Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer via “grafting from” RAFT polymerization

Article abstract of DOI:10.1016/j.cclet.2016.04.018

Organic/inorganic hybrid polymers have been widely studied for their potential use in nanocontainers and nanocarriers. In this article, one star-shaped hybrid polymer, polyhedral oligomeric silsesquioxane (POSS) grafted poly(N,N-(dimethylamino)ethyl metha

Full text of DOI:10.1016/j.cclet.2016.04.018

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CCLET 3682 1–5
Chinese Chemical Letters xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
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Original article
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Synthesis and characterization of silsesquioxane-cored star-shaped
hybrid polymer via ‘‘grafting from’’ RAFT polymerization
a,b
a,b
a,b,
a,b
a,b
5
6
Q1 Zhi-Wei Yu , Shu-Xi Gao , Kai Xu *, You-Xiong Zhang , Jun Peng
,
a,b
Ming-Cai Chen
a
7
8
Key Laboratory of Polymer Material for Electronics, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, China
China University of Chinese Academy of Sciences, Beijing 100049, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Organic/inorganic hybrid polymers have been widely studied for their potential use in nanocontainers
and nanocarriers. In this article, one star-shaped hybrid polymer, polyhedral oligomeric silsesquioxane
Received 23 February 2016
Received in revised form 5 April 2016
Accepted 14 April 2016
Available online xxx
(POSS) grafted poly(N,N-(dimethylamino)ethyl methacrylate) (POSS-g-PDMA), was synthesized via
reversible addition-fragmentation chain transfer polymerization (RAFT). The pH stimuli-responsive
character of POSS-g-PDMA in aqueous solution were also studied.
ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Star-polymer
RAFT
POSS
Graft from
Stimuli-responsive
9
1
0
1. Introduction
planar molecules can be expected due to such cage-structural 31
features. However, there are few articles studying the synthesis and 32
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Polyhedral oligomeric silsesquioxanes, represented by the
empirical formula (RSiO1.5 , can be regarded as organic/inorganic
hybrid materials at a molecular level [1–6]. Materials based on
silsesquioxanes have been proposed for applications in high-
performance materials. A typical silsesquioxane molecule that
possesses a cubic rigid (n = 8, T8) structure consisting of central
properties of POSS-containing star polymers.
33
)
n
So far, there are many other reports about star polymer synthesis 34
through ‘‘graft to’’ and ‘‘graft from’’ methods [29–31]. The advantage 35
of the ‘‘graft to’’ method is that both the core or backbone and side 36
chain can be firstly prepared by different living polymerization 37
techniques, but the grafting density of resulting polymer brushes is 38
often limited due to steric repulsion between bulky side chains. The 39
‘‘grafting from’’ approach enables preparation of star or brush- 40
polymerswithahighgraftingdensityandanarrow molecularweight 41
distribution[32].Inthispaper, wemainlyintroducedanovelstrategy 42
forsynthesisofastarpolymerwithocta-aminophenylsilsesquioxane 43
(OA-POSS)incorporatedintothepolymermatricestoeffectivelyform 44
responsive functional materials as a nucleus of a micelle. The OA- 45
POSS molecule was modified into a RAFT initiator (POSS-BSPA), 46
which was further used to prepare star-shaped organic/inorganic 47
hybrid polymer POSS-g-PDMA by ‘‘grafting from’’. Finally, the pH 48
8
inorganic core (Si O12) and organic moieties (R) at each of the eight
vertices is extensively studied for constructing hybrid polymers
with novel architectures [7,8], and for further enhancing the
mechanical and thermal properties of the hybrid polymers [9–
11]. With the flexible design of functional groups, a variety of
morphologies and hierarchical structures such as tadpole-shaped
[12,13] and star-shaped [14,15] polymers based on POSS has been
prepared by controlled radical polymerization methods [14,16–
18]. Star polymers with well-defined architectures have attracted
much attention because of their potential medical carriers [19–
24]. These molecular hybrids composed of a hydrophobic silses-
quioxane core and hydrophilic arms are studied as structural
amphiphiles with self-organizability and morphological tenability
[7,25–28]. Thus, unique characteristics distinctly different from
stimuli-response behavior of POSS-g-PDMA was also studied.
49
2. Experimental
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51
2.1. Materials
*
Corresponding author at: Key Laboratory of Polymer Material for Electronics,
Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou
10650, China.
Q2
N,N-(Dimethylamino)ethyl methacrylate (DMAEMA) was pur- 52
chased from Aladdin and purified by passing over a basic aluminum 53
5
http://dx.doi.org/10.1016/j.cclet.2016.04.018
1
001-8417/ß 2016 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Z.-W. Yu, et al., Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer
via ‘‘grafting from’’ RAFT polymerization, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.018

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Z.-W. Yu et al. / Chinese Chemical Letters xxx (2016) xxx–xxx
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oxide column. Other regents of analytical grade were all obtained
fromAldrich.Azobisisobuty-ronitrile(AIBN)wasrecrystallizedtwice
from ethanol before use. Dioxane was used without further
purification.
3. Results and discussion
80
3.1. Synthesis and characterization of silsesquioxane-cored star-
shaped hybrid polymer
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82
5
8
2.2. Synthesis of the POSS-g-PDMA
Scheme 1 illustrates the synthetic routes of the POSS-BSPA and
star POSS-g-(PDMA) polymer. BSPA can also be used in many RAFT
polymerization systems, especially grafting modification [33,34],
although BSPA is not a very effective CTA. POSS was grafted to a
RAFT agent via the reaction of the amino group of OA-POSS and
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70
3-Benzylsulfanylthiocarbonyl-sufanylpropionic acid (BSPA,
RAFT agent), OA-POSS, and POSS-BSPA agent were synthesized
in our laboratory (see supporting information).
1
DMAEMA (1.05 g, 0.0067 mol), POSS-BSPA agent (19.5 mg,
0.0065 mmol), and AIBN (1.0 mg, 0.0061 mmol) were introduced
into a Schlenk tube containing dioxane (3.0 mL). The reaction tube
was degassed by three freeze–pump–thaw cycles. The polymeriza-
tion was allowed to proceed under continuous stirring at 70 8C. The
polymerization was quenched by liquid nitrogen. The polymeriza-
tion mixture was purified by precipitation into cold petroleum
ether/diethyl ether (10:1) and arm polymer was also removed,
yielding the POSS-containing star polymer POSS-g-PDMA.
BSPCl. Compared with the H NMR spectra of BSPA and OA-POSS in
Fig. S1 in Supporting information, the peak shifts of methylene
protons moved from 2.84 ppm in BSPA to 2.60 ppm in POSS-BSPA
and the proton signal of the phenyl moved from 6.5–7.0 in OA-
POSS to 8.00–7.50 ppm in POSS-BSPA and fused with the phenyl
protons (–SCH
2
C
6
H
5
). In addition, the proton signal of the amino
groups of OA-POSS at 4.7–5.1 ppm completely disappeared. From
FT-IR spectra in Fig. S2 in Supporting information, compared with
OA-POSS, two new absorption bands could be discerned at
À1
À1
1
643 cm and 1050 cm in the spectrum of POSS-BSPA, which
7
1
2.3. Self-assembly of POSS-star polymer in aqueous solution
are assigned to the stretching vibration of carbonyl (–CONH–) and
C 55 S, respectively. Based on HNMR and FTIR spectra, we
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All the samples wereobtainedbydirectly dissolvingthepolymers
in aqueous solutions at pH 4.1 and formed 1 mg/mL solutions. The
solutions were stirred at least for 24 h to ensure the system reached
equilibrium. The pH of solutions was adjusted by hydrochloric acid
and sodium hydroxide aqueous solutions. The different pH solutions
of star polymers were stirred for 24 h and then measured.
Characterization and instrumentation and other synthesis are
listed in the Supporting information.
concluded the POSS-BSPA agent was synthesized successfully.
The POSS-BSPA macro-initiator was then used to prepare the POSS-
containing inorganic/organic hybrid DMAEMA via a ‘‘grafting
from’’ method according to Scheme 1.
The SEC traces in Fig. S3 in Supporting information showed
single and symmetrical peaks of POSS-g-PDMA and the poly-
dispersities were in the range of 1.57–1.65. As reaction time
increased, the chromatograms shift to lower elution volume.
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101
102
103
104
105
106
107
Scheme 1. The synthesis process of POSS-BSPA and POSS-g-PDMA.
Please cite this article in press as: Z.-W. Yu, et al., Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer
via ‘‘grafting from’’ RAFT polymerization, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.018

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Table 1
Results of the RAFT polymerization of DMAEMA using POSS-BSPA agent.
a
a
b
b
Time (h)
Conversion (%)
Mn
Mn
PDI
1
2
3
4
5
6
0.41
0.45
0.52
0.56
0.62
0.66
66 050
72 920
83 860
91 070
101 090
107 020
69 320
78 990
87 230
96 820
107 560
113 240
1.57
1.59
1.62
1.60
1.63
1.65
a
1
Estimated from H NMR spectrum.
b
Determined by GPC calibrated with poly(methyl methacrylate) (PMMA).
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
08
09
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Furthermore, there was almost no shoulder observed. This
indicated that irreversible terminations due to coupling reactions
between the star or linear chain radicals did not occur for the star
polymer [14,31]. The main results of the polymerizations by SEC
1
and H MNR are summarized in Table 1. From FT-IR spectra in
Fig. S4 in Supporting information, characteristic peaks at
À1
1720 cm mainly attributed to the carbonyl of PDMA segment
À1
were observed, and the peak at 3700–3100 cm
was the
Fig. 2. The diameter change in aqueous solution at different pH at 25 8C.
stretching vibration of benzene ring of POSS and BSPA parts,
and the other peaks were mainly fused with these of the PDMA
segment.
The kinetic plot of the star polymerization is shown in Fig. 1. A
linear relationship exists between ln(1/(1 À x)) (x was the
conversion of DMAEMA) and reaction time, indicating that the
concentration of chain radicals is constant. Thus, the polymeriza-
tion follows pseudo first-order kinetics.
the reversible change of the aggregate structure between ‘‘shrink’’ 138
and ‘‘stretch’’ of the complex micelle in response to alkali or acid 139
h
stimulation (inset of Fig. 2). However, D was increased when pH 140
was beyond 8.1 because PDMA became hydrophobic and the 141
solutions would become unstable. The unique properties of POSS- 142
g-PDMA are not observed for their linear counterparts in the our 143
experiment, in which there was no alkali or acid stimulation 144
because of no diameter change from DLS in different pH solutions 145
at 25 8C [7]. We tentatively attributed the self-assembly into 146
micelle of star polymer to the interaction of hydrophobic POSS cage 147
with hydrophilic PDMA arms and the strong tendency toward 148
1
24
3.2. The self-assembly of the star polymers POSS-g-PDMA
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The self-assembly of the resultant amphiphilic star polymers
was also investigated in this work. Such stimuli-responsive star
polymers have different hydrodynamic radii in the solution
compared to the linear polymers. Amphiphilic polymers based
on PDMA are well-known representatives in the study of the self-
assembly of stimuli-responsive polymers. In this work, the star-
shaped inorganic/organic hybrid POSS-g-PDMA was similar to an
amphiphilic block copolymer, since the POSS core was hydropho-
bic and the PDMA chain was hydrophilic.
densely packing due to its high symmetry [35].
149
To investigate the self-assembly morphology of POSS-g-PDMA, 150
AFM and high-resolution TEM were applied because they could 151
directly reveal the morphology. The AFM images of the POSS-g- 152
PDMA micelles are shown in Fig. 3, and we can calculate the size of 153
the micelle to be about 23 nm at pH 2.1 3.2, 6.2, and 7.3 at 25 8C. 154
The diameters measured by DLS in aqueous solution at 25 8C were 155
bigger than those measured by AFM, since DLS data directly 156
reflected the dimension of micelles in solution where the PDMA 157
chains could be well dispersed in water. However, for AFM 158
measurement, the micellar solution was spray-coated on the mica 159
surface, where POSS-g-PDMA micelles sharply shrunk during the 160
process of water evaporation, which resulted in a smaller diameter. 161
In addition, the theoretical diameter of the micelles could be 162
calculated according to the molecular weight of POSS-g-PDMA. The 163
The self-assembly solutions of POSS-g-PDMA in Fig. 2 were
measured by dynamic light scattering (DLS). As shown in Fig. 2, D
was about 52 nm at pH < 5.5, and the value (D ) of micelle
h
h
decreased as the pH increased from 6.2 to 8.1. This trend proved
calculated result showed the molecular diameter was about 30 nm 164
1
for a single POSS-g-PDMA (M
n
= 72 920, determined by H NMR), 165
which is smaller than that measured by DLS and larger than that of 166
micelle by AFM. From the TEM image in Fig. 3, it can be seen that 167
the size of the micelle was about 20 nm at pH 3.2 and 6.2, which 168
was almost the same as the results obtained by AFM at pH 3.2 and 169
6.2 within the allowable error. As for pH > 8.1, it was difficult to be 170
characterized by AFM and TEM for well dispersed individual 171
micelles because micelles would aggregate under the condition 172
(
see AFM images in Fig. S6 in Supporting information).
The
gives the
173
z
-potential is a good measure of colloidal stability. Table 2 174
z
-potentials under different pH conditions. The 175
z
-potentials of POSS-g-PDMA micelles were observed to increase 176
as pH decreased. The values of -potentials were about 59, 61, and 177
0 mV at pH 3.0, 4.1 and 5.5, 40 and 37 mV at pH 6.2 and 7.3, and 178
z
6
were 18 and 13 mV at pH 8.1 and 9.2. We could conclude that the 179
core-shell structures were formed, they had different stability at 180
different pH and was unstable at pH > 8.1 while being stable at 181
Fig. 1. Pseudo-first-order kinetic plot and polydispersities (PDI) of the
polymerization of DMAEMA in the presence of POSS-BSPA.
Please cite this article in press as: Z.-W. Yu, et al., Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer
via ‘‘grafting from’’ RAFT polymerization, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.018

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Fig. 3. The AFM and HRTEM images of the POSS-g-PDMA self-assembly micelles in aqueous solution at pH 2.1 (a), 3.2 (b, e), 6.2 (c, f) and 7.3 (d).
Table 2
-potential values in different pH aqueous solutions.
to the National Natural Science Foundation of China for its financial
support (Registry No. 21174162).
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211
z
pH
3.0
59
4.1
61
5.5
60
6.2
40
7.3
37
8.1
18
9.2
13
Potential (mV)
Appendix A. Supplementary data
212
Supplementarymaterialrelatedtothisarticlecanbefound, inthe
online version, at http://dx.doi.org/10.1016/j.cclet.2016.04.018.
213
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1
1
1
1
1
1
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pH < 6.2. The tertiary amine groups in PDMA shell were cationized
when solutions were acidic in the scheme inset of Fig. 2. The
cationic shells repelled each other, rendering this solution more
stable. On the contrary, under basic condition, there was no
repulsive interaction and the stability of solution was markedly
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In this paper, the star polymer POSS-g-PDMA was successfully
synthesized via RAFT polymerization; the polymer can self-
assemble into a hierarchical structure in aqueous solution. Driven
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Please cite this article in press as: Z.-W. Yu, et al., Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer
via ‘‘grafting from’’ RAFT polymerization, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.018

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Please cite this article in press as: Z.-W. Yu, et al., Synthesis and characterization of silsesquioxane-cored star-shaped hybrid polymer
via ‘‘grafting from’’ RAFT polymerization, Chin. Chem. Lett. (2016), http://dx.doi.org/10.1016/j.cclet.2016.04.018