A new family of thermo-responsive polymers based on poly[N-(4-vinylbenzyl)- N, N-dialkylamine]

Article abstract of DOI:10.1021/ma4002116

A new family of the thermo-responsive polymers based on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] with the pendent amine group as well as the doubly thermo-responsive triblock copolymer were synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization. These polymers showed the lower critical solution temperature (LCST) and/or the upper critical solution temperature (UCST) in alcohol and in the alcohol/water mixture. The polymer molecular weight, the polymer concentration, the cosolvent/nonsolvent and the deuterated solvent affecting the LCST and/or UCST were investigated, and their great influence on the LCST/UCST was demonstrated. The origin of the phase transition of poly[N-(4-vinylbenzyl)-N,N-dialkylamine] at LCST upon heating was investigated and the possible reason was proposed. The doubly thermo-responsive triblock copolymer of PVMA₅₃-b-PVEA ₁₀₈-b-PVMA₅₃ underwent phase transition at two LCST temperatures. The PVEA₁₀₈ block underwent the initial phase transition at the first LCST of 32.5 C to form core-corona micelles, and then the subsequent phase transition of the PVMA₅₃ block took place at the second LCST of 54.5 C to produce corona-collapsed micelles. The proposed polymers based on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] are anticipated to broaden the thermo-responsive polymer range and will be useful in polymer science.

Full text of DOI:10.1021/ma4002116

Article
pubs.acs.org/Macromolecules
A New Family of Thermo-Responsive Polymers Based on Poly[N‑(4-
vinylbenzyl)‑N,N‑dialkylamine]
Meihan Dan, Yang Su, Xin Xiao, Shentong Li, and Wangqing Zhang*
Key Laboratory of Functional Polymer Materials of the Ministry of Education, Institute of Polymer Chemistry, Nankai University,
Tianjin 300071, China.
S
* Supporting Information
ABSTRACT: A new family of the thermo-responsive polymers based on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] with the
pendent amine group as well as the doubly thermo-responsive triblock copolymer were synthesized by reversible addition−
fragmentation chain transfer (RAFT) polymerization. These polymers showed the lower critical solution temperature (LCST)
and/or the upper critical solution temperature (UCST) in alcohol and in the alcohol/water mixture. The polymer molecular
weight, the polymer concentration, the cosolvent/nonsolvent and the deuterated solvent affecting the LCST and/or UCST were
investigated, and their great influence on the LCST/UCST was demonstrated. The origin of the phase transition of poly[N-(4-
vinylbenzyl)-N,N-dialkylamine] at LCST upon heating was investigated and the possible reason was proposed. The doubly
thermo-responsive triblock copolymer of PVMA53-b-PVEA108-b-PVMA53 underwent phase transition at two LCST temperatures.
The PVEA108 block underwent the initial phase transition at the first LCST of 32.5 °C to form core−corona micelles, and then
the subsequent phase transition of the PVMA53 block took place at the second LCST of 54.5 °C to produce corona-collapsed
micelles. The proposed polymers based on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] are anticipated to broaden the thermo-
responsive polymer range and will be useful in polymer science.
inorganic additives37,38 can exert more or less influence on
LCST. For example, the LCST of PNIPAM in water can be
1. INTRODUCTION
Thermo-responsive polymers represent an important class of
tuned by the cosolvents of alcohol, acetone and dioxane, and
stimuli-responsive materials, which undergo phase transition at
the cosolvents usually lead to the LCST increase.33,34 On the
the lower critical solution temperature (LCST) or the upper
contrary, addition of the nonsolvent of octane in the poly(2-
critical solution temperature (UCST).1,2 Thermo-responsive
chloroethyl vinyl ether-alt-maleic anhydride) solution in n-butyl
polymers receive significant attention both in polymer science
acetate leads to the LCST decrease.30 The UCST-type
and in practical application.3−6 The polymers bearing the amide
group form the largest group of thermo-sensitive polymers,1,2
polymers undergo phase transition usually in a polar organic
solvent,11−17 although those such as polysulfobetaine,39 poly-
and poly(N-isopropylacrylamide) (PNIPAM) as well as
(N-acryloylasparaginamide)40 and poly(N-acryloyl glycina-
poly(N,N-diethylacrylamide) (PDEAAM), which has the
mide)41 in water are reported. The UCST is usually correlative
similar LCST of 32−33 °C in water, is the most popular.7−10
to the polymer molecular weight,14,16,40 and high polymer
Besides, the family of poly[oligo(ethylene glycol) (meth)-
molecular weight generally leads to high UCST. The reason is
acrylate] including poly[oligo(ethylene glycol) methyl ether
methacrylate], poly(2-[2-(2-methoxyethoxy)ethoxy]-
ascribed to the favorable polymer−polymer interaction in the
high-molecular-weight polymer, in which high temperature is
ethylmethacrylate) and poly[2-(2-methoxyethoxy)ethyl meth-
needed to disrupt the interaction.14,40 Besides the LCST-type
acrylate], which are very promising biocompatible polymers,
polymers and the UCST-type polymers, doubly thermo-
undergo UCST phase transition in aliphatic alcohols and
responsive polymers are also reported.42−47 These doubly
thermo-responsive polymers are usually diblock copolymers, in
which one block presents LCST whereas the other block
presents UCST or LCST, resulting in the double thermo-
response.42−46 For example, the schizophrenic diblock
copolymers of poly(N-isopropylacrylamide)-b-poly(3-[N-(3-
methacrylamidopropyl)-N,N-dimethyl] ammoniopropane sul-
fonate)42 and poly[3-dimethyl(methacryloyloxyethyl) ammo-
nium propane sulfonate]-b-poly[2-(N-morpholino)ethyl meth-
acrylate],43 the nonionic diblock copolymers of poly[oligo-
(ethylene glycol) methyl ether methacrylate]-b-poly(N-
exhibit LCST phase transition in water.11−17 Furthermore, the
phase transition of poly(propylene oxide),18,19 poly(2-alkyl-2-
oxazolines),20,21 poly(vinyl methyl ether),22,23 and the elastin-
like polypeptides (ELPs)24,25 in water, and poly(ethylene
oxide)26 as well as PNIPAM27 in ionic liquids (ILs) at LCST or
UCST have also been reported.
The LCST-type polymers such as PNIPAM and PDEAAM
are soluble in water at temperature below LCST and become
insoluble when temperature increases above LCST.7−10 It is
deemed that the hydrogen bonding between water molecules
and the polymer chains or the hydration of the polymer chains
by water molecules leads to the phase transition at
LCST.14,19,21,22,33−35 It is found that the polymer molecular
weight,16,28−30 the terminal,16,31,32 the polymer concentra-
tion,16,22,28−31 the cosolvent or nonsolvent,33−36 and the
Received: January 30, 2013
Revised: April 1, 2013
Published: April 9, 2013
© 2013 American Chemical Society
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isopropylacrylamide)44 and poly(benzyl methacrylate)-b-poly-
(N-isopropylacrylamide),45 and poly[2-(dimethylamino)ethyl
methacrylate-b-di(ethyleneglycol)methyl ether methacrylate]46
have been demonstrated to be the smart doubly thermo-
responsive polymers.
N-(4-Vinylbenzyl)-N,N-dibutylamine (VBA) (28.5 g, 70% yield).
1H NMR (CDCl3): δ = 7.36 (d, J = 8.0 Hz, 2H), 7.29 (d, J = 8.0 Hz,
2H), 6.72 (dd, J = 11.0 and 17.6 Hz, 1H), 5.74 (dd, J = 0.8 and 17.6
Hz, 1H), 5.22 (dd, J = 0.8 and 10.8 Hz, 1H), 3.57 (s, 2H), 2.42 (t, J =
8 Hz, 4H), 1.33 (m, 8H), 0.94 (t, J = 7.4 Hz, 6H). See the H NMR
spectra in Figure S1, Supporting Information.
1
Compared with millions of the general polymers showing
neither LCST nor UCST, the thermo-responsive polymers are
very limited, although some examples have been reported.1,2 In
this study, a new family of thermo-responsive polymers based
on poly[N-(4-vinylbenzyl)-N,N-dialkylamine] are synthesized
by reversible addition−fragmentation chain transfer (RAFT)
polymerization, and their thermo-response upon heating or
cooling in alcohol and in the alcohol/water mixture is
introduced, and the great influence on the LCST or UCST
by the polymer molecular weight, the polymer concentration
and the cosolvent/nonsolvent is demonstrated. These thermo-
responsive polymers bearing the amine group are much
different from the poly(acrylamide) analogies7−10,42−45 and
poly(2-alkyl-2-oxazolines)21,22 in the chemical composition,
and they are anticipated to be a new kind of thermo-responsive
polymers.
2.3. Synthesis of Poly[N-(4-vinylbenzyl)-N,N-dialkylamine].
The homopolymers of poly[N-(4-vinylbenzyl)-N,N-dialkylamine]
including poly[N-(4-vinylbenzyl)-N,N-dimethylamine] (PVMA), poly-
[N-(4-vinylbenzyl)-N,N-diethylamine] (PVEA) and poly[N-(4-vinyl-
benzyl)-N,N-dibutylamine] (PVBA) were prepared by RAFT polymer-
ization in 1,4-dioxane using BDMAT as RAFT agent and AIBN as
initiator, in which [VMA]o:[BDMAT]o:[AIBN]o = 540:3:1, [VEA]o:
[BDMAT]o:[AIBN]o = 540:3:1, and [VBA]o:[BDMAT]o:[AIBN]o=
360:3:1, respectively. Herein, the RAFT polymerization of VEA was
typically introduced. Into a 50 mL Schlenk flask with a magnetic bar,
VEA (11.2 g, 0.059 mol), BDMAT (0.093 g, 0.33 mmol), and AIBN
(18.1 mg, 0.11 mmol) dissolved in 1,4-dioxane (2.8 g) were added.
The flask content was initially degassed with nitrogen at 0 °C for 30
min, and then the polymerization was performed at 70 °C under
vigorous stirring. After a given time, the polymerization was quenched
by rapid cooling upon immersion of the flask in iced water. To detect
the monomer conversion, a drop of the polymerization solution
(about 0.1 mL) was diluted with CDCl3 (0.5 mL) and subjected to 1H
NMR analysis. The VEA monomer conversion was determined by
comparing the integral area of the characteristic signals of the VEA
monomer and the polymer of PVEA according to eq 1, in which I2.53 is
the integral area of the resonance signal at 2.53 ppm [PhCH2N-
(CH2CH3)2 in the VEA monomer and the PVEA polymer], and I5.20 is
the integral area of the resonance signal at 5.20 ppm (one proton of
PhCHCH2 in the VEA monomer), respectively. To collect the
polymer of PVEA, the flask content was added dropwise into the
ethanol/water mixture (6/5 by weight), washed twice with the
ethanol/water mixture, and then the precipitate was collected and
dried at room temperature under vacuum to afford pale yellow
powder.
2. EXPERIMENTAL SECTION
2.1. Materials. The chemical reagents including chloromethylstyr-
ene (CMS, >97%, Alfa), dimethylamine (DMA, 40 wt % aqueous
solution, Aladdin Chemistry Co. Ltd.), diethylamine (DEA, >99%,
Tianjin Ruijinte Chemical Reagent), n-dibutylamine (DBA, >98%,
Alfa), deuterated methanol-d4 (CD3OD, 99.8 atom % D, Innochem)
and methanol (CH3OH, >99.9%, Tianjin Kangkede Technology Co.
Ltd.) were used as received. The initiator of 2,2′-azobis-
(isobutyronitrile) (AIBN, >98%, Tianjin Ruijinte Chemical Reagent)
was recrystallized from alcohol. The RAFT agent of s,s′-bis(a,a′-
dimethyl-a″-acetic acid) trithiocarbonate (BDMAT, Scheme 1) was
synthesized as discussed elsewhere.47,48 All the other chemical reagents
were analytic grade and used as received. Deionized water was used in
the present study.
I2.53 − 4I5.20
conversion % =
× 100%
I2.53
(1)
Scheme 1. Chemical Structure of BDMAT
2.4. Synthesis of the Doubly Thermo-Responsive Triblock
Copolymer. The triblock copolymer of poly[N-(4-vinylbenzyl)-N,N-
dimethylamine]-b-poly[N-(4-vinylbenzyl)-N,N-diethylamine]-b-poly-
[N-(4-vinylbenzyl)-N,N-dimethylamine] (PVMA-b-PVEA-b-PVMA)
was prepared by sequential RAFT polymerization of VEA using
PVMA-TTC as macro-RAFT agent. Into a 25 mL Schlenk flask, VEA
(4.72 g, 0.025 mol), the PVMA-TTC macro-RAFT agent (PVMA106
-
TTC, 2.42 g, 0.14 mmol), AIBN (7.4 mg, 0.045 mmol) dissolved in
1,4-dioxane (2.3 g) were added. After PVMA106-TTC being dissolved
in the mixture, the flask content was initially degassed with nitrogen at
0 °C for 30 min, and then the polymerization was performed at 70 °C
for 12 h. The VEA monomer conversion at 60% was determined by 1H
NMR analysis just as introduced in section 2.3. The resultant triblock
polymer of PVMA53-b-PVEA108-b-PVMA53 was isolated by precip-
itation into petroleum ether, and dried at room temperature under
vacuum to afford pale yellow powder (5.2 g).
2.6. Characterization. The molecular weight and the polydisper-
sity index (PDI, PDI = Mw/Mn) of the synthesized polymers were
determined by gel permeation chromatography (GPC) equipped with
a Waters 600E GPC system, where THF or CHCl3 was used as eluent
and the narrow-polydispersity polystyrene was used as calibration
standard. The 1H NMR analysis was performed on a Bruker Avance III
400 MHz NMR spectrometer. For polymers dissolved in CDCl3, the
proton signal at δ = 7.26 ppm of the internal solvent was used as
standard; and for polymers dissolved in CD3OD, the chemical shift of
the methyl group of the residual CH3OH in CD3OD was locked at
3.36 ppm and the signal was used as reference. The LCST and UCST
of the thermo-responsive polymers were determined by turbidity
analysis on a Varian 100 UV−vis spectrophotometer equipped with a
thermo-regulator ( 0.1 °C) with the heating/cooling rate at 1 °C/
2.2. Synthesis of the N-(4-Vinylbenzyl)-N,N-dialkylamine
Monomers. The N-(4-vinylbenzyl)-N,N-dialkylamine monomers
were synthesized by the nucleophilic substitution reaction of CMS
with the secondary amine of DMA, DEA or DBA.49,50 Herein, the
synthesis of N-(4-vinylbenzyl)-N,N-dimethylamine (VMA) is typically
introduced. Into a flask CMS (30.5 g, 0.20 mol), the DMA aqueous
solution (20.0 mL, 0.90 g/mL), ethanol (200 mL) and K2CO3 (55.3 g,
0.40 mol) were added. The flask content was initially degassed by
nitrogen purge and then heated at 50 °C for 24 h with magnetic
stirring. The solvent was removed by rotary evaporation under vacuum
to obtain a crude product. The crude product was purified by column
chromatography using the solvent of petroleum ether, and distilled
under vacuum to afford a clear transparent liquid of VMA (29.0 g, 90%
1
yield). H NMR (CDCl3): δ = 7.36 (d, J = 8.0 Hz, 2H), 7.29 (d, J =
8.0 Hz, 2H), 6.71 (dd, J = 11.0 and 17.7 Hz, 1H), 5.71 (dd, J = 1.0 and
17.6 Hz, 1H), 5.21 (dd, J = 1.0 and 11.0 Hz, 1H), 3.42 (s, 2H), 2.24 (s,
1
6H). See the H NMR spectra in Figure S1, Supporting Information.
N-(4-Vinylbenzyl)-N,N-diethylamine (VEA) (16.0 g, 85% yield). 1H
NMR (CDCl3): δ = 7.34 (d, J = 8.2 Hz, 2H), 7.29 (d, J = 8.2 Hz, 2H),
6.71 (dd, J = 11.0 and 17.7 Hz, 1H), 5.72 (dd, J = 1.0 and 16.6 Hz,
1H), 5.20 (dd, J = 1.0 and 10.0 Hz, 1H), 3.55 (s, 2H), 2.52 (q, J = 7.1
1
Hz, 4H), 1.04 (t, J = 7.1 Hz, 6H). See the H NMR spectra in Figure
S1, Supporting Information.
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Scheme 2. Synthesis of N-(4-Vinylbenzyl)-N,N-dialkylamine and Poly[N-(4-vinylbenzyl)-N,N-dialkylamine]
min, in which the 500 nm wavelength was set for the homopolymer
solution and the 420 nm wavelength was set for the triblock
copolymer solution. The UCST and LCST values were determined at
50% transmittance or at the half of the maximal and minimal
transmittance. Dynamic light scattering (DLS) analysis was performed
on Nano-ZS90 (Malvern) laser light scattering spectrometer with He−
Ne laser at the wavelength of 633 nm at 90° angle, in which the
polymer concentration was set at 0.1 mg/mL. The transmission
electron microscopy (TEM) observation was performed using a
Tecnai G2 F20 electron microscope at an acceleration of 200 kV,
whereby a small drop of the preheated dispersion of the synthesized
polymer at a given temperature was deposited onto a piece of
preheated copper grid till the solvent was evaporated. The differential
scanning calorimetry (DSC) analysis was performed on a NETZSCH
DSC 204 differential scanning calorimeter under nitrogen atmosphere,
in which the sample was heated to 110 °C at the heating rate of 10
°C/min, cooled to 0 °C in 5 min, and then heated to 110 °C at the
heating rate of 10 °C/min.
Table 1. Summaries of the Synthesized Polymers of PVMA,
PVEA, PVBA, and PVMA-b-PVEA-b-PVMA
Mn (kg/mol)
time
(h)
convn
a
b
c
d
e
sample
(%)
Mn,th Mn,GPC Mn,NMR
PDI
PVMA106
PVEA9
16
2
59
5
17.4
2.0
8.7
2.0
2.8
3.7
4.0
7.1
8.3
8.1
18.0
21.4
−
−
1.30
1.12
1.14
1.26
1.29
1.30
1.36
1.32
1.35
PVEA25
PVEA40
PVEA54
PVEA86
PVEA104
PVBA72
4
14
22
30
48
58
60
60
5.0
6
7.8
8
10.5
16.6
20.0
17.9
37.8
−
−
−
−
46.2
12
16
16
12
PVMA53-b-PVEA108
b-PVMA53
-
a
The monomer conversion determined by 1H NMR analysis.
b
c
Theoretical molecular weight according to eq 2. Molecular weight
d
1
3. RESULTS AND DISCUSSION
determined by GPC analysis. Molecular weight determined by H
NMR analysis. The PDI (Mw/Mn) values determined by GPC
e
3.1. Synthesis of Poly[N-(4-vinylbenzyl)-N,N-dialkyl-
amine] and PVMA-b-PVEA-b-PVMA. Scheme 2 shows the
synthesis of the N-(4-vinylbenzyl)-N,N-dialkylamine monomers
by the nucleophilic substitution reaction of CMS with NHR2.
This nucleophilic substitution reaction affords the high yield of
VMA (90%) and VEA (85%) and the moderate yield of VBA
(70%). The chemical structure of the three monomers is
confirmed by 1H NMR analysis (Figure S1, Supporting
Information).
analysis.
From the 1H NMR spectra, the characteristic proton
resonances are clearly discerned. The molecular weight
Mn,NMR of PVMA106 can be determined by the characteristic
chemical shift at δ = 0.87 ppm corresponding to the RAFT
terminal and the signal at δ = 3.4 ppm corresponding to the
polymer chain (seeing the 1H NMR spectra for the low
molecular weight polymer of PVMA38 in Figure S4, Supporting
Information, in which the RAFT terminal can be clearly
discerned), and the Mn,NMR value is close to the theoretical
molecular weight Mn,th (Table 1). For PVEA and PVBA, the
chemical shift corresponding to the RAFT terminal is
overlapped with those of the polymer backbone and therefore
the polymer molecular weight cannot be accurately estimated
[monomer]0 × Mmonomer
Mn,th
=
× conversion + MRAFT
(2)
[RAFT]0
The polymers of PVMA, PVEA, and PVBA are prepared by
RAFT polymerization using BDMAT as RAFT agent and
AIBN as initiator (Scheme 2). Well-controlled RAFT polymer-
ization of VEA is achieved, which is indicated by the linear
increase in the polymer molecular weight with the monomer
conversion (Figure S2, Supporting Information) and the
relatively low PDI of the synthesized polymers (Figure S3,
Supporting Information). By quenching the RAFT polymer-
ization at a given time, six polymers of PVEA with different
molecular weight or polymerization degree (DP) were prepared
(Table 1). The samples of PVMA106 and PVBA72, in which the
subscript represents the DP, were also prepared by RAFT
polymerization. The molecular weight of the synthesized
polymers by GPC analysis (Mn,GPC) is lower than the
theoretical molecular weight Mn,th determined by the monomer
conversion according to eq 2,51 and the reason is possibly due
to the nonpolar polystyrene standard used in the GPC analysis.
The synthesized PVMA, PVEA and PVBA are characterized
by 1H NMR analysis, and Figure 1 shows the 1H NMR spectra
of the typical polymers of PVMA106, PVEA104 and PVBA72.
1
by H NMR analysis.
The triblock copolymer of PVMA53-b-PVEA108-b-PVMA53
was synthesized by RAFT polymerization of VEA employing
PVMA106-TTC as macro-RAFT agent. After 12 h polymer-
ization under [VEA]o:[PVMA-TTC]o:[AIBN]o = 540:3:1 at 70
°C, the 60% monomer conversion is achieved and the triblock
copolymer is prepared. The GPC analysis suggests the
molecular weight Mn,GPC at 18.0 kg/mol and the PDI at 1.35.
The theoretical molecular weight Mn,th of the triblock
copolymer is higher than Mn,GPC by GPC analysis (37.8 vs
18.0 kg/mol), and the reason is possibly due to the polystyrene
standard used in the GPC analysis. The PDI value of the
triblock copolymer is moderate, and the very slight shoulder at
the high molecular weight side (less elution time) in the GPC
traces is observed (Figure S5, Supporting Information),
suggesting there existing somewhat bimolecular radical
termination in the RAFT polymerization. Figure 1 shows the
1H NMR spectra of the triblock copolymer, from which the
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be determined. As shown in Table 1, the Mn,NMR of the triblock
copolymer is slightly higher than the theoretical molecular
weight Mn,th calculated by the monomer conversion.
The synthesized polymers are further confirmed by DSC
analysis (Figure S6, Supporting Information), in which the glass
transition temperature (Tg) at 35.0 °C corresponding to
PVEA104 and Tg at 56.7 °C corresponding to PVMA38 are
detected. For the PVMA53-b-PVEA108-b-PVMA53 triblock
copolymer, one peak of Tg at 42.2 °C between those of the
PVEA104 and PVMA38 homopolymers is observed, suggesting
the PVEA and PVMA blocks being miscible.
3.2. Solubility of Poly[N-(4-vinylbenzyl)-N,N-dialkyl-
amine] in Alcohols and in the Alcohol/Water Mixture.
The solubility of the three typical polymers of PVMA106
,
PVEA104 and PVBA72 in water and in alcohols is checked, and
the results are summarized in Table 2. PVMA106 is insoluble in
water, and is soluble in the five alcohols of methanol (MeOH),
ethanol (EtOH), 2-propanol (i-PrOH), n-propanol (n-PrOH),
and n-butanol (n-BuOH), and exhibits LCST in the methanol/
water mixture. PVBA72 is insoluble in water, methanol, ethanol
and 2-propanol, and shows UCST at 48.0 °C in n-propanol
(Figure S7, Supporting Information), and becomes soluble in n-
butanol. The solubility of PVEA104 in solvent is different from
either PVMA106 or PVBA72, and LCST in methanol and UCST
in 2-propanol are observed. The subsequent study is focused
initially on the solubility of PVEA, and the parameters including
the polymer molecular weight, the polymer concentration and
the cosolvent affecting on UCST or LCST are discussed. Then,
the solubility of PVMA and PVBA in alcohol and in the
alcohol/water mixture is discussed.
3.2.1. Solubility of PVEA in Alcohols and in the Alcohol/
Water Mixture. The temperature-dependent transmittance of
PVEA in methanol upon heating or cooling is shown in Figure
2. It shows that the thermo-responsive phase transition of
PVEA in methanol is reversible (Figure 2A), which is as similar
as the general LCST-type polymers as discussed elsewhere,12,16
although a very slight delay during the cooling process is
observed. The thermo-responsive phase transition is sharp
within a narrow temperature range of ∼3 °C especially for the
high DP polymers. The LCST of PVEA in methanol is DP-
dependent, and it decreases from 58.0 to 39.5 °C when the
polymer DP increases from 40 to 104. When DP decreases
below 25, PVEA becomes soluble in methanol. The LCST of
PVEA in methanol is also polymer-concentration-dependent,
and high LCST is generally observed in dilute polymer solution
(Figure 2B). For example, the LCST of PVEA54 in methanol
decreases from 50.7 to 47 and further to 44.3 °C when the
polymer concentration increases from 1.0 to 2.0 and further to
4.0 wt %. Besides, the polymer concentration effect on LCST is
serious for the low DP polymers. For example, for the low DP
polymer of PVEA40, it is soluble in methanol at 1.0 wt %
polymer concentration, meanwhile the LCST at 58.0 °C in the
2.0 wt % polymer solution and the LCST at 55.5 °C in the 4.0
wt % polymer solution are detected; whereas for the high DP
1
Figure 1. H NMR spectra of PVMA106 (A), PVEA104 (B), PVBA72
(C), and PVMA53-b-PVEA108-b-PVMA53 (D).
characteristic chemical shift due to the PVMA and PVEA blocks
can be clearly discerned. Based on the molecular weight Mn,NMR
of the PVMA106-TTC macro-RAFT agent and the area ratio of
the PhCH2N(CH3)2 signal at δ = 3.3 ppm to the PhCH2N-
(CH2CH3)2 signal at δ = 3.5 ppm, the DP of the PVEA block
and the molecular weight Mn,NMR of the triblock copolymer can
Table 2. Summaries of the Solubility of Poly[N-(4-vinylbenzyl)-N,N-dialkylamine] in Water and in Alcohols
sample
H2O
H2O/MeOH
MeOH
EtOH
i-PrOH
n-PrOH
n-BuOH
a
b
PVMA106
PVEA104
PVBA72
I
LCST
LCST
I
S
S
S
I
S
S
S
S
S
I
I
LCST
I
UCST
I
S
UCST
a
b
I represents the sample being insoluble. S represents the sample being soluble.
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deuterated solvents under the other same conditions,52
although the slight different LCST values in D2O/H2O are
reported for the thermo-responsive polymer of PNIPAM and
the elastin-like polypeptides (ELPs).25,53 The exact reason on
the great difference between the two LCST values in deuterated
methanol-d4 and in methanol needs further study. However, the
nonsolvent effect on LCST by the H2O impurity in deuterated
methanol-d4 and in methanol should be excluded out, since
very traced or no H2O in the two solvents is detected.
In order to gain close insight into the phase transition of
PVEA in alcohol upon heating, the 1H NMR spectra of
PVEA104 dissolved in CD3OD (2.0 wt %) at temperature
ranging from 5.0 to 40.0 °C are recorded, in which the integral
area of the characteristic signal is normalized by the signal at δ
= 3.36 ppm (CH3OH of the residual in CD3OD). Figure 4A
Figure 2. Temperature-dependent transmittance of PVEA in methanol
(A) and the LCST of PVEA with a given DP at different polymer
concentration (B).
polymer of PVEA104, the LCST values at 1.0, 2.0, and 4.0 wt %
polymer concentration are very close to each other.
As shown in Figure 3, PVEA shows much different LCST in
deuterated methanol-d4 (CD3OD) and in methanol (CH3OH).
The LCST of PVEA104 in CD3OD (2.0 wt %) is much lower
than that in CH3OH (23.0 °C vs 39.5 °C). To our best
knowledge, most of the thermo-responsive polymers exhibit
very similar LCST or UCST in the deuterated and non-
1
Figure 4. The H NMR spectra of PVEA104 in CD3OD at 10.0 and
35.0 °C (A), the signal of the N(CH2CH3)2 at different temperature
(B), and the temperature-dependent signals of the typical protons
adjacent to the nitrogen (N) atom and in the polymer backbone (C),
in which the polymer concentration is 2.0 wt %.
Figure 3. Temperature-dependent transmittance of PVEA104 in
methanol (CH3OH) and in deuterated methanol-d4 (CD3OD), in
which the polymer concentration is 2.0 wt %.
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Figure 5. Effect of the ethanol cosolvent (A) and the water nonsolvent (B) on the LCST of PVEA104 in methanol, in which the polymer
concentration is 2.0 wt %.
Figure 6. Temperature-dependent transmittance of PVEA104 in 2-propanol (A) and the DP-dependent UCST of PVEA (B), in which the polymer
concentration is 2.0 wt %.
shows the 1H NMR spectra recorded at 10.0 °C (below LCST
at 23.0 °C as indicated by Figure 3) and 35.0 °C (above
LCST), in which the characteristic signal weakening upon
heating at temperature above LCST is indicated, and therefore
the dehydration of the polymer chains at temperature above
LCST is confirmed. Figure 4B shows the temperature-
dependent signal at δ = 2.5 ppm corresponding to N-
(CH2CH3)2. At temperature below LCST, PVEA104 is soluble
and therefore the signal almost keeps constant. When the
temperature increases to the LCST at 23.0 °C, the signal
quickly decreases, indicating the polymer dehydration and the
phase transition. When the temperature further increases, the
signal decreases slightly. The slight signal decrease above LCST
suggests the compression of the dehydrated polymer chains
with the temperature increasing, which is as similar as the
molten globule states of PNIPAM in water.54 The temperature-
dependent signals at δ = 6.5 ppm (the ortho-H in the benzene
ring as indicated by the inset in Figure 4C), δ = 3.5 ppm
(PhCH2N), δ = 2.5 ppm [N(CH2CH3)2)], and δ = 1.1 ppm
[N(CH2CH3)2] are shown in Figure 4C. These three protons
adjacent to the nitrogen (N) atom and the proton in the
polymer backbone are chosen, since they can afford the
information on the hydrogen bonding between the polymer
chains and the solvent. When temperature increases above
LCST, the four signals weaken gradually. However, the signals
of PhCH2N and N(CH2CH3)2 decrease much faster than those
of N(CH2CH3)2 and the backbone proton. This suggests that
the segment adjacent to the PhCH2N and N(CH2CH3)2
groups in the polymer chains contributes the polymer
dehydration when the temperature increases above LCST.
The cosolvent/nonsolvent effect on the LCST of PVEA is
also checked. As shown in Table 2, PVEA is soluble in ethanol
and is insoluble in water. Thus, both the cosolvent of ethanol
and the nonsolvent of water are typically chosen to evaluate the
cosolvent/nonsolvent effect on the LCST of PVEA. Figure 5
shows the different effect on the LCST by the cosolvent of
ethanol and the nonsolvent of water. That is, the addition of the
cosolvent of ethanol leads to the LCST increase, and the
addition of the nonsolvent of water leads to the LCST decrease.
For example, the LCST of PVEA104 increases from 39.5 to 65.0
°C when the ethanol content in the ethanol/methanol mixture
increases from 0 to 50 wt %, and it becomes soluble and no
LCST is detected when the ethanol content is above 50 wt %.
On the contrary, the LCST of PVEA104 decreases from 36.0 to
1.2 °C when the water content in the water/methanol mixture
increases from 2 to 17 wt %. The similar cosolvent effect on
LCST is observed in the aqueous solution of poly(vinyl methyl
ether),34,46 and it is slightly different from that of the PNIPAM
aqueous solution, in which the LCST initially decreases with
the addition of the cosolvent and subsequently increases when
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the cosolvent is further added.33,34 Whatever, the great
cosolvent/nonsolvent effect on the LCST of PVEA as shown
in Figure 5 affords a convenient method to tune LCST at a
wide temperature range at 0−68 °C.
water, methanol, ethanol, and 2-propanol can act as nonsolvent
to tune the UCST of PVBA72 in n-propanol. As shown in
Figure 8, the addition of the 10 wt % cosolvent of n-butanol in
The insoluble-to-soluble transition of PVEA in 2-propanol
upon heating at UCST is also checked. The insoluble-to-soluble
transition at UCST is sharp and it generally takes place within a
narrow temperature range (Figure 6A). The UCST of PVEA in
2-propanol is also dependent on the polymer DP. There is a
positive correlation between UCST and DP, and the UCST
value increases from 14.1 to 43.5 °C when the DP of PVEA
increases from 25 to 104 (Figure 6B). For the low DP polymer
of PVEA9, it is soluble in 2-propanol and no UCST is detected.
These results are well consistent with the UCST behavior of
poly[oligo(ethylene glycol) methyl ether methacrylate] in
aliphatic alcohols,14,16 and the increase of UCST 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.
Figure 8. Cosolvent/nonsolvent effect on UCST of PVBA72 in n-
propanol, in which the polymer concentration is 2.0 wt % and the
content of the cosolvent or nonsolvent in the binary alcohol mixture is
10 wt %.
3.2.2. Solubility of PVMA in the Alcohol/Water Mixture. As
shown in Table 2, PVMA is soluble in methanol, and is
insoluble in water, and shows LCST in the methanol/water
mixture. Figure 7 shows that the LCST is firmly correlative to
n-propanol leads to the UCST decreasing from 48.0 to 29.0 °C,
and the addition of the 10 wt % nonsolvents leads to the UCST
increasing from 48.0 to 75.0 °C, respectively.
3.3. Doubly Thermo-Responsive Micellization of
PVMA-b-PVEA-b-PVMA in the Methanol/Water Mixture.
With the detailed solubility of the PVEA and PVMA
homopolymers in the alcohol/water mixture in hand, the
thermo-responsive micellization of the PVMA53-b-PVEA108-b-
PVMA53 triblock copolymer in the methanol/water mixture is
studied. It is found that the triblock copolymer is soluble in the
methanol/water mixture with water content below 20 wt %,
and neither LCST nor UCST is detected. With the water
content increasing to 20−33 wt %, LCST is detected and
micellization of the triblock copolymer takes place at
temperature above LCST. When the water content in the
methanol/water mixture further increases above 33 wt %, the
triblock copolymer becomes insoluble.
Figure 7. Temperature-dependent transmittance of PVMA106 in the
methanol/water mixture with different water content, in which the
polymer concentration is 2.0 wt %.
Figure 9 shows the temperature-dependent transmittance of
the PVMA53-b-PVEA108-b-PVMA53 triblock copolymer in the
the solvent character. In the methanol/water mixture with the
water content below 33 wt %, PVMA106 is soluble (100%
transmittance) and no LCST is detected. With the water
content increases from 40 to 50 wt %, LCST is detected and it
decreases from 51.5 to 19.5 °C. When the water content in the
methanol/water mixture further increases to above 60%,
PVMA106 becomes insoluble even when the temperature
decreases to 0 °C. These results suggest that the addition of
water in methanol leads to the phase transition of PVMA106 at
LCST, and the LCST decreases with the water content
increasing. This reason is due to water acting as the nonsolvent
to decrease the solubility of PVMA106 in the solvent, which is
very similar with that of PVEA104 demonstrated in Figure 5.
3.2.3. Solubility of PVBA in Alcohols. As shown in Table 2,
PVBA72 displays the UCST phase transition in n-propanol at
48.0 °C (Figure S7, Supporting Information), and is insoluble
in water, methanol, ethanol, and 2-propanol, and is soluble in n-
butanol. This suggests that n-butanol can act as cosolvent, and
Figure 9. Temperature-dependent transmittance of the PVMA53-b-
PVEA108-b-PVMA53 triblock copolymer in the methanol/water
mixture (75/25 by weight). The polymer concentration is 1.0 wt %.
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typical solvent of the methanol/water mixture with 25 wt %
water content. The triblock copolymer is molecularly soluble in
the solvent at temperature below 15.0 °C (stage I), as indicated
by the almost constant transmittance at ∼100%. With
temperature increasing above 15.0 °C, the transmittance
gradually decreases to 60% until at 50.0 °C (Stage II),
suggesting micellization of the triblock copolymer. This
micellization is confirmed by the DLS analysis of the triblock
copolymer solution at 10.0 °C below LCST (LCST1 at 32.5
°C) and at 40.0 °C above LCST1, in which the hydrodynamic
diameter (Dh) centered at ∼5 nm corresponding to the single
triblock copolymer chains and the Dh centered at 23 nm
corresponding to the triblock copolymer micelles are detected
(Figure 10). Note: the apparent Dh at 90° angle in the DLS
Besides, the micellar nanoparticles seem deformed during the
TEM observation due to the low Tg of the triblock copolymer
(Figure S6, Supporting Information). The size difference
between the DLS analysis and the TEM observation is due
to the TEM observation showing the dried aggregates and
whereas DLS analysis detecting the solvated micelles.
Interestingly, the soluble-to-insoluble phase transition of the
PVEA108 block in the solvent takes place at much higher
temperature than that of the reference homopolymer of
PVEA104 (32.5 vs ∼0 °C), although they have the similar DP.
The reason is due to the solvophilic PVMA53 block, which
increases the solubility and therefore the LCST of the PVEA108
block as similar as the block or random copolymers as discussed
elsewhere.59−64 When the temperature further increasing above
50.0 °C, the transmittance quickly decreases from 60% to 0%
(stage III), suggesting the shrinking of the triblock copolymer
micelles. The shrinking of the triblock copolymer micelles at
stage III is confirmed by the Dh decreasing from 23 to 20 nm
upon the micelles solution being heated from 40 to 55.0 °C
(Figure 10), although the size of the particles by the TEM
observation is very similar with each other under the two
conditions (Figure 11). It is expected that the shrinking of the
triblock copolymer micelles upon heating is due to the soluble-
to-insoluble transition of the PVMA53 corona at LCST (LCST2,
54.5 °C), and the soluble-to-insoluble transition of the PVMA53
corona leads to the formation of the corona-collapsed micelles
as indicated by the inset in Figure 9. The corona-collapsed
micelles can keep suspending in the methanol/water mixture at
temperature above LCST2 at dilute polymer concentration
(below 0.1 mg/mL) and the reason is possibly due to the
partial salvation of the micelles by methanol in the solvent of
the methanol/water mixture.58
Figure 10. Hydrodynamic diameter distribution f(Dh) of the triblock
copolymer in the methanol/water mixture (75/25 by weight) at 10.0
°C (A), 40.0 °C (B), and 55.0 °C (C).
Interestingly, the reference homopolymers of PVMA106 and
PVMA38 are soluble in the methanol/water mixture (75/25 by
weight) at this temperature range of 50−60 °C, which is
different from the soluble-to-insoluble transition of the
PVMA53 corona as discussed above. The reason why the
reference homopolymers of PVMA106 and PVMA38 are soluble
while the PVMA53 corona in the core−corona micelles
undergoes phase separation at LCST2 is discussed, and the
reason is ascribed to the hydrophobic PVEA108 block, which
decreases the solubility of the PVMA53 corona in the solvent
and leads to the phase transition of the PVMA53 corona at
LCST2. The phase transition of the PVMA53 corona is very
similar with the soluble-to-insoluble transition of poly(N,N-
dimethylacrylamide),59 which is hydrophilic and generally
shows no LCST, in water. It was found, through the
copolymerization of N,N-dimethylacrylamide with a hydro-
phobic monomer of N-tert-butylacrylamide (TBA), a thermo-
responsive copolymer of poly(N,N-dimethylacrylamide-co-N-
tert-butylacrylamide) (PDMA-co-PTBA) was obtained and the
phase transition of the PDMA segment at LCST was
observed.59
The structure of the triblock copolymer micelles is further
discussed. On the basis of the composition of the triblock
copolymer and the size of the single micellar nanoparticles by
TEM, the core radius (Rc) at 5.3 nm and the shell thickness
(Rs) of the single micellar nanoparticles at 1.2 nm can be
appreciatively estimated by assuming the well-defined core−
shell structure of the nanoparticles. The Rc values of the
micelles in the solvent of the methanol/water mixture (75/25
by weight) by DLS analysis are larger than that of the dried
single micellar nanoparticles (5.3 nm) by TEM observation
analysis is afforded herein, and the real Dh should be larger than
the apparent values. Since the PVMA53 block is soluble and the
PVEA108 block is insoluble in the methanol/water mixture at
stage II, the triblock copolymer micelles are expected to have an
insoluble PVEA108 core and a solvophilic PVMA53 corona as
indicated by the inset shown in Figure 9.55−57 The TEM image
shown in Figure 11A and Figure S8, Supporting Information,
Figure 11. TEM images of the triblock copolymer micelles formed at
40 °C (A) and 55 °C (B) in the methanol/water mixture (75/25 by
weight).
indicates the micellar nanoparticles, in which the single
nanoparticles (13 nm) corresponding to the triblock copolymer
micelles and the nanoparticle-clusters (40−100 nm depending
on the number of aggregated nanoparticles) are observed. The
formation of the nanoparticle-clusters is possibly due to the
small size and therefore the high surface energy of the micellar
nanoparticles in the sampling process for the TEM observation.
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(8.2 nm vs 5.3 nm; seeing the calculation detail in the
Supporting Information). Therefore, it is expected the
preferential adsorption of methanol in the core of the micelles
occurring in the binary solvent mixture of methanol/water.58
The preferential adsorption of methanol is due to the PVEA108
core being more affinitive with methanol than that with water
in the methanol/water mixture.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures S1−S10 showing the 1H NMR spectra of the
monomers, the RAFT polymerization kinetics, the GPC traces,
1
the H NMR spectra, the DSC thermograms of synthesized
polymers, the temperature dependent transmittance of PVBA72
in n-propanol, the pH-dependent transmittance of the poly[N-
(4-vinylbenzyl)-N,N-dialkylamine] aqueous solutions, and the
TEM image of the triblock copolymer micelles. This material is
available free of charge via the Internet at http://pubs.acs.org.
Based on the results and discussion mentioned above, the
doubly thermo-responsive micellization of PVMA53-b-PVEA108
-
b-PVMA53 is confirmed. It is soluble in the methanol/water
mixture below LCST1 (Stage I), forms core−corona micelles
containing a PVEA core and a PVMA corona through the
soluble-to-insoluble transition of the PVEA block at temper-
ature above LCST1 (stage II), and further forms corona-
collapsed micelles through the soluble-to-insoluble transition of
the PVMA block at temperature above LCST2 (stage III).
3.4. pH-Response of Poly[N-(4-vinylbenzyl)-N,N-dia-
lkylamine]. Besides the thermo-response, the pH-response of
the homopolymers of poly[N-(4-vinylbenzyl)-N,N-dialkyl-
amine] and the PVMA53-b-PVEA108-b-PVMA53 triblock copoly-
mer in aqueous solution is also demonstrated (Figures S9 and
S10, Supporting Information). Compared with the pH-
responsive polymer of poly(4-vinyl pyridine) at pH = 5,65 the
phase transition of poly[N-(4-vinylbenzyl)-N,N-dialkylamine]
occurs at more basic condition at pH = 5−8, and the reason is
the pendent amine group being more basic than pyridine.
AUTHOR INFORMATION
■
Corresponding Author
*(W.Z.) E-mail: wqzhang@nankai.edu.cn. Telephone: 86-22-
23509794. Fax: 86-22-23503510.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by National Science Foundation of China
(Nos. 21074059 and 21274066) is gratefully acknowledged.
REFERENCES
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