Producing bimodal molecular weight distribution polymers through facile one-pot/one-step RAFT polymerization

Article abstract of DOI:10.1002/pola.26212

Well-defined bimodal molecular weight distribution (MWD) polystyrene and polystyrene-b-poly(acrylonitrile) were successfully synthesized using a pair of mono/difunctional trithiocarbonate RAFT agents 1 and 2 via one-pot RAFT polymerization. The kinetics of RAFT polymerization for styrene in bulk with a molar ratio of [St]₀:[AIBN]₀:[1]₀:[2] ₀ = 1200:1:2.5:2.5 was studied at 75°C. The results indicated that the system showed excellent controllability and living characteristics to both higher and lower molecular weight fractions, providing an efficient and facile way to producing bimodal MWD (co)polymers with both controlled molecular weight (MW) and MWD in molecular level, and the plausible mechanism was discussed in this work.

Full text of DOI:10.1002/pola.26212

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Producing Bimodal Molecular Weight Distribution Polymers Through
Facile One-Pot/One-Step RAFT Polymerization
Hongjuan Jiang, Lifen Zhang, Jian Qin, Wei Zhang, Zhenping Cheng, Xiulin 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, China
Correspondence to: Z. Cheng (E-mail: chengzhenping@suda.edu.cn) or X. Zhu (E-mail: xlzhu@suda.edu.cn)
Received 7 February 2012; accepted 7 June 2012; published online
DOI: 10.1002/pola.26212
ABSTRACT: Well-defined bimodal molecular weight distribution
(MWD) polystyrene and polystyrene-b-poly(acrylonitrile) were
successfully synthesized using a pair of mono/difunctional tri-
thiocarbonate RAFT agents 1 and 2 via one-pot RAFT polymer-
ization. The kinetics of RAFT polymerization for styrene in bulk
with a molar ratio of [St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:2.5 was
studied at 75ꢀC. The results indicated that the system showed
excellent controllability and ‘‘living’’ characteristics to both
higher and lower molecular weight fractions, providing an effi-
cient and facile way to producing bimodal MWD (co)polymers
with both controlled molecular weight (MW) and MWD in
molecular level, and the plausible mechanism was discussed
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in this work. 2012 Wiley Periodicals, Inc. J Polym Sci Part A:
Polym Chem 000: 000–000, 2012
KEYWORDS: bimodal polymer; kinetics (polym.); trithiocarbon-
ate; living radical polymerization; one-pot/one-step method; re-
versible addition-fragmentation chain transfer (RAFT)
INTRODUCTION The molecular weight (MW) and molecular
weight distribution (MWD) of polymers have the key influ-
ence over their properties, such as physical properties, me-
chanical properties, and processing performance. Polymer
resins with higher molecular weight (HMW) have higher
strength but are difficult to process. On the contrary, lower
molecular weight (LMW) polymer resins are endued with
good toughness and rheological properties but lower rigidity.
For some special end-use, mechanical properties and good
processability are both needed. In order to synchronously
possess these characteristics, bimodal MWD polymers, which
contain both HMW and LMW fraction polymers, effectively
balance the processing performance (LMW component) and
mechanical performance (HMW component) of materials
under extreme conditions.1,2
bimodal MWD polymer resins.3 More recently, an alternative
strategy that combines conversional and living radical poly-
merization (LRP)11–20 can also be used to produce bimodal
MWD polymers.3,21 For example, Lenzi et al.3 produced bi-
modal MWD polymer resins of polystyrene and poly(butyl
acrylate) using nitroxide-mediated polymerization combined
with conventional free radical polymerization. Wan et al.21
reported formation of bimodal MWD poly(methyl methacry-
late) using controlled radical polymerization catalyzed by
CuCl/bis(2-dimethyl-aminoethyl) ether (BDE) complex in
aqueous medium. It must be emphasized, however, that all
these strategies mentioned earlier have failed to synthesize
bimodal MWD polymers with both controlled MW and MWD
at the same time, which has been a great challenging topic
until now.
A traditional way to produce bimodal MWD polymer resins
is physical blending, namely blending of different polymer
resins during the final processing stage. However, in this
case, the final performance of the polymer materials might
be affected by the much less efficient mixing of the polymer
chains. Therefore, there are incentives to produce polymer
materials with bimodal MWD polymers at the reaction stage,
which provides a common principle: mixing of different poly-
mer materials at the molecular level.3 Till now, a large num-
ber of techniques (e.g., two-step reaction,4,5 using chain
transfer agent6,7 or cross-linking agent,8,9 employing mixed
catalysts10) have been made available for the production of
Reversible addition-fragmentation chain transfer (RAFT) po-
lymerization, one of the most efficient techniques of LRP, has
been widely used for a variety of monomers, providing an ef-
ficient way to synthesize well-defined unimodal MWD poly-
mers with designed structures and good controlled MW and
narrow MWD.22–29 However, whether it is possible to synthe-
size bimodal MWD polymers via the RAFT process remains
to be answered. Barner-Kowollik and coworkers simulated a
RAFT process where poly-RAFT species and a small molecule
RAFT agent could grow simultaneously in the same polymer-
ization system.30 Very recently, we have reported a novel
strategy for the facile synthesis of well-defined bimodal
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SCHEME 1 Structures of trithiocarbonates 1 and 2.
MWD (co)polymers via one-pot RAFT polymerization using a
pair of mono- and difunctional dithiocarbamate RAFT
agents.31 Inspired by these results, in this work, we extend
this strategy to another type of RAFT agents, a pair of mono-
and difunctional trithiocarbonates to synthesize bimodal
MWD (co)polymers. Here, monofunctional dibenzyl trithio-
carbonates (1) and difunctional dibenzyl trithiocarbonates
(2) (the structures are shown in Scheme 1) were simultane-
ously added to the RAFT polymerization of the styrene sys-
tem, and well-defined bimodal MWD polymers with both
controlled MW and MWD were obtained successfully.
General Procedure for RAFT Polymerization of St
A typical bulk RAFT polymerization with a molar ratio of
[St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:2.5 was as follows: A
mixture was obtained by adding AIBN (1.19 mg, 0.0073
mmol), 1 (5.26 mg, 0.019 mmol), 2 (9.10 mg, 0.019
mmol), St (1.0 mL, 8.70 mmol) to a dried ampoule with a
stir bar. The ampoule was thoroughly bubbled with argon
for 20 min to eliminate the dissolved oxygen in the solu-
tion. Then the ampoule was flame-sealed and transferred
into an oil bath held by a thermostat at 75ꢀC to polymer-
ize under stirring. After the desired polymerization time,
the ampoule was cooled by immersing it into iced water.
Afterwards, it was opened and the contents were dis-
solved in THF (ꢂ2 mL); the polymerization product of St
was dissolved in DMF (ꢂ2 mL) and precipitated into a
large amount of methanol (ꢂ200 mL). The polymer
obtained by filtration was dried under vacuum until con-
stant weight at 35ꢀC. The conversion of the monomer was
determined gravimetrically.
EXPERIMENTAL
Materials
Unless otherwise specified, all chemicals were purchased
from Shanghai Chemical Reagents Co. (Shanghai, China). The
monomers, acrylonitrile (AN, þ99%), and styrene (St,
þ99%) were passed through a neutral alumina column and
stored at ꢁ18ꢀC; 2,20-Azobisisobutyronitrile (AIBN, chemi-
cally pure) was purified by crystallization from chloroform/
methanol. a, a0-dibromo-p-xylene (þ98%) was purchased
from TCI and used as received. Benzyl mercaptan (þ96%)
was purchased from Aldrich and used without further purifi-
cation. Benzyl bromide (þ98%), triethylamine (Et3N, þ99%),
carbon disulfide (CS2, þ99%), anisole (þ99%), N, N-dime-
thylformamide (DMF, þ99%), tetrahydrofuran (THF, þ99%),
and all other chemicals were used as received unless
mentioned.
Typical Procedures for Chain-Extension Reaction and
Block Copolymerization Using PS as Macro-RAFT Agent
The PS sample (Mn, GPC ¼ 9640/28,180 g molꢁ1, Mw/Mn
¼
1.19/1.12) obtained with the molar ratio of [St]0:[AIBN]0:
[1]0:[2]0 ¼ 1200:1:2.5:2.5 was used as the macro-RAFT
agent, St, or acrylonitrile (AN) as the monomer and AIBN as
the initiator with a molar ratio of [monomer]0:[AIBN]0:[ma-
cro-RAFT agent]0 ¼ 1200:1:2.5. The polymerization proce-
dure is as follows: PS (174.7 mg, 0.019 mmol) was dissolved
in 1.0 mL of anisole or DMF, and then the predetermined
quantities of AIBN (1.19 mg, 0.0073 mmol) and the mono-
mer St (1 mL, 8.70 mmol) or AN (1 mL, 15.2 mmol) were
added. The rest of the procedure was the same as that
described previously.
Synthesis of Dibenzyl Trithiocarbonate (1)
It was prepared according to literature.32 1H NMR (399.97
MHz, CDCl3, d): 7.41–7.24 (m, 10 H, Ar H); 4.64 (s, 4 H,
ArCH2).
Synthesis of Carbonotrithioic Acid, S, S’-[1,4-
Phenylenebis(methylene)] S, S’-Dibenzyl Ester (2)
Benzyl mercaptan (2.00 g, 14.70 mmol) was added to a
stirred suspension of K3PO4 (3.44 g, 16.18 mmol) in acetone
(50 mL) followed by stirring for 10 min. CS2 (3.36 g, 44.12
mmol) was then added and the solution turned bright yel-
low. After stirring for 10 min, a, a0-dibromo-p-xylene (1.94 g,
7.35 mmol) acetone solution was added, and an instant pre-
cipitation of KBr and 2 was observed. After stirring for 15
min, the suspension was filtered and the cake was washed
with deionized water and acetone several times. Then, a yel-
low solid was obtained. 1H NMR (399.97 MHz, CDCl3, d):
7.42–7.16 (m, 14H, Ar H); 4.63–4.57 (d, 8H, ArCH2); 13C
NMR (100.57 MHz, CDCl3, d): 217.4 (C¼¼S), 129.6 (Ar C),
129.5 (Ar C), 124.4 (Ar C), 124.1 (Ar C), 123.5 (Ar C), 122.6
(Ar C), 36.4 (CAS), 35.8 (CAS).
Aminolysis of Polymer
A solution of PS (0.2 g, Mn, GPC ¼ 14,980 g/mol, Mw/Mn
¼
1.53) and 1 mL of hexylamine in THF (2 mL) was stirred at
ambient temperature for 24 h under a nitrogen atmosphere.
Then the mixture was dropped into a large amount of meth-
anol, and a white solid was precipitated. The solid was
washed with water to neutral and re-dissolved in 2 mL of
THF, followed by a period of 5 h at 50ꢀC with stirring in the
presence of acetic acid (1 mL) and zinc powder (0.5 g). A
white solid was obtained after precipitation from methanol,
washing with water to neutral, and drying in a vacuum.33
White powder: yield 0.12 g; Mn,gpc ¼ 13,060/5260 g/mol;
Mw/Mn ¼ 1.09/1.12.
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TABLE 1 RAFT Polymerization of St with Different Amounts of Trithiocarbonates 1 and 2a
b
Entry
1/2
Time (h)
Con. (%)
Mn, th (g/mol)
Mn, GPC (g/mol)
Mw/Mn
1
2
3
a
2.0/2.0
2.5/2.5
3.0/3.0
23
23
23
69.4
66.2
57.1
14,730/29,370
11,310/22,540
10,140/20,200
9,910/28,680
12,850/35,420
8,680/24,740
1.20/1.08
1.19/1.08
1.17/1.10
[St]0:[AIBN]0:[1]0:[2]0 ¼ 1200/1/x/x (x ¼ 2.0, 2.5, 3.0); tempera-
¼ 2 ꢃ {[St]0/([1]0 þ 2 ꢃ [2]0)} ꢃ Mw,
ꢃ conversion % þ
St
ture ¼ 75ꢀC, in bulk.
Mw,2.
b
Mn,
(lower MW fraction)
¼
{[St]0/([1]0
þ
2
ꢃ
[2]0)}
ꢃ
Mw, thꢃ conversion % þ Mw,1, Mn,th (higher MW fraction)
St
Characterization
2.5 at 75ꢀC. The linearity of the plot indicates that the poly-
merization was approximately first-order with respect to the
monomer concentration and that the number of active spe-
cies remained constant during the polymerization process.
Evolution of Mn with conversion was found to be linear as
shown in Figure 1(b). At the same time, experimental Mw/
The number-average molecular weight (Mn, GPC) and molecu-
lar weight distribution (Mw/Mn) of the polymers were deter-
mined using a Waters 1515 gel permeation chromatograph
(GPC) equipped with a refractive index detector (Waters
2414), using HR1, HR2, and HR4 (7.8 ꢃ 300 mm2, 5-mm
beads’ size) columns with measurable molecular weights in
the range of 102 ꢁ 5 ꢃ 105 g molꢁ1. THF was used as an
eluent at a flow rate of 1.0 mLminꢁ1 operated at 30ꢀC. The
GPC samples were injected using a Waters 1515 plus auto-
sampler and calibrated with polystyrene standards from
Waters. For PAN, DMF þ 0.05 mol/L LiBr was used as an
eluent at a flow rate of 0.8 mLminꢁ1 operated at 30ꢀC. The
GPC samples were injected using a Waters 1515 plus auto-
sampler and calibrated with polystyrene standards from
Mn, values were narrow, indicating
a well-controlled
polymerization process. All the evidences demonstrated the
features of ‘‘living’’/controlled radical polymerization of this
polymerization system.
1
Waters. H and 13C NMR spectra were recorded on an Unity
1
Inova 400 spectrometer (399.97 MHz for H, 100.57 MHz for
13C). All spectra were obtained in CDCl3 or (CD3)2SO at am-
bient temperature. Chemical shifts are given in ppm and are
referenced to SiMe4 (d 0.00; H, 13C).
1
RESULTS AND DISCUSSION
Polymerization of St
In this work, we successfully obtained bimodal polymers
where both higher and lower molecular weight chains
possessed narrow MWDs with AIBN as the initiator; simulta-
neously we added 1 and 2—two kinds of CTAs—to the poly-
merization system in bulk at 75ꢀC. The experimental results
shown in Table 1 demonstrate the effect of concentration of
trithiocarbonates 1 and 2 on the polymerization of styrene.
It is obvious that in the three cases, bimodal MWD polysty-
rene (PS) with narrow MWDs (Mw/Mn ¼ 1.08–1.20) can be
obtained with molecular weights close to their correspond-
ing theoretical values, indicating that it is an effective and
facile strategy to produce bimodal MWD polymers. In addi-
tion, from Table 1, the polymerization was better controlled
with the molar ratio of [St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:
2.5:2.5, and so this feed ratio was intensively investigated in
this work.
In order to further investigate the polymerization behavior,
the kinetics of RAFT polymerization of St was studied in
detail. Figure 1(a) shows the kinetic plots of polymerization
of St in bulk initiated by AIBN in the presence of RAFT
agents with a ratio of [St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:
FIGURE 1 ln([M]0/[M]) as a function of time (a) and average-
number molecular weight (Mn,
) and molecular weight
GPC
distribution (Mw/Mn) versus conversion (b) for the bulk RAFT
polymerization of St. Polymerization conditions: [St]0:[AIBN]0:
[1]0:[2]0 ¼ 1200:1:2.5:2.5, VSt ¼ 1 mL, temperature ¼ 75ꢀC.
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FIGURE 2 GPC curves of obtained polystyrenes using different
molar ratios of 1/2. (a) [St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1.66:2.5:5;
(b) [St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:2.5; (c) [St]0:[AIBN]0:[1]0:
[2]0 ¼ 1200:1.33:5:2.5; VSt ¼ 1 mL, in bulk, temperature ¼ 75ꢀC.
FIGURE 3 1H NMR spectrum of PS (Mn,
¼ 9640/28,180
GPC
g molꢁ1, Mw/Mn ¼ 1.19/1.12) with CDCl3 as a solvent and TMS
as a internal standard. Polymerization conditions: [St]0:[AIBN]0:
[1]0:[2]0 ¼ 1200:1:2.5:2.5, VSt ¼ 1.0 mL, in bulk, temperature ¼
75ꢀC.
Figure 2 shows the GPC curves of the obtained polystyr-
enes via RAFT polymerization with different molar ratios
of 1/2 (1:2, 1:1, and 2:1, respectively) using St as the
monomer and AIBN as the initiator at 75ꢀC. From Figure
2, it can be seen that two peaks (representing higher mo-
lecular weight and lower molecular weight polymer chains,
respectively) were discernable and that the corresponding
peak areas shifted with the molar ratios, indicating that
the proportion of lower and higher molecular weight poly-
mer chains can be adjusted by the feed ratio of RAFT
agents 1 and 2. After further changing of the molar ratio
of 1 and 2 (Table 2), lower and higher molecular weight
exhibited the same multiple relationship, while the peak
coverage areas changed from 0.0% to 74.5% for the lower
molecular weight fraction. In addition, it can be seen that
the experimental mass concentrations were close to their
corresponding theoretical values from 4.8% to 83.3%.
Thus, the proportion of lower and higher molecular weight
polymer chains could be easily adjusted by changing the
feed molar ratio of 1 and 2.
Analyses of (Co)polymer Structures and Chain-Extension
or Block Copolymerization
To investigate the ‘‘living’’ features of RAFT polymerization
for the synthesis of bimodal MWD polymers, the obtained PS
(Mn, GPC ¼ 9640/28,180 g molꢁ1, Mw/Mn ¼ 1.19/1.12) was
analyzed by 1H NMR spectroscopy. The chemical shifts at
6.10–7.46 ppm (a in Fig. 3) corresponded to the aromatic
protons of the RAFT agent moieties and PS chains. The
chemical shifts at 1.63–2.39 ppm (b and d in Fig. 3) are
assigned to the methylene protons of the RAFT agent moi-
eties and methyne protons in the PS main chains. The
TABLE 2 Effect of Feed Molar Ratio of Trithiocarbonates 1 and 2 on RAFT Polymerization of Sta
Rb
Time (h)
Con. (%)
Mn, th (g/mol)
Mn, GPC (g/mol)
Mw/Mn
mth (%)
mGPC (%)
c
d
Entry
1e
2f
3g
4h
5i
6j
7k
a
1/1
23
23
23
23
23
23
23
66.2
55.1
74.4
69.8
51.5
58.4
50.9
11,310/22,540
11,290/22,500
8,540/17,010
8,580/17,090
13,130/26,200
10,010/19,950
10,860/21,650
12,850/35,420
16,600/36,190
12,910/25,110
16,900
1.19/1.08
1.19/1.06
1.17/1.05
1.55
33.3
20
40.9
29.3
19.2
0.0
1/2
1/4
11.1
4.8
1/10
2/1
11,930/31,970
9,580/31,610
8,420/22,890
1.10/1.17
1.21/1.17
1.24/1.18
50
47.0
60.4
74.5
4/1
66.7
83.3
10/1
d
Polymerization temperature ¼ 75ꢀC, in bulk.
R ¼ [1]0/[2]0.
mGPC (%) ¼ ALMW/(AHMW þ ALMW) ꢃ 100%, mass concentration (%) of
b
c
PS with lower molecular weight calculated by the corresponding peak
area (A) measured by GPC.
mth (%)
¼
R/(R
þ
2)
ꢃ
100%, theoretical mass concentration (%)
e–k
of PS with lower molecular weight calculated by the feed molar
[St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:2.5, 2400:1.66:2.5:5, 2400:2.99:2.5:1, 2400:
ratio R.
2.86:1.0:10, 2400:1.33:5:2.5, 2400:2.00:10:2.5, and 2400:1.74:10:1, respectively.
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FIGURE 4 GPC curves of before and after chain extension
using PS prepared by RAFT polymerization. Original PS:
[St]0:[AIBN]0:[1]0:[2]0 ¼ 1200:1:2.5:2.5, VSt ¼ 1 mL, in bulk, tem-
perature ¼ 75ꢀC, time ¼ 18 h, conversion ¼ 62.0%. Chain
extended PS: [St]0:[AIBN]0:[PS]0 ¼ 1200:1:2.5, VSt ¼ 1 mL, Vani-
¼ 1 mL, temperature ¼ 75ꢀC, time ¼ 24 h.
FIGURE 6 1H NMR spectrum of PS-b-PAN (Mn,
¼ 20,620/
sole
GPC
61,070 g molꢁ1, Mw/Mn ¼ 1.20/1.07) with (CD3)2SO as a solvent
and TMS as an internal standard. Polymerization conditions:
[AN]0:[AIBN]0:[PS]0 ¼ 1200:1:2.5, VAN ¼ 1.0 mL, VDMF ¼ 1.0 mL,
temperature ¼ 75ꢀC.
chemical shifts at 0.92–1.63 ppm (c in Fig. 3) are assigned to
the methylene protons in the PS main chains. The chemical
shifts at 3.74 ppm (e in Fig. 3) are attributed to the methyne
protons in the chain of PS because of the influence of S
atom, which indicates that the trithiocarbonate moieties
were successfully maintained in the polymer chains. There-
fore, the obtained PS can be used as macro-RAFT agents to
conduct chain-extension reaction and block copolymeriza-
shown in Figure 5, bimodal block copolymer PS-b-PAN was
also obtained successfully. This block copolymer (Mn, GPC
¼
20,620/61,070 g molꢁ1, Mw/Mn ¼ 1.20/1.07) was analyzed
by 1H NMR spectroscopy. The chemical shifts at 6.10–7.46
ppm (a in Fig. 6) corresponded to the aromatic protons of
the RAFT agent moieties and PS main chains. The chemical
shifts at 1.63–1.93 ppm (b in Fig. 6) were assigned to the
methyne protons in the PS main chains. The chemical shifts
at 0.92–1.63 ppm (c in Fig. 6) were assigned to the methyl-
ene protons in the PS main chains. The chemical shifts at
3.00–3.26 ppm (d in Fig. 6) were assigned to the methyne
protons in the PAN chains. The chemical shifts at 1.93–2.26
ppm (e in Fig. 6) were assigned to the methylene protons
in the PAN chains. The chemical shift at 3.38 ppm (f in
Fig. 6) is attributed to the methyne protons in the chain of
PAN because of the influence of S atom, which indicates
that after block copolymerization the trithiocarbonate moi-
eties were still maintained in the polymer chains. The suc-
cessful chain extension reaction and block copolymerization
further verified the ‘‘living’’ features of RAFT polymeriza-
tion of St.
tion. The PS (Mn,
¼ 9640/28,180 g molꢁ1, Mw/Mn
¼
GPC
1.19/1.12) obtained in the presence of thermal free radical
initiators (AIBN) was used as the macro-RAFT agents, St and
acrylonitrile (AN) as the monomers, AIBN as the initiator,
with
agent]0 ¼ 1200:1:2.5. There was a peak shift from the
macro-RAFT agents to the chain-extended PS with Mn, GPC
a molar ratio of [monomer]0:[AIBN]0:[macro-RAFT
¼
23,850/67,060 g/mol and Mw/Mn ¼ 1.20/1.10 (Fig. 4). As
Plausible Polymerization Mechanism for the Bimodal
MWD Polymers
As simulated by Barner-Kowollik30 and reported by our pre-
vious work,31 the propagating radicals with different chain
length could grow simultaneously in a same RAFT process.
In this work, two RAFT agents, one monofunctional dibenzyl
trithiocarbonate 1 and another difunctional trithiocarbonate
2 were simultaneously added into the RAFT polymerization
system. According to the RAFT mechanism using trithiocar-
bonates as the RAFT agents,34 monofunctional dibenzyl
FIGURE 5 GPC curves before and after block copolymerization
with bimodal PS as macro-RAFT agent. Polymerization condi-
tions: [AN]0:[AIBN]0:[PS]0 ¼ 1200:1:2.5, VAN ¼ 1.0 mL, VDMF
¼
1.0 mL, temperature ¼ 75ꢀC, time ¼ 24 h.
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SCHEME 2 Plausible polymer structures with different trithiocarbonates 1 and 2.
HMW chains was almost three times (not twice) those of
LMW chains in most cases, which indicated that the mecha-
nism is more complicated than the hypothesis as reported by
our previous work, where a pair of mono- and difunctional
dithiocarbamate RAFT agents was used.31 To further investi-
gate the possible mechanism, the polymerization of St was
conducted using difunctional trithiocarbonate 2 as the RAFT
agent solely at 75ꢀC. As shown in Figure 7, it was found that
a bimodal MWD polymer with Mn, GPC ¼ 13,060/5260 g/mol
and Mw/Mn ¼ 1.09/1.12 was obtained after aminolysis of the
obtained PS (Mn, GPC ¼ 14,980 g/mol, Mw/Mn ¼ 1.53, poly-
merization condition: [St]0/[AIBN]0/[2]0 ¼ 600:0.67:2.5, poly-
merization time ¼ 15 h), which indicated that there exist sev-
eral kinds of moieties of PS chains with different molecular
weights from polymers with structures 3, 4, and 5, as shown
in Scheme 2. This is attributed to the fact that the polymers
obtained from the difunctional RAFT agent having symmetri-
cal trithiocarbonate should contain not only polymer 4 but
also shorter polymer 3 and longer polymers (i.e., structure 5)
because scrambling of the growing chain would occur during
polymerization. On the other hand, from the experimental
results mentioned earlier, simultaneously adding RAFT agents
1 and 2 to the polymerization system can actually result in
bimodal MWD polymers with HMW chains being almost three
times those of LMW chains and low molecular weight distri-
butions; therefore, it is possible that polymers 3 and 5 are
main product in this case, as shown in Scheme 3.
FIGURE 7 GPC curves of before and after aminolysis of the
polymer (PS). Polymerization condition: [St]0/[AIBN]0/[2]0
600:0.67:2.5, temperature ¼ 75ꢀC.
¼
trithiocarbonate 1 should give rise to polymer with structure
3; Similarly, difunctional trithiocarbonate 2 should grow in
four directions, yielding polymers with structure 4, as shown
in Scheme 2. However, in this work, the molecular weights of
SCHEME 3 Schematic diagram illustrating the process for preparing bimodal MWD polymers.
6
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13 Wang, J. S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117,
CONCLUSIONS
5614–5615.
Bimodal MWD polymers were successfully synthesized using
a pair of mono/difunctional trithiocarbonate RAFT agents 1
and 2 via one-pot RAFT polymerization. The system showed
excellent controllability and ‘‘living’’ characteristics to both
higher and lower molecular weight fractions, providing an ef-
ficient and facile way to producing bimodal MWD polymers
with both controlled MW and MWD in the molecular level.
14 Ouchi, M.; Terashima, T.; Sawamoto, M. Chem. Rev. 2009,
109, 4963–5050.
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ACKNOWLEDGMENTS
18 Hawker, C. J.; Bosman, A. W.; Harth, E. Chem. Rev. 2001,
101, 3661–3688.
The financial support from the National Natural Science Foun-
dation of China (Nos. 20974071 and 21174096), the Special-
ized Research Fund for the Doctoral Program of Higher
Education (No. 20103201110005), the Project of International
Cooperation of the Ministry of Science and Technology of China
(No. 2011DFA50530), the Project Funded by Jiangsu Key Lab
Carbon Based Functional Materials & Devices and the Project
Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions (PAPD) is gratefully
acknowledged.
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