Cobalt bipyridine bisphenolate complex in controlled/living radical polymerization of vinyl monomers

Article abstract of DOI:10.1021/ma5018764

Cobalt(II) bipyridine bisphenolate (Co^II(BpyBph)) was applied to mediate the controlled/living radical polymerization of vinyl acetate (VAc), methyl acrylate (MA), and other vinyl monomers such as N-vinylpyrrolidone (NVP) and acrylonitrile (AN). The living characters of linear increased molecular weight with monomer conversion, relatively narrow molecular weight distribution, and the synthesis of block copolymer demonstrated by the formation of PVAc-b-PNVP were all observed. The polymerization of VAc was proposed to be mediated by the degenerative transfer (DT) process, but the polymerization of MA should be mainly controlled by the reversible termination (RT) process since the equilibrium constant (K_eq = [Co^III-R]/[Co^II][R^?]) between cobalt(II) and organocobalt(III) was measured as 8.6 × 10⁷ M^-1. The correlation between K_eq and the redox potential (E_1/2) of cobalt complexes was preliminarily observed as a higher E_1/2 leading to a larger K_eq.

Full text of DOI:10.1021/ma5018764

Article
pubs.acs.org/Macromolecules
Cobalt Bipyridine Bisphenolate Complex in Controlled/Living Radical
Polymerization of Vinyl Monomers
Yi-Chien Lin, Yi-Liang Hsieh, Yuan-Deng Lin, and Chi-How Peng*
Department of Chemistry and Frontier Research Center on Fundamental and Applied Sciences of Matters, National Tsing Hua
University, Hsinchu 30013, Taiwan
S
* Supporting Information
ABSTRACT: Cobalt(II) bipyridine bisphenolate (CoII(BpyBph))
was applied to mediate the controlled/living radical polymerization of
vinyl acetate (VAc), methyl acrylate (MA), and other vinyl monomers
such as N-vinylpyrrolidone (NVP) and acrylonitrile (AN). The living
characters of linear increased molecular weight with monomer
conversion, relatively narrow molecular weight distribution, and the
synthesis of block copolymer demonstrated by the formation of
PVAc-b-PNVP were all observed. The polymerization of VAc was
proposed to be mediated by the degenerative transfer (DT) process,
but the polymerization of MA should be mainly controlled by the
reversible termination (RT) process since the equilibrium constant (Keq = [CoIII−R]/[CoII][R•]) between cobalt(II) and
organocobalt(III) was measured as 8.6 × 107 M−1. The correlation between Keq and the redox potential (E1/2) of cobalt
complexes was preliminarily observed as a higher E1/2 leading to a larger Keq.
Cobalt complexes mediated radical polymerization
(CMRP)5,15 is a subcategory of organometallic mediated
radical polymerization (OMRP)26−34 and controls the polymer-
ization through the pathways of reversible termination (RT)
and/or degenerative transfer (DT) as shown in Scheme 1.7,12,35
INTRODUCTION
■
Cobalt complexes were amazed by their unique performance in
polymerization processes such as catalytic chain transfer
polymerization1−3 and, more recently, controlled/living radical
polymerization (C/LRP).4−10 Cobalt complexes of
CoII(TMP),11−13 CoII(acac)2,5,6,14−17 and CoII(Salen*)18−20
(Figure 1) were applied to control the radical polymerization of
Scheme 1. (a) Reversible Termination (RT) and (b)
Degenerative Transfer (DT) Mechanisms That Occurred in
Cobalt Mediated Controlled/Living Radical Polymerization
Figure 1. Cobalt complexes used in controlled/living radical
polymerization: (a) cobalt(II) tetramesitylporphyrin, CoII(TMP);
(b) cobalt(II) acetylacetonate, CoII(acac)2; and (c) cobalt(II) [N,N′-
bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine],
CoII(Salen*).
The RT and DT mechanisms are distinguished by the source
and concentration of radicals. When the radicals are dissociated
from the dormant species (CoIII−P) and the radical
concentration is dominated by the equilibrium of deactivator
(CoII) and dormant species ([P•] = [CoIII−P]/([CoII] × Keq)),
the polymerization is described to be controlled by reversible
termination. On the other hand, when the radicals are
vinyl acetate (VAc), methyl acrylate (MA), and other functional
monomers. Particularly the control of VAc polymerization is
difficult to achieve by using other transition metal catalysts or
chain transfer agents except the proper RAFT agent.21 The
attraction of poly(vinyl acetate) (PVAc) is mainly from its wide
application in adhesive22 and the transformation to poly(vinyl
alcohol),23−25 an important biocompatible material, via
hydrolysis.
Received: September 10, 2014
Revised: October 16, 2014
Published: October 29, 2014
© 2014 American Chemical Society
7362
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
Scheme 2. Synthesis of Bipyridine Bisphenolate Ligand (BpyBph-H2) Followed by the Formation of Cobalt(II) Bipyridine
Bisphenolate (CoII(BpyBph))
a
Table 1. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) with Different Monomer to Cobalt Ratio
b
c
d
c
entry
1
[VAc]/[Co]
700
time (min)
conv (%)
Mn,exp [10−3 g mol−1
]
Mn,th [10−3 g mol−1
]
PDI
180
240
360
180
210
300
13.8
30.6
53.9
17.3
28.6
55.2
10.2
22.3
30.6
28.0
40.3
8.3
18.4
32.5
20.9
34.5
1.12
1.20
1.48
1.14
1.29
1.66
2
1400
61.6
66.5
a
b
c
General conditions: [CoII]0/[AIBN]0 = 1/20, [VAc]0 = 10.68 M in bulk at 60 °C. Conversion was detected by 1H NMR. Mn was determined by
d
gel permeation chromatography (GPC) with polystyrene as standard. Mn,th = ([M]0/[CoII]0) × conversion × MW of monomer.
exclusively from the initiator and the radical concentration is
mainly determined by the initiator concentration ([I]), the rate
constants for radicals to enter solution (ki), and the radical
termination rate constant (kt) ([P•] = (ki[I]/2kt)1/2), the
control of polymerization is achieved by degenerative transfer.12
Practically, the radical source may hardly be defined since the
radicals came from both external initiators and dormant species
in many cases, and thus the radical concentration becomes a
better indication to distinguish these two mechanisms. In the
literature, C/LRP of MA mediated by CoII(TMP)11,12,36,37 and
CoII(Salen*)18,19 was dominated by the RT process, but C/
LRP of VAc mediated by CoII(TMP),38 CoII(Salen*),19 and
5,8
CoII(acac)2 was controlled via the DT process.
Herein, cobalt(II) [2,2′-[2,2′]bipyridinyl-6-ylbis-4,6-di-tert-
Figure 2. First-order kinetics plots of VAc polymerization mediated by
butylphenolate] (CoII(BpyBph)), which has a low spin (S =
CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/
1/2) metal center coordinated by two nitrogen and two oxygen
[AIBN]0/[VAc]0 = (a) 1/20/700 and (b) 1/20/1400; [VAc]0 = 10.68
with a square planar structure,39 was selected to mediate the
M.
polymerization of vinyl acetate, methyl acrylate, and other vinyl
monomers for expansion of the system and more compre-
kinetic plots is proper to the rate of polymerization and is equal
hensive understanding of CMRP. The cobalt(II) bipyridine
to kp[R•].37,40 The polymerization approached 53.9% monomer
bisphenolate complex was synthesized by following the
reported method as summarized in Scheme 2.39 The
conversion in 6 h with the molecular weight not only linearly
CoII(BpyBph) was generated by the reaction of cobalt acetate
increasing versus conversion but also matching the theoretical
values calculated by the assumption of one chain per mediator.
The polymerization reaction was stopped due to the high
viscosity. The molecular weight distribution was slightly broad
and the ligand, which was obtained from a halogen−metal
exchange of the bromophenol and tert-butyllithium followed by
nucleophilic addition to the bipyridine. The cobalt complex and
the ligand were characterized by 1H NMR and UV−vis spectra.
as indicated by the polydispersity index (PDI = Mw/Mn) equal
The results of polymerization showed that CoII(BpyBph) can
to 1.48 (Figure 3a). Poly(vinyl acetate) with higher molecular
weight equal to 61 600 was obtained by raising the monomer to
control the radical polymerization of vinyl acetate, methyl
cobalt ratio (Table 1, entry 2). The similar length of induction
acrylate, acrylonitrile, and N-vinylpyrrolidone as a new control
system for CMRP.
period was due to the same ratio of [CoII]0/[AIBN]0, and the
change of slope in kinetic plots was attributed to the different
[AIBN]0 (Figure 2b). The control fashion demonstrated by the
linearity of increasing molecular weight with conversion and the
molecular weight distribution was not significantly affected by
the change of monomer to cobalt ratio (Figure 3).
The concentration of AIBN is critical to the CMRP of vinyl
acetate since it dominates the radical concentration during the
polymerization. A higher radical concentration gives a faster,
more economical polymerization process,37 but a lower radical
concentration provides a better control to the polymeric
product.41 The VAc polymerization was thus carried out with
different initial concentration of AIBN at 60 °C (Table 2). An
RESULTS AND DISCUSSION
Polymerization of Vinyl Acetate. The CoII(BpyBph)
■
mediated vinyl acetate (VAc) radical polymerization was
performed under the condition of CoII(BpyBph)/AIBN/VAc
= 1/20/700 at 60 °C in bulk (Table 1, entry 1). A 120 min
induction period followed by the first-order polymerization
process (ln(M0/Mt) = kp[R•]t) was observed (Figure 2a). This
induction period is characteristic of CMRP and was rationalized
as the time required to convert cobalt(II) complexes and
radicals to organocobalt(III) complexes.12,37 The slope in
7363
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
Figure 4. First-order kinetics plots of VAc polymerization mediated by
CoII(BpyBph) in bulk at 60 °C under the conditions of [CoII]0/
[AIBN]0 = (a) 1/10, (b) 1/20, and (c) 1/40; [VAc]0 = 10.68 M.
Figure 3. Plots of conversion vs Mn vs PDI for VAc polymerization
mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of
[CoII]0/[AIBN]0/[VAc]0 = (a) 1/20/700 and (b) 1/20/1400; [VAc]0
= 10.68 M.
induction period followed by a linear first-order kinetic plots
was observed in all polymerizations, but the length of induction
period decreased and the slope of kinetic plots raised with the
higher AIBN concentration (Figure 4), which can be
rationalized as that the increased radical concentration speeded
up the conversion of cobalt(II) to organocobalt(III) species
and enhanced the propagation rate. The control of VAc
polymerization demonstrated by the linearity of molecular
weight growth, deviation of molecular weight from the
theoretical values, and the PDI values was not significantly
influenced by the change of AIBN concentration (Figure 5).
Comparing to CoII(TMP)38,42 and CoII(Salen*),19
CoII(BpyBph) can mediate VAc polymerization with a higher
radical concentration and thus gives a more rapid and
economical polymerization process.
The temperature effect was also considered, and therefore,
vinyl acetate radical polymerization was mediated by
CoII(BpyBph) at different temperature from 50 to 70 °C
(Table 3). An induction period followed by a linear first-order
kinetic plots was still observed in all polymerizations, but the
length of induction period decreased and the slope of kinetic
plots raised with the increased temperature (Figure 6), which
should be mainly attributed to the higher radical concentration
generated by the accelerated dissociation of AIBN. The half-life
of AIBN is shortened from 74 to 4.8 h when the temperature
changed from 50 to 70 °C.40,43 The increased temperature also
changed the kinetic parameters such as formation rate constant
of organocobalt(III) and propagation rate constant that would
Figure 5. Plots of conversion vs Mn vs PDI for VAc polymerization
mediated by CoII(BpyBph) in bulk at 60 °C under the conditions of
[CoII]0/[AIBN]0 = (a) 1/10, (b) 1/20, and (c) 1/40; [VAc]0 = 10.68
M.
affect the induction time and polymerization rate as well.
Besides, the raised temperature led to a lower viscosity, and
thus the polymerization can approach a higher monomer
conversion. The temperature effect was not significant to the
molecular weight distribution but caused the molecular weight
deviation to the polymeric products formed at 50 and 70 °C
(Figure 7). The positive Mn deviation that occurred at 50 °C
was suspected to be relevant to the slow exchange process
a
Table 2. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) with Different Initial Concentration of AIBN
b
c
d
c
entry
1
[AIBN]/[Co]
10
time (min)
conv (%)
Mn,exp [10−3 g mol−1
]
Mn,th [10−3 g mol−1
]
PDI
300
420
630
180
240
360
90
20.0
32.4
53.0
13.8
30.6
53.9
20.6
33.3
59.7
14.5
22.0
29.0
10.2
22.3
30.6
13.6
22.0
35.5
12.1
19.5
31.9
8.3
1.14
1.34
1.55
1.12
1.20
1.48
1.18
1.40
1.56
2
3
20
40
18.4
32.5
12.4
20.1
36.0
120
200
a
b
c
1
General conditions: [VAc]0 = 10.68 M in bulk at 60 °C. Conversion was detected by H NMR. Mn was determined by gel permeation
d
chromatography (GPC) with polystyrene as standard. Mn,th = ([M]0/[CoII]0) × conversion × MW of monomer.
7364
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
a
Table 3. Polymerization of Vinyl Acetate Mediated by CoII(BpyBph) at Different Temperature
b
c
d
c
entry
1
temp (°C)
t1/2,AIBN (h)
time (min)
conv (%)
Mn,exp [10−3 g mol−1
]
Mn,th [10−3 g mol−1
]
PDI
50
74
675
840
1020
180
240
360
60
17.3
28.6
36.7
13.8
30.6
53.9
20.0
32.9
62.0
18.2
27.6
33.6
10.2
22.3
30.6
9.7
10.5
17.2
22.1
8.3
1.18
1.37
1.45
1.12
1.20
1.48
1.17
1.37
1.68
2
3
60
70
15
18.4
32.5
12.1
19.8
4.8
80
17.8
140
27.4
37.4
a
b
c
1
General conditions: [CoII]0/[AIBN]0 = 1/20, [VAc]0 = 10.68 M in bulk. Conversion was detected by H NMR. Mn was determined by gel
d
permeation chromatography (GPC) with polystyrene as standard. Mn,th = ([M]0/[CoII]0) × conversion × MW of monomer.
concentration did not significantly impact the control process is
still being studied.
Polymerization of Methyl Acrylate. The CoII(BpyBph)
was also applied to the C/LRP of methyl acrylate since
CoII(TMP) and CoII(Salen*) showed a good control to MA
polymerization. CoII(BpyBph) mediated MA polymerization
was performed under the condition of CoII(BpyBph)/AIBN/
MA = 1/10/700 at 60 °C in benzene with [MA]0 = 5.47 M and
reached 82.6% monomer conversion in 240 min with a 90 min
induction period (Figure 8). Instead of a linear increased
Figure 6. First-order kinetics plots of VAc polymerization mediated by
CoII(BpyBph) in bulk under the conditions of [CoII]0/[AIBN]0/
[VAc]0 = 1/20/700, [VAc]0 = 10.68 M, at (a) 50, (b) 60, and (c) 70
°C.
Figure 8. First-order kinetics plots of MA polymerization mediated by
CoII(BpyBph) in benzene at 60 °C under the conditions of [CoII]0/
[AIBN]0/[MA]0 = 1/10/700, [MA]0 = 5.47 M.
kinetic plots in VAc polymerization, the kinetic plots of MA
polymerization increased as a curvature, which was also
observed in CoII(TMP)37 and CoII(Salen*)19 mediated MA
polymerization and correlated to the equilibrium constant (Keq)
of cobalt(II) and organocobalt(III). The kinetic simulation
indicated that the curvature becomes more obvious when Keq
decreases.37 The living characters of a linear increased
molecular weight with conversion and narrow molecular weight
distribution demonstrated by PDI values below 1.17 were
observed (Figure 9). However, the molecular weight measured
by GPC was slightly larger than the theoretical values calculated
by the assumption of one chain per mediator. The molecular
weight deviation associated with the curvature of kinetic plots
suggested that CoII(BpyBph) could mediate the MA polymer-
ization via the reversible termination pathway, and possibly not
all cobalt(II) complexes were converted to organocobalt(III)
species during the induction period.
Figure 7. Plots of conversion vs Mn vs PDI for VAc polymerization
mediated by CoII(BpyBph) in bulk under the conditions of [CoII]0/
[AIBN]0/[VAc]0 = 1/20/700, [VAc]0 = 10.68 M, at (a) 50, (b) 60,
and (c) 70 °C.
between the propagating radicals in solution and the dormant
radicals on the organocobalt(III) species. A negative molecular
weight deviation was observed after 44% conversion in the VAc
polymerization at 70 °C and could be due to the chain transfer
process that was accelerated by the increasing of temperature.
CoII(BpyBph) has been demonstrated to be able to mediate
the controlled/living radical polymerization of vinyl acetate
with molecular weight that increased linearly with conversion
and matched the theoretical values. The mediation of VAc
polymerization with high radical concentration is the feature of
the CoII(BpyBph) system, but why the increased radical
UV−vis spectroscopy was thus used to follow the trans-
formation of cobalt species during the induction period (Figure
7365
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
Figure 11. Change of [CoII(BpyBph)] and [CoIII(BpyBph)−R]
during the induction period in the polymerization of methyl acrylate
under the conditions of [CoII]0/[AIBN]0/[MA]0 = 1/10/700,
[AIBN]0 = 0.078 M, in benzene at 60 °C.
Figure 9. Plots of conversion vs Mn vs PDI for MA polymerization
mediated by CoII(BpyBph) in benzene (MA:benzene = 1:1 (v/v)) at
60 °C under the conditions of [CoII]0/[AIBN]0/[MA]0 = 1/10/700,
[MA]0 = 5.47 M. The black dotted line was the theoretical molecular
weight calculated by the assumption of one chain per cobalt complex
([CoII]0), and the red dashed line was the theoretical molecular weight
calculated by the equilibrium concentration of organocobalt(III)
species ([CoIII]eq).
slightly larger than that for CoII(Salen*) (2.4 × 107 M−1)19 but
smaller than the value for CoII(TMP) (8.7 × 109 M−1).37
The equilibrium of cobalt(II) and organocobalt(III) species
(eq 1) is crucial to CMRP and determines the control
mechanisms of the polymerization. When the equilibrium
10). The concentration of CoII(BpyBph) was estimated by the
absorbance at 550 nm with extinction coefficient equal to 1723
extremely favors the site of organocobalt(III) (Keq
>
1012),8,19,42 the reverse reaction, Co−C bond homolysis that
is the key step for reversible termination, can hardly happen,
and thus DT becomes the only possible control mechanism.
When the equilibrium constant declines to 1010−107,19,37 the
radical could be dissociated from the organocobalt(III) so that
the contribution of RT to the control process becomes more
significant. The discussion of the change of equilibrium was
focused on the stability of radical species,37,44 but the
transformation of the oxidation state of cobalt species should
also play an important role so that the redox potentials of
selected cobalt complexes used in CMRP were compared to the
cobalt(II)/organocobalt(III) equilibrium constants in MA
polymerization (Table 4). The complexes of CoII(Salen*)
Table 4. Equilibrium Constant and Redox Potential of
Cobalt Complexes in CH2Cl2 Solution
Figure 10. UV−vis spectrum illustrating the formation of the
CoIII(BpyBph)−R complex during the induction period of the methyl
acrylate polymerization ([CoII]0/[AIBN]0/[MA]0 = 1/10/700,
[AIBN]0 = 0.078 M in benzene at 60 °C).
complex
Keq (M−1
)
E1/2 (V)
ref
a
CoII(BpyBph)
CoII(Salen*)
CoII(TMP)
8.6 × 107
2.4 × 107
8.7 × 109
−0.01
0.01
39
a
19, 45
b
0.60
37, Supporting Information
a
b
In the presence of 0.1 M n-Bu4NClO4. In the presence of 0.1 M n-
M−1 cm−1, and the concentration of CoIII(BpyBph)−R was
calculated by the absorbance at 550 nm with extinction
coefficient equal to 4557 M−1 cm−1 and at 839 nm with
extinction coefficient equal to 962 M−1 cm−1 (Supporting
Information). Changes in the electronic spectra during the
induction period for CoII(BpyBph) mediated MA polymer-
ization demonstrated that part of CoII(BpyBph) was converted
to the CoIII complex (PMA−CoIII(BpyBph)) and the ratio of
[CoII]eq/[CoIII]eq was 0.38 (Figure 11), which gave an estimate
for the theoretical molecular weight calculated by [CoIII]eq
(Mn,th = ([M]0/[CoIII]eq) × conversion × MW of monomer)
highly agreeing with the results of MA polymerization mediated
by CoII(BpyBph) complexes (Figure 9, red dashed line). The
value of [CoII]eq/[CoIII]eq combined with a radical concen-
tration of 3.1 × 10−8 M obtained from the kinetic plots ([R•] =
slope/kp(333 K) of MA = 3.98 × 10−4/13000)37,43 indicates
the equilibrium constant of cobalt(II) and organocobalt(III)
species in MA polymerization as 8.6 × 107 M−1, which is
Bu4NPF6. Potential values given in V versus the Fc/Fc+ reference
electrode, T = 298 K.
and CoII(BpyBph) have the E1/2 as 0.0145 and −0.01 V39 with
the Keq equal to 2.4 × 107 and 8.6 × 107 M−1, respectively.19
However, the values of E1/2 and Keq for CoII(TMP) are much
larger as 0.60 V (Supporting Information) and 8.7 × 109 M−1.37
Although the order of E1/2 values (CoII(BpyBph) <
CoII(Salen*) < CoII(TMP)) did not strictly match that of
Keq (CoII(Salen*) < CoII(BpyBph) < CoII(TMP)), the trend
for the interplay of Keq and E1/2 can still be observed. The
higher redox potential of the cobalt(II)/cobalt(III) leads to a
larger equilibrium constant and thus drives the control
mechanism of CMRP to degenerative transfer process. More
systematic study of CMRP with varied cobalt complexes would
be required to build an in-depth understanding of the
correlation between different factors and the cobalt equilibrium,
like what was reported for the copper ATRP system.46
7366
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
a
Table 5. Polymerization of Different Monomers Mediated by CoII(BpyBph) with the Initiation of AIBN
b
c
d
c
entry
monomer
solvent
time (min)
conv (%)
Mn,exp [10−3 g mol−1
]
Mn,th [10−3 g mol−1
]
PDI
1
2
3
NVP
DMF
180
300
105
150
87.9
76.3
92.0
75.1
79.9
51.6
10.04
68.4
28.3
72.9
1.36
1.35
1.54
1.89
AN
DMF
NIPAM
DMF
e
f
4
styrene
benzene
6.3
54.8
a
b
c
General conditions: [CoII]0/[AIBN]0 = 1/10, [monomer]0 = 5.47 M at 60 °C. Conversion was detected by 1H NMR. Mn was determined by gel
d
permeation chromatography (GPC) in DMF with poly(ethylene oxide) (PEO) calibration corrected by the Mark−Houwink equation. Mn,th
=
e
f
([monomer]0/[CoII]0) × conversion × MW of monomer. Polymerization was carried out at 90 °C. Mn was determined by THF gel permeation
chromatography (GPC) with polystyrene as standard.
Figure 12. GPC traces for polymerization under the following condition of [CoII]0/[AIBN]0/[monomer]0 = 1/10/700, [monomer]0 = 5.47 M,
monomer = (a) NVP, (b) AN, (c) NIPAM, and (d) Sty.
Keq
with Mn equal to 15 000 and PDI equal to 1.21. The chain
extension from PVAc to PNVP was carried out in DMF at 60
CoII(L) + R• XooY CoIII(L)−R
(1)
Keq = [CoIII(L)−R]/([CoII(L)][R•])
Polymerization of Other Monomers. The CoII(BpyBph)
°C to approach 20% conversion in 40 min with Mn equal to 21
000 and PDI equal to 1.30 (Figure 13). The success in block
copolymer synthesis associated with the observation of the
was further used in the radical polymerization of acrylonitrile
linear increased molecular weight versus conversion and a
(AN), N-vinylpyrrolidone (NVP), N-isopropylacrylamide
moderate polydispersity demonstrated that the CoII(BpyBph)
(NIPAM), and styrene (Sty) (Table 5). The induction period
mediated VAc polymerization fulfilled the requirements of
followed by a linear first-order kinetic plots was observed in the
controlled/living radical polymerization.
polymerization of AN and NVP (Figures SI1 and SI2), and the
control of polymerization was demonstrated by the linear
increased molecular weight versus conversion with smoothly
shifted GPC traces (Figure 12a,b) and the relatively low
polydispersity. However, the significant deviation of molecular
weight, broad molecular weight distribution, and the fixed GPC
traces (Figure 12c,d) indicated that CoII(BpyBph) is not a good
mediator for controlled/living radical polymerization of
NIPAM and Sty.
Synthesis of Block Copolymer. The formation of block
copolymers is one of the most important features of controlled/
living radical polymerization, and thus organocobalt(III)
complexes of PVAc−CoIII(BpyBph) was used as the macro-
initiators to initiate the NVP polymerization for the preparation
of block copolymer of poly(vinyl acetate) and poly(N-
vinylpyrrolidone) (PVAc-b-PNVP). The macroinitiator was
the product of CoII(BpyBph) mediated VAc polymerization
Figure 13. GPC traces of (a) PVAc macroinitiator, Mn = 15 000, PDI
= 1.21, and (b) PVAc-b-PNVP, Mn = 21 000, PDI = 1.30. PVAc-b-
PNVP was synthesized under the condition of [macroinitiator]0/
[NVP]0 = 1/700 in DMF solution at 60 °C with [NVP]0 = 2.92 M.
7367
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
under nitrogen. The gray mixture was stirred and heated at 60 °C to
start the polymerization. The monomer conversion was followed by
1H NMR. The polymerizations of methyl acrylate, N-vinylpyrrolidone,
acrylonitrile, N-isopropylacrylamide, and styrene were performed with
the same procedure.
Synthesis of Block Copolymer. Macroinitiators of
CoIII(BpyBph)−PVAc (conversion = 29.5%, Mn = 15 000, PDI =
1.21) were prepared by general polymerization procedure as [CoII]0/
[AIBN]0/[VAc]0 = 1/10/700 with [VAc]0 = 10.68 M at 60 °C in bulk,
and the unreacted monomers were removed after the polymerization
by high vacuum. The flask with macroinitiators was then filled with
nitrogen, followed by the addition of DMF (9 mL) and N-
vinylpyrrolidone (3.6 mL, 35 mmol) solution dissolving CoII(BpyBph)
(31.1 mg, 0.05 mmol). The mixture was stirred and heated at 60 °C to
start the polymerization.
CONCLUSION
■
The cobalt(II) bipyridine bisphenolate complex has been
demonstrated as a good mediator for the controlled/living
radical polymerization of vinyl acetate, methyl acrylate, N-
vinylpyrrolidone, and acrylonitrile by fulfilling the living
characters of linear increased molecular weight with monomer
conversion, relatively narrow molecular weight distribution, and
the generation of block copolymer. The VAc polymerization
was proposed to be mediated by CoII(BpyBph) via a
degenerative transfer process and could be accelerated by the
increasing of [AIBN]0 with no significant influence to the
control efficiency. The control mechanism of MA polymer-
ization should be the reversible termination process as
demonstrated by the measurement of equilibrium constant
(Keq = [CoIII−R]/[CoII][R•]) between cobalt(II) and
organocobalt(III) that equals to 8.6 × 107 M−1. A positive
correlation of reduction potential of cobalt(II) complexes and
equilibrium constant in MA polymerization was preliminarily
observed and could be used to evaluate the control property of
cobalt complexes in C/LRP.
ASSOCIATED CONTENT
* Supporting Information
Experimental data of each polymerization, H NMR spectrum
■
S
1
of PVAc-b-PNVP block copolymer, GPC traces, UV−vis
spectrum and numerical method, and CV measurement of
CoII(TMP). This material is available free of charge via the
Internet at http://pubs.acs.org.
EXPERIMENTAL SECTION
■
Materials. Solvents were dried by refluxing at least 24 h over
sodium/benzophenone (toluene, THF, ether) and CaH2 (CH2Cl2,
benzene). Vinyl acetate (>99%, ACROS) was degassed by three
freeze−pump−thaw cycles and distilled under reduced pressure.
Methyl acrylate (>98%, ACROS), N-vinylpyrrolidone (>98%,
ACROS), acrylonitrile (>99%, ACROS), and styrene (>99%, Aldrich)
were passed through the basic Al2O3 to remove the inhibitor. N-
Isopropylacrylamide (97%, Alderich) was purified by repeated
recrystallization in a mixture of toluene/hexane (70:30, v/v) and
dried in a vacuum. Dimethylformamide (DMF, Alfa Aesar) and 2,2′-
azobis(isobutyronitrile) (AIBN, Showa) were used as received.
Bromophenols and bipyridine were purchased from Aldrich and
used without further purification. Deuterated solvents (Aldrich) were
dried over molecular sieves.
AUTHOR INFORMATION
Corresponding Author
*E-mail: chpeng@mx.nthu.edu.tw (C.-H.P.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the National Science Council, Taiwan (NSC 102-
2113-M-007-007-MY2), for support of this research.
■
REFERENCES
■
(1) Gridnev, A. J. Polym. Sci., Part A: Polym. Chem. 2000, 38, 1753−
1766.
Characterization. 1H NMR spectroscopy used to identify the
ligand structure, and monomer conversion was measured on a Varian
Unity INOVA 400 MHz spectrometer using CDCl3, CD3OD, and
DMSO-d6 as solvent at 298 K. Chemical shifts of proton spectra are
reported as follow: δ 7.24 ppm (CDCl3), δ 3.31 ppm (CD3OD), and δ
2.49 ppm (DMSO-d6). The UV−vis spectrum were measured by a
SHIMADZU UV-1800 instrument from 1100 to 190 nm. Gel
permeation chromatography (GPC) was carried out by THF eluent
using the Ultimate 3000 liquid chromatograph equipped with a 101
refractive index detector and three Shodex columns (Shodex KF-802,
KF-803, and KF-804) or by DMF−LiBr eluent using the Waters 600
liquid chromatograph equipped with a 101 refractive index detector
and styragel HR columns (HR2; HR5). The signal was collected by a
DIONEX Shodex RI-101 refractometer (RI) detector and an UltiMate
3000 variable wavelength detector operated at 254 nm for the THF
and the DMF system. The calibration was based on narrow linear
poly(styrene) standard for the THF system and poly(ethylene oxide)
standard for the DMF system. The molar masses of the PNVP and
PAN were corrected using the Mark−Houwink equation with the
parameters for PNVP (K = 6.910 × 10−5 dL g−1, a = 0.7236) and PAN
(K = 3.87 × 10−6 dL g−1, a = 0.9088).47
(2) Gridnev, A. A.; Ittel, S. D. Chem. Rev. 2001, 101, 3611−3660.
(3) Gridnev, A. A.; Ittel, S. D.; Wayland, B. B.; Fryd, M.
Organometallics 1996, 15, 5116−5126.
(4) Wayland, B. B.; Sherry, A. E.; Bunn, A. G. J. Am. Chem. Soc. 1993,
115, 7675−7684.
(5) Debuigne, A.; Caille, J. R.; Jerome, R. Angew. Chem., Int. Ed. 2005,
44, 1101−1104.
(6) Kaneyoshi, H.; Matyjaszewski, K. Macromolecules 2005, 38,
8163−8169.
(7) Poli, R. Angew. Chem., Int. Ed. 2006, 45, 5058−5070.
́ ̂ ́ ̂
(8) Debuigne, A.; Poli, R.; Jerome, C.; Jerome, R.; Detrembleur, C.
Prog. Polym. Sci. 2009, 34, 211−239.
(9) Kumar, K.; Gnanou, Y.; Champouret, Y.; Daran, J. C.; Poli, R.
Chem.Eur. J. 2009, 15, 4874−4885.
(10) Zhao, Y.; Yu, M.; Fu, X. Chem. Commun. 2013, 49, 5186−5188.
(11) Wayland, B. B.; Poszmik, G.; Mukerjee, S. L.; Fryd, M. J. Am.
Chem. Soc. 1994, 116, 7943−7944.
(12) Wayland, B. B.; Peng, C.-H.; Fu, X.; Lu, Z.; Fryd, M.
Macromolecules 2006, 39, 8219−8222.
(13) Peng, C.-H.; Fryd, M.; Wayland, B. B. Macromolecules 2007, 40,
6814−6819.
Synthesis of Cobalt Bipyridine Bisphenolate Complex,
CoII(BpyBph). The cobalt complex was synthesized according to
published methods.39 To a 10 mL ethanol solution of Co(OAc)2 (1
equiv) was added a (BpyBph)H2 (1 mmol) under refluxing conditions;
the solution was stirred for 2 h and filtered with methanol.
CoII(BpyBph) solid was obtained in 60% yield.
(14) Debuigne, A.; Willet, N.; Jerome, R.; Detrembleur, C.
Macromolecules 2007, 40, 7111−7118.
́ ̂
(15) Debuigne, A.; Champouret, Y.; Jerome, R.; Poli, R.;
Detrembleur, C. Chem.Eur. J. 2008, 14, 4046−4059.
(16) Morin, A. N.; Detrembleur, C.; Jerome, C.; De Tullio, P.; Poli,
R.; Debuigne, A. Macromolecules 2013, 46, 4303−4312.
(17) Kermagoret, A.; Debuigne, A.; Jerome, C.; Detrembleur, C. Nat.
Chem. 2014, 6, 179−187.
General Procedure for Polymerization. [CoII(BpyBph)] (55.9
mg, 0.09 mmol) and AIBN (298.6 mg, 1.8 mmol) were placed in a 50
mL Schlenk flask and degassed by three vacuum/nitrogen cycles. Dry,
degassed vinyl acetate (5.9 mL, 63 mmol) was then added by syringe
7368
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369

Macromolecules
Article
(18) Sherwood, R. K.; Kent, C. L.; Patrick, B. O.; McNeil, W. S.
Chem. Commun. 2010, 46, 2456−2458.
(19) Liao, C.-M.; Hsu, C.-C.; Wang, F.-S.; Wayland, B. B.; Peng, C.-
H. Polym. Chem. 2013, 4, 3098−3104.
(20) Chiang, L.; Allan, L. E.; Alcantara, J.; Wang, M. C.; Storr, T.;
Shaver, M. P. Dalton Trans. 2014, 43, 4295−4304.
(21) Benaglia, M.; Chen, M.; Chong, Y. K.; Moad, G.; Rizzardo, E.;
Thang, S. H. Macromolecules 2009, 42, 9384−9386.
(22) Khan, U.; May, P.; Porwal, H.; Nawaz, K.; Coleman, J. N. ACS
Appl. Mater. Interfaces 2013, 5, 1423−1428.
(23) Shaffer, M. S. P.; Windle, A. H. Adv. Mater. 1999, 11, 937−941.
(24) Chiellini, E.; Corti, A.; D’Antone, S.; Solaro, R. Prog. Polym. Sci.
2003, 28, 963−1014.
(25) Rhim, J. W.; Park, H. B.; Lee, C. S.; Jun, J. H.; Kim, D. S.; Lee,
Y. M. J. Membr. Sci. 2004, 238, 143−151.
(26) Le Grognec, E.; Claverie, R.; Poli, R. J. Am. Chem. Soc. 2001,
123, 9513−9524.
(27) Percec, V.; Popov, A. V.; Ramirez-Castillo, E.; Monteiro, M.;
Barboiu, B.; Weichold, O.; Asandei, A. D.; Mitchell, C. M. J. Am. Chem.
Soc. 2002, 124, 4940−4941.
(28) Stoffelbach, F.; Poli, R.; Richard, P. J. Organomet. Chem. 2002,
663, 269−276.
(29) Asandei, A. D.; Moran, I. W. J. Am. Chem. Soc. 2004, 126,
15932−15933.
(30) Yamago, S. Chem. Rev. 2009, 109, 5051−5068.
(31) Yamago, S.; Iida, K.; Yoshida, J. J. Am. Chem. Soc. 2002, 124,
2874−2875.
(32) Yamago, S.; Ray, B.; Iida, K.; Yoshida, J.; Tada, T.; Yoshizawa,
K.; Kwak, Y.; Goto, A.; Fukuda, T. J. Am. Chem. Soc. 2004, 126,
13908−13909.
(33) Stehling, U. M.; Malmstrom, E. E.; Waymouth, R. M.; Hawker,
C. J. Macromolecules 1998, 31, 4396−4398.
(34) Poli, R. Eur. J. Inorg. Chem. 2011, 10, 1513−1530.
(35) Yoshikawa, C.; Goto, A.; Fukuda, T. Macromolecules 2003, 36,
908−912.
(36) Wayland, B. B.; Basickes, L.; Mukerjee, S.; Wei, M.; Fryd, M.
Macromolecules 1997, 30, 8109−8112.
(37) Peng, C.-H.; Li, S.; Wayland, B. B. Inorg. Chem. 2009, 48, 5039−
5046.
(38) Hsu, C.-S.; Yang, T.-Y.; Peng, C.-H. Polym. Chem. 2014, 5, 3867.
(39) Arora, H.; Philouze, C.; Jarjayes, O.; Thomas, F. Dalton Trans.
2010, 39, 10088−10098.
(40) Odian, G. G.; Odian, G. Principles of Polymerization; Wiley-
Interscience: New York, 2004; Vol. 3.
(41) Zhong, M.; Matyjaszewski, K. Macromolecules 2011, 44, 2668−
2677.
(42) Peng, C.-H.; Scricco, J.; Li, S.; Fryd, M.; Wayland, B. B.
Macromolecules 2008, 41, 2368−2373.
(43) Brandrup, J.; Immergut, E. H.; Grulke, E. A.; Abe, A.; Bloch, D.
R. Polymer Handbook; Wiley & Sons: New York, 1999; Vol. 89.
(44) Dong, H.; Hou, T.; Zhao, Y.; Fu, X.; Li, Y. Comput. Theor. Chem.
2012, 1001, 51−59.
(45) Kochem, A. l.; Kanso, H.; Baptiste, B.; Arora, H.; Philouze, C.;
Jarjayes, O.; Vezin, H.; Luneau, D.; Orio, M.; Thomas, F. Inorg. Chem.
2012, 51, 10557−10571.
(46) Tang, W.; Kwak, Y.; Braunecker, W.; Tsarevsky, N. V.; Coote,
M. L.; Matyjaszewski, K. J. Am. Chem. Soc. 2008, 130, 10702−10713.
(47) Mori, S. Anal. Chem. 1983, 55, 2414−2416.
7369
dx.doi.org/10.1021/ma5018764 | Macromolecules 2014, 47, 7362−7369