Simple Iron Halides Enable Electrochemically Mediated ATRP in Nonpolar Media

An electrochemically controlled atom transfer radical polymerization (eATRP) was successfully carried out with a minimal amount (ppm-level) of FeBr3 catalyst in a nonpolar solvent, specifically anisole. Traditionally, nonpolar media have been advantageous for Fe-based ATRP, but their low conductivity has hindered any electrochemical application. This study introduces the application of electrocatalytic methods in a highly nonpolar polymerization medium. Precise control over the polymerization was obtained by employing anhydrous anisole with only 400 ppm of FeBr3 and applying a negative overpotential of 0.3 V. Additionally, employing an undivided cell setup with two simple iron wire electrodes resulted in a significant 15-fold reduction in electrical resistance compared to traditional divided cell setups. This enabled the production of polymers with a dispersity of ≤1.2. Lastly, an examination of kinetic and thermodynamic aspects indicated that the ppm-level catalysis was facilitated by the high ATRP equilibrium constant of Fe catalysts in nonpolar environments.


Instrumentation
Electrochemical measurements were carried out in a 7-neck glass cell under an Ar atmosphere; an Autolab PGSTAT 30 or 30N potentiostat/galvanostat (EcoChemie, The Netherlands) run by a PC with GPES or NOVA software (EcoChemie) was used.The working electrode, counter electrode and reference electrode used during voltammetric investigations (either CV and LSV) were a 3 mm diameter GC disk (Tokai GC-20), a Pt ring and Ag/AgI/0.1 M n-Bu4NI in DMF, respectively.Before each experiment, the GC disk was cleaned by polishing with a 0.25-µm diamond paste, followed by ultrasonic rinsing in ethanol for 5 min.The reference electrode was always calibrated with ferrocene (Fc), which was added at the end of each experiment as an internal standard, and all potentials are reported versus the ferrocenium/ferrocene (Fc + /Fc) redox couple.Compensation of resistance was applied during all CV experiments (ca.80% resistance compensated), unless otherwise noted.No compensation was applied during electrosynthesis (eATRP) experiments.eATRP experiments were carried out with a Pt mesh (Alfa Aesar, 99.9 % metals basis, area = ca.10 cm 2 , unless otherwise noted) working electrode or an iron wire (Sigma-Aldrich, 99%) and the same reference electrode used in cyclic voltammetry.Before each experiment, the Pt mesh was electrochemically activated in 0.5 M H2SO4 by cycling the potential from -0.7 V to 1 V vs Hg/Hg2SO4 at a scan rate of 0.2 V s -1 (60 cycles).For the polymerizations carried out in a divided cell, a graphite rod was used as counter electrode.The rod was separated from the working solution by a glass frit filled with a solution of Et4NBF4 0.1 M in DMF and a methylcellulose gel saturated with Et4NBF4.For the polymerizations carried out in an undivided cell, iron wire and aluminum wire were used as electrodes without any activation step.Metal wire electrodes were 12 cm long, for a total area of 6 cm 2 .Gel permeation chromatography (GPC) was used to determine the number average molecular weight (Mn) and dispersity (Ɖ) of polymers prepared by eATRP.The GPC instrument was Agilent 1260 Infinity, equipped with a refractive index (RI) detector and two PLgel Mixed-D columns (300 mm, 5 µm) connected in series.The column compartment and RI detector were thermostated at 70 °C and 50 °C, respectively.The eluent was DMF containing 10 mM LiBr, at a flow rate of 1 mL/min.Before injection, the samples were filtered through alumina over a PTFE membrane of 200 nm pore to remove any particulate material and the iron catalyst.The column system was calibrated with 12 linear poly(methyl methacrylate) standards (Mn = 540-2210000).Monomer conversion was determined by 1 H-NMR spectroscopy with a 200 MHz Bruker Avance instrument, using CDCl3 as a solvent.UV-Vis spectra were recorded with an Agilent Cary 5000 spectrophotometer by using 10 mm optical path length quartz cuvettes.

S2.1 eATRP
A thermostated 7-neck electrochemical cell, flushed with an inert gas, was loaded with anisole/MMA (50:50, v/v) + 0.2 M n-Bu4NBF4, the desired amount of iron catalyst and n-Bu4NBr.After recording a CV of the catalyst, the initiator EPBA was injected, and another CV was recorded.Polymerization was then started by applying the selected applied potential (Eapp), and samples were withdrawn periodically to measure monomer conversion, and Mn and Ɖ of the polymer.

S2.2 Estimation of [Fe III ]/[Fe II ] during eATRP
To determine KATRP, kact and kdeact, the evolution of Fe III and Fe II concentrations was monitored during a typical eATRP experiment in a divided cell a Pt WE and a graphite CE.This was done by periodically stopping electrolysis and immediately recording a linear sweep voltammogram under stirring to create steady state conditions.As expected, well-defined waves, showing both anodic and cathodic limiting currents, were observed (see Fig. 4, main text).The anodic and cathodic limiting currents, ILa and ILc, respectively, are given by: 1 where n is the number of exchanged electrons, F is Faraday's constant, A is the area of the electrode, and m Fe II and m Fe III are mass-transfer coefficients of Fe II Br 4 2− and Fe III Br 4 − , respectively.Dividing S1 by S2, with the assumption that the two Fe species have approximately the same mass-transfer coefficient, we obtain: Combining the above equation with the mass balance for iron, [Fe III ] 0 = [Fe III ] + [Fe II ], enables to calculate the concentration of both iron species present in solution.

S4. Determination of the final concentration of iron and characterization of the process
Final iron concentration (   6ℎ ) in polymerization solution (Table 1, entry 9 of the main text) has been determined by spectrophotometric titration of Fe III (50 mM in anisole) with an Agilent Cary 5000 spectrophotometer and 10 mm optical path length quartz cuvette, using the standard addition method.To prepare the sample for analysis, the polymerization media was diluted 50 times (1:49) in anhydrous anisole.An UV-VIS spectrum of this solution was recorded (Figure S5).Then, standard addition of various known aliquots of FeBr3 was performed using a 200 µL micropipette.A UV-Vis spectrum was recorded after every addition, and the final catalyst concentration was calculated from the linear equation obtained by fitting the experimental absorbance at 475 nm (S4).The final Fe concentration in the polymerization solution, after considering the 50x dilution, was determined as 3.25 ± 0.07 mM (corresponding to a release of 1.23 mM additional Fe).This final concentration represents a 65% increase from the initial concentration of 1.88 mM.

Estimation of Faradaic efficiency for process with sacrificial anode
Using a sacrificial Fe anode setup within a non-separated cell configuration, two reactions could occur at the anode.First, the oxidation of the Fe surface takes place, whereby Fe 3+ is liberated into the polymerization solution as per half-reaction (S5).Second, Fe II Br 4 2− (produced at the cathode can reach the anode and undergo oxidation), as described in equation (S6).This last reaction represents a "parasitic" cycle whereby Fe II Br 4 2− is unproductively cycled between the two electrodes (effectively "shorting" the system).These cyclic processes are unproductive but inevitable when using a non-separated cell configuration.Overall, reaction S6 lowers the faradic yield for the generation of new Fe ions from the sacrificial anode.
The faradic yield is computed as follows.Given the initial and final concentrations of iron (  =0 and   =6ℎ ), the theoretical charge required to produce 1.23 mM of catalyst in solution previously calculated by titration ( ℎ ) can be obtained as follows: The total consumed charge during the 6-hour polymerization process was   = 12.8 .The Faradic yield was therefore:

S5. Quantification of ohmic resistance with different setups
During an electrochemical process, the potential difference (ΔV) existing between the CE and the WE arises from the combination of two distinct contributions (Equation S9): i) a potential drop due to the solution resistance (iR); ii) a contribution stemming from the potentials of the semi-reactions at the two electrodes (∆E), including overpotentials related with the electrode reactions (e.g.activation and mass transfer overpotentials).A high cell resistance, particularly when coupled with the utilization of a low-polarity polymerization medium, proves disadvantageous in the context of bulk electrolysis processes.A substantial ohmic component translates to dissipated energy as heat.
In the configuration employing a separate-cell CE (as employed in experiments 1 to 7 of the main text), the ΔV recorded at the onset of the reaction was approximately 90 V. Given that ∆E ≪ iR, we can estimate the cell resistance by approximating ∆ = .Accounting for an initial current of 1.2 mA during the initial stages of polymerization the system resistance is 75 KΩ.
A setup employing sacrificial Fe anode was used to obviate the resistance associated with the presence of the porous septum dividing the anodic and cathodic compartments.The ΔV existing between CE and WE in this setup was approximately 5 V. Calculated resistance in this case, ignoring again ∆E in equation S9, the maximum solution resistance is 5 KΩ.The sacrificial anode configuration thus enables overall energy saving, even in the presence of useless cycles (Equation S6), owing to the diminished system resistance.Moreover, in the lab scale this setup remains applicable with inexpensive potentiostats with low potential compliance.

Figure S5 .
Figure S5.(a) UV-vis spectra of the polymerization solution diluted 50 times (black line) and of subsequent addition of 10, 100, 50, 50 µL stock solution; (b) linear fit of the absorbance registered at 475 nm.