A new thermo-responsive block copolymer with tunable upper critical solution temperature and lower critical solution temperature in the alcohol/water mixture

Article abstract of DOI:10.1002/pola.26854

The multi-thermo-responsive block copolymer of poly[2-(2-methoxyethoxy) ethyl methacrylate]-block-poly[N-(4-vinylbenzyl)-N,N-diethylamine] (PMEO ₂MA-b-PVEA) displaying phase transition at both the lower critical solution temperature (LCST) and

Full text of DOI:10.1002/pola.26854

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A New Thermo-responsive Block Copolymer with Tunable
Upper Critical Solution Temperature and Lower Critical
Solution Temperature in the Alcohol/Water Mixture
Yang Su, Meihan Dan, Xin Xiao, Xiaohui Wang, Wangqing Zhang
Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry,
Nankai University, Tianjin 300071, China
Correspondence to: W. Zhang (E-mail: wqzhang@nankai.edu.cn)
Received 24 February 2013; accepted 24 June 2013; published online 19 July 2013
DOI: 10.1002/pola.26854
ABSTRACT: The multi-thermo-responsive block copolymer of
poly[2-(2-methoxyethoxy)ethyl methacrylate]-block-poly[N-(4-
vinylbenzyl)-N,N-diethylamine] (PMEO2MA-b-PVEA) displaying
phase transition at both the lower critical solution temperature
(LCST) and the upper critical solution temperature (UCST) in
the alcohol/water mixture is synthesized by reversible addition-
fragmentation chain transfer polymerization. The poly[2-(2-
methoxyethoxy)ethyl methacrylate] (PMEO2MA) block exhibits
the UCST phase transition in alcohol and the LCST phase tran-
sition in water, while the poly[N-(4-vinylbenzyl)-N,N-diethyl-
amine] (PVEA) block shows the UCST phase transition in
isopropanol and the LCST phase transition in the alcohol/water
mixture. Both the polymer molecular weight and the co-sol-
vent/nonsolvent exert great influence on the LCST or UCST of
the block copolymer. By adjusting the solvent character includ-
ing the water content and the temperature, the block copoly-
mer undergoes multiphase transition at LCST or UCST, and
various block copolymer morphologies including inverted
micelles, core-corona micelles, and corona-collapsed micelles
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KEYWORDS: block copolymers; lower critical solution tempera-
ture (LCST); micelles; poly[N-(4-vinylbenzyl)-N,N-diethylamine];
stimuli-sensitive polymers; upper critical solution temperature
(UCST)
lated drugs are triggered with the body temperature increas-
ing. Up to now, only a few of UCST-type thermo-responsive
polymers such as polysulfobetaines22 in water, poly(methyl
methacrylate)23,24 in the alcohol/water mixture and PNI-
PAM25 in ionic liquid have been discussed. The third kind of
thermo-responsive polymers are those displaying phase tran-
sition in solvent at both LCST and UCST.28–41 These doubly
thermo-responsive polymers are usually diblock copoly-
mers,28–38 in which one block presents LCST, whereas the
other block presents UCST, resulting in the double thermo-
response. For example, the schizophrenic diblock copolymers
of poly(N-isopropylacrylamide)-block-poly(3-[N-(3-methacry-
lamidopropyl)-N,N-dimethyl] ammoniopropane sulfonate)31 and
poly[3-dimethyl(methacryloyloxyethyl) ammonium propane
sulfonate]-block-poly[2-(N-morpholino)ethyl methacrylate],32 the
nonionic diblock copolymer of poly[oligo(ethylene glycol) methyl
ether methacrylate]-block-poly(N-isopropylacrylamide),33 pol-
y(benzyl methacrylate)-block-poly(N-isopropylacrylamide),34
INTRODUCTION During the past two decades, the thermo-
responsive polymers have received great interest because of
their various applications.1,2 Generally, three kinds of
thermo-responsive polymers are classified. The first kind of
thermo-responsive polymers are those displaying phase tran-
sition in solvent at the lower critical solution temperature
(LCST).3–21 These LCST-type thermo-responsive polymers are
molecularly soluble in solvent at temperature below LCST
and become insoluble when temperature increases above
LCST. Among the LCST-type thermo-responsive polymers,
poly(N-isopropylacrylamide) (PNIPAM) exhibiting the LCST
ꢀ
phase transition in water at 32 C may be the most focused
one.17–21 The second kind of thermo-responsive polymers
are those displaying phase transition in solvent at the upper
critical solution temperature (UCST).22–27 These UCST-type
thermo-responsive polymers are molecularly soluble in sol-
vent at temperature above UCST and become insoluble when
temperature decreases below UCST, which is just different
from those of the LCST-type thermo-responsive polymers.
The UCST-type polymers are expected to be very promising
for the drug delivery purposes, as the release of encapsu-
and
poly(ethylene
oxide)-block-poly(N-isopropylacryla-
mide),35 are demonstrated to be the doubly thermo-
responsive polymers. Besides the block copolymers, some
Additional Supporting Information may be found in the online version of this article.
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random copolymers based on poly(2-oxazoline)s such as
poly[(2-ethyl-2-oxazoline)-co-(2-phenyl-2-oxazoline)]
agent of cumyl dithiobenzoate (CDB) was synthesized as dis-
cussed elsewhere.61 All the other chemical reagents were
analytic grade and used as received. Deionized water was
used in this study.
[P(EtOx-co-PhOx)]
and
poly[(2-ethyl-2-oxazoline)-co-(2-
nonyl-2-oxazoline)] [P(EtOx-co-NonOx)] are found to display
phase transition in the water/ethanol mixture at both UCST
and LCST.39,40
Synthesis of the N-(4-vinylbenzyl)-N,N-diethylamine
(VEA) Monomer
To prepare suitable thermo-responsive block copolymers dis-
playing both UCST and LCST phase transition for a given pur-
pose, the initial selection of the suitable polymers exhibiting
phase transition at LCST or UCST and the subsequent synthe-
sis strategy such as atom transfer radical polymerization,42–45
nitroxide-mediated polymerization,46 single-electron transfer
living radical polymerization,47–49 and reversible addition-
fragmentation chain transfer (RAFT) polymerization33,36,50–53
should be involved. The family of poly[oligo(ethylene glycol)
(meth)acrylate] including poly[oligo(ethylene glycol) methyl
ether methacrylate] (POEGMA), poly(2-[2-(2-methoxyethox-
y)ethoxy]ethyl methacrylate) (PMEO3MA) and poly[2-(2-
methoxyethoxy)ethyl methacrylate] (PMEO2MA), which are
very promising biocompatible polymers, undergo phase tran-
sition either in aliphatic alcohols at UCST or in water at
LCST.33,54–59 It is found that the LCST of POEGMA in water is
correlative to the number of the ethyleneoxide units, and the
polymer with longer ethyleneoxide side chain usually has
higher LCST.54–56 Besides, the UCST of POEGMA in isopropa-
nol increases with the polymer molecular weight increasing.59
Furthermore, the LCST or UCST of POEGMA is also affected by
the solvent character,59 enabling a convenient tuning of LCST
or UCST by an appropriate choice of solvent. Owing to the
biocompatible character and the tunable phase transition tem-
perature, the POEGMA based block copolymers have aroused
much interest in polymer science.33,54–59
Into a flask, K2CO3 (27.6 g, 0.20 mol), CMS (15.3 g, 0.10
mol), DEA (14.6 g, 0.20 mol), and CHCl3 (100 mL) were
added. The flask content wasꢀ initially degassed by nitrogen
purge and then heated at 50 C for 24 h with magnetic stir-
ring. The resultant mixture was filtered, washed with water
(100 mL 3 3), and then the organic phase was collected.
After dried over anhydrous magnesium sulfate, the solvent
was removed by rotary evaporation to obtain a crude prod-
uct. Finally, the crude product was purified by column chro-
matography using the solvent of petroleum ether and
distilled under vacuum to afford a colorless or pale yellow
1
liquid of VEA (16.0 g, 85% yield). H NMR (CDCl3): d 5 7.34
(d, J 5 8.2 Hz, 2H), 7.29 (d, J 5 8.2 Hz, 2H), 6.71 (dd, J 5 11.0
and 17.7 Hz, 1H), 5.72 (dd, J 5 1.0 and 16.6 Hz, 1H), 5.20
(dd, J 5 1.0 and 10.0 Hz, 1H), 3.55 (s, 2H), 2.52 (q, J 5 7.1
Hz, 4H), 1.04 (t, J 5 7.1 Hz, 6H).
Synthesis of PVEA by RAFT Polymerization
A typical polymerization is introduced. Into a Schlenk flask,
VEA (3.79 g, 20.0 mmol) and the mixture of CDB (40.8 mg,
0.15 mmol) and AIBN (8.21 mg, 0.050 mmol) dissolved in
toluene (2.0 mL) were added. After removal of oxygen by
three freeze-pump-thaw cycles, the polymerization was per-
formed at 75 ꢀC for 18 h under vigorous stirring and
quenched by immersing the flask in iced water. The synthe-
sized polymer of PVEA was precipitated into the ethanol/
water mixture (6/5 by weight) and dried at room tempera-
ture under vacuum. To detect the VEA monomer conversion,
a drop of the polymerization solution (0.1 mL) was directly
diluted with CDCl3 and subjected to 1H NMR analysis. The
VEA monomer conversion was determined according to eq
(1), in which I2.5 is the integral area of the proton resonance
signal at d 5 2.5 ppm [PhCH2N(CH2CH3)2 in the VEA mono-
mer and the PVEA polymer] and I5.2 is the integral area of
the proton resonance signal at d 5 5.2 ppm (one of
PhCH@CH2 in the VEA monomer), respectively.
In our recent work, a new family of thermo-responsive poly-
mer based on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] has
been reported.60 In this contribution, the multi-thermo-
responsive diblock copolymer of poly[2-(2-methoxyethox-
y)ethyl methacrylate]-block-poly[N-(4-vinylbenzyl)-N,N-dieth-
ylamine] (PMEO2MA-b-PVEA) are synthesized by RAFT
polymerization. The multi-thermo-responsive micellization of
the PMEO2MA-b-PVEA block copolymer in the isopropanol/
water mixture at UCST and LCST is also demonstrated.
Besides, the parameters such as the polymer molecular
weight and the polymer chemical composition affecting the
UCST and LCST of the thermo-responsive polymers are
investigated.
I
2:524I5:2
Conversion % 5
3 100%
(1)
I2:5
Synthesis of PMEO2MA by RAFT Polymerization
Typically, into a Schlenk flask, MEO2MA (4.52 g, 24.0 mmol)
and the mixture of CDB (0.163 g, 0.60 mmol) and AIBN
(16.4 mg, 0.10 mmol) dissolved in toluene (5.0 mL) were
added. The flask content was degassed by three freeze-
pump-thaw cycles, then the polymerization was performed
at 65 ꢀC for 8 h under vigorous stirring and quenched by
immersing the flask in iced water. The synthesized polymer
was precipitated in n-hexane and dried at room temperature
under vacuum. To detect the monomer conversion, a drop of
the polymerization solution was dropped into CDCl3 and
EXPERIMENTAL
Materials
The monomer of 2-(2-methoxyethoxy)ethyl methacrylate
(MEO2MA, 95%, Aldrich) was purified by passing through a
column of basic alumina. The chemical reagents including
diethylamine (DEA, >99.5%, Tianjin Chemical Corp.), chloro-
methylstyrene (CMS, >97%, Alfa), K2CO3 (>99%, Tianjin
Chemical Corp.) were used as received. 2,20-Azobis(isobutyr-
onitrile) (AIBN) was recrystallized from ethanol. The RAFT
1
subjected to H NMR analysis. The monomer conversion was
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SCHEME 1 The schematic synthesis of PVEA (A), PMEO2MA (B), and PMEO2MA-b-PVEA (C).
calculated according to eq (2), in which I4.3 and I4.1 are the
integral area of the proton resonance signals at d 5 4.3 ppm
(COOCH2) in the remaining monomer and at d 5 4.1 ppm in
the synthesized polymer of PMEO2MA, respectively.
extracted with CH2Cl2 (50 mL 3 3). The organic phase was
collected and dried over anhydrous magnesium sulfate, and
then the solvent was removed by rotary evaporation to
afford the PMEO2MA-b-PVEA block copolymer.
I4:1
Conversion % 5
3 100%
(2)
Characterization
The H NMR analysis was performed on a Bruker Avance III
I
4:31I4:1
1
400 MHz NMR spectrometer using CDCl3 as solvent. The
molecular weight and its distribution or the polydispersity
index (PDI, PDI 5 Mw/Mn) of the synthesized polymers were
determined by gel permeation chromatography (GPC)
equipped with a Waters 600E GPC system, where THF was
used as eluent and the narrow-polydispersity polystyrene
was used as calibration standard. The differential scanning
calorimetry (DSC) analysis was performed on a NETZSCH
DSC 204 differential scanning calorimeter under nitrogen
Synthesis of PMEO2MA-b-PVEA by RAFT Polymerization
The PMEO2MA-b-PVEA block copolymer was prepared by
RAFT polymerization using the synthesized PMEO2MA as
macromolecular RAFT agent (macro-RAFT agent). Typically,
into a Schlenk flask, the macro-RAFT agent of PMEO2MA
(Mn,th 5 5.8 kg/mol, 0.464 g, 0.080 mmol), VEA (1.51 g, 8.0
mmol), and AIBN (3.28 mg, 0.020 mmol) dissolved in tolu-
ene (1.0 mL) were added. After oxygen being removed by
three freeze-pump-thaw cycles, the flask content was heated
ꢀ
atmosphere, in which the sample was heated to 80 C at the
ꢀ
at 80 C for 8 h, and then the polymerization was quenched
heating rate of 10 ꢀC/min, cooled to 280 ꢀC in 5 min, and
then heated to 80 ꢀC at the heating rate of 10 ꢀC/min. The
LCST and UCST of the thermo-responsive polymers were
determined by turbidity measurement at 500 nm on a Varian
100 ultraviolet-visible spectrophotometer equipped with a
thermo-regulator (60.1 ꢀC) with the heating/cooling rate at
by immersing the flask in iced water. The monomer conver-
sion was determined by 1H NMR analysis as similarly as
introduced in the Synthesis of PVEA by RAFT Polymerization
section. The polymerization mixture was first diluted with
the ethanol/water mixture (10 mL, 6/5 by weight) and then
the unreacted monomer was removed by dialysis against the
ethanol/water mixture (6/5 by weight) for 3 days (molecu-
lar weight cutoff: 7000 Da). After dialysis, water (100 mL)
was added into the polymer solution, and the polymer was
1
ꢀC/min. The UCST and LCST values were determined at
50% transmittance or at the half of the maximal and mini-
mal transmittance. Dynamic light scattering (DLS) analysis
was performed on
a Nano-ZS90 (Malvern) laser light
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TABLE 1 Experimental Details and Summary of the Synthesized Polymers of PVEA, PMEO2MA, and PMEO2MA-b-PVEA
Mn (kg/mol)
Conv. (%)a
Mn,th
Mn,GPC
Mn,NMR
PDIe
b
c
d
Entry
Polymer
[M]0:[CTA]0:[I]0
Time (h)
A1
A2
A3
B1
B2
B3
C1
C2
C3
C4
C5
PVEA20
150:3:1
400:3:1
800:3:1
240:6:1
500:6:1
1000:6:1
200:4:1
400:4:1
800:4:1
400:4:1
400:4:1
11
18
24
8
40.3
36.3
27.0
73.2
56.0
53.3
52.1
48.7
37.9
44.4
41.4
4.1
2.3
4.5
1.10
1.08
1.15
1.21
1.23
1.24
1.29
1.36
1.38
1.36
1.37
PVEA48
9.5
6.1
10.8
14.8
6.2
PVEA72
14.0
5.8
9.0
P(MEO2MA)29
5.6
P(MEO2MA)47
8
8.9
7.9
9.7
P(MEO2MA)89
8
17.0
10.7
15.3
20.1
17.2
24.9
13.2
9.5
17.5
12.3
16.8
22.5
20.1
27.0
P(MEO2MA)29-b-PVEA26
P(MEO2MA)29-b-PVEA49
P(MEO2MA)29-b-PVEA72
P(MEO2MA)47-b-PVEA44
P(MEO2MA)89-b-PVEA42
8
8
11.4
12.8
12.3
17.2
8
8.5
10
a
d
The monomer conversion determined by 1H NMR analysis.
Theoretical molecular weight determined by monomer conversion.
The molecular weight determined by GPC analysis.
The molecular weight determined by 1H NMR analysis.
b
c
e
The PDI (Mw/Mn) value determined by GPC analysis.
scattering spectrometer with He-Ne laser at the wavelength
of 633 nm at 90ꢀ angle. The transmission electron micros-
copy (TEM) observation was performed using a JEOL 100CX-
II electron microscope at an acceleration of 100 kV, whereby
a small drop of the preheated dispersion of the synthesized
polymer was deposited onto a piece of preheated copper
grid till the solvent was evaporated at a given temperature.
terminal group and the chemical shift at d 5 2.5 ppm corre-
sponding to the polymer main chain, the molecular weight
Mn,NMR of the synthesized PVEA are calculated and summar-
ized in Table 1 (Entries A1–A3, Table 1). It shows that the
Mn,NMR of PVEA is very close to the theoretical molecular
weight Mn,th, and it is larger than Mn,GPC
½monomer ꢁ0 3 Mmonomer
3 conversion 1 MRAFT (3)
.
M
n;th 5
½RAFT ꢁ0
RESULTS AND DISCUSSION
PMEO2MA was synthesized by the RAFT polymerization
[Scheme 1(B)] as discussed elsewhere.64 Three samples of
P(MEO2MA)29, P(MEO2MA)47, and P(MEO2MA)89 with the
theoretical DP at 29, 47, and 89 were synthesized (Entries
B1–B3, Table 1). The synthesized polymers are characterized
by GPC analysis (Supporting Information Fig. S2) and 1H
NMR analysis [Fig. 2(B)], from which Mn,NMR is calculated
based on the characteristic chemical shift at d 5 7.88 ppm
corresponding to the terminal group and the chemical shift
Synthesis of PVEA, PMEO2MA, and PMEO2MA-b-PVEA
The VEA monomer was synthesized by the nucleophilic sub-
stitution reaction of CMS with DEA as discussed elsewhere,62
and its chemical structure was confirmed by 1H NMR analy-
sis (Supporting Information Fig. S1). The PVEA polymer was
synthesized via RAFT polymerization of VEA in toluene using
CDB as chain transfer agent and AIBN as initiator
[Scheme 1(A)]. By varying the molar ratio of the monomer
to the RAFT agent (VEA/CDB), three polymers of PVEA20
,
PVEA48, and PVEA72 with the theoretical molecular weight
(Mn,th) at 4.1, 9.5, and 14.0 kg/mol and with the theoretical
polymerization degree (DP) at 20, 48, and 72, in which Mn,th
is calculated by the monomer conversion according to eq
3,63 were synthesized at 27%–40% monomer conversion
(Entries A1–A3, Table 1). From the symmetrical and unimo-
dal GPC traces shown in Figure 1, the molecular weight
Mn,GPC and the PDI values are obtained. The Mn,GPC values by
GPC analysis are slightly smaller than Mn,th by the monomer
conversion, and the reason is possibly due to the polystyrene
standard used in the GPC analysis. The PDI value of the syn-
thesized polymers locates at 1.08–1.15. Figure 2(A) shows
1
the H NMR spectra of the typical PVEA48 polymer, in which
the characteristic proton chemical shifts of the PVEA main
chain and the terminal group originated from the RAFT
agent of CDB are clearly observed. Based on the characteris-
tic chemical shift at d 5 7.82 ppm corresponding to the
FIGURE 1 The GPC traces of the PVEA polymers.
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moderate, possibly suggesting somewhat bimolecular radical ter-
mination in the RAFT polymerization. Besides, the tailing in GPC
traces is also ascribed to the column adsorption of the block
copolymer and this is expected to be the main reason, since the
1
polymer molecular weight by H NMR analysis is quite close to
the theoretical one as discussed subsequently. Figure 2(C) shows
the typical 1H NMR spectra of P(MEO2MA)29-b-PVEA49, from
which both the characteristic signals corresponding to the
PMEO2MA block and the PVEA block are discerned. Based on
the area ratio of the characteristic chemical shift at d 5 2.5 ppm
to that at d 5 4.1 ppm and the molecular weight of the
PMEO2MA macro-RAFT agent, the molecular weight Mn,NMR of
the PMEO2MA-b-PVEA block copolymers are determined and
summarized in Table 1. It shows that the Mn,NMR values are
quite close to the theoretical molecular weight Mn,th and are
larger than Mn,GPC. The block copolymer is further confirmed by
DSC analysis, in which the glass transition temperature (Tg) at
229.3 ꢀC corresponding to the PMEO2MA block and the Tg at
27.4 ꢀC corresponding to the PVEA block are observed (Support-
ing Information Fig. S3).
Solubility of PMEO2MA in Alcohol
The solubility of the synthesized PMEO2MA in three aliphatic
alcohols of methanol (MeOH), ethanol (EtOH), and isopropa-
nol (iPrOH) is investigated and summarized in Figure 4.
PMEO2MA is soluble in methanol at temperature below the
ꢀ
boiling point (65 C), while it exhibits the UCST phase transi-
tion in ethanol and isopropanol. As shown in Figure 5, the
UCST in either ethanol or isopropanol is positively correlated
to the polymer DP or the polymer molecular weight. For
example, the UCST in ethanol increases from 13.4 ꢀC
[P(MEO2MA)29] to 17.1 ꢀC [P(MEO2MA)47] and further to
20.6 ꢀC [P(MEO2MA)89], and the UCST in isopropanol
increases from 27.8 ꢀC [P(MEꢀO2MA)29
]
to 30.5 ꢀC
[P(MEO2MA)47] and further to 33.3 C [P(MEO2MA)89], when
the polymer DP increases from 29 to 89. At all the UCST
phase transitions, the sharp conversion at a narrow tempera-
ture range of 2 ꢀC is observed. These results are well con-
sistent with those of POEGMA in aliphatic alcohols reported
FIGURE 2 The 1H NMR spectra of PVEA48 (A), P(MEO2MA)29
(B), and P(MEO2MA)29-b-PVEA49 (C).
at d 5 4.1 ppm corresponding to the main chain of
PMEO2MA, and the results are summarized in Table 1.
The PMEO2MA-b-PVEA block copolymers were prepared by
sequential RAFT polymerization using PMEO2MA as macro-
RAFT agent [Scheme 1(C)]. Five block copolymers of
P(MEO2MA)29-b-PVEA26, P(MEO2MA)29-b-PVEA49, P(MEO2MA)29-
b-PVEA76, P(MEO2MA)47-b-PVEA44, and P(MEO2MA)89-b-PVEA42
were synthesized (Entries C1–C5, Table 1). Figure 3 shows the
GPC traces of the typical macro-RAFT agent of P(MEO2MA)29
and the synthesized block copolymers, from which the shift to
less elution time with the increase in the DP of the PVEA block
is observed, indicating the chain extension of the PVEA block.
The slight shoulder at the high-molecular weight side (less elu-
tion time) in all the GPC traces of the block copolymers is
observed and the PDI of the block copolymers (1.29–1.38) is
FIGURE 3 The GPC traces of the typical P(MEO2MA)29 macro-
RAFT agent and the PMEO2MA-b-PVEA block copolymers.
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FIGURE 4 Summary of the solubility of P(MEO2MA)29, PVEA48, and P(MEO2MA)29-b-PVEA49 in the alcohol/water mixture.
by Roth et al.,59 and the UCST increasing with the polymer
molecular weight can be ascribed to the favorable polymer–
polymer interaction in the polymer with high molecular
weight, in which high temperature is needed to disrupt the
interaction.
dependent on the DP or the molecular weight of the PVEA
polymer. For example, the polymer with low DP (PVEA20) is
molecularly soluble in methanol and no detectable LCST is
observed. However, the LCST of PVEA48 at 53.1 ꢀC is
detected and the LCST of PVEA72 further decreases to
44.5 ꢀC [Fig. 6(A)]. As to the UCST in isopropanol, the poly-
mer with low DP (PVEA20) is molecularly soluble at temper-
ature below the solvent boiling temperature; and theꢀUCST
Solubility of PVEA in Alcohol
The solubility of PVEA in three aliphatic alcohols of metha-
nol, ethanol, and isopropanol is investigated. It is found that
the solubility of PVEA in these three aliphatic alcohols is
much different (Fig. 4). In ethanol, PVEA is molecularly solu-
ble at temperature below the solvent boiling point and nei-
ther LCST nor UCST is detected; in methanol, the LCST phase
transition is detected; in isopropanol, the UCST phase transi-
tion is found. Figure 6 shows all the phase transitions occur-
ring at a narrow temperature range of 2 ꢀC. Besides, both
the LCST in methanol and the UCST in isopropanol are
ꢀ
of PVEA48 at 24.3 C and the UCST of PVEA72 at 36.8 C are
detected, respectively [Fig. 6(B)].
It is generally accepted that the UCST increasing with the
increasing polymer molecular weight is ascribed to the
favorable polymer–polymer interaction in the polymer chains
with high molecular weight.27,59 However, the LCST decreas-
ing with the increasing polymer molecular weight is due to
the less favorable combinatorial entropy in the binary
FIGURE 5 The transmittance versus temperature plots of PMEO2MA in the solvent of ethanol (A) and isopropanol (B). The concen-
tration of polymer is 2.0 wt %.
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FIGURE 6 The transmittance versus temperature plots of PVEA in the solvent of methanol (A) and isopropanol (B). The polymer
concentration is 2.0 wt %.
mixture.29 Compared with PNIPAM,17–21 PVEA seems inter-
esting as its LCST can be tuned within a relatively wide
range just by varying the polymer molecular weight.
Herein, the solubility of P(MEO2MA)29 and PVEA48 in the
alcohol/water mixture is taken as the typical example to
study the co-solvent or nonsolvent effect on their UCST or
LCST. For the sake of briefness, the alcohol/water mixture
with the water content at X wt % is called the X wt % alco-
hol/water mixture. Figure 4 summarizes the solubility of
P(MEO2MA)29 and PVEA48 in the alcohol/water mixture.
P(MEO2MA)29 is soluble in methanol and shows UCST phase
transition in ethanol or isopropanol, which is just introduced
above. When the water content in the isopropanol/water
mixture slightly increases from 0 to 1 wt %, the UCST of
P(MEO2MA)29 decreases from 27.8 to 17.8 ꢀC [Fig. 7(A)].
Similar co-solvent effect on the UCST of poly[oligo(ethylene
glycol) (meth)acrylate] in alcohol is also reported,58,59 and
the reason is possibly due to the strong hydrogen bonding
between water and the P(MEO2MA)29 chains leading to the
formation of a hydration shell around the polymer to
increase the polymer solubility. When the water content is
above 5 wt %, P(MEO2MA)29 becomes molecularly soluble in
the methanol/water, ethanol/water, and isopropanol/water
Solubility of PMEO2MA and PVEA in the
Water/Alcohol Mixture
For thermo-responsive polymers, the LCST or UCST generally
can be tuned by addition of co-solvent or nonsol-
vent.40,58,59,65–67 For example,ꢀ the LCST of PNIPAM in water
decreases from 32.5 to 27.5 C when 35 vol % methanol is
added and then increases with the further addition of metha-
nol.65 For POEGMA, the UCST in aliphatic alcohols increases
with the chain length of the alcohol.59 Besides, the UCST of
POEGMA in isopropanol can be tuned by addition of co-
ꢀ
solvent or nonsolvent, and it decreases from 35.6 to 23.1 C
when 1 vol % co-solventꢀ of water is added, while it
increases from 35.6 to 46.7 C when 50 vol % nonsolvent of
hexane is added.59 The reason is generally ascribed to the
variation of the solvation shell around the polymer chains by
the (co)nonsolvent.40,42,43
FIGURE 7 The temperature dependent transmittance of P(MEO2MA)29 (A) and PVEA48 (B) in the isopropanol/water mixture. The
polymer concentration is 2.0 wt %.
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below 5 wt %, P(MEO2MA)29-b-PVEA49 is soluble at temper-
ature above UCST and it forms micelles at temperature
below UCST. In the isopropanol/water mixture with water
content at 5–20 wt %, P(MEO2MA)29-b-PVEA49 is soluble
and neither UCST nor LCST is detected. In the isopropanol/
water mixture with water content at 25–35 wt %,
P(MEO2MA)29-b-PVEA49 is soluble at temperature below
LCST and it self-assembles into micelles at temperature
above LCST. In the isopropanol/water mixture with water
content at 40–90 wt %, the P(MEO2MA)29-b-PVEA49 block
copolymer forms micelles. All these block copolymer micelles
are stable, and no deposition is observed. In the isopropa-
nol/water mixture with water content above 95 wt %, the
P(MEO2MA)29-b-PVEA49 block copolymer precipitates in the
solvent. Clearly, the solubility or micellization of the
P(MEO2MA)29-b-PVEA49 block copolymer in the isopropanol/
water mixture is firmly dependent on the solubility of the
PMEO2MA and PVEA blocks except those in the isopropanol/
water mixture with 30–35 wt % water content, in which the
P(MEO2MA)29-b-PVEA49 block copolymer is soluble below
LCST whereas the reference homopolymer of PVEA48 is
insoluble. This is not surprising and the reason is due to the
hydrophilic P(MEO2MA)29 block increasing the solubility of
the PVEA49 block and therefore the P(MEO2MA)29-b-PVEA49
block copolymer. In the subsequent study, the thermo-
responsive micellization of the P(MEO2MA)29-b-PVEA49 block
copolymer in the isopropanol/water mixture with water con-
tent at 0%, 25%–35%, and 40–90 wt % is discussed. The
isopropanol/water mixture with water content at 5–20 wt%
or above 95 wt % is not chosen, as the P(MEO2MA)29-b-
PVEA49 block copolymer is soluble in the former solvent and
precipitates in the later solvent.
FIGURE 8 The temperature dependent transmittance of
P(MEO2MA)29-b-PVEA49 in the isopropanol/water mixture. The
polymer concentration is 2.0 wt %.
mixtures and no LCST or UCST is observed. When the water
content further increases, LCST in the three alcohol/water
mixtures is detected, and the LCST in the isopropanol/water
ꢀ
mixture decreases from 53.5 to 15.0 C when the water con-
tent increases from 75 to 100 wt % [Fig. 7(A)]. The similar
co-solvent effect on the solubility of p(OEGMeA-co-OEG-
PhA)58 and poly(trimethylene ether) glycol66 in water are
also reported.
Different from the co-solvent effect on LCST,40,58,66 the addi-
tion of the nonsolvent of octane in the poly(2-chloroethyl
vinyl ether-alt-maleic anhydride) solution in n-butyl acetate
leads to the LCST decreasing, and the reason was due to the
nonsolvent disturbing the solvation shell around the poly-
mer chains, resulting in the decreased solubility of the poly-
mer in the solvent.67 The similar nonsolvent effect on the
solubility of PVEA is also observed. As shown in Figure
7(B), PVEA48 is soluble in isopropanol at temperature
above the UCST at 24.3 ꢀC. However, the nonsolvent of
water greatly changes the solubility of PVEA48 in isopropa-
nol. That is, LCST instead of UCST is detected and the LCST
of PVEA48 in the isopropanol/water mixture decreases from
As shown in Figure 8, the UCST phase transition of
P(MEO2MA)29-b-PVEA49 in pure isopropanol at 23.2 ꢀC is
observed. At the temperature above the UCST, P(MEO2MA)29
-
b-PVEA49 is molecularly soluble (100% transmittance), and
micellization occurs when temperature decreases below
UCST (0% transmittance). The ꢀUCST of the P(MEO2MA)29-b-
PVEA49 block copolymer (23.2 C) is slightly lower than that
of the P(MEO2MA)29 homopolymer (27.8 ꢀC) andꢀis close to
that of the reference PVEA48 homopolymer (24.3 C). This is
possibly due to the soluble PVEA49 block enhancing the solu-
bility of the P(MEO2MA)29 block and therefore results in a
lower UCST than that of the P(MEO2MA)29 homopolymer.
The DLS analysis of the block copolymer dispersion/solution
confirms the block copolymer micelles with the narrow-
distributed apparent hydrodynamic diameter Dh 5 151 nm at
temperature below UCST and the single polymer chains with
Dh 5 5 nm at temperature above UCST [Fig. 9(A)]. As the
UCST of the P(MEO2MA)29 reference in isopropanol is higher
than that of the PVEA48 reference (27.8 ꢀC vs. 24.3 ꢀC), the
micelles are expected to contain a solvophilic PVEA49 corona
and a desolvated P(MEO2MA)29 core as those of the double
hydrophilic block copolymers.68–73 Herein, these core-corona
micelles are called inverted micelles or inverse micelles,35 as
the DP of PVEA49 corona is larger than that of the
P(MEO2MA)29 core.
ꢀ
52.8 to 7.3 C when the water content increases from 15 to
25 wt %.
Thermo-Responsive Micellization of PMEO2MA-b-PVEA
in the Isopropanol/Water Mixture
With the detailed solubility of the PMEO2MA and PVEA
homopolymers in the alcohol/water mixture in hand, the
thermo-responsive micellization of PMEO2MA-b-PVEA in the
typical solvent of the isopropanol/water mixture is studied.
The solvent of the isopropanol/water mixture is chosen, as
the variable solubility of PMEO2MA and PVEA in this solvent
is demonstrated. The solubility of the typical block copoly-
mer of P(MEO2MA)29-b-PVEA49 in the isopropanol/water
mixture is checked and summarized in Figure 4. In isopropa-
nol or the isopropanol/water mixture with water content
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FIGURE 9 The hydrodynamic diameter distribution f(Dh) of P(MEO2MA)29-b-PVEA49 in the solvent of isopropanol (A), the 30 wt %
isopropanol/water mixture (B), and the 80 wt % isopropanol/water mixture (C).
In the 25–35 wt % isopropanol/water mixture, the thermo-
responsive micellization of the P(MEO2MA)29-b-PVEA49 block
copolymer occurs at temperature above LCST (Fig. 8). As the
P(MEO2MA)29 reference is soluble in the 25–35 wt % isopro-
panol/water mixture as discussed above, the micellization
should be ascribed to the LCST phase transition of the
PVEA49 block. It is found that the LCST of the PVEA49 block
is correlative to the water content in the isopropanol/water
mixture. That is, the LCST of P(MEO2MA)29-b-PVEA49 in the
isopropanol/water mixture decreases from 34.5 ꢀC to 21.5
ꢀC and further to 8.0 ꢀC when the water content increases
from 25 to 30 wt % and further to 35 wt % (Fig. 8), which
is as similar as the nonsolvent effect on the LCST of PVEA
shown in Figure 7(B). Compared with the PVEA48 reference,
the LCST of P(MEO2MA)29-b-PVEA49 is much higher (for
example, 34.5 ꢀC vs. 7.3 ꢀC in the 25 wt % isopropanol/
water mixture), as the solvophilic P(MEO2MA)29 block
increases the solubility of the P(MEO2MA)29-b-PVEA49 block
copolymer in the isopropanol/water mixture as discussed
elsewhere.68–73 Based on the different solubility of the
P(MEO2MA)29 and the PVEA49 blocks in the 25–35 wt % iso-
propanol/water mixture, the block copolymer micelles
should contain a solvophilic P(MEO2MA)29 corona and a des-
olvated PVEA49 core, which is just different from the
inverted micelles formed in isopropanol. This micellization is
confirmed by the DLS analysis of the P(MEO2MA)29-b-
PVEA49 block copolymer in the 30 wt % isopropanol/water
ꢀ
mixture typically at temperature of 10 C (below LCST) and
at temperature of 30 ꢀC (above LCST), in which Dh 5 5 nm
corresponding to the single polymer chains and apparent
Dh 5 129 nm corresponding to the core-corona micelles are
indicated [Fig. 9(B)]. The TEM image shown in Figure 10(A)
also confirms the formation of micelles with the average size
centered at 35 nm. Clearly, the size of the micelles by TEM
observation is much smaller than that by DLS analysis, and
the reason is due to the TEM observation showing the dried
aggregates while DLS analysis detecting the solvated
micelles.
In the 40–90 wt % isopropanol/water mixture, the PVEA49
block is insoluble and the P(MEO2MA)29 block is either solu-
ble or shows the LCST phase transition (Fig. 8), and there-
fore the core-corona micelles containing a desolvated PVEA49
core and a P(MEO2MA)29 corona are prepared in this sol-
vent. The core-corona micelles prepared in the 40–70 wt %
isopropanol/water mixture are different from those in the
75–90 wt % isopropanol/water mixture. For example, on
heating the core-corona micelles prepared in the 50 wt %
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FIGURE 10 The TEM images and size-distribution of the P(MEO2MA)29-b-PVEA49 micelles in the 30 wt % isopropanol/water mix-
ture at 30 ꢀC (A) and in the 80 wt % isopropanol/water mixture at 20 ꢀC (B) and 50 ꢀC (C).
isopropanol/water mixture, the transmittance keeps almost
constant; whereas for the core-corona micelles prepared in
the 75–90 wt % isopropanol/water mixture, the transmit-
tance decreases with the temperature increasing. This sug-
gests that these core-corona micelles prepared in the 75–
90 wt % isopropanol/water mixture are thermo-responsive.
Based on the solubility of the PMEO2MA and PVEA homo-
polymers shown in Figure 4, the thermo-response of the
core-corona micelles is ascribed to the LCST phase transi-
tion of the P(MEO2MA)29 corona. This LCST phase transi-
tion of the P(MEO2MA)29 block leads to the P(MEO2MA)29
block collapsing on the PVEA49 core to form the corona-
collapsed micelles. This transition of the core-corona
micelles to corona-collapsed micelles is confirmed by TEM
observation and DLS analysis of the micelles in the 80 wt
% isopropanol/water mixture at temperature below and
above LCST of the P(MEO2MA)29 corona. Figure 9(C) indi-
cates the apparent Dh decreasing from 156 nm to 128 nm
when the temperature increases from 20 ꢀC to 50 ꢀC, sug-
gesting the micelles shrinking on heating. The TEM images
[Fig. 10(B,C)] show that the P(MEO2MA)29-b-PVEA49
micelles in the 80 wt % isopropanol/water mixture at tem-
perature of 20 ꢀC (below LCST) and 50 ꢀC (above LCST)
have the same size at 44 nm. The difference between the
DLS analysis and the TEM observation is due to the TEM
observation showing the dried aggregates while DLS analy-
sis detecting the solvated micelles, and it indicates the
LCST phase transition of the P(MEO2MA)29 corona to
corona-collapsed micelles in the 75–90 wt % isopropanol/
water mixture. As similar as the P(MEO2MA)29 homopoly-
mer [Fig. 7(A)], the LCST of the P(MEO2MA)29 corona is
also correlative to the water content in the isopropanol/
water mixture, and it generally decreases with the water
content although the LCST of the P(MEO2MA)29 corona in
the core-corona micelles is slightly lower than that of the
P(MEO2MA)29 homopolymer.
SCHEME 2 Schematic solubility of the P(MEO2MA)29-b-PVEA49
block copolymer in the isopropanol/water mixture.
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Based on the above discussion, the thermo-responsive micel-
lization of the P(MEO2MA)29-b-PVEA49 block copolymer is
summarized in Scheme 2. It indicates the phase transitions
from soluble P(MEO2MA)29-b-PVEA49 to inverted micelles in
isopropanol at temperature below UCST, to core-corona
micelles in the 30–35 wt % isopropanol/water mixture at
temperature above LCST, to core-corona micelles in the 40–
70 wt % isopropanol/water mixture, to corona-collapsed
micelles in the 75–90 wt % isopropanol/water mixture at
temperature above LCST, and further to insoluble polymer in
the isopropanol/water mixture with water content above 95
wt %.
block is similar with each other while the DP of the
PMEO2MA block increases from 29 to 89.
As shown in Figure 11(A), there is a positive correlation
between the DP of the PMEO2MA or PVEA block and the
UCST of the block copolymer in isopropanol. For example,
for the block copolymers in Group 1, the UCST increases
ꢀ
ꢀ
ꢀ
from 22.6 C to 23.2 C and further to 27.8 C when the DP
of the PVEA block increases from 26 to 49 and further to
76; for theꢀblock copolymers in Group 2, theꢀUCST increases
ꢀ
from 23.2 C to 26.7 C and further to 30.6 C when the DP
of the PMEO2MA block increases from 29 to 47 and further
to 89. The present positive correlation between the UCST of
the block copolymer and the DP of the PMEO2MA or PVEA
block is due to the strong polymer–polymer interaction in
the high-molecular weight polymer as discussed above.
The Effect of Block Chain Length on the Thermo-
Responsive Micellization of PMEO2MA-b-PVEA
Herein, the effect of the block chain length on the thermo-
responsive micellization of the PMEO2MA-b-PVEA block
copolymer is investigated. To fulfill this investigation, five
In the 30 wt % isopropanol/water mixture, the increasing
DP of PVEA leads to the LCST of the PMEO2MA-b-PVEA block
copolymer decreasing, whereas the increasing DP of
PMEO2MA leads to the LCST increasing [Fig. 11(B)]. For
example, for the block copolymers in Group 1, the LCST
decreases from 46.2 ꢀC to 21.5 ꢀC and further to 12.6 ꢀC
when the DP of PVEA increases from 26 to 49 and further to
76; for theꢀ block copolymers in Group 2, the LCST increases
block copolymers of P(MEO2MA)29-b-PVEA26, P(MEO2MA)29
-
b-PVEA49, P(MEO2MA)29-b-PVEA76, P(MEO2MA)47-b-PVEA44
,
and P(MEO2MA)89-b-PVEA42 are synthesized. Clearly, these
five block copolymers can be classified in two groups. In the
first group of P(MEO2MA)29-b-PVEA26
, P(MEO2MA)29-b-
PVEA49, and P(MEO2MA)29-b-PVEA76 (Group 1), the DP of
the PMEO2MA block is kept at the constant of 29 while the
DP of the PVEA block increases from 26 to 76. In the second
ꢀ
ꢀ
from 21.5 C to 29.2 C and further to 46.0 C when the DP
of PMEO2MA increases from 29 to 47 and further to 89.
These results suggest that both the increase in the DP of the
solvophilic PMEO2MA and the decrease in the DP of the
group of P(MEO2MA)29-b-PVEA49, P(MEO2MA)47-b-PVEA44
,
and P(MEO2MA)89-b-PVEA42 (Group 2), the DP of the PVEA
FIGURE 11 Transmittance versus temperature plots for the PMEO2MA-b-PVEA block copolymers in the isopropanol/water mixture.
The polymer concentration is 2.0 wt %.
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thermo-responsive PVEA block increase the LCST of the
PMEO2MA-b-PVEA block copolymer in the 30 wt % isopro-
panol/water mixture. Similar results are also observed for
the random or block copolymers of P(EtOx-co-PhOx),40
P(MEO2MA-co-OEGMA),56 poly(ethylene oxide)-block-poly(N-
isopropylacrylamide),73 and poly(ethylethylene phosphate)-
block-poly[2-(dimethylamino)ethyl methacrylate],43 in which
the hydrophilic block content increasing or the hydrophobic
block content decreasing leads to the LCST increasing. As
similar as the P(MEO2MA)29-b-PVEA49 block copolymer, the
other four block copolymers self-assemble into core-corona
micelles, in which the PVEA block forms the core and the
PMEO2MA block forms the corona, in the 30 wt % isopropa-
nol/water mixture at temperature above LCST. The TEM
observation confirms the formation of micelles (Supporting
Information Fig. S4), and the size of the micelles of the five
block copolymers is 26 nm [P(MEO2MA)29-b-PVEA26], 35 nm
[P(MEO2MA)29-b-PVEA49], 45 nm [P(MEO2MA)29-b-PVEA76],
prepared by RAFT polymerization. The solubility of
PMEO2MA, PVEA, and PMEO2MA-b-PVEA in aliphatic alco-
hols and in the isopropanol/water mixture is investigated.
PMEO2MA is soluble in methanol, shows UCST phase transi-
tion in ethanol and isopropanol, and shows LCST phase
transition in the alcohol/water mixture. The UCST of
PMEO2MA is firmly dependent on the polymer DP, and it
increases with the DP increase. The LCST of PMEO2MA in
the alcohol/water mixture is dependent on the water con-
tent, and it decreases with the water content increase. PVEA
exhibits LCST phase transition in methanol, is soluble in
ethanol, shows UCST phase transition in isopropanol, and
shows LCST phase transition in the isopropanol/water
mixture, respectively. The polymer molecular weight and the
co-solvent/nonsolvent exert great influence on the LCST or
UCST of PVEA. By combining the thermo-responsive
PMEO2MA and PVEA through RAFT polymerization, the
multi-thermo-responsive PMEO2MA-b-PVEA block copoly-
mer is prepared. The PMEO2MA-b-PVEA block copolymer
undergoes phase transition from soluble polymer to inverted
micelles in isopropanol at temperature below UCST, to core-
corona micelles in the 30–35 wt % isopropanol/water
mixture at temperature above LCST, to core-corona micelles
in the 40–70 wt % isopropanol/water mixture, to corona-
collapsed micelles in the 75–90 wt % isopropanol/water
mixture at temperature above LCST, and further to insoluble
polymer in the isopropanol/water mixture with water
content above 95 wt % just by changing the water content in
the solvent mixture or by adjusting the solvent temperature.
32 nm [P(MEO2MA)47-b-PVEA44], and 29 nm [P(MEO2MA)89
-
b-PVEA42], respectively. It indicates that the micelle size
increases with the DP of the PVEA block while decreases
with the DP of the PMEO2MA block.
Finally, the DP effect on the thermo-response of the
PMEO2MA corona in the core-corona micelles prepared in
the 80 wt % isopropanol/water mixture is investigated
using the Group 1 block copolymers as typical example. As
discussed above, core-corona micelles containing the PVEA
core and the PMEO2MA corona are formed in the 80 wt %
isopropanol/water mixture. The TEM images shown in Sup-
porting Information Figure S5 confirm the formation of
micelles at temperature either below or above LCST of the
PMEO2MA corona, in which the size of the micelles
increases from 30 to 44 to 80 nm when the DP of the PVEA
block increases from 26 to 49 and further to 76 (Note: the
size of the micelles at temperature below LCST and above
LCST by TEM observation is very similar). Figure 11(C)
indicates that the LCST of the core-corona micelles of
P(MEO2MA)29-b-PVEA49 and P(MEO2MA)29-b-PVEA76 is
almost the same as that of the reference P(MEO2MA)29
homopolymer. However, for the core-corona micelles of
P(MEO2MA)29-b-PVEA26 containing a short PVEA block, its
LCST is lower than that of the P(MEO2MA)29 reference
(32.4 ꢀC vs. 38.6 ꢀC). The reason of the different LCST of
the P(MEO2MA)29 corona in the core-corona micelles is not
very clear, and it is expected that the interaction between
the PMEO2MA block and the PVEA block should be
involved. Besides, the phase transition of the P(MEO2MA)29
corona in the core-corona micelles is not as sharp as that of
the reference P(MEO2MA)29 homopolymer, and the reason
is possibly due to the steric repulsion between the tethered
P(MEO2MA)29 chains in the core-shell particles as discussed
elsewhere.74
The
multi-thermo-responsive
micellization
of
the
P(MEO2MA)29-b-PVEA49 block copolymer is checked by DLS
analysis and TEM observation and the tunable thermo-
response at LCST or UCST is demonstrated.
ACKNOWLEDGMENTS
The financial support by National Science Foundation of China
(Nos. 20974051 and 21074059) and PCSIRT (IRT1257) is
gratefully acknowledged.
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