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May 27, 2016 - In addition, a kinetic model based on the method of moments was also constructed to explain the mismatch in Mn and Mn,theo. Simulation ...
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Kinetic Insights into the Iron-Based Electrochemically Mediated Atom Transfer Radical Polymerization of Methyl Methacrylate Jun-Kang Guo, Yin-Ning Zhou, and Zheng-Hong Luo* Department of Chemical Engineering, School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, P. R. China S Supporting Information *

ABSTRACT: An iron- and methyl methacrylate (MMA)-based electrochemically mediated atom transfer radical polymerization (eATRP) system was developed for the first time. Kinetic behaviors, including the effect of applied potential and catalyst loading, were systematically investigated. Results indicated that with more negative electrode potential, the polymerization rate increased until the mass transport limitation was reached. However, reduction of the catalyst loading had adverse effects on polymerization behaviors, such as decreased polymerization rate and increased molecular weight distributions (Mw/Mn). In addition, a kinetic model based on the method of moments was also constructed to explain the mismatch in Mn and Mn,theo. Simulation results showed that slow initiation significantly influenced on the kinetic behaviors in this system. Iron(II) bromide-catalyzed normal ATRP, iron(III) bromidecatalyzed eATRP, and copper(II) bromide-catalyzed eATRP were conducted to compare and elucidate their respective polymerization reaction kinetic characteristics qualitatively. This work expanded the scope of eATRP from a copper-based system to an iron-based system in terms of polymerization kinetics, with the hope of promoting the widespread application of this method.

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agent (SARA) ATRP,15−17 photoinduced ATRP (photoATRP), 18−20 and electrochemically mediated ATRP (eATRP),17,21−29 have been developed. These environmentally friendly and convenient techniques, which can drastically decrease catalyst loading and improve the tolerance of system to air, greatly promote the application of ATRP techniques in chemistry and chemical engineering. As an important component of polymer reaction engineering, polymerization kinetics has been widely explored. The features of ATRPs can be recognized simply and accessibly by thoroughly analyzing polymerization kinetics. For example, Xue et al.30 reported the iron(III)-catalyzed ATRP with a phosphorus ligand but without an additional reducing agent and analyzed the dynamic characteristics to facilitate the application of this method. Rabea et al.7,8 focused on the polymerization behaviors of methyl methacrylate (MMA) at high conversion and proposed a new method to improve the monomer conversion and polymer quality. Zhu et al.9,31,32 completed a series of studies on the kinetics of ICAR and AGET ATRP using iron catalysts and investigated the effect of various reaction conditions, including temperature, catalyst loading, ligands, and initiators. Matyjaszewski et al.15,33,34 and Buback et al.35,36 significantly contributed to the development of kinetic studies for ironbased ATRP systems, especially for the following three catalytic

INTRODUCTION As versatile controlled radical polymerization techniques, reversible deactivation radical polymerizations (RDRPs) provide access to specialized polymeric materials with precisely tailored architectures, low polydispersity, and well-preserved functionality.1,2 An example of extensively employed RDRPs is atom transfer radical polymerization (ATRP) that mediates fast activation/deactivation equilibrium between active radical species and dormant ones by catalyst-based transition metal complexes.3 Catalysts play a crucial role in polymerization systems under a dynamic equilibrium: low-oxidation-state catalysts activate dormant macromolecular chains to generate high-oxidation-state deactivators and active propagating chains. A wide variety of transition metal complexes, such as multidentate-amines-type copper complexes, phosphines-type iron complexes, half-metallocene-type ruthenium complexes, and other metal complexes, can be employed as active ATRP catalysts.4,5 Compared to other transition metals, the iron catalyst is more attractive on an industrial development perspective because of its lower price and better sustainability owing to the abundant reserves of Fe on Earth. Furthermore, iron-mediated systems with low toxicity and excellent biocompatibility have great potential applications in the field of biomedical science.4−6 In recent years, several improved ATRP techniques, including initiators for continuous activator regeneration (ICAR) ATRP,7−11 activators regenerated by electron transfer (ARGET) ATRP,12−14 supplemental activator and reducing © XXXX American Chemical Society

Received: May 16, 2016 Revised: May 23, 2016

A

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analyze the kinetic behaviors. Furthermore, the polymerization behaviors of three ATRP techniques, namely, FeBr2-catalyzed normal ATRP, FeBr3-catalyzed eATRP, and CuBr2-catalyzed eATRP, were investigated to elucidate their respective polymerization kinetic characteristics qualitatively. Our study aimed to probe the kinetics of polymerization and to expand the scope of eATRP, with hope of promoting the engineering application of this method.

systems: ppm level catalyst system, additional-ligand-free system, and high-pressure system. Compared with other newly developed ATRP techniques, eATRP has been extensively investigated because of its outstanding features. Scheme 1 shows that the regeneration Scheme 1. Mechanism of Iron-Catalyzed eATRP

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EXPERIMENTAL SECTION

Materials. Most chemical reagents were purchased from commercial sources. Tris(2,4,6-methoxyphenyl)phosphine (TMPP, 95%, Adamas), tetra(n-butyl)ammonium bromide (TBABr, 98%, Adamas), tetra(n-butyl)ammonium hexafluorophosphate (TBAPF6, 99%, Adamas), silver nitrate (AgNO3, 99.0%, Sigma-Aldrich), iron(III) bromide (FeBr3, 98%, Alfa), iron(II) bromide (FeBr2, 98%, Alfa), copper(II) bromide (CuBr2, 99%, Adamas), hexamethylated tris(2aminothyl) amine (Me6TREN, 99%, Alfa Aesar), ethyl 2-bromoisobutyrate (Eib-Br, 98%, Alfa), and methyl cellulose (Acros) were used as received. Methyl methacrylate (MMA, Sinopharm Chemical Reagent Co. (SCRC), 99%) was rinsed with 10 wt % aqueous NaOH solution and dried with anhydrous MgSO4 overnight prior to use. Acetonitrile (MeCN, 99.9%, Adamas) was distilled with CaH2 thoroughly to remove residual water. Instrumentation. 1H NMR (Bruker Avance III HD 400, 400 MHz NMR spectrometer) was used to measure the monomer conversion. A size exclusion chromatograph (SEC, Tosoh Corporation) equipped with two HLC-8320 columns (TSK gel Super AWM-H, pore size: 9 μm; 6 × 150 mm, Tosoh Corporation) and a double-path, double-flow a refractive index detector (Bryce) under 30 °C was used for measuring the number-average molecular weights (Mn) and dispersity of polymers with DMF (0.01 mol/L LiBr) as the eluent at a flow rate of 0.6 mL/min using a linear PMMA standard. Besides, the cyclic voltammetries and constant potential electrolysis were carried out on a CHI660E potentiostat (Shanghai, China). Experiments were performed in a three-electrode cell with a platinum (Pt) disk for cyclic voltammetry (CV) and Pt gauze for constant potential electrolysis as working electrode, a Pt wire as counter electrode, and a Ag/Ag+ electrode as reference electrode. The reference electrode was inserted into a glass jacket full of 0.1 M TBAPF6 in acetonitrile, ending in a luggin capillary filled with salt bridge which is made of methyl cellulose gel saturated with TBAPF6 to prevent contamination of reference electrolyte. A glass frit and salt bridge were also used to separate the cathodic and anodic compartments to avoid the reoxidation of low-oxidation state catalyst. Equimolar FeBr3 and TMPP (CuBr2 and Me6TREN for coppercatalyzed eATRP), whose total amount varied in terms of experimental reactions, were added initially to the dried cell, followed with a 30 mL solution of 4.7 M MMA in MeCN containing 0.1 M TBABr

of activators is accomplished by single electron reduction process; its reduction rate is related to the potential applied to an electrode. The concentration ratio of an activator and a deactivator in an equilibrium state can be accordingly regulated and controlled by changing the electrode potential. In this way, the polymerization rate is expected to be artificially regulated by predetermined electrolysis program.37 Furthermore, the transition metal catalyst can be reutilized through diverse electrochemical techniques.23 Thus, copper-based eATRP has been thoroughly investigated and used as a versatile method to synthesize various polymeric materials with well-defined structures. For instance, Magenau et al.23 examined the eATRP kinetics of butyl acrylate (BA) catalyzed by Cu(II) under various formulations and electrochemical conditions and obtained optimized reaction conditions. However, kinetic studies on eATRP by using iron catalysts have yet to be performed. An iron(III) bromide (FeBr3)-based complex was used to catalyze the eATRP of MMA in this study, which is the first work on iron-based eATRP. This study extends eATRP systems from copper-based systems to iron-based systems. The effects of applied potential and catalyst loading were systematically examined to obtain reaction features and optimal process parameters. A kinetic model was also developed to Table 1. eATRP of MMA in Different Reaction Systems entry

applied potential

[M]0:[I]0:[C]0:[L]0a

time (h)

c

no electrolysis Epc + 0.1 V Epc Epc − 0.1 V Epc − 0.2 V Epc − 0.1 V Epc − 0.1 V Epc − 0.1 V Epc− 0.1 V no electrolysis

100:1:1:1 100:1:1:1 100:1:1:1 100:1:1:1 100:1:1:1 100:1:0.25:0.25 100:1:0.5:0.5 100:1:0.75:0.75 100:1:1:1 100:1:1:1

24 12 11.5 11.8 11.5 12 15 12 5.5 9

1 2 3 4 5 6 7 8 9d 10e

conversionb (%) 46.8 56.7 60.2 58.7 9.9 23.7 37.1 63.5 76.8

Mn,GPC (g/mol) no polymer 10400 11250 13500 13600 14630 16500 11500 10400 14100

Mw/Mn 1.39 1.41 1.39 1.39 1.55 1.50 1.41 1.29 1.41

a M = MMA; I = Eib-Br; C = catalyst; L = ligand. bThe monomer conversion was determined by 1H NMR spectroscopy from the intensity ratio of the OCH3 group signals of the polymer (3.60 ppm) over the monomer (3.75 ppm). cThe catalytic system used were equimolar quantities of FeBr3 and TMPP. dCuBr2-catalyzed eATRP. eFeBr2-catalyzed normal ATRP.

B

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Macromolecules supporting electrolyte. Before eATRP was conducted, CV measurements were performed at a scan rate of 0.1 V/s to determine the redox properties of the iron complexes. Afterward, eATRP was conducted at a constant potential based on CV results. Reaction liquid was purged with N2 for 15 min before measurements were carried out. During polymerization, the cell was maintained at 55 °C with vigorous stirring. Samples were withdrawn periodically for characterization. Before each experiment was conducted, the Pt disk electrode was polished with 0.05 μm aluminum oxide powder and sonicated in ethanol. Pt gauze was rinsed with chloroform to remove the attached polymer and successively sonicated in dilute H2SO4 and ethanol. Normal ATRP. The normal ATRP of MMA with formulation of [MMA]0:[Eib-Br]0:[FeBr2]0:[TMPP]0 = 100:1:1:1 in MeCN was performed as follows. Appropriate amounts of FeBr2 (61 mg, 0.28 mmol) and TMPP (99.7 mg, 0.28 mmol) were loaded in a flask; afterward, a 6 mL mixed solvent of MMA and MeCN with a 1:1 volume ratio was added. The catalyst is well dissolved under stirring. The flask was sealed with a rubber stopper and deoxygenated by three freeze−pump−thaw cycles. Finally, the initiator (Eib-Br, 40.8 μL, 0.28 mmol) was injected under the protection of N2. The reaction mixture was intensely stirred and maintained at 55 °C during polymerization.

results of CV obtained in the presence (solid blue) and absence (dashed black) of initiator. The potential of reductive peak (Epc) is −0.48 V vs Ag+/Ag reference electrode. All of the following potentials reported in this study are compared with this criterion unless otherwise stated. In Figure 1, the FeBr3/ TMPP catalytic system shows a typical quasi-reversible behavior, whose ratio of peak currents between oxidative peak and reductive peak is about 1. However, the cathodic current fails to exhibit a significant change when the initiator is added to this system (Figure 1), which is out of our expectation. In Cu(II)-catalyzed eATRP systems, the current intensity of the reductive peak is greatly enhanced when the initiator is added as the catalytic electrochemical−chemical (EC′) reaction occurs.21,22,26 This finding is likely attributed to the relatively low activation rate in Fe catalytic systems, especially in strongly polar solvents.42 With a slow activation step, the reduced catalyst is insufficient to react with an initiator and to convert into high-oxidation-state deactivators during one CV. As sketched in Figure 1, the linear sweep voltammogram (LSV) in the presence of initiator under convection was also carried out to estimate the diffusion-controlled limiting potential. Our results revealed that the threshold appeared at near ca. −0.53 V. This finding provides helpful insight into the effect of the applied potential on kinetic analysis. Effect of Applied Potential (Eapp). The effect of Eapp on polymerization kinetics was investigated after the electrochemical characteristics of the reaction system were established. The significance of Eapp is presented in Figure 2 by varying applied potential from −0.38 to −0.68 V with an identical formulation: [MMA] 0 :[Eib-Br] 0 :[FeBr 3 ] 0 :[TMPP] 0 = 100:1:1:1. Figure 2A illustrates the close relationship between the apparent polymerization rate and Eapp. With the electrode potential becoming more negative, the polymerization rate increases notably. However, although enhanced polymerization is detected with increasing overpotential, the growth rate decreases or even stops increasing when Eapp exceeds −0.58 V, as seen in Figure 2A. This finding is in agreement with previous studies in copper-catalyzed eATRP.21 A reasonable explanation is briefly discussed below by combining electrochemical theory with polymerization kinetics. Under the assumption of a high initial deactivation rate, the impact of terminations can be ignored in an ATRP system. And on the basis of quasi-steady-state approximation,43 the apparent polymerization rate can be approximately expressed as eq 1, where P·i and PiX represent the active and dormant chains, respectively. As shown in eq 1, the value of apparent polymerization rate is directly proportional to the concentration ratio of activator and deactivator. On the other hand, this ratio can be regulated by Eapp in an eATRP system; thus, the polymerization rate and Eapp are associated with each other in this system.

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RESULTS AND DISCUSSION Characterization and Control Study. The role of phosphine in ATRP systems is complicated because it can serve as a ligand, chain initiation assistant, or reducing agent.38−41 It was reported that FeBr3 could be reduced by excessive phosphines to generate the low-oxidation state activator and initiate the polymerization.40 To ensure the nature of electrochemical reduction over iron catalyst, a control study was conducted to exclude the regeneration of activator through other pathways. As listed in Table 1 (entry 1), the equimolar ratio of FeBr3 and the phosphorus ligand, TMPP, was employed as the catalytic system. After 24 h, no polymerization occurred in this reaction system, while the following experiments with identical formulation under electrolysis, polymers with well-defined structure (Mw/Mn ≈ 1.4) were synthesized, verifying the electrochemical generation of activator. Prior to eATRP, the redox potentials of the catalyst complexes were identified through CV. Figure 1 depicts the

kapp = k p[Pi] = Figure 1. Cyclic voltammetry of FeBr3/TMPP complex (0.28 mM) in 50% (v/v) MMA/MeCN ([MMA]0 = 4.7 M) with a scan rate of 0.1 V/s in the presence (solid blue) and absence (dashed black) of Eib-Br (0.28 mM). Linear sweep voltammogram (solid red) was obtained with an identical formulation containing Eib-Br under stirring. The values annotated correspond to the electrolysis potentials employed in following eATRP experiments.

II k pKATRP[PX][Fe /L] i

[Fe IIIX/L]

(1)

In an electrochemical process, the activation energy needed for reduction of metal ions is strongly dependent on the potential applied on the electrode. According to the Butler− Volmer equation,44 the generation rate of activator vnet can be expressed as eq 2: C

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Figure 2. Kinetics of FeBr3-catalyzed eATRP as a function of applied potential: (A) semilogarithmic kinetic plot; (B) evolution of Mn and Mw/Mn versus monomer conversion.

⎧ ⎡ − αF ⎤ (Eapp − E θ ′)⎥ vnet = k0⎨COS exp⎢ ⎣ RT ⎦ ⎩ ⎡ (1 − α)F ⎤⎫ (Eapp − E θ ′)⎥⎬ − COS exp⎢ ⎣ RT ⎦⎭

Remarkably, Mw/Mn values are maintained at approximately 1.4 (Table 1, entries 2−5) for polymerization at different Eapp, except the sample points at low monomer conversion for −0.38 and −0.48 V (Figure 2B). This is likely caused by the longer induction period when small overpotential is used, which is consistent with the experimental data shown in Figure 2A. All these findings imply that a more negative potential obtained before the mass transport limit is reached corresponds to a shorter induction period and a higher production efficiency. Mw/Mn, which is a quality index of polymer products, remains constant. This finding provides a useful guidance for industrial scale production. Unless otherwise stated, Eapp at −0.58 V (Epc − 0.1 V) is chosen and used as the optimum reaction condition for the following studies in this system to achieve an integrative consideration. Effect of Catalyst Loading. Figure 3 shows the CVs for eATRP systems at different concentrations of catalyst (10 000,

(2)

where k0 is the standard rate constant, α is the transfer coefficient, F is Faraday’s constant, R is the gas constant, T is the temperature, Eθ′ is the formal potential, and CSO, CSR are the concentration of high-oxidation-state and low-oxidation-state catalysts on the surface of electrode, respectively. The second item in the equation is actually sufficiently small for an eATRP system, and it can generally be neglected. Therefore, the equation form approaches an exponential equation, which helps explain the correlation between apparent polymerization rate and applied potential. In dynamics, a more negative potential applied to the cathode corresponds to the faster regeneration of an activator, as indicated by Eapp from −0.38 to −0.58 V (Figure 2A). However, the apparent reduction rate is also affected by the mass transfer process because of the existence of concentration gradient. In an eATRP system, the deactivator near the electrode is reduced to a low-oxidation-state activator through single electron reduction after an appropriate potential is imposed on the cathode. The generated activator diffuses from electrode surface to bulk under convection and initiates polymerization. Meanwhile, the deactivator diffuses in the opposite direction, that is, from bulk solution to the electrode surface, to supplement the consumed high-oxidation-state catalyst. Once the mass transport limit is reached, the consumption rate of the deactivator is equal to its diffusion rate from the bulk solution to the electrode surface, and diffusion becomes a rate-limiting step. As a result, polymerization kinetics unlikely varies when Eapp increases from −0.58 to −0.68 V (Figure 2A). Figure 2B presents the evolution of Mn and Mw/Mn with monomer conversion. The molecular chains grow gradually as the monomer is consumed, and the Mw/Mn values remain low during the whole polymerization. These findings confirm the good controllability of this iron-catalyzed polymerization system. It is noteworthy that the molecular weights deviate from the theoretical values, and this deviation may be associated with the ATRP initiation step;13 a detailed analysis on this topic is available in the following simulation section.

Figure 3. Cyclic voltammograms for systems under different concentrations of catalyst.

7500, 5000, and 2500 ppm, respectively). These curves indicate the same half-wave potential E1/2. The currents increase as the catalyst loading increases, indicative of lager current with higher concentration of catalyst in electrolysis experiments. Furthermore, the peak currents of reduction and oxidation are approximately equal in each curve, signifying good reversibility for all these systems. D

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Figure 4. Kinetics of FeBr3-catalyzed eATRP as a function of catalyst loading: (A) semilogarithmic kinetic plot; (B) evolution of Mn and Mw/Mn versus monomer conversion.

Table 2. Elementary Reactions for Iron-Based Electrochemically Mediated ATRP elementary reaction

rate coefficienta

equation

13

k in

6.03 × 101

49

P•i + M → P•i + 1

kp

7.25 × 102

50

ATRP equilibrium

II + • III + PX i + [Fe /L] XooY Pi + [Fe X/L]

7.04 × 10 2.53 × 107

this work

electrochemical activator (re)generation

[Fe IIIX/L]+ + e− ⎯→ ⎯ [Fe II /L]+ + X −

k r,e

2.83 × 10−3

this work

P•i + M ⎯⎯⎯→ Pi + P1•

3.74 × 10−2

51

9

2.00 × 10

52

2.00 × 109

52

8.81 × 107

53, 54

9.79 × 106

53, 54

P0X + [Fe II /L] XooooY P•0 + [Fe IIIX/L]+

initiation

kda0

P•0 + M → P1• propagation

ka

kda

k trM

transfer to monomer

P•0 +

termination

k t0 P•0 ⎯→ ⎯ P0P0

k t1

P•0 + P•i → P0Pi

a

ref

6.05 × 100 6.30 × 107

+ ka0

The units for all rate coefficients are M

−1

P•i +

k tc P•j →

P•i +

k td P•j ⎯→ ⎯ Pi

Pi + j

+ Pj

−1

s , except that of kr is expressed in s

The amount of catalyst loading has been extensively investigated because it hampers the industrialization of ATRP techniques. Thus, the following experiments are performed to examine the influence of catalyst loading on polymerization kinetics in iron-catalyzed eATRP systems. Figure 4 presents the first-order kinetic plot and evolutions of Mn and Mw/Mn versus monomer conversion using different concentrations of catalyst. Figure 4A suggests that the polymerization rate is facilitated by larger amount of catalyst loading. When the catalyst loading is larger, the concentration of activator, which can be regenerated, is higher; as a result, a more rapid polymerization occurs. For example, the final monomer conversion with a change in catalyst loading molar ratio from 0.25 to 1 increases by about 6 times from 9.9% to 60.2% (Table 1, entries 4 and 6). In addition, the linear growth of ln[(M)0/(M)t] as time demonstrates an almost constant concentration of radical chains according to eq 1. This finding indicates that the equilibrium between active chains and dormant chains are well maintained. Of course, the termination cases can never be neglected in ATRP systems, while the impact of termination becomes less because of diffusional limitations.45

1

−1/2

.

Furthermore, the catalyst loading has a significant impact on the evolution of Mn and Mw/Mn versus monomer conversion (Figure 4B). When the formulation [MMA]0:[Eib-Br]0: [FeBr3]0:[TMPP]0 = 100:1:0.25:0.25 is used, for example, the Mw/Mn value increases remarkably at low monomer conversion. When the same potential is applied, the activator-to-deactivator ratio increases more rapidly in the beginning of electrolysis for a low catalyst concentration system. Thus, the deactivation effect weakens, leading to a great quantity of radical termination. Nevertheless, we are looking for access to new ways for decreasing catalyst loading without negative impact on polymerization behaviors. Kinetic Model for In-Depth Insight into Kinetic Characteristics. Kinetic modeling is a powerful tool in the research of kinetic study, as it can describe the detailed information on the entire polymerization. Combined with our previously developed model, a kinetic model of iron-catalyzed eATRP using the method of moments was established to obtain an in-depth understanding of the kinetic characteristics in this work.37,46−48 The relevant elementary reactions are listed in Table 2, including initiation, propagation, activation/deactivaE

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Figure 5. Comparison of kinetic experimental data (points) and simulation results (lines) with slow initiation and fast initiation respectively: (A) semilogarithmic kinetic plot; (B) evolution of Mn and Mw/Mn versus monomer conversion.

Figure 6. Kinetics of FeBr2-catalyzed normal ATRP, FeBr3-catalyzed eATRP, and CuBr2-catalyzed eATRP: (A) semilogarithmic kinetic plot; (B) evolution of Mn and Mw/Mn versus monomer conversion.

ization with slow ATRP initiation, leading to a slow apparent propagation rate and reduced controllability on polymerization as illustrated by increased Mw/Mn values (Figures 5A,B). Besides, the almost linear increase in molecular weights (Mn) with conversion can also be explained because the number of total polymer chains continuously increases. Comparison of Normal ATRP, FeBr3-Based eATRP, and CuBr2-Based eATRP. As a newly developed ATRP technique, the applications of eATRP is not as wide as other ATRP techniques, including the iron-catalyzed eATRP proposed for the first time in this study. Hence, it is worthwhile to distinguish the FeBr2-catalyzed normal ATRP, FeBr3-catalyzed eATRP, and CuBr2-catalyzed eATRP for better understanding of their respective characteristics. Figure 6 shows the kinetic results of these three different kinds of ATRP. A typical ligand, hexamethylated tris(2aminothyl)amine (Me6TREN), is selected for CuBr2-catalyzed eATRP, and the experiment proceeds under the formulation of [MMA]0:[Eib-Br]0:[CuBr2]0:[Me6TREN]0 = 100:1:1:1 with electrode potential applied also at Epc − 0.1 V. By contrast, FeBr2-catalyzed normal ATRP occurs under the traditional condition of [MMA] 0 :[Eib-Br] 0 :[FeBr 2 ] 0 :[TMPP] 0 = 100:1:1:1. All these experiments were carried out at 55 °C. Figure 6A shows the inherent high activity for the Cucatalyzed ATRP system, whose polymerization rate is faster

tion equilibrium, chain transfer to monomer, radical termination, and electrochemical reduction of catalyst. However, chain elimination, chain transfer to polymer, and chain-length dependence of reaction rate constants are not taken into account because of their limited contributions on controlled radical polymerization under the present experimental conditions.55,56 The detailed kinetic equations and differential moment equations for all species in this system are listed in the Supporting Information. It should be stressed that the activation/deactivation equilibrium coefficient KATRP for initiator (ka0/kda0) is much lower than that of dormant macromolecular chains (ka/kda) in Table 2; this result is consistent with literature reports, which reveal that the initiation step is very slow in Eib-Br/ methacrylates system.11,49,57 The importance of slow initiation is validated in Figure 5. Figure 5 illustrates the comparison of the simulation results with slow initiation and fast initiation and the experimental data. With slow initiation, the model provides an excellent description of the experimental data, matching particularly in terms of the significant deviation of molecular weight with the theoretical one. However, the simulation results turn out to be completely inconsistent with the experimental data when the same activation rate is used for the initiator as for the dormant macromolecular chains. In fact, new polymer chains are evidently generated continuously during polymerF

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catalyzed eATRP system maintained a good living polymerization feature. In summary, this work expanded the scope of eATRP from a copper-based system to an iron-based system and investigated the polymerization kinetics on the basis of polymerization reaction kinetics. Considering that this study is the first to develop an iron-based eATRP system, we should improve the system in terms of several aspects, such as further decreasing the catalyst loading, and we have been exerting efforts to improve the proposed system.

than the other two Fe-catalyzed systems. Compared with FeBr3-catalyzed eATRP, almost no induction period is necessary for the Cu-catalyzed ATRP system. This feature can be attributed to the fast activation of the Cu-catalyzed ATRP system. Furthermore, the FeBr2-catalyzed normal ATRP system yields a relatively high apparent polymerization rate (Figure 6A) because almost no high-oxidation-state catalyst is found in the system, and the deactivation effect is extremely weak in the early period of polymerization. As a result, high concentrations of radical chains are produced. However, the cost of fast polymerization rate for normal ATRP system is the loss of controllability. The evolution of Mn and Mw/Mn versus monomer conversion sketched in Figure 6B clearly confirms the speculation. The Mw/Mn values become slightly higher in the FeBr2-catalyzed normal ATRP system compared with those of the two other systems. This condition is especially true at the beginning of polymerization. This finding implies a high concentration of radical chains, a resultant massive chain termination, and loss of end functionality. By contrast, the controllability of eATRP systems is much better due to its unique mechanism. The activator is regenerated gradually, with the proportion of activator to deactivator increasing slowly, until the dynamic activation/deactivation equilibrium is established. As a result, the concentration of active species is maintained at a low level throughout the polymerization. And the Mw/Mn values for FeBr3-catalyzed eATRP and CuBr2catalyzed eATRP maintain low and stable, suggesting the superiority of eATRP over normal ATRP.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01022. Detailed kinetic model for the iron-based electrochemically mediated ATRP (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-21-54745602; Fax +86-2154745602 (Z.-H. Luo.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21276213) and the National High Technology Research and Development Program of China (No. 2013AA032302) for supporting this work. Support from Center for High Performance Computing, Shanghai Jiao Tong University is appreciated.

CONCLUSION This work mainly aimed to extend the application of ATRP techniques by developing the iron-based eATRP system on the basis of polymerization reaction kinetics. The electrochemical characteristics of this catalytic system were studied, and the nature of electrochemical regeneration of activator was verified by control studies. The kinetic behaviors, including the effect of applied potential and catalyst loading, were systematically studied. A reasonable explanation for the varied polymerization rates with the applied potential was provided by combining electrochemical theory and polymerization kinetics. Our results showed that the applied potential elicited a promotion effect on the polymerization rate before the mass transfer limit was reached. Once the electrochemical reduction process was controlled by diffusion, the increase in overpotential did not affect the polymerization system. However, catalyst loading adversely affected polymerization behaviors, such as decreased polymerization rate and increased Mw/Mn. A kinetic model based on the method of moments was also developed to explain the mismatch in Mn and Mn,theo, whose results revealed that slow initiation had a significant influence on the kinetic behaviors in this system. What is more, FeBr2-catalyzed normal ATRP, FeBr3-catalyzed eATRP, and CuBr2-catalyzed eATRP were conducted to compare and elucidate their respective polymerization kinetic characteristics qualitatively. Considering the inherent high activity of copper-based catalyst, we found that the kinetic behaviors of the CuBr2-catalyzed eATRP system were slightly more efficient than those of iron-mediated systems. However, iron-based catalysts provide several advantages, including low price, good sustainability, low toxicity, and excellent biocompatibility. FeBr2-catalyzed normal ATRP yielded a high propagation rate with a relatively low controllability because of large amounts of low-oxidation-state catalysts at the beginning of polymerization, while the FeBr3-

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DOI: 10.1021/acs.macromol.6b01022 Macromolecules XXXX, XXX, XXX−XXX