Synthesis of liquid-crystalline star polymers with sulfonyl groups in the central core and selective degradation of their cores by base

Article abstract of DOI:10.1080/15421406.2013.876171

Star polymers with sulfonyl groups in the central core were prepared by ATRP and the degradation of their cores was investigated. The core of the star polymer was efficiently and selectively decomposed by piperidine. Furthermore, the star polymer with mes

Full text of DOI:10.1080/15421406.2013.876171

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Synthesis of Liquid-Crystalline Star
Polymers with Sulfonyl Groups in the
Central Core and Selective Degradation
of their Cores by Base
Yumiko Nakaa, Hitomi Kawamuraa & Takeo Sasakia
a Department of Chemical, Faculty of Science, Tokyo University of
Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, Japan
Published online: 27 May 2014.
To cite this article: Yumiko Naka, Hitomi Kawamura & Takeo Sasaki (2014) Synthesis of
Liquid-Crystalline Star Polymers with Sulfonyl Groups in the Central Core and Selective
Degradation of their Cores by Base, Molecular Crystals and Liquid Crystals, 593:1, 141-150, DOI:
10.1080/15421406.2013.876171
To link to this article: http://dx.doi.org/10.1080/15421406.2013.876171
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Mol. Cryst. Liq. Cryst., Vol. 593: pp. 141–150, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.876171
Synthesis of Liquid-Crystalline Star Polymers
with Sulfonyl Groups in the Central Core
and Selective Degradation of their Cores by Base
YUMIKO NAKA,∗ HITOMI KAWAMURA,
AND TAKEO SASAKI
Department of Chemical, Faculty of Science, Tokyo University of Science,
1-3 Kagurazaka, Shinjuku-ku, Tokyo, Japan
Star polymers with sulfonyl groups in the central core were prepared by ATRP and
the degradation of their cores was investigated. The core of the star polymer was
efficiently and selectively decomposed by piperidine. Furthermore, the star polymer
with mesogens in the side chain showed almost the same LC-isotropic phase transition
temperature as the resulting degradation product, demonstrating the independence of
polymer architecture on LC properties.
Keywords Atom transfer radical polymerization; degradable initiators; liquid crystals;
star polymers; sulfonyl groups
Introduction
Recently, well-defined polymers with sophisticated architectures have found increasing
attention and applications in functional materials [1]. In particular, star polymers composed
of a single branch point as the core and two or more linear chains (arms) are the simplest
models of branched polymers, and such polymers containing functional arms have been
prepared in hopes of attaining both functionality and unique properties arising from the
polymer architecture [2–4]. Star polymers can be classified into core-shell structured star
polymers and multi-arm star polymers based on the size of the central core. Core-shell
structured star polymers, which have a large number of arms tethered to a macroscopic
core of cross-linked or aggregated polymers, have been applied as drug carriers [5–7]
and reaction sites [8], for example, because they behave like ultrasoft colloidal particles
in solution [9] and functionality can be introduced in both the macroscopic core and
arms. The relationship between polymer architecture and properties has been explored
from both theoretical and experimental standpoints in multi-arm star polymers with small
molecule cores. Chen et al. reported that the incorporation of a bulky core in an eight-
arm star polymer inhibited the aggregation of polyfluorene arms as compared with linear
polymers, resulting in improvement of thermal and optoelectronic properties in organic
light-emitting diodes [10]. Kricheldorf et al. and Polk et al. demonstrated that a liquid
crystalline (LC) star polymer with mesogens in the main chain showed low crystallinity
∗Address correspondence to Yumiko Naka, Department of Chemical, Faculty of Science, Tokyo
University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan. Tel.: +81-3-5228-8277.
E-mail: sasaki@rs.kagu.tus.ac.jp
141

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Y. Naka et al.
due to its branched structure [11, 12]. In a side-chain LC polymer, it was observed that
the transition temperature was prone to decrease with increasing branching. Interestingly,
it was recently proven using well-defined branched polymers that the broad LC-isotropic
phase transition observed in general polymers resulted from the limited miscibility of the
mixture of branched structures rather than the wide polydispersity of molecular weight
[13–16]. Generally, side-chain LC polymers have been studied as advanced functional
materials due to their responsiveness to external stimuli induced by self-organizing na-
ture, cooperative motion, anisotropy and so on [17]. Furthermore, investigating the ef-
fects of polymer architecture on LC properties is particularly relevant in side-chain LC
polymers. However, it is difficult to directly explore slight influences of polymer archi-
tecture on LC properties in side-chain LC polymers because there are multiple factors
that affect the properties, such as molecular weight, type of end group, polydispersity, and
tacticity.
Degradable polymers have been extensively studied in the field of polymer science [18].
Degradability is generally introduced via the monomer repeating units, the cross-linkers,
or the initiated fragment in the polymer. We previously reported the depolymerization of
poly(olefin sulfone)s possessing sulfonyl groups as degradable groups in the main chain
[19]. The depolymerization was induced by the base abstraction of protons on carbon
atoms adjacent to the sulfonyl group and the subsequent chain reaction. These reactions
took place at ambient temperature in solution or films. Accordingly, strongly electron-
withdrawing sulfonyl groups may act as cleavable groups under mild conditions, although
they are not generally known as labile groups [18]. If well-defined star polymers with
sulfonyl groups in the core are cleaved at a precise central location, linear chains with
identical properties as one arm of the primary star polymers are produced, enabling us to
obtain a better understanding of the relationship between the polymer architecture and LC
properties without complicating factors such as polydispersity or tacticity. In this study,
four-arm star polymers with sulfonyl groups in the core were prepared by atom transfer
radical polymerization (ATRP), and their degradability was investigated by treatment with
bases. Furthermore, LC properties of a star polymer possessing cyanobiphenyl mesogens
in the side chain were compared with those of the linear polymer obtained by degradation
of the corresponding four-arm star polymer.
Experimental Methods
Synthesis of Initiator with Sulfonyl Groups
Tetrakis[(2-hydroxyethylthio)methyl]methane (Compound 1). A solution of 1,3-dibromo-
2,2-bis(bromomethyl)propane (4.0 g, 52 mmol) in 200 mL of methanol was added dropwise
over 30 min with stirring to a mixture of 2-mercaptoethanol (4.0 g, 52 mmol) and 11 M
NaOH (7.1 mL, 77 mmol). The reaction mixture was stirred at 90 ◦C for 3 hr. The solvent
was evaporated under vacuum, and the residue was poured in water and extracted with
diethyl ether. The combined ether extracts were washed with saturated NaClaq, dried over
anhydrous MgSO4, and filtered. The filtrate was evaporated under vacuum.
Tetrakis{[2-(2-bromoisobutyryloxy)ethylthio]methyl}methane (Compound 2). Unpurified
compound 1 (1.7 g, 4.6 mmol) was dissolved in dry tetrahydrofuran (T◦HF) and triethylamine
(9.3 mL, 92 mmol). The solution was slowly added dropwise at 0 C to a solution of 2-
bromoisobutyryl bromide (21 g, 92 mmol) in THF and the reaction mixture was stirred at
room temperature overnight. After the reaction, the solvent was evaporated under vacuum.

Star Polymers with Sulfonyl Groups in the Core
143
The product dissolved in chloroform was washed with saturated NaHCO3aq, dried over
anhydrous MgSO4, and filtered. The oil was purified by column chromatography (silica
gel, chloroform as eluent) to yield 4.8 g (> 12%) of light yellow oil. 1H NMR (300 MHz,
CDCl3, δ): 1.96 (s, 24H), 2.80 (s, 8H), 2.87 (t, J = 6.8 Hz, 8H), 4.33 (t, J = 6.8 Hz, 8H).
Tetrakis{[2-(2-bromoisobutyryloxy)ethylsulfonyl]methyl}methane (Compound 3). Com-
pound 2 (2.0 g, 2.1 mmol) was dissolved in 2 mL of a 50–50 mixture of glacial acetic
acid and acetic anhydride, and then 30% hydrogen peroxide (2.0 g, 17 mmol) was added
to the solution. The reaction mixture was stirred at 60 ◦C for 10 hr. After the reaction, the
product was extracted with chloroform, and then washed with saturated NaHCO3aq. The
chloroform solution was dried over anhydrous MgSO4 and filtered. Removal of the solvent
gave a residue which was dried at room temperature for a long time to afford a white solid.
1H NMR (500 MHz, CDCl3, δ): 1.95 (s, 24H), 3.49 (t, J = 5.6 Hz, 8H), 4.23 (s, 8H), 4.63
(t, J = 5.6 Hz, 8H); GCMS (FAB) m/z: [M]+ calcd for C28H38N2O3, 1100.56; found, 1101.
Anal. calcd. for C28H38N2O3: C 31.65, H 4.40, O 23.26, Br 29.04, S 11.65; found: C 31.55;
H 4.03.
Synthesis of Star Polymers
Star-PMMA. Star polymers were prepared by ATRP using compound 3. To remove the
stabilizer (hydroquinone, 0.005%), commercially available methyl methacrylate (MMA,
98%) was washed with an aqueous solution of sodium sulfite (pH 4), sodium hydrox-
ide (5%), and sodium chloride (5%), then stirred over anhydrous magnesium sulfate
overnight and distilled under vacuum. Copper (I) bromide [Cu(I)Br, Wako Pure Chemical
Industries Ltd.] as catalyst was purified as described in the literature [20]. 1,1,4,7,10,10-
Hexamethyltriethlyenetetramine (HMTETA, Aldrich) and dry THF were used without
further purification. After compound 3 (0.100 g, 0.092 mmol), MMA (3.87 g, 38.7 mmol),
Cu(I)Br (13.2 mg, 0.093 mmol), HMTETA (25.3 μL, 0.093 mmol), and THF (1.5 mL)
were mixed, the mixture was degassed using freeze-pump-thaw cycles and sealed under
nitrogen, then stirred for 30 min at room temperature. The reaction was carried out in a
preheated oil bath at 90 ◦C for 3 hr. Next, the ampule was opened and quickly placed in an
ice bath. The polymer solution was passed through a column (silica gel) with chloroform as
eluent to remove the catalyst, and star-PMMA was precipitated in a large excess of hexane
and finally dried under vacuum. A white solid product was obtained in a yield of 67%.
Star-PCB11. An LC star polymer was prepared by ATRP in a similar manner as
above. First, an LC monomer, 11-[4-(4-cyanobiphenyl)oxy]undecyl methacrylate (CB11),
was synthesized by the Scho¨tten-Baumann reaction between methacryloyl chloride and
1
(4-cyano-4ꢀ-biphenyloxy)undecyl-1-ol. H NMR (300 MHz, CDCl3, δ): 1.23–1.84 (m,
34 H), 3.65 (t, J = 7 Hz, 2H), 4.03 (t, J = 7 Hz, 4H), 4.16 (t, J = 7 Hz, 2H), 5.81
(dd, J = 10, 2 Hz, 1H), 6.12 (dd, J = 17, 10 Hz, 1H), 6.40 (dd, J = 17, 2 Hz, 1H), 6.98 (m,
4H), 7.86 (m, 4H); Anal. calcd. for C28H35NO3: C 77.56, H 8.14, N 3.23; found: C 77.24,
◦
H 8.02, N 2.96. The DSC thermograms indicated phase transition peaks at 72 and 74 C
upon heating, and 63 ◦C upon cooling at a scanning rate of 3 ◦C/min. Compound 3 (2.6 mg,
2.4 × 10−6 mol), CB11 (0.201 g, 0.46 mmol), Cu(I)Br (4.9 mg, 0.035 mmol), HMTETA
(9.4 μL, 0.035 mmol), and THF (2.0 mL) were mixed in a 50 mL ampule equipped with
a reflux condenser, which was degassed and filled with nitrogen. The mixture was stirred
◦
at 80 C for 24 hr and purified in a similar manner as above. A white solid product was
obtained in a yield of 65%.

144
Y. Naka et al.
Scheme 1. Synthetic route of an initiator with sulfonyl groups.
Results and Discussion
A multi-site ATRP initiator with sulfone groups was synthesized in three steps as
shown in Scheme 1: introduction of sulfide (-S-) by the reaction between a halide and
thiol, addition of bromo end groups by Scho¨tten-Baumann reaction, and oxidation of

Star Polymers with Sulfonyl Groups in the Core
145
O
O
Br
Br
R
n
O
O
O
O
O
R
O
O
O
S
S
S
S
O
O
O
O
O
O
O
O
O
O
Br
S
O
S
O
Br
Br
S
O
S
O
Br
n
O
O
O
O
n
O
O
O
O
R
O
O
O
O
O
O
Br
O R
n
Br
O
CH3
Star-PMMA
Star-PCB11
CN
R =
O
11
Scheme 2. Synthetic route of star polymer by ATRP.
sulfide [21–23]. In the first step, a simple compound with four halides, 1,3-dibromo-2,2-
bis(bromomethyl)propane, was reacted with 2-mercaptoethanol in an aqueous alkaline
solution. The obtained product (compound 1) was used for the next reaction without pu-
rification because of the ill-smelling unreacted starting material, 2-mercaptoethanol. Thus,
a large excess of 2-bromoisobutyryl bromide was added to the system to complete the
reaction, and the obtained product was purified by column chromatography on silica gel.
Finally, the sulfide was oxidized using a mixture of glacial acetic acid and acetic anhydride
1
with hydrogen peroxide. The introduction of sulfone groups was confirmed by H-NMR
1
and IR spectra. In the H-NMR spectrum, resonances from the two neighboring protons
of the sulfide shifted over 1.4 and 0.6 ppm after the reaction on account of the strong
electron-withdrawing sulfone group [Fig.1(a)]. Furthermore, two strong peaks correspond-
ing to sulfone groups at 1130 and 1280 cm−1 (O S O) appeared in the FT-IR spectrum
[Fig.1(b)].
Star poly(methyl methacrylate) (star-PMMA) with a unimodal peak [Mn = 39,000 and
Mw/Mn = 1.19 (GPC)] was prepared by ATRP using compound 3 with four initiation sites
(Scheme 2). To investigate the decomposition behavior of star-PMMA, the polymer (26 mg)
was dissolved in chloroform (2 mL), and then base was added to the polymer solution. After
stirring for 24 hr at room temperature, the polymer was precipitated in a large excess of
hexane to remove the base. First, piperidine, which promotes effective depolymerization
in poly(olefin sulfone)s, was selected as the base. Various amounts of piperidine (10.0,
5.0, 2.0, 0.99, 0.55, 0.09, 0.04, and 0.01 mmol) were added to the star-PMMA solution. In
the products obtained by addition of over 0.04 mmol of piperidine, GPC peaks appeared
at longer elution times than that of the original polymer as shown in Fig. 2(a), indicating
decomposition of the star polymers by piperidine. Next, decomposition behavior by other
common bases such as pyridine and triethylamine was investigated [Fig.2(b)]. When star-
PMMA was treated with sufficient quantities of pyridine (10 mmol), the GPC elution time
did not change. On the other hand, part of the star PMMA decomposed in polymers treated
with triethylamine. Upon treating the original polymer (Mn = 39,000, Mw/Mn = 1.19) with

146
Y. Naka et al.
Figure 1. Confirmation of introduction of sulfonyl groups. (a) NMR spectra of compounds 2 and 3.
(b) IR spectra of compound 3.
piperidine under the same conditions, a polymer with well-defined molecular weight Mn
of 16,000 and Mw/Mn of 1.22 was formed. The 1H-NMR spectrum of the cleaved polymer
agreed closely with that of the original polymer, with the chemical structure of the repeating
units remaining unchanged (Fig. 3). These results suggest that selective cleavage of the
central core occurs in star-PMMA with sulfonyl groups without decomposition of the main
polymer chain.
Next, an LC polymer bearing cyanobiphenyl moieties in the side chain (star-PCB11)
was synthesized by ATRP. The resulting polymer with Mn of 34,000 and narrow molar-mass
dispersity of 1.26 was decomposed under the same conditions as star-PMMA. As shown
in Fig. 4, the degradation product showed a unimodal peak in the GPC curve and small
Mn of 16,000 (Mw/Mn = 1.22). 1H-NMR and IR spectra of the polymer were unchanged
after addition of piperidine (Fig. 5). From these results, the decomposition of both the main
chain and side chain was confirmed. Mesomorphic properties of the original star-PCB11
and degradation product were studied by DSC and polarizing microscopy (POM). Both

Star Polymers with Sulfonyl Groups in the Core
147
Figure 2. GPC curves of original star-PMMA and its degradation product. (a) To a star-PMMA
(26 g) solution was added 0.01, 0.04, 0.09, 0.55, 0.99, 2.0, 5.0, and 10.0 mmol of piperidine. (b)
Star-PMMA upon addition of pyridine, piperidine, and triethylamine in chloroform solution.
Figure 3. NMR spectra of star-PMMA and its degradation product.
Figure 4. GPC curves of original star-PCB11 and its degradation product.

148
Y. Naka et al.
Figure 5. IR spectra of star-PCB11 and its degradation product.
the polymers showed focal conic texture in the POM studies, corresponding to a smectic
A phase. The glass transition (Tg) and LC-isotropic phase transition temperatures (TLC-iso
)
of the degraded product were almost the same as those of the original star-PCB11 (Fig. 6).
The polymer obtained by the degradation of the branched polymer is completely identical
to the arm of the branched polymer itself (Table 1). This means that only the effects of the
branched structure of the polymer on Tg and TLC-iso contribute to the result. Thus, it can be
Figure 6. DSC curves (a) and polarizing optical micrographs (b) of star-PCB11 and its degradation
product.

Star Polymers with Sulfonyl Groups in the Core
149
Table 1. Glass-Transition Temperatures of Star-PMMA, the Degradation Product of Star-
PMMA, Star-PCB11, and the Degradation Product of Star-PCB11
Star-PMMA
Star-PCB 11
Before degradation
After degradation
118
118
51
48
concluded that molecular weight and branched structures of the polymers had no effect on
the transition temperature.
Conclusions
A new compound that possesses four ATRP initiation sites and degradable sulfonyl groups
was synthesized. The compound was designed to study decomposition of polymers by bases.
The four-arm star poly(methacrylate) with well-defined molecular weight was prepared by
ATRP using the obtained initiator. The addition of triethylamine or piperidine to the solution
of the star poly(methyl metacrylate) at room temperature produced linear polymers. On
the other hand, addition of pyridine did not induce decomposition of the star polymer even
when an excessive amount was used. Moreover, liquid-crystalline (LC) properties of the star
polymer with cyanobiphenyl mesogens in the side chain were investigated. They showed
almost the same LC-isotropic phase transition temperature as the corresponding linear
arm polymer with the same polydispersity and tacticity, exhibiting no effect of polymer
architecture on LC properties.
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