Controlled bimodal molecular-weight-distribution polymers: Facile synthesis by RAFT polymerization

Article abstract of DOI:10.1002/chem.201103914

The RAFT agents RAFT-1 and RAFT-2 were used for RAFT polymerization to synthesize well-defined bimodal molecular-weight-distribution (MWD) polymers. The system showed excellent controllability and "living" characteristics toward both the higher- and lower

Full text of DOI:10.1002/chem.201103914

FULL PAPER
DOI: 10.1002/chem.201103914
Controlled Bimodal Molecular-Weight-Distribution Polymers: Facile
Synthesis by RAFT Polymerization**
Jian Qin, Lifen Zhang,* Hongjuan Jiang, Jian Zhu, Zhengbiao Zhang, Wei Zhang,
Nianchen Zhou, Zhenping Cheng,* and Xiulin Zhu*[a]
Abstract: The RAFT agents RAFT-
1 and RAFT-2 were used for RAFT
polymerization to synthesize well-de-
fined bimodal molecular-weight-distri-
bution (MWD) polymers. The system
showed excellent controllability and
“living” characteristics toward both the
higher- and lower-molecular-weight
fractions. It is important that bimodal
higher-molecular-weight (HMW) poly-
mers and block copolymers with both
(MW) and MWD could be prepared
easily due to the “living” features of
RAFT polymerization. The strategy re-
alized a mixture of higher/lower-molec-
ular-weight polymers at the molecular
level but also preserved the features of
living radical polymerization (LRP) of
the RAFT polymerization.
well-controlled
molecular
weight
Keywords: living radical polymeri-
zation · molecular-weight distribu-
tion · polymers · radical reactions ·
RAFT
Introduction
lar level. Therefore, the performance of the polymers is
greatly influenced by the extent of mixing. 2) A two-step re-
action[2] is another strategy for synthesizing bimodal MWD
polymers. In a two-step reaction, one kind of polymer would
be synthesized first under certain polymerization conditions.
The reaction conditions would then be changed in the
second reaction step to obtain another kind of molecular-
weight polymer. 3) A mixed-hybrid-catalyst system in
a single reactor is a commonly used method for the prepara-
tion of homogeneous bimodal MWD polymers on the mo-
lecular level. But the systems usually include expensive and
toxic metal catalysts, such as cobalt complexes,[3] Ziegler–
Natta[4] or metallocene catalyst components, and so on.[5]
These catalysts are hard to eliminate completely and there-
fore seriously affect the material properties. 4) Chain-trans-
fer[6] or crosslinking[7] agents can also be used as modifiers
in polymerization for the synthesis of bimodal MWD poly-
mers. Generally, the HMW fraction is created by adding
a certain amount of chain-transfer or crosslinking agents to
the reaction system.
Actually, if the molecular weights for both HMW and
LMW polymers can be adjusted during the polymerization
according to the requirements of the application, it will pro-
vide a powerful tool for the synthesis of bimodal MWD
polymers. However, all of the strategies mentioned above
have failed to synthesize bimodal MWD polymers with both
controlled MW and MWD at the same time, which has been
a great challenge until now.
As we know, living/controlled radical polymerization
(LRP)[8] provides a simple way to achieve well-defined poly-
mers with both controlled MW and MWD. However, few
documents about the synthesis of bimodal MWD polymers
are available for LRP techniques. Lenzi and co-workers[9]
prepared bimodal MWD polymer resins of polystyrene and
poly(butyl acrylate) by using nitroxide-mediated polymeri-
The physical/mechanical properties and processing perfor-
mance of polymers are greatly affected by their molecular
weights (MWs) and molecular-weight distributions
(MWDs). Polymers with higher molecular-weight (HMW)
have higher strength but are difficult to process. On the con-
trary, lower-molecular-weight (LMW) polymers are endued
with good toughness and rheological properties but possess
lower rigidity. For some special applications, desirable me-
chanical properties and process abilities are both needed. In
order to synchronously possess these characteristics, bimodal
MWD polymers, which contain both HMW and LMW frac-
tion polymers, effectively balance the processing perfor-
mance (LMW component) and mechanical performance
(HMW component) of materials under extreme condi-
tions.[1]
Until now, a number of approaches have been proposed
to produce bimodal MWD polymers. 1) Physical blending is
a traditional way to mix higher- and lower-molecular-weight
polymers after the two components have been synthesized
separately. This method is difficult to carry out on a molecu-
[a] J. Qin, Dr. L. F. Zhang, H. J. Jiang, Dr. J. Zhu, Dr. Z. B. Zhang,
Dr. W. Zhang, Dr. N. C. Zhou, Dr. Z. P. Cheng, Dr. X. L. Zhu
Jiangsu Key Laboratory of Advanced Functional Polymer Design
and Application, Department of Polymer Science and Engineering
College of Chemistry, Chemical Engineering and Materials Science
Soochow University, Suzhou 215123 (P. R. China)
Fax : (+86)512-65882787
Fax : (+86)512-65112796
E-mail: chengzhenping@suda.edu.cn
xlzhu@suda.edu.cn
zhanglifen@suda.edu.cn
[**] RAFT: Reversible addition–fragmentation chain-transfer.
Chem. Eur. J. 2012, 18, 6015 – 6021
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6015

zation combined with conventional free radical polymeri-
zation. Wan and co-workers[10] reported the formation of bi-
modal MWD poly(methyl methacrylate) by using controlled
radical polymerization catalyzed by a CuCl/bis(2-dimethyl-
aminoethyl) ether (BDE) complex in an aqueous medium.
Unfortunately, these reports with LRP methods have not
overcome the drawbacks mentioned above; that is, they
failed to synthesize bimodal MWD polymers with both con-
trolled MW and MWD, due to the concurrent presence of
conventional free radical polymerization[11] in these systems.
Therefore, only one kind of molecular weight, which was
produced by living radical polymerization, could be con-
trolled during the whole polymerization process.
Results and Discussion
Synthesis of bimodal molecular-weight-distribution polystyr-
ene: Figure 1 shows the GPC curves of the polystyrene (PS)
A facile LRP method, reversible addition–fragmentation
chain-transfer (RAFT) polymerization,[12] has been devel-
oped as a vital technique to synthesize well-defined poly-
mers with designed structures since its first advent in 1998.
Lovestead and co-workers studied the chain-length depend-
ence of the termination-rate coefficient for disparate aver-
age-size radicals generated by a RAFT agent and a macro-
RAFT species in the same system.[13] For the first time, in
this work, the RAFT polymerization method has been de-
veloped to establish a new strategy for synthesizing bimodal
MWD polymers with both higher- and lower-molecular-
weight fractions controlled in MW and MWD by a one-step/
one-pot method. Herein, we describe the use of a pair of
RAFT agents (a monofunctional RAFT agent: {2-bromo-2-
methyl-propionic acid 4-[2-(carbazole-9-carbodithioate)-2-
methyl-propionyloxy]phenyl ester} (BMCCDP, RAFT-1),
and a difunctional RAFT agent: {1,4-[2-(carbazole-9-carbo-
dithioate)-2-methyl-propionic acid] phenyl ester} (BCCDP,
RAFT-2; Scheme 1) with similar structure in the same poly-
merization system.
Figure 1. GPC curves of obtained polystyrenes with different molar ratios
of RAFT-1/RAFT-2. [St]0/ACHTUNGRTEN[NNUG AIBN]0/ACHUTNRTGEGN[NUN RAFT-1]0/CATHUNGTREN[NUNG RAFT-2]0 ratios of
a) 1200:1.66:2.5:5; b) 1200:1:2.5:2.5; c) 1200:1.33:5:2.5; temperature=
758C; volume of styrene (VSt)=3.0 mL.
obtained by this strategy in the presence of different molar
ratios of RAFT-1/RAFT-2 (1:2, 1:1, and 2:1, respectively) by
using styrene (St) as the monomer and 2,2’-azobisisobutyro-
nitrile (AIBN) as the initiator at 758C. From Figure 1, it can
be seen that two peaks (representing the HMW and LMW
polymers, respectively) were discernable, which indicated
that bimodal MWD polymers were successfully obtained.
To further investigate the polymerization behavior for this
strategy, the kinetics of the RAFT polymerization of St
were studied first. Figure 2a shows the kinetic plot for the
RAFT polymerization of St with
a molar ratio for
[St]0/A[AIBN]0/A[RAFT-1]0/A[RAFT-2]0 of 1200:1:2.5:2.5.
G
E
N
A
linear plot with respect to the monomer concentration and
without an obvious induction period at the beginning of the
polymerization indicated that the polymerizations were ap-
proximately first order and the concentration of propagating
radicals remained constant during the polymerization. It can
be seen from Figure 2b that both of the number-average
molecular weights (Mn,GPC) of the resultant polymers in-
creased linearly with monomer conversion, from
3850 gmolÀ1 to 12250 gmolÀ1 for the LMW polymer and
from 8550 gmolÀ1 to 27900 gmolÀ1 for the HMW polymer,
respectively. These values were consistent with the corre-
sponding theoretical ones. Moreover, very low values for
the weight-average molecular weight (Mw)/Mn (<1.10) were
obtained for both HMW and LMW polymers. All these re-
sults suggest that RAFT-1 and RAFT-2 were effective
RAFT agents for the synthesis of bimodal MWD polymers.
Scheme 1. Synthetic route to BMCCDP and BCCDP. THF: tetrahydro-
furan; TEA: triethylamine; BMPB: 1,4-(2’-bromo-2’-methylpropionato)-
benzene; DMSO: dimethylsulfoxide.
Thermal RAFT polymerization of St and RAFT polymeri-
zation of methyl acrylate and acrylonitrile: Thermal poly-
merization without the AIBN initiator was carried out at
1108C. The result is shown in entry 1 of Table 1. The poly-
merization could still be well-conducted to produce bimodal
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RAFT Polymerization
FULL PAPER
(Mn,th) at lower ratios of mono-
mer/RAFT agents, the Mw/Mn
ratio (HMW and LMW frac-
tions, respectively) of the poly-
mers remained low (<1.09). An
increase in the ratio of mono-
mer/RAFT agents resulted in
good consistency between the
Mn,GPC and Mn,th values (entry 5
in Table 1). AN was another
monomer that could be used
for the preparation of bimodal
MWD polymers with this pair
of RAFT agents (entry 6 in
Table 1).
Analysis of chain end and chain
extension: To evaluate the
“living” characteristics of the
RAFT polymerization for the
synthesis of bimodal MWD
polymers, 1H NMR spectrosco-
py was used to analyze the
chain end of the PS prepared in
the presence of thermal free-
radical initiator (AIBN). As il-
lustrated in Figure 3, a peak at
Figure 2. ln([M]0/[M]) as a function of time and Mn,GPC and Mw/Mn values versus conversion for the bulk
RAFT polymerization of St in the presence of a pair of RAFT agents RAFT-1/RAFT-2 (a and b) or in the
presence of RAFT-1 or RAFT-2 as individual RAFT agents, respectively (c and d). Polymerization conditions:
a
and b) [St] /
d) [St]0/A[AIBN]0/A[RAFT-1]0 =600:0.33:2.5, [St] /
perature=758C.
[AIBN] /
E
R
VSt =3.0 mL, temperature=758C; c and
G
d=4.50–4.85 ppm
(f
in
Figure 3) corresponded to the
methenyl protons of the styrene
unit, which were attached to
Table 1. Polymerizations of MA and AN and thermal polymerization of St with mon-
ofunctional BMCCDP and difunctional BCCDP as RAFT agents.
À
the polymer chain ends linked to the S C(=S) moi-
eties. The characteristic signals at d=8.3, 7.9, and
7.3 ppm (a–c in Figure 3) were assigned to the pro-
tons of the carbazole group at the chain end. The
peak at d=1.0 ppm (d in Figure 3) corresponded to
the methyl protons connected to the ester group.
The signal at d=3.77 ppm (e in Figure 3) was at-
tributed to the methyl group at another chain end;
this deviates from the chemical shift (d=1.0 ppm, d
in Figure 3) of the other methyl groups because of
the electron-attracting functionality of the Br atom.
Entry Ratio[a]
t
Conv.[b] Mn,th
G
M
G
Mw/Mn
ACHTUNGTNER(NUNG LMW/
HMW)
[h]
[%]
HMW)
/HMW)
[gmolÀ1
[gmolÀ1
1[d]
2[e]
3[e]
4[e]
5[e]
6[f]
1200:0:2.5:2.5 28
91.0 15800/31000
79.6 6100/11650
86.8 12600/24600
98.0 27600/54700
55700/113500 64600/176000
84.0 7700/14950 23000/44600
14700/30400
10100/21000
22900/46300
40900/86000
1.08/1.04
1.04/1.08
1.04/1.09
1.06/1.09
1.15/1.25
1.03/1.07
600:1:2.5:2.5
7.5
1200:1:2.5:2.5 9.3
2400:1:2.5:2.5 19
4800:1:2.5:2.5 7.0 ꢀ100
1200:1:2.5:2.5 2.0
[a] Ratio of [Monomer] /
[c] Mn,th(LMW) ={[M]0/A([BMCCDP]0 +2ꢁ[BCCDP]0)}ꢁMw(M) ꢁconversion%+
Mw(BMCCDP) Mn,th (HMW) =2ꢁ{[M]0/A
sion%+Mw(BCCDP) [d] Thermal polymerization of St; temperature=1108C, VSt
0 ACTHNUGTRENU[NGN AIBN]0/ACHTUNRTGEG[NNNU BMCCDP]0/ACHTGUNTREN[NUNG BCCDP]0. [b] Conv.: conversion.
CHTUNGTRENNUNG
;
CHTUGNTRNEN(GNU [BMCCDP]0 +2ꢁ[BCCDP]0)}ꢁMw(M) ꢁconver- Signals at d=1.2–2.2 and 6.2–7.2 ppm (h and g in
.
=
Figure 3, respectively) were assigned to styrene
units in polymers. Furthermore, the ratio of the in-
tegrals of peaks Ha and Hf was 2:1. These results
suggest that the PS chains obtained were capped by
the BMCCDP and BCCDP moieties at the polymer
3.0 mL. [e] Solution polymerization of MA; MA/anisole=1:1 (v/v), temperature=
758C. [f] Solution polymerization of AN; AN/DMSO=1:2 (v/v), temperature=758C.
MWD polymers, even in the absence of the conventional
free-radical initiator (AIBN).
chain ends, a result that is consistent with the mechanism of
RAFT polymerization.
Monomers such as methyl acrylate (MA) and acrylonitrile
(AN) were also used for the preparation of bimodal MWD
polymers to validate the extensive application of this strat-
egy for other monomers. The results of the polymerization
of MA are shown in entries 2–5 of Table 1. Although the
Mn,GPC values of the polymers (HMW and LMW fractions,
respectively) were slightly higher than the theoretical values
The chain-extension reaction is another way to verify the
functionality of the polymers. Therefore, the obtained PS
was used as a macro-RAFT agent to polymerize fresh St
monomer. Figure 4 shows the GPC curves of the original
and chain-extended polymers of PS. The bimodal elution
peaks of the macro-RAFT agent obviously shifted to the
chain-extended PS with Mn,GPC =28000/62000 gmolÀ1 and
Chem. Eur. J. 2012, 18, 6015 – 6021
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Z. P. Cheng, L. F. Zhang, X. L. Zhu et al.
Preparation of bimodal molecular-weight-distribution block
copolymers: According to the mechanism of RAFT poly-
merization, the polymers obtained are “living”. The chain-
end analysis and chain-extension experiment verified the
successful attachment of RAFT-agent moieties in the bimo-
dal MWD polymers (see Figures 2 and 3). To further con-
firm the “living” features of the obtained bimodal MWD
polymers, poly(methyl acrylate) (PMA) and poly(acrylonit-
rile) (PAN) prepared by the method mentioned above
(Table 1) were used as macro-RAFT agents to synthesize
block copolymers with other fresh monomers. Solvents were
added to obtain a homogeneous reaction system. Figure 5
Figure 5. GPC curves before and after block copolymerization with bimo-
dal PSt and PMA as macro-RAFT agents. The polymerization conditions
were the same as those listed in Table 2. PNIPAM: poly(N-isopropylacry-
lamide).
Figure 3. 1H NMR spectrum of PS (Mn,GPC =3850/8550 gmolÀ1, Mw/Mn =
1.06/1.05) obtained in the absence of oxygen with CDCl3 as a solvent and
tetramethylsilane (TMS) as an internal standard. Polymerization condi-
tions:
[St] /
N
N
G
VSt =
3.0 mL, temperature=758C.
shows the GPC curves before and after block copolymeriza-
tion. The obtained polymers were used as bimodal macro-
RAFT agents, with AIBN as the initiator and MA, N-isopro-
pylacrylamide (NIPAM), and St as the monomers at 758C.
Four different bimodal MWD block copolymers, PS-b-
PMA, PS-b-PNIPAM, PS-b-PAN, and PMA-b-PS were ob-
tained successfully. Obviously, the bimodal elution peaks of
the macro-RAFT agents shifted to those of the correspond-
ing block copolymers, and the results are listed in Table 2.
From Table 2, we can see that good control over the MW
and MWD could be attained during block copolymerization.
The bimodal peak remains good, with narrow polydisper-
sities (<1.14). This is the first time that bimodal MWD
block copolymers with both well-defined MW and MWD
have been synthesized, which demonstrates the advantages
of this strategy over the other reported methods mentioned
above.
Figure 4. GPC curves before and after chain extension with PS as
macro-RAFT agent. Polymerization conditions for original PS:
[St]0/A[AIBN]0/A[BMCCDP]0/A[BCCDP]0 =1200:1:2.5:2.5, St =3.0 mL,
time=2 h, conversion=15.6%, temperature=758C. Polymerization con-
ditions for chain-extended PS: [St]0/A[AIBN]0/[PS]0 =2400:1:2.5, VSt
3.0 mL, time=34 h, conversion=68.8%, temperature=758C.
a
G
E
E
V
G
=
Effect of the molar ratio of mono/difunctional RAFT agents
on bimodal MWD PS: As discussed above, all chain ends of
the polymers prepared by mono/difunctional RAFT agents
have equal chances to grow further. In this work, different
molar ratios of RAFT-1 to RAFT-2 (1:100–100:1) were used
in the polymerization to control the proportions of higher-/
lower-molecular-weight fractions in the bimodal MWD poly-
low Mw/Mn values (1.08/1.07). The successful chain-extension
reaction further verified the “living” features of the RAFT
polymerization of St.
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RAFT Polymerization
FULL PAPER
Table 2. Preparation of bimodal MWD block copolymers by RAFT polymerization with bimodal MWD
macro-RAFT agents.
the experimental mass concen-
tration for the lower molecular
weight from the GPC results
was 51.8%, which is close to
the theoretical value of 50%.
[c]
Entry Copolymers Ratio[a]
t
Conv.
Mn,GPC[b] (LMW/
Mn,GPC (LMW/
Mw/Mn
(LMW/
HMW)
[h] [%]
HMW) [gmolÀ1
]
HMW) [gmolÀ1
]
1[d]
2[d]
PS-b-PMA 2400:1:2.5 50.5
86.3 2900/5750
33000/80300
19700/35700
1.12/1.14
1.02/1.07
PS-b-
PNIPAM
PS-b-PAN
1200:1:2.5 50.5 ꢀ100
2900/5750
Synthesis of higher-molecular-
weight PS: The synthesis of
HMW polystyrenes by using
this pair of RAFT agents was
also attempted. The retention
of functional chain ends is cru-
cial in synthesizing HMW poly-
mers. A HMW PS with narrow
MWD was obtained. The poly-
merization rate decreased with
an increase in the molar ratio
3[e]
4[d]
2400:1:2.5 9.3
49.7 4850/9950
27.8 10100/21000
15000/31200
19400/35000
1.06/1.07
1.02/1.08
PMA-b-PS 2400:1:2.5 70.0
[a] Ratio of [Monomer] /
N
macro-RAFT agents. [b] Molecular weights of the first blocks (macro-RAFT agents). [c] Molecular weights of
block copolymers. [d] Anisole used as solvent, monomer/anisole=1:1 (v/v), temperature=758C, molecular
weights measured by GPC in THF. [e] DMSO used as solvent, monomer/DMSO=1:1 (v/v), temperature=
758C, molecular weights measured by GPC in N,N-dimethylformamide (DMF) as Mn,GPC/2.5 with acceptable
errors.[14]
Table 3. Effect of molar ratio of mono/difunctional RAFT agents on RAFT polymerization of styrene.
of [St]0/ACHTNUTRGENN[UG RAFT-1]0/ACHTUNGTERN[NUGN RAFT-2]0,
[b]
[c]
Entry Ratio[a]
t
Conv.
Mn,th (LMW/HMW) Mn,GPC (LMW/
Mw/Mn (LMW/ mth
HMW)
mGPC
[%]
due to the corresponding lower
concentration of AIBN initia-
tor, as expected. As shown in
Table 4, both higher- and lower-
molecular-weight fractions had
low Mw/Mn values (ꢀ1.2), even
[h] [%]
[gmolÀ1
]
HMW) [gmolÀ1
]
[%]
1[d]
2[e]
3[f]
4[g]
5[h]
6[i]
1:2
1:3
1:20
1:50
1:100
2:1
3:1
20:1
50:1
26
26
23.8
16
16
70
70
40.2
40
75.0 15650/30700
82.8 12450/24350
76.5 9950/19400
5600/10600
5600/10600
80.5 20700/41000
84.9 17600/34700
66.1 15650/30750
76.7 8000/15450
83.3 8800/17050
14080/33470
10680/25900
7200/18900
3750/10200
3850/9550
22250/48200
20150/42850
20900/43700
10300/22900
7900
1.10/1.06
1.09/1.05
1.11/1.06
1.07/1.06
1.06/1.06
1.09/1.05
1.08/1.05
1.07/1.03
1.10/1.02
1.14
20.0
14.3
2.5
1.0
0.5
50.0
60.0
90.9
96.1
98.0
29.4
26.4
15.7
12.5
11.0
51.8
58.0
87.9
92.7
100
ꢀ100
ꢀ100
the
MW
was
up
to
in
7[j]
8[k]
9[l]
871000 gmolÀ1
(entry 8
Table 4). The Mn,GPC values
were close to the predicted the-
10[m] 100:1
40
[a] Ratio of [RAFT-1] /
0 ACHTUNGTRENNUNG[RAFT-2]0. Polymerization conditions: VSt =3.0 mL; temperature=758C. [b] mth =Ra- oretical values. The above re-
tio/(Ratio+2)ꢁ100%=the theoretical mass concentration (%) of PS with lower molecular weight calculated
ACHTUNGTRENNUNG
sults demonstrate that RAFT
polymerization with this pair of
RAFT agents, without any addi-
tional treatments, could synthe-
size bimodal HMW polymers
with excellent controllability.
It is worth pointing out that
from the feed molar ratio. [c] mGPC =ALMW/(AHMW+ALMW)ꢁ100%=the mass concentration (%) of PS
with lower molecular weight calculated from the corresponding peak areas (A) measured by GPC. [d–
m] [St] /
G
N
N
d) 2400:1.66:2.5:5,
e) 2400:2.33:2.5:7.5,
f) 4800:5.46:1:20,
g) 4800:13.5:1:50, h) 9600:26.8:1:100, i) 2400:1.33:5:2.5, j) 2400:1.66:7.5:2.5, k) 4800:2.9:20:1, l) 4800:6.93:50:1,
m) 9600:13.6:100:1, respectively.
mers. As shown in Table 3, bimodal MWD polystyrenes
with low Mw/Mn values (HMW and LMW fractions, respec-
tively) could be obtained even though the ratios of RAFT-
1 to RAFT-2 were from 1:100 to 50:1. The Mn,GPC values of
the polymers were close to the Mn,th values (HMW and
LMW fractions, respectively). However, the peak of the
HMW fraction became weak if the ratio of RAFT-1 to
RAFT-2 was increased to 100:1
the HMW is almost twice the LMW for all of the obtained
bimodal MWD polymers in our work. This is because this
strategy is based on the idea that a RAFT polymerization is
carried out in the concurrent presence of two different
RAFT agents, one monofunctional RAFT agent (RAFT-1)
and one difunctional RAFT agent (RAFT-2). According to
the mechanism of RAFT polymerization, polymers with the
(entry 10 in Table 3). In addi-
tion, it can also be seen that
each fraction of higher or lower
molecular weights in the bimo-
dal MWD polymers can be con-
trolled by changing the molar
ratio of RAFT-1 to RAFT-2
(Table 3). The experimental
mass concentrations were close
to the corresponding theoretical
ones. For example, when a ratio
of 2:1 for RAFT-1 to RAFT-2
Table 4. Bimodal MWD PS with higher molecular weight from RAFT polymerization.
Entry
Ratio[a]
t [h]
Conv. [%]
Mn,th
Mn,GPC (LMW/
Mw/Mn (LMW/
HMW)
(LMW/
HMW) [gmolÀ1
]
HMW) [gmolÀ1
]
1
2
3
4
5
6
7
8
2400:1:2.5:2.5
3600:1:2.5:2.5
4800:1:2.5:2.5
6000:1:2.5:2.5
9600:1:2.5:2.5
12000:1:2.5:2.5
36000:1:2.5:2.5
48000:1:2.5:2.5
33.5
33.5
33
65.5
65
24
53
24
70.9
65.0
61.5
63.8
54.6
61.0
45.3
50.7
24250/47950
33100/65700
41600/82700
53800/107100
73400/146300
102100/203700
227000/453000
338000/675000
22800/49000
31100/66900
38800/84400
46800/107800
64700/149900
88000/237000
146000/407000
231000/871000
1.07/1.05
1.07/1.05
1.07/1.06
1.10/1.10
1.09/1.10
1.18/1.13
1.16/1.11
1.36/1.08
was used (entry 6 in Table 3), [a] Ratio of [St]0/ACHTUNGTNERN[UNG AIBN]0/AHCTUNTREG[NNGUN RAFT-1]0/CAHTGUNRTEN[NNGU RAFT-2]0. Polymerization conditions: VSt =3.0 mL, temperature=758C.
Chem. Eur. J. 2012, 18, 6015 – 6021
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6019

Z. P. Cheng, L. F. Zhang, X. L. Zhu et al.
À
À À
À
were passed through an alumina column and stored at À188C. 2,2-Azobi-
sisobutyronitrile (AIBN, chemically pure), dimethylsulfoxide (DMSO,
+99%), tetrahydrofuran (THF, +99%), anisole (+99%), and all other
chemicals were used as received unless mentioned. 2-Bromo-2-methyl-
propionyl bromide (98%) was purchased from Aldrich Chemical Co.
structure of R ACHTUNGTRENNUNG[monomer]n S C(=S) Z could be obtained
À À
by using a monofunctional RAFT-1 with the structure R S
À
C(=S) Z. Similarly, difunctional RAFT-2, with the structure
À
À À À À
À
Z (S=)C S R S C(=S) Z, can be used to synthesize poly-
À
À À À À
mers with the structure Z (S=)C S [monomer]n
G
R
Characterizations: The number-average molecular weight (Mn,GPC) and
molecular weight distribution (Mw/Mn) of the polymers were determined
by using a Waters 1515 gel permeation chromatograph equipped with
a Waters 2414refractive-index detector, with HR1, HR2, and HR4 col-
umns (7.8ꢁ300 mm2, 5 mm bead size) with measurable molecular weights
in the range of 102–5ꢁ105 gmolÀ1. THF was used as the eluent at a flow
rate of 1.0 mLminÀ1, at 308C. The GPC samples were injected by using
a Waters 1515 plus autosampler and calibrated with polystyrene stand-
ards from Waters (PMA was calibrated with poly(methyl methacrylate)
standards). For PAN, DMF with added LiBr (0.05 molLÀ1) was used as
the eluent at a flow rate of 0.8 mLminÀ1, also at 308C. In this case, the
GPC samples were injected by using a Waters 1515 plus autosampler and
calibrated with polystyrene standards from Waters. 1H NMR spectra
were recorded on an Inova 400 MHz NMR instrument with CDCl3 or
(CD3)2SO as the solvent and tetramethylsilane (TMS) as the internal
standard at ambient temperature.
À À
À
ACHTUNGTRENNUNG
prepared with difunctional RAFT-2 were twice those of
polymers prepared with monofunctional RAFT-1. The key
to preparing bimodal MWD polymers is that the structures
of the pair of RAFT agents should have equal chances to be
addition and fragmentation radicals. In order to confirm the
hypothesis, RAFT-1 or RAFT-2, respectively, was used as
a RAFT agent in the polymerization of St. The results are
shown in Figure 2c and d. As determined from the kinetic
plots shown in Figure 2c, the apparent rate constant of the
polymerization, kp
Rp: polymerization rate; M: monomer; Pn: propagating radi-
cal) with difunctional RAFT-2 ([St]0/A[AIBN]0/A[RAFT-2]0 =
app
(Rp =Àd[M]/dt=k
[Pn·][M]=kpapp[M];
pACHTUNGTRENNUNG
G
CHTUNGTRENNUNG
600:0.66:2.5) was calculated as 0.0782 sÀ1, which was approx-
imately twice that of monofunctional RAFT-1 (0.0395 sÀ1)
under the same polymerization conditions (the same ratio of
À
Synthesis of 1,4-[2-(carbazole-9-carbodithioate)-2-methyl-propionic acid]
phenyl ester (BCCDP): Carbazole (1.67 g, 0.01 mol) was added to a sus-
pension of KOH (0.56 g, 0.01 mol) in DMSO (25 mL) under vigorous stir-
ring. The solution was stirred for 3 h at 308C, and then carbon disulfide
(0.76 g, 0.01 mol) was added dropwise. The resultant reddish solution was
stirred for 5 h at 308C, and then 1,4-(2’-bromo-2’-methylpropionato)ben-
zene (BMPB; 2.04 g, 0.005 mol; synthesized according to reference [15])
was added. The mixture was stirred for 24 h at 308C and was then
poured into a large amount of deionized water (ꢀ300 mL). A yellow
solid was obtained. The crude product was purified by washing with ace-
tone three times. 1H NMR ((CD3)2SO, 400 MHz): d=1.99 (s, 12H), 7.31
(s, 4H), 7.41–7.45 (m, 4H), 7.51–7.55 (m, 4H), 8.22–8.24 (d, 4H), 8.37–
8.39 ppm (d, 4H).
[AIBN]0/ACHTUNGTRENNUNG[S C(=S) segments]0; [St]0/ACHTNURGTEG[NNUN AIBN]0/ACHTNUGTNER[NUGN RAFT-1]0 =
600:0.33:2.5). From Figure 2d, it can be seen that all of the
molecular weights obtained from GPC results were consis-
tent with the corresponding theoretical ones in three cases.
À
These results confirmed that all S C(=S) groups in this pair
of RAFT agents have the same “living” for the RAFT poly-
merization and, therefore, result in bimodal MWD polymers
in the presence of this pair of RAFT agents by the one-step/
one-pot method.
Synthesis of 2-bromo-2-methyl-propionic acid 4-[2-(carbazole-9-carbodi-
thioate)-2-methyl-propionyloxy]phenyl ester (BMCCDP): BMCCDP was
synthesized by using a similar method to that used for BCCDP, except
that the amount of BMPB was increased to 6.12 g (0.015 mol). The crude
product was purified first by column chromatography on silica oxide with
a mixture of petroleum ether and ethyl acetate (10:1) as the eluent and
then by recrystallization three times from acetone. 1H NMR (CDCl3,
400 MHz): d=2.06–2.11 (d, 12H), 7.19–7.22 (d, 2H), 7.28–7.30 (d, 2H),
7.39–7.43 (m, 2H), 7.48–7.52 (m, 2H), 8.03–8.05 (d, 2H), 8.45–8.48 ppm
(d, 2H).
Conclusion
Bimodal MWD polymers were successfully synthesized by
using a pair of mono/difunctional RAFT agents in RAFT
polymerization. The system showed excellent controllability
and “living” characteristics for both higher- and lower-mo-
lecular-weight fractions. The Mn,GPC values of the polymers
(for the HMW and LMW fractions, respectively) were close
to the theoretical Mn,th values. The Mw/Mn ratio (for the
HMW and LMW fractions, respectively) of the polymers
was kept low (<1.10) during the polymerization process. Bi-
modal HMW polymers and block copolymers with both
well-controlled MW and MWD could be prepared easily
due to the “living” features of RAFT polymerization. The
strategy produced a mixture of higher-/lower-molecular-
weight polymers at the molecular level but also preserved
the features of LRP of RAFT polymerization.
Typical bulk RAFT polymerization of St: AIBN (3.5 mg, 0.022 mmol),
RAFT-1 (37.5 mg, 0.055 mmol), RAFT-2 (42.0 mg, 0.055 mmol), and St
(3.0 mL, 26.2 mmol) were added in that order to a dried ampoule under
stirring. The ampoule was thoroughly bubbled with argon for 20 min to
eliminate the dissolved oxygen in the solution. The ampoule was then
flame sealed and transferred into an oil bath, held at 758C by a thermo-
stat, to allow polymerization in the mixture under stirring. After the de-
sired polymerization time, the ampoule was cooled by immersing it into
iced water. Afterwards, it was opened and the contents were dissolved in
THF (2 mL); the contents from the polymerization of AN were dissolved
in DMF (2 mL). The products were then precipitated in a large amount
of methanol (ꢀ200 mL). The polymer obtained by filtration was dried
under vacuum until constant weight was achieved at 508C. The conver-
sion of monomer was determined gravimetrically. The procedures for
MA and AN polymerization and for the block copolymerization were
similar to that of St polymerization except that a certain amount of ani-
sole or DMSO was added to the reaction system to form a homogeneous
solution. In addition, for block copolymerization, the macro-RAFT
agents were used in place of RAFT-1 and RAFT-2.
Experimental Section
Chain extension of the PS macro-RAFT agent with St: Predetermined
quantities of AIBN and PS (obtained by RAFT polymerization of St
with RAFT-1 and RAFT-2 agents at 758C) were dissolved in St (3.0 mL)
in a dried ampoule. The rest of the procedure was the same as that de-
Materials and reagents: Unless otherwise specified, all chemicals were
purchased from Shanghai Chemical Reagents Co. (Shanghai, China). The
monomers, methyl acrylate (MA, +99%), N-isopropylacrylamide
(NIPAM, +99%), acrylonitrile (AN, +99%), and styrene (St, +99%),
6020
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 6015 – 6021

RAFT Polymerization
FULL PAPER
scribed above. The chain-extension polymerization was carried out under
stirring at 758C.
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Received: December 14, 2011
Published online: April 4, 2012
Chem. Eur. J. 2012, 18, 6015 – 6021
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
6021