TEMPO‐Modified Polymethacrylates as Mediators in Electrosynthesis – Redox Behavior and Electrocatalytic Activity toward Alcohol Substrates

Abstract

Homogeneous catalysts (“mediators”) are useful for tuning selectivity in organic electrosynthesis. However, they can have a negative impact on the overall mass and energy balance if used only once or recycled inefficiently. In a previous work, we introduced the polymediator concept, in which soluble redox‐active polymers catalyze the electrochemical reaction, allowing for recovery by dialysis or pressure‐driven membrane filtration. Using anodic alcohol oxidation as a test case, it was shown that TEMPO‐modified polymethacrylates (TPMA) can serve as efficient and reusable mediators. In the present study, the properties of a TPMA sample with well‐defined molecular weight distribution were studied using cyclic voltammetry and compared to low‐molecular TEMPO species. The non‐catalytic profiles of TPMA are shaped by diffusive and adsorptive processes, whereby the latter only become pronounced at low mediator concentrations and high scan rates. Electrocatalytic studies suggest that under the applied conditions, TPMA‐catalyzed alcohol oxidation is a predominantly homogeneous process. The homogeneous kinetics are determined rather by the mediator potential than by steric influences of the polymer backbone.

TEMPO‐Modified Polymethacrylates as Mediators in Electrosynthesis: Influence of the Molecular Weight on Redox Properties and Electrocatalytic Activity

“…UV-vis spectroscopic studies confirmed that PTM • HCl initially contains intact thiobenzoylthio end groups, which are cleaved off during reaction to TPMA. [13] End group removal under alkaline and oxidative conditions is also in agreement with literature reports on the stability of thiobenzoylthio moieties. [21] Further details on preparation, purification, and characterization of the polymers are provided in the Supporting Information.…”

“…Concept of indirect electrolysis using polymediators and polyelectrolytes. [12,13] A) Preparative-scale anodic oxidation of alcohols using polymediators (TPMA) and polyelectrolytes (PTE). [12] B) Polyelectrolyte and Noxyls under investigation in previous works.…”

“…This deviation results from a superposition of adsorptive and diffusive processes, i. e., from charge transfer to both dissolved and adsorbed polymer chains, and has been studied in detail in our previous work. [13] Consequently, the square root dependency of j p,a at low scan rates indicates that at large time scales, the major part of the charge is transferred in diffusive processes. On the other hand, the adsorptive fraction of j p,a that increases linearly with v [24] becomes more pronounced at higher scan rates.…”

“…The same effect has already been observed in our previous study on 3.6-TPMA. [13] Consequently, polymer adsorption seems to be a general effect that occurs independently from the M n value of the polymer.…”

“…It should be noted that c TU is kept constant (5 mM) and that for the polymer samples, the treatment renders apparent diffusion coefficients that refer to individual TEMPO units rather than to entire polymer chains. [12,13,17] To exclude influences of adsorption, only the v range dominated by the diffusive process is evaluated (5-250 mV s À 1 , see Figure 3C). The resulting D values are summarized in Table 2 for the selected examples and in Figure 4 for all studied N-oxyl species.…”