Electrochemical control over an ATRP
When one considers the status of ATRP at the end on 2010, after exemplification of ARGET and ICAR ATRP1 and a true emulsion ATRP process,2 one could ask what targets remain to improve in order to make ATRP more industrially compatible? The answers include, as always; reduce costs, elimination/reduction of reagents, define conditions that allow simplified reaction conditions e.g. low levels of O2 permissible, and if possible provide additional degrees of polymerization control.
So what's next?
It is a new environmentally friendly procedure for controlling the ratio of activator to deactivator in an ATRP by electrochemical procedures. Electrochemical methods offer multiple readily adjustable parameters, e.g. applied current, potential, and total charge passed, to manipulate polymerization rates by selective targeting of redox-active catalytic species which will provide industrially scalable procedures that allow for the creation of even more complex and specialized materials.
Cyclic voltammetric (CV) studies of copper complexes suitable for catalyzing atom transfer radical polymerization have been used for over a decade to measure the activity of copper based catalyst complexes in an ATRP.3-4 It was found that the E1/2 value for the redox couple CuI/CuII strongly depends on the nature of the ligand and the halogen, Figure 1.
As the number of coordination sites on the ligand increases, the reducing power of the corresponding CuI complex also increases. With the same ligand, CuCl complexes usually have a more negative potential, lower redox potential, than CuBr complexes. The possibility of using CV electrochemistry as a screening method for selecting ATRP catalysts was subsequently discussed and implemented.5-6 Electrochemistry has even been suggested as a means to remove the copper from the reaction medium by electrodeposition.7
However application of an electrochemical stimulus (i.e. electrolysis), which can be uniquely paired with redox based catalyst systems employed for an atom transfer radical polymerization (ATRP), had been generally overlooked. An initial report exploring the possibility of using an electrochemically-triggered initiation of controlled polymerization sequence during a CV cycle involving activation of alkyl- and benzyl halide initiators by an electrogenerated FeIISalen complex was reported in 2009.8 The reactions between the formed FeIISalen and several organic halides were investigated within the time scale of cyclic voltammetry measurements. A fast reaction was observed between electrogenerated FeIISalen catalyst complex and ATRP initiator molecule in the presence of styrene at 110 0C. Polymerization was initiated and the polydispersity of the resulting low molecular weight polystyrene appeared to depend on the initial FeIIISalen/FeIISalen ratio. However this work was apparently not continued or expanded.
The proposed mechanism of ATRP mediated through electrochemical (re)generation of activators is shown below in Figure 2.
Figure 2. Schematic of proposed mechanism for electrochemical control over an ATRP
The schematic demonstrates the dynamic modulation of polymerization rates through electrochemical means, essentially a specific targeted fraction of the air stable CuIIBr2/Me6TREN catalyst complexes can be reduced to form the CuIBr/Me6TREN activator electrochemically to invoke or trigger a controlled polymerization. In absence of mass transport limitations, the rate of reduction is dictated by the applied potential (Eapp) allowing one to finely tune the polymerization rate by balancing the ratio of CuI:CuII and thereby fine tuning of the polymerization rate. Further to this point, electrochemical methods allow a lower oxidation state catalyst (CuIBr/Me6TREN) to be reverted back to its original higher oxidation state, by simply shifting Eapp to more positive values and in doing so providing a means to deactivate an ongoing polymerization and control temperature exotherms. This dormant time may also be used for modification of polymer or injection of additional reagents/monomers. This procedure for electrochemically mediated ATRP has been given the abbreviation eATRP.
The initial paper that truly studied electrochemical control over an ATRP9 demonstrated that the dynamic modulation of polymerization rates through electrochemical provided a means to allow "on demand" polymerization initiation, cessation, and rejuvenation of a controlled/living radical polymerization process. Applying electricity to the system provided more precise control over the reaction. Electrochemical methods provide significant improvements in almost instantaneous control over an ATRP by offering additional adjustable control parameters, e.g. current, potential, and total charge passed, to manipulate polymerization activation/deactivation, polymerization rates by selective targeting of a selected ratio of redox-active catalytic species. The computer-controlled battery allows one to manipulate the ATRP process in real-time by changing the current or voltage. Moreover, advantages of systems such as ARGET ATRP are maintained by achieving ppm concentrations of catalyst1,10 and tolerance to ambient O2,11 while simultaneously offering an environmentally friendly alternative by elimination of chemical reducing agents and catalyst removal/recycle through electrodeposition.7
A schematic of the equipment used for this initial eATRP study is shown in Figure 3.
Figure 3. Five necked flask with three electrodes used for e-ATRP: the working electrode, the counter electrode, and the reference electrode which were a 3 mm Pt disk (Gamry Instruments), a Pt mesh, and an Ag|Ag+, which contained a 0.1 M AgNO3 andTBAPF6 filling solution in MeCN separated from the working solutions by a porous Vycor tip, respectively.
As noted above applying the appropriate charge is a requirement for a well controlled electrochemical ATRP reaction. In the initial example reduction of the CuIIBr2/Me6TREN complex was accomplished through application of a cathodic current. Prior to the electrolysis experiments, cyclic voltammograms (CVs) of methyl acrylate (MA) and acetonitrile (MeCN) over a potential range of 1.5 volts, from 0.0 to -1.5 V versus Ag|Ag+ reference electrode were acquired to ensure the absence of any redox process that might interfere with the catalyst reduction. After confirming the electrochemical stability of the reaction medium an additional CV in the presence of added CuIIBr2/Me6TREN was acquired to identify the potential window appropriate for accurate manipulation of the oxidation states of the redox-active catalyst in electrolysis experiments. A reversible peak couple attributed to the monoelectronic reduction of the CuIIX2/L complex was observed, as shown in Figure 4. The half wave potential of the copper catalyst (CuII/CuI) was calculated to be E1/2 = -0.69 V versus Ag|Ag+.
Figure 4: Cyclic voltammetry of 1.0 mM CuIIBr2/Me6TREN recorded at v = 0.5 V/s-1 in 50 % (v/v) MA in MeCN with 0.1 M supporting electrolyte.
The application of a -0.66 V potential to the reaction medium resulted in the rapid consumption of monomer, reaching nearly 80 % conversion within four hours, Figure 5A blue points. Linear first order kinetic behavior was observed during the polymerization, Figure 5B, indicating a constant concentration of propagating species, and an excellent correlation persisted between theoretical and experimental molecular weight values as the polymerization progressed. Molecular weight values increased linearly with conversion and molecular weight distributions decreased gradually with increasing monomer conversion reaching a minimum value of approximately Mw/Mn = 1.06. Since the rate of an ATRP is defined by ratio of [CuIBr2/Me6TREN] and [CuIIBr2/Me6TREN], and therefore depends on the extent to which CuIIBr2/Me6TREN is reduced and consequently the Eapp. Two additional and separate polymerizations were conducted with a 30 and 60 mV increase of the Eapp to -0.69 and -0.72 V, respectively. An enhanced polymerization rate was observed at more negative potentials whereas more positive potentials provided a slower rate of polymerization, Figure 5A.
Figure 5:(A) Monomer conversion with respect to time, and (B) number average molecular weight (Mn) and Mw/Mn with respect to conversion as a function of applied potential. Polymerizations conducted in 50% (v/v) MA in MeCN at 25° C with a total reaction volume of approximately 12 mL. [MA]0 = 5.55 M; [MA]0:[EBP]0:[Me6TREN]0:[CuIIBr2]0 = 500:1:0.025:0.025.
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