Synthesis of tadpole-shaped POSS-containing hybrid polymers via "click chemistry"

Article abstract of DOI:10.1016/j.polymer.2010.03.028

Copper-catalyzed alkyne-azide "click chemistry" is applied in the preparation of tadpole-shaped ("monochelic") POSS-end functional hybrid polymers by combining with ATRP and RAFT polymerization. Alkyne-functionalized ATRP initiator and RAFT agent were respectively synthesized and applied in the preparation of alkyne-terminal poly(methyl methacrylate) and polystyrene. The tadpole-shaped POSS-containing hybrid polymers are easily obtained by the click reaction with an azido-functional POSS molecule. This presents a novel and effective method to prepare POSS-containing hybrid polymers.

Full text of DOI:10.1016/j.polymer.2010.03.028

Polymer 51 (2010) 2133e2139
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Polymer
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Synthesis of tadpole-shaped POSS-containing hybrid polymers via
“click chemistry”
,
b
,
a,
*
Weian Zhang a , Axel H.E. Müller
*
a Makromolekulare Chemie II, Universität Bayreuth, D-95440 Bayreuth, Germany
b Department of Chemistry, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Copper-catalyzed alkyne-azide “click chemistry” is applied in the preparation of tadpole-shaped (“mono-
chelic”) POSS-end functional hybrid polymers by combining with ATRP and RAFT polymerization. Alkyne-
functionalized ATRP initiator and RAFTagent were respectively synthesized and applied in the preparation of
alkyne-terminal poly(methyl methacrylate) and polystyrene. The tadpole-shaped POSS-containing hybrid
polymers are easily obtained by the click reaction with an azido-functional POSS molecule. This presents
a novel and effective method to prepare POSS-containing hybrid polymers.
Received 23 January 2010
Received in revised form
9 March 2010
Accepted 12 March 2010
Available online 20 March 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Click chemistry
Controlled radical polymerization
POSS hybrid
1. Introduction
using a POSS-based methacrylate monomer via ATRP [17,18]. POSS-
containing star-shaped hybrid polymers were prepared using the
Polyhedral oligomeric silsesquioxanes (POSS) have attracted
great attention in materials science, since POSS, as unique nanosized
agents, can be introduced into polymer matrices to effectively
enhance polymeric properties such as thermal and oxidation resis-
tance, and reduce flammability [1e8]. A typical POSS molecule
(formula [RSiO1.5]n, where n ¼ 8,10,12) has a cage-like inorganic core
in the range of a nanometer, surrounded by organic corner groups,
which endow the POSS molecule with a higher solubility in organic
solvents and reactivity in the preparation of the POSS-containing
hybrid materials [9,10]. In the past reports on the preparation of
POSS-containing hybrid polymers, the POSS molecules were mostly
used as an enhancing agent to be incorporated into the polymeric
matrices by chemical copolymerization, crosslinking or physical
blending [11e16].
Recently, more and more attention has focused on novel archi-
tectures of POSS-containing hybrid polymers using controlled/living
radical polymerization technique such as atom transfer radical
polymerization (ATRP) and reversible addition-fragmentation chain
transfer (RAFT) polymerization. The POSS molecules can be modified
into monomers, initiators, or chain transfer agents (CTAs) for the
living polymerization. For example, POSS-containing homopolymers,
linear and star-shaped block copolymers, have been synthesized
octa-bromo-functionalized POSS molecule as an ATRP initiator
[19,20]. The mono-bromo-functionalized POSS molecule was also
used as ATRP initiator to synthesize tadpole-shaped (“monochelic”,
also named “hemi-telechelic” [21]) POSS-containing hybrid polymers
[22,23]. RAFT polymerization, as a very useful living radical poly-
merization technique in polymer science, is widely used to design
many novel polymeric materials with well-defined architectures
[24e26]. We recently modified a RAFT chain transfer agent with
a POSS molecule to prepare ‘tadpole-shaped” POSS-containing
hybrid polymers via RAFT polymerization [27,28]. The polymeriza-
tion can be well-controlled by the POSS-containing RAFT agent.
Although a typical POSS molecule (molecular weight w 1000 g/mol)
is small compared to the polymer it is attached to, its presence can
cause important changes in the polymer properties. As an example,
POSS-functional poly(acrylic acid) forms aggregates in water [27].
“Click chemistry” has been proven to be a fast and efficient
approach to prepare polymers with novel architectures [29e31].
Currently, the copper-catalyzed Huisgen 1,3-dipolar cycloaddition
between azidesand alkynes isthemostpopularclickreactionasitcan
be performed under mild reaction conditions and it has a good
tolerance of functional groups [32,33]. When click chemistry is
combined with the living/controlled polymerization technique, it
becomes more powerful in constructing novel polymeric architec-
tures [34e37].
* Corresponding authors. Tel.: þ49 921 55 3399; fax: þ49 921 55 3393.
E-mail addresses: wazhang@ecust.edu.cn (W. Zhang), axel.mueller@uni-
bayreuth.de (A.H.E. Müller).
So far, click chemistry has been mostly used to functionalize the
end-group or pendant groups of polymers or to construct polymers
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.03.028

2134
W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
with well-defined structures, and the click reaction occurred
between a polymer chain and small organic molecules, or between
two different polymer chains. We recently showed that the copper-
catalyzed click reaction between alkyne-functional POSS molecules
and mono-, di- and penta-functional azido-terminal polymers
made by ATRP proceeds smoothly to form monochelic (tadpole-
shaped), and di-telechelic (dumbbell-shaped) linear hybrid poly-
mers as well as penta-telechelic, star-shaped hybrids [21].
Here, we show that an inversion of the procedure is possible.
First, we synthesized an azido-terminal POSS molecule. This was
then “clicked” to alkyne-terminal polymers, which were prepared
using both an alkyne-terminal ATRP initiator and RAFT agent. We
compare the efficiency between ATRP- and RAFT-synthesized
polymers. Our results show that the click reaction is a useful
alternative to using POSS-functional initiators or chain transfer
agents.
Elemental analysis calcd for C31H69N3O12Si8 (901): C, 41.34; H, 7.72;
N, 4.67, Found: C, 42.25; H, 6.44; N, 4.08, Melting point: 241ꢀC.
2.1.3. Synthesis of propargyl 2-bromoisobutyrate (PBiB)
PBiB was prepared according to literature [38]. 1H NMR (CDCl3,
ppm): 4.71 (m, 2H, CH^CCH2OOC(CH3)2Br), 2.50 (t, 1H,
CH^CCH2OOC(CH3)2Br) and 1.89 (s, 6H, CH^CCH2OOC(CH3)2Br).
2.1.4. Preparation of alkyne-terminated poly(methyl methacrylate)
via ATRP (alkyne-PMMAATRP
)
A round-bottom glass flask (20 mL) with a magnetic stirring bar
was charged with methyl methacrylate (4.5 mL, 42.2 mmol),
PMDETA (0.208 mL, 1.0 mmol), PBiB (0.205 g, 1.0 mmol), and anisole
(4.5 mL) was purged with argon. After 20 min, CuCl (0.10 g,1.0 mmol)
was added under argon, and the flask was purged with argon for
another 5 min. The polymerization was carried out at 40ꢀC. Small
samples (about 0.1 ml) were taken out from the reaction flask to
check the conversion, which was measured by 1H NMR. At the end of
the polymerization reaction, the reaction solution obviously became
viscous. Final conversion determined by 1H NMR reached 57.5%. The
reaction was stopped by opening to the air, and THF was added. After
passing through a basic alumina column, the solution was concen-
trated by a rotary evaporator. Afterwards, it was precipitated into
cold n-hexane to remove the residual monomer and other impuri-
2. Experimental section
2.1. Materials
Trisilanolheptaisobutyl polyhedral oligomeric silsesquioxane
(POSS-(OH)3) was purchased from Hybrid Plastics Company. Styrene
and methyl methacrylate were kindly donated by BASF SE, and were
passed through a silica column to remove the inhibitor. Azobisiso-
butyronitrile (AIBN) was recrystallized from ethanol. CuCl and CuBr
were respectively purified by stirring with acetic acid overnight.
After filtration, it was washed with ethanol and ether, and then dried
in vacuum oven at room temperature overnight. Other regents in
analytical grade were all obtained from Aldrich. Tetrahydrofuran
(THF) was distilled from a purple sodium ketyl solution.
ties. The polymer was dried under vacuum at 50ꢀC for 24 h (Mn,GPC
2770 g/mol, Mw/Mn ¼ 1.25).
¼
2.1.5. Preparation of tadpole-shaped POSS-containing hybrid poly
(methyl methacrylate) via CuBr-catalyzed click coupling (POSS-
PMMAATRP
)
A round-bottom glass flask (20 mL) with a magnetic stir bar was
charged with alkyne-PMMAATRP (0.50 g, 0.18 mmol), POSS-N3 (0.32
g, 0.36 mmol), PMDETA (0.075 mL, 0.36 mmol) and dioxane (10 mL)
was purged with argon. After 20 min, CuBr (0.052 g, 0.36 mmol)
was added under argon, and the flask was purged with argon for
another 5 min. The reaction was carried out at 50ꢀC. After 24 h, the
flask was opened, and THF was added to quench the reaction. The
solution was passed through a basic alumina column, and was
concentrated by a rotary evaporator. Afterwards, it was precipitated
into 500 mL of n-hexane to remove non-reactive POSS-N3 and other
impurities. The resulting product was dried under vacuum at 50ꢀC
for 24 h (Mn,GPC ¼ 5290 g/mol, Mw/Mn ¼ 1.21).
2.1.1. Synthesis of (3-chloropropyl)heptaisobutyl polyhedral
oligomeric silsesquioxane (POSS-Cl)
POSS-(OH)3 (5.0 g, 6.3 mmol) was dissolved with 100 mL of
absolute THF in two-neck flask with a magnetic stirring bar, and 1.76
mL of dry triethylamine was added. 3-Chloropropyltrichlorosilane
(1.97 mL,12.6 mmol) was quickly charged into the flask with ice-bath
using an argon-purged syringe. The mixture was allowed to stir for 1
h at 0ꢀC and 5 h at room temperature. After removing the salt
byproduct by filtration, the solutionwas concentrated to about 20 mL
by a rotary evaporator. The condensed solution was precipitated into
200 mL of acetonitrile twice, and the resulting product was dried
under vacuum at 50ꢀC for 24 h to give 4.80 g of POSS-Cl, 85.0% yield.
1H NMR (CDCl3, ppm): 3.54 (t, 2H, eSieCH2CH2CH2Cl), 1.88 (m,
9H, eSieCH2CH(CH3)2, eSieCH2CH2CH2Cl), 0.98 (d, 42H, eSieCH2CH
(CH3)2), 0.76 (m, 2H, eSieCH2CH2CH2Cl), 0.63 (q, 14H, eSieCH2CH
2.1.6. Preparation of alkyne-terminated polystyrene via ATRP
(alkyne-PSATRP
)
The preparation of alkyne-PSATRP is similar to that of alkyne-
PMMAATRP (Mn,GPC ¼ 8350 g/mol, Mw/Mn ¼ 1.16). Here, PMDETA/
CuBr/CuBr2 was used as catalyst at a ratio of 1.0:0.9:0.05.
(CH3)2). 13C NMR,
d (TMS, ppm): 47.65, 26.07, 24.25, 22.87, and 10.16.
Elemental analysis calcd for C31H69ClO12Si8 (894): C, 41.65; H, 7.78.
Found: C, 41.95; H, 7.65. Melting point: 266.3ꢀC.
2.1.7. Preparation of tadpole-shaped POSS-containing hybrid
polystyrene via CuBr-catalyzed click coupling (POSS-PSATRP
)
2.1.2. Synthesis of (3-azidopropyl)heptaisobutyl polyhedral
The preparation of POSS-PSATRP is similar to that of POSS-
oligomeric silsesquioxane (POSS-N3)
PMMAATRP (Mn,GPC ¼ 10,210 g/mol, Mw/Mn ¼ 1.15).
POSS-Cl (3.0 g, 3.4 mmol) and sodium azide (2.2 g, 34 mmol)
were reacted in 26.5 mL mixed solution of DMF/THF (v/v,1/2) at 80ꢀC
for 48 h. The reaction mixture was cooled to room temperature, and
the solvents were removed under pressure by heating evaporation.
The product was dissolved in 50 ml dichloromethane, and washed
with 250 mL of saturated NaCl aqueous solution twice. The organic
layer was separated, dried over MgSO4, and filtered. The solution was
concentrated to obtain 2.8 g of POSS-N3, 92% yield. 1H NMR (CDCl3,
ppm): 3.26 (t, 2H, eSieCH2CH2CH2Cl), 1.88 (m, 7H, eSieCH2CH
(CH3)2) 1.73 (t, 2H, eSieCH2CH2CH2Cl), 0.98 (d, 42H, eSieCH2CH
(CH3)2), 0.71 (m, 2H, eSieCH2CH2CH2Cl), 0.63 (q, 14H, eSieCH2CH
2.1.8. Preparation of S-1-dodecyl-S0-(
trithiocarbonate (DDAT)
a,a a
0-dimethyl- 00-acetic acid)
DDAT was prepared according to the literature [39]. 1H NMR
(CDCl3, ppm): 0.90 (t, 3H, e(CH2)11CH3); 1.20e1.48 (m, 18H, e(CH2)9
CH3); 1.64e1.83 (m, 8H, eSC(CH3)2COOH and eCH2(CH2)9CH3), and
3.30 (t, 2H, eCH2(CH2)10CH3).
2.1.9. Preparation of alkyne-terminated S-1-dodecyl-S0-(
a, -
a0
dimethyl-
a
00-acetic ester chloride) trithiocarbonate (alkyne-DDAT)
The preparation of alkyne-DDAT has reported by Brittain et al.
(CH3)2). 13C NMR,
d
(TMS, ppm): 54.02, 26.06, 24.26, 22.84, and 9.70.
[40]. 1H NMR (CDCl3, ppm): 0.87 (t, 3H, e(CH2)11CH3); 1.14e1.46

W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
2135
Scheme 1. Synthesis of (3-azidopropyl)heptaisobutyl polyhedral oligomeric silsesquioxane (POSS-N3).
(m, 18H, e(CH2)9CH3); 1.55e1.80 (m, 8H, eSC(CH3)2COOH and
eCH2(CH2)9CH3); 2.46 (t, 1H, eOCH2C^CH), 3.26 (t, 2H,
eCH2(CH2)10CH3), 4.69 (m, 2H, eOCH2C^CH).
added. The solution was precipitated into 250 mL of hexane. The
polymer was filtered and dried under vacuum at 50ꢀC for 24 h
(Mn,GPC ¼ 6860 g/mol, Mw/Mn ¼ 1.52).
2.1.10. Preparation of alkyne-termined poly(methyl methacrylate)
2.1.11. Preparation of tadpole-shaped POSS-containing hybrid poly
(methyl methacrylate) via CuBr-catalyzed click coupling (POSS-
using DDAT-alkyne via RAFT polymerization(alkyne-PMMARAFT
)
A round-bottom glass flask (20 mL) with a magnetic stir bar was
charged with MMA (3.20 mL, 30 mmol), DDAT-alkyne (0.10 g, 0.26
mmol), AIBN (0.014 g, 0.085 mmol) and 6.0 mL of toluene. The
mixture solution was purged with argon for 30 min, and then the
glass tube was sealed under argon. The polymerization was carried
out in a thermostated oil bath at 70ꢀC. Small samples (about 0.1 ml)
were taken out from the reaction flask at intervals to check the
conversion measured by 1H NMR. At the end of the polymerization
reaction, the polymerization was stopped by plunging the tube into
ice water. The polymerization flask was opened, and THF was
PMMARAFT)
A round-bottom glass flask (20 mL) with a magnetic stir bar was
charged with alkyne-PMMARAFT (0.5 g, 0.073 mmol), POSS-N3 (0.13
g, 0.14 mmol), PMDETA (0.030 mL, 0.14 mmol) and dioxane (10 mL)
was purged with argon. After 20 min, CuBr (0.021 g, 0.14 mmol) was
added under argon, and the flask was purged with argon for
another 5 min. The reaction was carried out at 50ꢀC. After 24 h, the
flask was opened, and THF was added to quench the reaction. The
solution was passed through a basic alumina column, and was
concentrated by a rotary evaporator. Afterwards, then precipitated
into 500 mL n-hexane. The precipitated solution was kept in cool-
ing-room (4ꢀC) overnight, and was centrifuged at 3000 rpm for 10
min. The resulting product was dried under vacuum at 50ꢀC for 24 h
(Mn,GPC ¼ 9080 g/mol, Mw/Mn ¼ 1.46).
2105 cm-1
3174 cm-1
POSS-N3
a
b
a
POSS-Cl
f
d
c
O
Si
O
Si
POSS-(OH)3
O
O
O
Cl
e
Si
Si
O
O
O
Si
O
O
Si
O
O
Si
Si
c
b,e
f
d
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
ppm
5
4
3
2
1
Fig. 2. 1H NMR of POSS-Cll oligomeric silsesquioxane (POSS-Cl).
Fig. 1. FT-IR of POSS-(OH)3, POSS-Cl and POSS-N3.

2136
W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
2.1.13. Preparation of tadpole-shaped POSS-containing hybrid
polystyrene via CuBr-catalyzed click coupling (POSS-PSRAFT
The preparation of POSS-PSRAFT is similar to that of POSS-
PMMARAFT (Mn,GPC ¼ 4550 g/mol, Mw/Mn ¼ 1.11).
a
b
)
f
d
a
c
O
Si
O
Si
O
N3
O
O
e
Si
Si
O
O
2.2. Methods
O
Si
O
O
Si
O
O
Si
1H and 13C nuclear magnetic resonance spectroscopy (NMR)
measurements were carried out on a Bruker AC-250 instrument
with tetramethylsilane (TMS) as an internal reference.
Fourier transform infrared spectroscopy (FT-IR) measure-
ments were conducted on a PERKINeELMER Spectrum One FT-
IR spectrophotometer equipped with an ATR sampling unit
(25ꢀC).
Si
c
f
b
d
e
Elemental analyses were performed on a Vario Elementar EL III.
Gel permeation chromatography (GPC). The molecular weights
and molecular weight distribution were measured by conventional
5
4
3
2
1
ppm
Fig. 3. 1H NMR of POSS-N3.
GPC. Column set: 5 m
m SDV gel, 102, 103, 104, and 105 Å, 30 cm each
(PSS, Mainz). Detectors used are RI and UV operated at 254 nm.
Polystyrene and PMMA standards, respectively (PSS, Mainz), with
narrow molecular weight distribution were used for the calibration
of the column set, and tetrahydrofuran (THF) was used as the eluent
at a flow rate of 1 mL/min.
2.1.12. Preparation of alkyne-terminated polystyrene via RAFT
polymerization (alkyne-PSRAFT
The preparation of alkyne-PSRAFT is similar to that of alkyne-
PMMARAFT (Mn,GPC ¼ 3470 g/mol, Mw/Mn ¼ 1.10).
)
Scheme 2. Synthesis of tadpole-shaped PMMA and polystyrene, respectively, via click chemistry.

W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
2137
Table 1
respectively (Scheme 2). The alkyne-functionalized typical ATRP
initiator propargyl 2-bromoisobutyrate (PBiB) was synthesized
according to Tsvarevski et al. [38], and it was respectively applied to
synthesize alkyne-terminated PMMA and PS via ATRP in presence of
Cu(I)/PMDETA as catalyst (Figure S1, Supporting Information). In this
case, we attempt to prepare alkyne-terminal PMMA with a lower
molecular weight, so that the cycloaddition reaction can be easily
detected. As we expected, alkyne-terminal PMMA with a short chain
was obtained (see Table 1). Although the stretching peak of alkyne
group is very weak in the FT-IR spectrum of alkyne-PMMA (Fig. 4), the
1H NMR clearly showed the signals of alkyne protons at 4.68 and 2.50
ppm (Fig. 5a).
GPC characterization of PMMA and polystyrenes before and after click reaction.
c
c
Conv., x (%)
103 Mn,th,
g/mol
103 Mn,GPC
g/mol
,
Mw/Mn
Alkyne-PMMAATRP
POSS-PMMAATRP
Alkyne-PSATRP
POSS-PSATRP
Alkyne-PMMARAFT
POSS-PMMARAFT
Alkyne-PSRAFT
POSS-PSRAFT
57.5
e
2.64a
3.54b
8.38a
9.28b
5.37a
6.27b
3.05a
3.95b
2.8
5.3
8.4
10.2
6.9
9.1
1.25
1.21
1.16
1.15
1.52
1.46
1.10
1.11
34.0
e
42.7
e
32.5
e
3.5
4.6
a
Mn,th ¼ [M]0x/[I]0 ꢂ Mmonomer þ Minitiator, where x is the monomer conversion.
b
c
The click cycloaddition of azide/alkyne was performed at 50ꢀC for
24 h using CuBr/PMDETA as catalyst and dioxane as solvent. To
complete the coupling reaction, the initial molar ratio of POSS-N3 to
alkyne-PMMA was 2:1. The unreactive POSS-N3 is removed by
precipitating the resulting product into n-hexane, since POSS-N3 is
well soluble in n-hexane. The click reaction was confirmed by 1H
NMR and FT-IR. Since the molecular weight of the PMMA block is
very low, the proton signals of the POSS-containing PMMA (POSS-
PMMAATRP) are clearly displayed in Fig. 5b. Except for the over-
lapping region of the proton signals about from 0.68 to 2.2 ppm, the
Estimated as Mn,th ¼ Mn,th(alkyne-Polymer) þ Mazido-POSS
.
Measured by GPC calibrated with linear PMMA and PS standards, respectively.
3. Results and discussion
3.1. Synthesis of Azido-POSS
To prepare the azido-functionalized POSS molecule, (3-chloro-
propyl)heptaisobutyl polyhedral oligomeric silsesquioxane (POSS-Cl)
was first synthesized as a precursor (Scheme 1). Trisilanolheptaiso-
butyl polyhedral oligomeric silsesquioxane (POSS-(OH)3) was corner-
capped with 3-chloropropyltrichlorosilane to afford POSS-Cl, The
chlorine group of POSS-Cl was converted to the azido group using
sodium azide in a mixture solution of DMF/THF (v/v, 1/2) at 80ꢀC for
other signal peaks are easily to be identified. The signals at
ppm (k) and 0.62 ppm (c) are respectively assigned to the methyl
d
¼ 3.61
1
24 h, which was confirmed by FT-IR and H NMR, respectively. The
broad peak at about 3174 cmꢁ1, corresponding to the stretching
vibration of three silica-hydroxyl groups, completely disappeared,
and a new peak appeared at 2105 cmꢁ1, which is ascribed to the
asymmetric stretching of the azido group (Fig. 1). The 1H NMR
also clearly reveals that POSS-Cl was successfully prepared
(Fig. 2). The characteristic signals at
d
¼ 3.54 and 0.98 ppm are
respectively assigned to the resonance of the methylene protons
(eSieCH2CH2CH2Cl) and methyl protons (eSiCH2CH(CH3)2) of POSS-
Cl. Compared to the 1H NMR spectrum of POSS-Cl and POSS-N3
(Fig. 3), it can be seen that the peak of the methylene protons shifts
from 3.54 ppm in the spectrum of POSS-Cl to 3.26 ppm in that of
POSS-N3.
3.2. ATRP using an alkyne-functional initiator
The alkyne-terminated polymers were prepared using an alkyne-
functionalized ATRP initiator and a RAFT chain transfer agent,
3282 cm-1
-1
1093 cm
POSS-PMMAATRP
alkyne-PMMAATRP
PBiB
4000
3500
3000
2500
2000
1500
1000
Wavenumber (cm-1)
Fig. 4. FT-IR of PBiB ATRP initiator, alkyne-PMMAATRP and POSS-PMMAATRP
.
Fig. 5. 1H NMR of alkyne-PMMAATRP (a) and POSS-PMMAATRP (b).

2138
W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
Fig. 6. Evolution of GPC chromatograms for alkyne-PMMAATRP and POSS-PMMAATRP (a); alkyne-PSATRP and POSS-PSATRP (b).
protons (eCOOCH3) and methylene protons (eSiCH2CH(CH3)2,
eSiCH2CH2CH2e). The integral ratio of these two peaks is well equal
to the theoretical value, which confirms the click reaction is
Table 1 shows that the molecular weight of the POSS-functional
polymermeasured by GPC is always higher than the theoreticalvalue.
We believe that the POSS-functional polymers have a different
hydrodynamic volume as compared to their corresponding precur-
sors. A similar observation was made in our previous work and
Mather’s work [4,27].
complete. Moreover, the signals at
d
¼ 5.20 ppm (g) and 4.35 ppm (f)
of methylene protons adjacent to the triazole ring can be clearly
discerned in the enlarged inset. FT-IR also confirmed that POSS-
PMMAATRP was successfully prepared. Compared to the FT-IR spec-
trum of alkyne-PMMAATRP, a new band appears at 1093 cmꢁ1 in the
spectrum of POSS-PMMAATRP, which is assigned to the stretching
vibration of SieOeSi (Fig. 4).
3.3. RAFT polymerization using an alkyne-functional chain transfer
agent
The preparation procedure of “tadpole-shaped” POSS-containing
hybrid polystyrene using ATRP (POSS-PSATRP) is similar to that of the
Alkyne-terminal polymers were also prepared using an alkyne-
functionalized RAFT agent (Scheme 2). We chose S-1-dodecyl-S0-(
a,
a
0-dimethyl- 00-acetic acid) trithiocarbonate (DDAT), as the
a
above POSS-PMMAATRP. The alkyne-functionalized polystyrene
(alkyne-PSATRP) was prepared using propargyl 2-bromoisobutyrate
(PBiB) as initiator and PMDETA/CuBr/CuBr2 as catalyst. The synthetic
steps were characterized by FT-IR and 1H NMR, which are respec-
tively shown in Figures S2 and S3 (Supporting Information). In the
FT-IR spectrum of POSS-PSATRP, it is also seen that a new peak
appears at 1110 cmꢁ1, which corresponds to the stretching vibration
of SieOeSi. In the 1H NMR spectrum of POSS-PSATRP, the signals of
precursor for this RAFT agent (alkyne-DDAT), since DDAT has a high
chain transfer efficiency and has been used in many RAFT poly-
merization systems [43,44]. In addition, it contains a carboxylic
group, which can be easily functionalized into other novel RAFT
agents [45]. Alkyne-DDAT was synthesized from DDAT and prop-
argyl alcohol [40]. 1H NMR and FT-IR verified that alkyne-DDAT
with high purity was successfully synthesized (Figures S7 and S8).
the characteristic protons derived from POSS are shown at
ppm and 0.62 ppm.
d
¼ 0.98
The signals at
d
¼ 4.69 ppm (b) and 2.45 ppm (c) are respectively
ascribed to the resonance of the alkyne group protons. The
stretching vibration peak at 3306 cmꢁ1 of the alkyne group clearly
shows in the FT-IR spectrum.
The GPC traces of the alkyne- and POSS-functional polymers are
shown in Fig. 6. In both cases the eluograms of the POSS-functional
polymers shift to a lower elution volume, which indicates an
increased molecular weight. However, in contrast to the GPC curve
of alkyne-PMMAATRP, the curve of POSS-PMMAATRP is not perfectly
symmetrical, showing a small shoulder towards lower elution
volumes. In the case of PS, the same shoulder appears in both
traces. This shoulder is assumed to stem from the radicaleradical
coupling of polymer chains during polymerization. However, we
cannot exclude that the alkyne group undergoes a coupling reac-
tion with a radical chain end. A similar shoulder was observed by
Tsarevsky et al. and Laurent and Grayson [38,41], when using an
alkyne-functional ATRP initiator.
In the case of PMMA one might argue that the shoulder in the
POSS-functional polymers is due to a side reaction in the click
reaction. In fact, adding CuBr/PMDETA as the catalyst for the
alkyne-azide cycloaddition will also form radicals at the chlorine-
terminated chain end. These chain ends might undergo coupling.
For that reason we removed the chlorine by using dodecylthiol as
a chain transfer agent (for details, see Supporting Information). This
did not lead to any improvement (Figure S4). Thus, coupling of the
chlorine-terminal chain ends during the click reaction is not the
reason for the shoulder. We prepared the silyl-protected ATRP
initiator, 3-(trimethylsilyl)propargyl 2-bromoisobutyrate (TMS-
PBiB), according to Grayson’s work [42] (see 1H NMR in Figure S5).
Again, this did not lead to an improvement (Figure S6).
Fig. 7. 1H NMR of POSS-PMMARAFT
.

W. Zhang, A.H.E. Müller / Polymer 51 (2010) 2133e2139
2139
Fig. 8. Evolution of GPC chromatograms for alkyne-PMMARAFT and POSS-PMMARAFT (a); alkyne-PSRAFT and POSS-PSRAFT (b).
The RAFT polymerizations of MMA and styrene were carried out
References
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shown above for the polymers made by ATRP. The reaction was also
performed in dioxane, and the feed ratio of POSS-N3 to alkyne-
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solution filtered through an alumina column, and was then
precipitated in excess n-hexane. Then precipitated solution was
kept at 4ꢀC overnight, and was centrifuged to afford the resulting
product. The click reaction was also verified by FT-IR and 1H NMR.
The peak of the SieOeSi stretching vibration respectively appears
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signals of the methylene protons originating from POSS
((eSieCH2CH(CH3)2), eSieCH2CH2CH2e) at 0.62 ppm (c, d). The
signals of the methylene protons adjacent to the triazole ring still
can be identified in the enlarged graph.
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Acknowledgement
This work was supported by the National Natural Science
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Appendix. Supplementary information
Supplementary information associated with this article can be
found in the on-line version, at doi:10.1016/j.polymer.2010.03.028.