Selective Bond Cleavage in RAFT Agents Promoted by Low-Energy Electron Attachment

Abstract

Radical polymerization with reversible addition-fragmentation chain transfer (RAFT polymerization) has been successfully applied to generate polymers of well-defined architecture. For RAFT polymerization a source of radicals is required. Recent work has demonstrated that for minimal side-reactions and high spatio-temporal control these should be formed directly from the RAFT agent or macroRAFT agent (usually carbonothiosulfanyl compounds) thermally, photochemically or by electrochemical reduction. In this work, we investigated low-energy electron attachment to a common RAFT agent (cyanomethyl benzodithioate), and, for comparison, a simple carbonothioylsulfanyl compound (dimethyl trithiocarbonate, DMTTC) in the gas phase by means of mass spectrometry as well as quantum chemical calculations. We observe for both compounds that specific cleavage of the C-S bond is induced upon low-energy electron attachment at electron energies close to zero eV. This applies even in the case of a poor homolytic leaving group (^. CH₃ in DMTTC). All other dissociation reactions found at higher electron energies are much less abundant. The present results show a high control of the chemical reactions induced by electron attachment.

Keywords: RAFT agents; dimethyl trithiocarbonate; dissociative electron attachment; low-energy electron; radical polymerization.

Figure 1

(a) Proposed reactions relevant for activation of RAFT agents by eRAFT (“Electrochemistry pathway”) and standard methods (“Standard RAFT pathway”) in solution which leads to the formation of a polymer (either in active or dormant status) upon addition of monomers M. (b)  and (c)  show the molecular structures of cyanomethyl benzodithioate (CMBDT) and dimethyl trithiocarbonate (DMTTC), respectively.

Figure 2

Intensity map of anions formed upon electron attachment to dimethyl trithiocarbonate, DMTTC. The map shows the intensity of m/z‐selected anions as a function of initial electron energy between ≈0 and 8 eV.

Figure 3

Intensity map of anions formed upon electron attachment to cyanomethyl benzodithioate, CMBDT. The map shows the intensity of m/z‐selected anions as a function of initial electron energy between ≈0 and 12 eV. The energy region below ≈2 eV is omitted for m/z 32 (S⁻) due to the presence of yield from an impurity.

Figure 4

Thresholds (in eV) of the experimentally observed dissociation reactions of DMTTC, left, and CMBDT, right, calculated at M06‐2X/Def2‐TZVP level of theory. The weakest DEA channel for DMTTC, SCSS⁻ with a calculated threshold of 1.81 eV, is not shown.