Electrochemical Behavior of Thiocarbonylthio Chain Transfer Agents for RAFT Polymerization

Abstract

Electrochemical activation of thiocarbonylthio reversible addition−fragmentation chain transfer (RAFT) agents (S=C(Z)S-R) is explored as a potential method for initiating RAFT polymerization under mild conditions without producing initiator-derived byproducts. Herein we apply cyclic voltammetry to establish a predominant reduction mechanism, where electrochemical reduction is coupled to an irreversible first-order chemical reaction. Structure-dependent trends in cyclic voltammograms (CVs), and comparison to absorption spectra, clarify the role of R-and Z-groups in determining reduction processes. The major reduction peak moves to more cathodic potentials in the series dithiobenzoates > trithiocarbonates > heteroaromatic dithiocarbamates > xanthates ∼ N-alkyl-N-aryldithiocarbamates, due to the Zgroup influence on thiocarbonyl bond reactivity. More active (electron-withdrawing, radical stabilizing) R-groups shift the reduction peak anodically, in part due to their influence on the rate of the coupled chemical reaction. Analysis of CVs across a range of scan rates revealed that kinetic control over the reduction mechanism is influenced by both the charge transfer rate and chemical reaction rate.

Progress and Perspectives Beyond Traditional RAFT Polymerization

“…Instead, electroreduction appears to irreversibly cleave TCTs at the weak C-S bond to generate anionic fragments, which undergo a variety of side reactions without producing any long-lived radical species. [51,52] To circumvent this effect, electroactive mediators have shown promise for generating radicals in the presence of a reducing electric field, which can initiate and sustain eRAFT indirectly. [53] In addition, a mediated approach using electroactivated oxidizing agents has also been successfully applied by the Fors and Yan groups for indirect TCT oxidation, leading to wellcontrolled cationic RAFT polymerization (vide infra) with excellent temporal regulation.…”

Initiation of RAFT Polymerization: Electrochemically Initiated RAFT Polymerization in Emulsion (Emulsion eRAFT), and Direct PhotoRAFT Polymerization of Liquid Crystalline Monomers

“…[20,21] Electrochemically initiated RAFT polymerization, dubbed eRAFT, has recently attracted attention for similar reasons. [22][23][24][25][26] Electrochemically Initiated RAFT Polymerization (eRAFT) in Emulsion…”

“…We believed that the use of eRAFT polymerization in a dispersed medium might overcome some of the existing limitations to eRAFT, namely the passivation of the working electrode by radical species [22] and various side-reactions involving the RAFT agent at the working electrode. [25] Our approach to electroactive initiation was adapted from a commonly used redox initiation system that comprises ferric sulfate hydrate (Fe 2 III (SO 4 ) 3 .xH 2 O), EDTA, and sodium formaldehyde sulfoxylate (SFS) as reductant, and ammonium persulfate (APS) as oxidant and source of sulfate radical anion as initiating radicals, according to Scheme 1.…”

Selektive Bindungsspaltung in RAFT Agenzien durch niederenergetische Elektronenanlagerung

“…Die exergonische Reaktion wird zusätzlich gefördert, da sie aufgrund der Abwesenheit einer Barriere bei Elektronenenergien nahe 0 eV induziert werden kann, wo die Elektroneneinfang‐Effizienz ein Maximum hat. Aufgrund dieser günstigen Reaktionseigenschaften im Zusammenhang mit der C‐S‐Bindungsspaltung erwarten wir, dass diese schnelle einfache Bindungsspaltung auch in Lösung eine relevante Reaktion sein wird, allerdings in Konkurrenz zu Sekundärreaktionen, [31] die in der vorliegenden Gasphasenstudie nicht untersucht werden können. Interessanterweise sind DMTTC und andere nicht‐substituierte primäre und sekundäre Alkyltrithiocarbonate jedoch schlechte RAFT‐Agenzien bei der Polymerisation.…”