A novel approach to branched polymers via latent thiol chain-transfer polymerization

Article abstract of DOI:10.1246/cl.2011.1378

A novel sodium 4-vinylbenzyl thiosulfate (VBTS) monomer was designed and can be prepared effectively via the Bunte salt reaction in room temperature, which can be conveniently used as the latent chain-transfer monomer in the radical polymerization to prep

Full text of DOI:10.1246/cl.2011.1378

doi:10.1246/cl.2011.1378
1378
Published on the web November 26, 2011
A Novel Approach to Branched Polymers via Latent Thiol Chain-transfer Polymerization
Jinqiang Jiang,* Xiuli Jia, Xin Chen, Yiwu Zong, Hongwu Zhang, Ling Lin, Xiaoya Liu, and Mingqing Chen*
School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P. R. China
(Received September 8, 2011; CL-110749; E-mail: jiangjq@jiangnan.edu.cn, mqchen@jiangnan.edu.cn)
One vinyl group
H2
C
A novel sodium 4-vinylbenzyl thiosulfate (VBTS) monomer
was designed and can be prepared effectively via the Bunte salt
reaction in room temperature, which can be conveniently used as
the latent chain-transfer monomer in the radical polymerization
to prepare branched polymers.
CH
Chain growth center
H2
C
H2C
H2C
CH
CH
CH
Na2S2O3
r.t.
DMSO
AIBN
*
CH2
CH2
CH2
SH
*
S2
VBTS
O3Na
Cl
One transfer site
SH
One transfer site
AB*
Branched and hyperbranched polymers have captured
considerable attention over the past few years because of their
attractive features similar to dendrimers, such as high branching,
multiple end groups, improved solubility, lower solution
viscosity, and three-dimensional globular structure.1 Typically,
branched polymers can be synthesized conveniently via the
random branch-on-branch propagation of ABn-type monomers
in large scale with relatively low cost. One of the most
noteworthy and versatile approaches to branched polymers is
self-condensing vinyl polymerization (SCVP) of the AB*
inimer.2 And since Fréchet proposed this concept in 1995, this
approach has been greatly expanded to various types of
controlled polymerizations, such as reversible addition-frag-
mentation chain-transfer polymerization (RAFT), nitroxide-
mediated radical polymerization (NMRP), and atom-transfer
radical polymerization (ATRP).3-5 However, many of those
methods for synthesis of branched polymers also have signifi-
cant drawbacks, including the need for complex monomers and
harsh reaction conditions.6
Transfer chain
growth center
S
S
S
S
monomer conc.
time
h
BTS
S2O3Na
CH2
S
VBTS/BTS/BM/AIBN/St
mg/mL
LPSBM
0/0/1/1/100
0/1/0/1/100
0/0/0/1/100
1/0/0/1/100
1
1
1
1
24
24
24
LPSBTS
LPSAIBN
BPSt1-t8
CH2
SH
BM
5, 7, 9, 12, 15, 18, 21, 24
Scheme 1. The basic concept of the latent thiol chain-transfer
branched polymerization (LTCTBP).11
Branched polymers can also be prepared via conventional
free radical polymerization that uses only inexpensive and
readily available starting materials.7 The strategy involves the
conventional free radical polymerization of vinyl monomers in
the presence of a small amount of chain-transfer monomers or
divinyl comonomer and thiols (-SH). However, it is very
difficult to prepare a stable chain-transfer monomer containing
-SH transfer group since the -SH group can easily react with
vinyl under thermal condition or light irradiation. The optical
alternative is to use latent sulfanyl groups (for example, the
sodium thiosulfonyl group shown in Scheme 1) which can be
effectively activated to release -SH transfer center and form
branch points during the polymerization. We named this novel
and convenient approach to branched polymers the latent thiol
chain-transfer branched polymerization (LTCTBP).
Scheme 1 outlines the basic concept of the named thiol
chain-transfer branched polymerization. The synthesis of the
chain-transfer monomer, i.e., sodium 4-vinylbenzyl thiosulfate
(VBTS), was conveniently prepared via the Bunte salt reaction
in room temperature.8 As expected, a white precipitate can be
easily collected after the mixture of 4-vinylbenzyl chloride and
the aqueous sodium thiosulfate solution for 12 h. Figure 1 shows
the 1H NMR spectrum of the VBTS monomer. The peaks of
sodium benzyl thiosulfate and vinyl groups can be clearly
observed at 4.17-4.31, 5.27-5.30, 5.80-5.86, and 6.71-6.81
ppm. Furthermore, the sodium thiosulfate group can be
Figure 1. 1H NMR spectrum of the sodium 4-vinylbenzyl
thiosulfate (VBTS).
effectively deprotected to release the active -SH in DMSO or
DMF solvent or acidified by HCl, which can afford the VBTS
monomer the latent chain-transfer ability in the free radical
polymerization.8
To investigate the branched polymerization, DMSO was
chosen as the polymerization solvent, more especially as the
deprotection agent of the sodium thiosulfate group. As listed in
Scheme 1, eight branched polymers of BPSt1-t8 were designed
and prepared via one-pot synthesis at 70 °C by using VBTS as
chain-transfer monomer, styrene (St) as common monomer, and
AIBN as initiator with a constant ratio of VBTS/AIBN/St =
1/1/100. And for the VBTS chain-transfer monomer, there are
two different on going polymerizations, i.e., the common radical
polymerization and latent chain-transfer polymerization, at the
same time. So it would be very difficult to investigate the
deprotection-transfer step. For this reason, we also introduced
Chem. Lett. 2011, 40, 1378-1380
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1379
10
8
Table 1. The solution characteristics of the obtained polymers
10.0
7.5
5.0
2.5
0.0
LPS
AIBN
5h BPS
7h BPS
9h BPS
t1
t2
t3
BPS
t8
MW(GPC) PDI
MW(LS)
[©]
LPS
¡ © 10
BT
¹1
¹1
¹1
/104 g mol (GPC) /104 g mol /mL g
LPS
BTS
12h BPS
15h BPS
18h BPS
21h BPS
24h BPS
LPSBT
LPSBTS
LPSAIBN
BPSt1
BPSt2
BPSt3
BPSt4
BPSt5
BPSt6
BPSt7
BPSt8
2.23
3.68
4.67
2.99
2.95
3.49
3.75
4.18
5.28
5.01
6.34
3.078
1.616
1.642
1.626
1.587
1.646
1.702
1.793
1.967
1.937
2.185
2.61
3.90
4.93
3.10
3.08
3.76
4.18
4.60
6.24
5.89
7.86
16.2
0.585 « 0.004
0.640 « 0.006
0.624 « 0.005
0.584 « 0.006
0.550 « 0.005
0.552 « 0.005
0.559 « 0.006
0.574 « 0.005
0.567 « 0.005
0.550 « 0.005
0.554 « 0.004
t4
t5
t6
t7
t8
21.4
24.3
17.5
16.9
19.0
19.7
20.9
23.6
23.2
25.3
6
12
16
20
24
28
32
Retention Time/min
4
2
0
12
16
20
24
28
32
Retention Time/min
the analog chain-transfer compounds, sodium benzyl thiosulfate
(BTS) and benzylthiol (BT) to prepare linear polymers of
LPSBTS and LPSBT, respectively. All the polymers were
obtained without gelation and purified using the same method
described in the experimental section and dried in vacuum oven
for 12 h at 40 °C.
A Wyatt triple detector system equipped with a WATERS
1515 pump, a WATERS 2414 detector, a Wyatt miniDAWN
TRISTAR detector, a Wyatt VicoStar viscometer detector, and
Wyatt ASTRA Chromatography Software was used to determine
the molecular weights, PDIs, and other solution characteristics
of the obtained polymers. Additionally, all the polymers were
also characterized by DSC and NMR.
Table 1 summarizes the typical characteristics of the
obtained polymers. It is known that Mark-Houwink shape
parameter ([©] = KM¡) of the branched polymers has lower
values than those of their linear counterparts.6 As expected, the
PDI and ¡ values of LPSBTS are very near those of LPSAIBN
(which was prepared without any chain-transfer agent), which
indicates the ongoing deprotection-transfer step of the sodium
thiosulfonyl group in DMSO solvent. This can also be
demonstrated from their GPC curves. And when transferred by
the VBTS monomer, the branched polymers of BPSt1-t8 gained
surprisingly low PDI values ranging between 1.587 and 2.185.
As expected, the molecular weights of all branched polymers
measured by GPC were much lower than those measured
via light scattering (an absolute method for molecular weight
determination). The obtained Mark-Houwink ¡-values range
between 0.550 and 0.584 (Table 1, column 6). Which is
consistent with the compact and globular shape of branched
polymer compared to their linear counterpart.
Figure 2 compares the GPC curves of the branched
polymers vs. the varied polymerization time. As expected, the
leading peak shifted forward and the molecular weight distri-
bution broadened along with the polymerization time. And
interestingly, all the branched polymers gained high Gaussian-
shaped curves without any shoulders or tailing peaks as the
linear polymer LPSAIBN and LPSBTS did. The observed PDI
values are quite lower (Table 1, column 3), which is very
surprising for branched polymers prepared via one-pot polymer-
ization.
Figure 2. The GPC curves of the branched polymers along
with polymerization time; the inset is the GPC curves of three
linear polymers and a branched polymer BPSt8.
100
95
100
75
50
25
0
Decrease
Decrease
90
85
80
5
10
15
20
25
Polymerization time/h
Figure 3. Plots of the branching magnitude of g¤ (red one) and
the glass-transition temperature Tg (blue one) on polymerization
time.
linear analogs due to the compact character of the branched
structure.9 More specifically, the value of the exponential in the
Mark-Houwink relation for branched polymer is apparently
smaller than that for the linear one.10 And the effect of
architectural branching can also be expressed by the contracting
factor, g¤ which is defined as g¤ = [©]b/[©]l, where [©]b and [©]l
are the obtained intrinsic viscosity of branched polymers in
solvent and the calculated one according to the Mark-Houwink
equation ([©] = KM¡), respectively.9,10 Here, we use the
observed Mark-Houwink parameters of LPSAIBN (Kl =
3.042 © 10¹2 mL g¹1, ¡l = 0.624) and the absolute molecular
weight MW(LS) of branched polystyrene (column 4, Table 1)
measured by lighter scattering to calculate the [©]l values. As
shown in Figure 3, the magnitude of g¤ decreased linearly along
with the polymerization time and the branched polymer BPSt8
gained the lowest g¤ value of 73%, which indicates that much
more VBTS monomers were copolymerized into the polymer
chains to form branches along with the polymerization time. The
decrease of the glass-transition temperature (Tg) on the varied
polymerization time in Figure 3 also demonstrates this branch
growth.
It is well known that the solution properties of branched
polymers, such as MW dependence of intrinsic viscosity ([©])
and the radius of gyration, are remarkably different from their
Chem. Lett. 2011, 40, 1378-1380
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

1380
In summary, the novel sodium 4-vinylbenzyl thiosulfate
4
5
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(VBTS) monomer was designed and can be prepared effectively
via the Bunte salt reaction in room temperature. And through the
deprotection-transfer reaction of VBTS monomer in the radical
polymerization, the named latent thiol chain-transfer branched
polymerization (LTCTBP) can be approached conveniently. It is
easy to envision that this general and convenient branched
polymerization can readily be applied to many vinyl monomers.
Therefore, as an extension of this work, we are currently
preparing hyperbranched polymers with other functional vinyl
monomers.
6
7
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This work was supported by the Natural Science Foundation
of China (Grant No. 50973044), the Qing Lan Project of Jiangsu
Province, China and the Scientific and Fundamental Research
Funds for the Central Universities, China (No. JUSRP31003).
8
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Chem. Lett. 2011, 40, 1378-1380
© 2011 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/