Article
pubs.acs.org/Macromolecules
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.
■
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
© 2016 American Chemical Society
Received: May 16, 2016
Revised: May 23, 2016
Published: May 27, 2016
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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
■
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.
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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.
■
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:
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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.
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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/deactiva4042
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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 polymer4043
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Macromolecules
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.
■
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)
■
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.
■
■
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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