Thermoresponsive Sulfone and Sulfoxide-Containing Polyacrylamides

Article abstract of DOI:10.1002/bkcs.12341

A series of thermoresponsive polyacrylamides containing sulfide, sulfone, or sulfoxide was successfully prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. The thermal properties of polyacrylamides with sulfone or sulfoxide, which have not been studied before, were investigated for the first time. While poly[N-(2-(propylsulfonyl)ethyl)acrylamide] (P2SO2) with sulfone groups was water insoluble, poly[N-(2-(propylsulfinyl)ethyl)acrylamide] (P2SO) with sulfoxide groups was water soluble, indicating that the sulfoxide moiety is more hydrophilic than sulfone. By controlling the end of the polymer chain, we synthesized poly[N-(2-(ethylsulfonyl)ethyl)acrylamide] (P1SO2) and poly[N-(2-(butylsulfinyl)ethyl)acrylamide] (P3SO), which exhibit an lower critical solution temperature (LCST) in water around 24–26°C. For adjusting a wide range of LCST, we synthesized the random copolymers by controlling the initial feed ratio of hydrophobic M2SO2 and hydrophilic M2SO. The amphiphilic block copolymers were also synthesized and assembled in water to yield the micelles. After adding H₂O₂ to this micellar solution, the hydrophobic block was oxidized and converted to a hydrophilic block, leading to the transformation of micelles to unimers.

Full text of DOI:10.1002/bkcs.12341

Article
DOI: 10.1002/bkcs.12341
BULLETIN OF THE
J. Choi and H.-i. Lee
KOREAN CHEMICAL SOCIETY
Thermoresponsive Sulfone and Sulfoxide-Containing Polyacrylamides
*
Jieun Choi and Hyung-il Lee
Department of Chemistry, University of Ulsan, Ulsan, Republic of Korea.
E-mail: sims0904@ulsan.ac.kr
*
Received April 29, 2021, Accepted June 3, 2021
A series of thermoresponsive polyacrylamides containing sulfide, sulfone, or sulfoxide was successfully
prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization. The thermal proper-
ties of polyacrylamides with sulfone or sulfoxide, which have not been studied before, were investigated
for the first time. While poly[N-(2-(propylsulfonyl)ethyl)acrylamide] (P2SO2) with sulfone groups was
water insoluble, poly[N-(2-(propylsulfinyl)ethyl)acrylamide] (P2SO) with sulfoxide groups was water sol-
uble, indicating that the sulfoxide moiety is more hydrophilic than sulfone. By controlling the end of the
polymer chain, we synthesized poly[N-(2-(ethylsulfonyl)ethyl)acrylamide] (P1SO2) and poly[N-
(
2-(butylsulfinyl)ethyl)acrylamide] (P3SO), which exhibit an lower critical solution temperature (LCST)
ꢀ
in water around 24–26 C. For adjusting a wide range of LCST, we synthesized the random copolymers
by controlling the initial feed ratio of hydrophobic M2SO2 and hydrophilic M2SO. The amphiphilic block
copolymers were also synthesized and assembled in water to yield the micelles. After adding H O to this
2
2
micellar solution, the hydrophobic block was oxidized and converted to a hydrophilic block, leading to
the transformation of micelles to unimers.
Keywords: Sulfide, Sulfone, Sulfoxide, Thermoresponsive, Polyacrylamide, RAFT, LCST
ꢀ
Introduction
polymers with the LCSTs lower than 100 C if an appropriate
10
hydrophilic/hydrophobic balance is achieved. The most well-
known and extensively researched thermoresponsive polymers
are poly(N-isopropylacrylamide) (PNIPAm) and its derivatives.
PNIPAm has a LCST in the range of 30–35 C in water.
The most interesting feature of PNIPAm is that its LCST is
very close to that of the human physiological temperature,
Stimuli-responsive polymers have been widely investigated
over the past few decades because of their numerous appli-
cations in the biomedical field. They undergo physical or
chemical changes in response to small variations under
environmental conditions, such as temperature, pressure,
pH, chemicals, redox, light, and so on. Polymers show a
sharp change in properties upon a small or modest change
in temperature. A lower critical solution temperature
ꢀ
1–4
11
which could be very advantageous. However, changing the
N-substituent group of PNIPAM results in the largest and most
documented thermoresponsive polymers of the polyacrylamide
family. Our previous study on thermo-sensitive fluorinated
polyacrylamide has indicated that their thermal properties
can be controlled. Therefore, we try to control the LCST
behavior, by sulfonation of thermoresponsive polyacrylamide.
A study on the comparison of the thermoresponsive behavior
of sulfone and sulfoxide in polyacrylamide has not been
systematically conducted before.
Organosulfur compounds, especially those containing
sulfone or sulfoxide, attract special interest in medicinal
chemistry and gives significance to many kinds of biologi-
cal activity, meaning that the SO group could be a hydro-
For example, sulfoxide-containing
polymer showed no cytotoxicity toward human embryonic
kidney cells (HEK 293). Furthermore, the oxidation-
responsive thioethers have been investigated as platforms
Sulfone-containing
poly(NIPAm) could be used in electrodeposition and cell
adhesion/detachment of human umbilical vein endothelial
cells (HUVECs). Considering this potential, we adjusted
the LCST by changing the structure of the acrylamide
(
LCST), which can be defined as a critical temperature at
which the polymeric solution shows a phase separation, is
the important feature of thermoresponsive polymers. The
polymers with a LCST have one phase below a critical tem-
perature, but they become insoluble upon heating above
12
5
–7
their LCST. Common LCST-type polymers show rapid,
sharp, and reversible phase transitions in response to tem-
perature changes. These types of polymers have become
more important in recent years as they have a wide range
of applications in drug delivery, bio-engineering, sensors,
8
and other advanced materials.
13–15
There are many known examples of LCST-type thermo-
responsive polymers. Poly(N,N-dimethylaminoethyl methacry-
late) (PDMAEMA) has been reported to show an LCST
transition at around 50 C. Moreover, derivatives of 2-oxazoline
POx) are also widely used as thermoresponsive polymers. The
gen bond receptor.
16
ꢀ
17,18
(
for reactive oxygen species (ROS).
ꢀ
POx family has an LCST ranging from 23 to 75 C, which
9
depends on the side chain. A combination of poly(ethylene
19
glycol) (PEG) or oligo(ethylene glycol) (OEG) with other struc-
tural motifs will afford water-soluble thermoresponsive (co)
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Article
BULLETIN OF THE
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KOREAN CHEMICAL SOCIETY
polymer with sulfone and sulfoxide. Theoretically, sulfone
would be more hydrophilic than sulfoxide because
sulfone has two oxygens, which can make hydrogen bond-
ing with water molecules, while sulfoxide has only one
oxygen. However, this study showed that sulfoxide moiety is
more water-soluble than sulfone moiety. This could be due to
the partial negative charge on the oxygen atom of sulfoxide
groups, which favors the formation of the hydrogen bonds
with water molecules, giving rise to the hydrophilicity of the
Results and Discussion
P1SO2 and P3SO exhibit an LCST in water around
4–26 C, which is comparable to body temperature. A
ꢀ
2
schematic of the strategy employed in this study is illustrated
in Scheme 1. A series of homopolymers (P1SO2 and P3SO)
and random copolymers (P2SO2-co-P2SO) were synthesized
via reversible addition-fragmentation chain transfer (RAFT)
polymerization from sulfur-containing acrylamide mono-
mers (M1SO2, M2SO2, M2SO, and M3SO), using
20
2
-dodecylsulfanylthiocarbonylsulfanyl-2-methylpropiionic
sulfoxide.
0
acid (DMP) as the chain transfer agent (CTA) and 2,2 -
azobisisobutyronitrile (AIBN) as the initiator. All cases of
RAFT polymerization under the molar ratio of [monomer]:
Herein, we synthesized the series of sulfur-containing
polyacrylamide, controlling the degree of sulfonation.
The thermal behavior was tuned by adjusting the degree
of polymerization (DP), molecular weight distribution
[
DMP]:[AIBN]:[trioxane], at 200:1:0.1:10 with the total
monomer concentration at 0.2 g/mL, was fixed. The forma-
(MWD), and environmental conditions such as concentra-
1
tion of these polymers was confirmed by H NMR spectros-
tion and the addition of salt. In addition, to give a wide
range of LCST, random copolymers were synthesized with
different initial ratios. We also synthesized amphiphilic
block copolymers that formed micelles in water. Subse-
quently, H O was added into the polymer aqueous solution
copy and the number average molecular weights (Mn) was
determined by gel permeation chromatography (GPC)
Table 1; Supporting Information Figures. S2 and S3). How-
ever, the apparent molecular weight obtained by GPC was
higher than the theoretical molecular weight calculated from
(
2
2
causing the micelles to become unimers.
Scheme 1. Synthesis of thermo-responsive sulfur-containing monomers and homopolymers by RAFT polymerization.
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Table 1. Results from synthesis of homopolymers (P1SO2 and P3SO) and random copolymers (P2SO2:P2SO) by RAFT polymerization.
b
d
M
(g/mol)
n,theory
M
n,app
a
1
c
ꢀ
Monomer (M)
a:b
Polymer
P1SO2-FRP
P2SO2-FRP
P2SO-FRP
P3SO-FRP
P1SO260
P1SO2100
P1SO2200
P3SO50
Conv of M (%)
M
n, HNMR (g/mol)
(g/mol)
PDI
LCST ( C)
M1SO2
M2SO2
M2SO
M3SO
M1SO2
—
—
—
—
—
33
47
69
24
59
69
—
—
—
—
—
—
—
46 000
39 200
22 500
33 800
31 500
35 600
69 800
14 700
23 200
40 400
40 900
20 200
30 200
1.78
2.11
1.91
2.09
1.10
1.13
1.21
1.18
1.29
1.34
1.21
1.20
1.19
26
—
—
25
26
26
24
25
25
24
10
35
62
—
—
—
—
—
—
—
—
12 400
17 900
39 900
9870
23 900
42 500
—
10 100
14 500
24 400
6780
21 700
31 500
—
M3SO
—
P3SO120
P3SO200
M2SO2:M2SO
75:25
0:50
5:75
P2SO2 -co-P2SO
23
7
7
5
2
P2SO243-co-P2SO47
P2SO2 -co-P2SO
76
—
—
—
—
2
4
a
1
Determined by H NMR spectroscopy.
b
c
d
Theoretical molecular weight determined from monomer conversions.
Calculated from H NMR spectroscopy using end group analysis.
Apparent number-average molecular weight and PDI determined by DMF GPC with PMMA calibration.
1
Figure 1. Plots of transmittance as a function of temperature measured for (a) 10 mg/mL aqueous solutions of P3SO homopolymers with
different MWs and MWDs, (b) different concentrations of P3SO200 in water (c) 10 mg/mL aqueous solutions of P3SO200 with NaCl
concentrations, and (d) 10 mg/mL aqueous solutions of P3SO200 homopolymer with a heating and a cooling cycle.
the monomer conversion, due to the differences in hydro-
dynamic volumes of sulfur-containing polyacrylamides, and
PMMA standards.
At first, we focused on the homopolymer (P3SO) since it
has thermosensitive properties. The general trend observed
from previous reports is that the LCST is lower for the
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2
1
higher molecular weight (MW) polymers. Additionally,
the LCST transition became extensive with broad molecular
(b)). Similarly, the concentration of the P3SO solution was
not related to the LCST value. Note that, salt content can
influence the LCST of a thermoresponsive polymer in addi-
tion to the MW, MWD, and concentration. For this reason,
the concentration of NaCl on the thermoresponsiveness of
an aqueous solution of P3SO200 was investigated. The addi-
tion of salt generally results in a decrease of the cloud point
temperature (“salting out” effect), as a function of the salt
2
2
weight distribution (MWD). Figure 1(a) indicates that the
transmittance of the 1 wt % P3SO solution, with different
chain lengths, synthesized by RAFT polymerization
decreases sharply at a certain temperature, owing to the tur-
bidity of the solutions when precipitation occurred. The
cloud point of thermoresponsive polymers in water was
determined at 50% transmittance point in the heating curve.
We found that MW had little effect on the thermal property
of the P3SO. We also investigated the cloud point of poly-
disperse P3SO (Mn = 33 800, PDI = 2.09) synthesized by
free-radical polymerization (FRP). Interestingly, the phase
transition of P3SO-FRP occurs at a similar temperature
compared to that of P3SO, synthesized by RAFT. More-
over, no significant differences in the LCST are observed
for molecular weight and molecular weight distribution.
We determined the LCST values of the polymer solutions
at different concentrations. The cloud point of the P3SO60
solution had no change at 0.5, 1, and 2 mg/mL (Figure 1
23,24
concentration.
However, Figure 1(c) shows the cloud
point of an aqueous P3SO200 solution had no difference
after the salt was added. We have also investigated the
reversibility of thermoresponsive behavior. Figure 1(d)
showed P3SO200 had a very sharp transition when heated,
but a broad hysteresis was observed when cooled. How-
ever, there are no differences in the heating and cooling
cycle of P3SO homopolymer. The P1SO2 homopolymer
was also investigated in the same way as the P3SO homo-
polymer (Supporting Information Figure S4). Moreover,
there are no significant differences in the thermal properties
of the P1SO2 homopolymer. The major disadvantage of
PNIPAm is the strong hysteresis of the thermal solubility
transition, which is due to the formation of intramolecular
25
hydrogen bonds in the collapsed state. Therefore, we can
overcome this weakness of PNIPAm with P1SO2 and
P3SO homopolymers.
First, we have to synthesize the sulfur-containing poly-
acrylamide, which has a wide range of LCSTs because
P1SO and P3SO have limited LCSTs. In general, the LCST
values depend on the introduction of comonomers that
influence the hydrophilic/hydrophobic balance of the
Scheme 2. Synthesis of thermoresponsive sulfur-containing ran-
dom copolymers by RAFT polymerization.
Figure 2. (a)The cloud point for 10 mg/mL aqueous solutions of P2SO2-co-P2SO with three different initial feed ratios, (b) GPC traces of
1
series of P2SO2-co-P2SO, and the deconvoluted H NMR spectrum of P2SO2-co-P2SO with initial feed ratio of (c) 75:25, (d) 50:50, and
(e) 25:75 for the quantification of the similar final structure.
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Scheme 3. Synthesis of an amphiphilic block copolymer P2SO120-b-P1SO2120, subsequent postpolymerization modification to yield
P2SO120-b-P1SO2120
.
1
Figure 3. (a) Overlaid GPC traces of P2S120-CTA, P2SO120-b-P1SO2120, and P2SO120-b-P1SO2120, H NMR spectra of (b) P2SO120-b-
P1SO2120, (c) P2SO120-b-P1SO2120
.
copolymer. P2SO2 is a hydrophobic homopolymer and
P2SO is a hydrophilic homopolymer. Therefore, it is
expected that the cloud point of the random copolymers
can be tuned by adjusting the fraction of each monomer
component in the copolymer chains. By controlling the ini-
tial feed ratio of M2SO2 and M2SO (75:25, 50:50, and
with three different compositions, were prepared (Scheme
2). The successful formation of these polymers was charac-
terized by gel permeation chromatography (GPC) and H
NMR spectroscopy (Figure 2(b) and Supporting
Information S5). From the ratio of these peak areas, the
final incorporation ratio of P2SO2/P2SO was similar to the
initial feed ratio (Figure 2(c)–(e)). The cloud point
1
25:75), a series of P2SO2-co-P2SO random copolymers,
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Figure 4. (a) Schematic diagram of the formation/disruption of micelles for adding H O , (b) hydrodynamic size distributions of a
2
2
0
0
.025 wt % aqueous solution of P2SO120-b-P1SO2120 after adding H O depending on the time, and representative AFM phase images of
2 2
.005 wt % of P2SO120-b-P1SO2120 micellar solutions spin-coated on mica (c) before and (d) after adding H O .
2
2
temperatures of the polymer samples were determined in an
aqueous solution at a concentration of 10 mg/mL (Figure 2
Scheme 3 shows the synthetic strategy used in this study.
The thermoresponsive copolymer block of P1SO2 was
extended from hydrophobic P2S macro-CTA, via RAFT,
with a feed ratio of 200:1 (P1SO2:P2S macro-CTA). The
molecular weight and molecular weight distribution of
P2SO120-b-P1SO2120 were determined on a GPC DMF
line, using PMMA standards (Mn = 59 900 g/mol, Mw/
Mn = 1.20) (Figure 3(a)), showing a clear shift to a high-
(a)). These three copolymers which have an initial monomer
feed ratio of 75:25, 50:50, and 25:75 were found to have
ꢀ
transition temperatures of 9.8, 35.5, and 62.1 C, respec-
tively. This indicated that the incorporation of P2SO2 had
caused an increase in the hydrophobicity of the resulting
copolymers. Also, the LCST increased with increasing P2SO
composition because P2SO is water soluble. In other words,
by tuning the initial ratio of random copolymers, sulfur-
containing polyacrylamide with a wide range of LCSTs, even
similar to body temperature, could be obtained.
molecular
weight
region.
The
appearance
of
─(O═S═O)─CH ─ protons of P1SO2, at 3.01–3.25 ppm,
2
and ─NH─CH ─ protons of P1SO2, at 3.76–4.04 ppm,
2
confirmed the successful synthesis of P2SO120-b-P1SO2120
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1
by H NMR spectroscopy (Figure 3(b)). After polymeriza-
polyacrylamide with sulfone and sulfoxide, we can conclude
that the sulfoxide moiety is more hydrophilic than the sulfone
moiety. We also adjusted the solubility of the polyacrylamide,
with sulfone and sulfoxide, in water by controlling the length
of the polymer chain end. We also indicated that PNIPAm,
with a strong hysteresis for the thermal solubility transition,
can be overcome with P1SO2 and P3SO homopolymers. To
tailor a wide LCST range, including an LSCT similar to body
temperature, random copolymers with three different initial
feed ratios of hydrophilic and hydrophobic monomers were
synthesized. In addition, P2S-b-P1SO2 self-assembled into
tion, the sulfide groups of P2SO120-b-P1SO2120 were
converted to sulfoxide groups, by reacting with H O , lead-
ing to the formation of P2SO120-b-P1SO2120
spectroscopy provided evidence for the post-polymerization
modification of P2SO120-b-P1SO2120. The peak (c, d, e)
representing the ─CH ─(S═O)─CH ─CH ─ protons
shifted downfield in the spectrum (Figure 3(b)). The GPC
traces in Figure 3(a) also showed that a small change in the
apparent molecular weight occurred after the post-
polymerization modification had been performed.
P2S is a hydrophobic block and P1SO2 exhibits an LCST
in water at 25 C. Thus, P2SO120-b-P1SO2120 is expected to
exhibit an amphiphilic self-assembly behavior and forms
micelles below the LCST of P1SO2. Here, the hydrophobic
P2S block forms the inner core of the aggregates, whereas the
2
1
2
.
H NMR
2
2
2
ꢀ
micelles in water at 20 C. However, after adding H O , the
2
2
ꢀ
sulfide of the P2S block changed to sulfoxide and the micelles
became unimers. Indeed, sulfur-containing polyacrylamides
are promising for a variety of applications in biomedicine and
biotechnology, because the organosulfur compound has the
potential for bioactivity.
ꢀ
outer shell consists of hydrophilic P1SO2 (at 20 C). We inves-
tigated the transformation of P2SO120-b-P1SO2120 micelles
which were prepared under mild oxidation conditions (hydro-
Acknowledgments. This work was supported by Univer-
sity of Ulsan Research Fund of 2021.
ꢀ
gen peroxide, acetic acid, 20 min, 0 C) to oxidize all sulfides
26,27
to sulfoxides.
Hydrogen peroxide converted hydrophobic
P2S into hydrophilic P2SO.
Therefore, after adding hydrogen peroxide, the P2SO120
b-P1SO2120 micelles became unimers which eventually
would not aggregate at 20 C. The overall process is sche-
Supporting Information. Additional supporting informa-
tion may be found online in the Supporting Information
section at the end of the article.
-
ꢀ
matically depicted in Figure 4(a). DLS was used to obtain
the size distributions for P2SO120-b-P1SO2120 before and
after the addition of H O . The average hydrodynamic diam-
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