which means that the polymer is not grown via a homolytic cleavage of the ATRP initiator. It has recently been shown that this is also a possibility induced via an electrochemical reductive process [37]. Though, the XPS-data indicates that the amount of copper in the polymer layer is lower than 0.1%, the presence of it is obvious from a discoloration, which would not result solely from the polymer.…”

Cu(0)-Mediated Living Radical Polymerization: A Versatile Tool for Materials Synthesis

“…18 In addition, Coote, Guliashvili, and co-workers demonstrated that neither homolytic nor heterolytic bond dissociation applied to the reductive cleavage of C−X in alkyl halides and that a radical anion is not formed during this process, suggesting that the whole SET-LRP mechanism has to be revisited. 276 Although the literature is quite complete for the polymerization in DMSO, the reports regarding aqueous polymerizations are still preliminary and limited. The major difference between DMSO and water is the extent of disproportionation which in the presence of pure water is reported to be ∼100%.…”

Reversible-Deactivation Radical Polymerization in the Presence of Metallic Copper. A Critical Assessment of the SARA ATRP and SET-LRP Mechanisms

“…160 However, the process requires potentials that are more than 1 V more negative than the standard reduction potential of the alkyl halide, and under such highly reducing conditions, radicals would also be reduced to anions, which is not consistent with the radical mechanism of both SARA ATRP and SET-LRP. 160 To determine if the A comparison of the E p values measured at both electrodes with E o points out that the heterogeneous electron transfer, ET, involves high overpotentials (0.5−1.0 V), typical of ET processes concerted with bond rupture. 160 In addition, E p at Cu is 0.5 V more positive than E p at GC, which is often considered to be the best approximation to an outer-sphere donor, i.e., an electrode acting by an OSET mechanism.…”