Synthesis and characterization of novel liquid crystalline polystyrene

Article abstract of DOI:10.1246/cl.130267

The paper introduces a simple method to synthesize novel liquid crystalline polystyrene (LC-PS) containing a mesogenic unit at a side chain by hydrosilylation through poly[(4-vinylphenyl)dimethylsilanol] (PVPDMS) and liquid crystal monomer cholesteryl 4-[4-(undec-10-enoyloxy)benzoyloxy]-phenyladipate (M). The LC-PS had high purity, high molecular weight, narrow distribution, good thermal stability, and typical thermotropic liquid crystallinity (S_A phase), as shown by ¹HNMR analyses, size exclusion chromatography (SEC) thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction (XRD). The glass-transition temperature (T_g) of LCPS was much lower than that of PVPDMS, while the clearing point (T_i) of LC-PS significantly increased compared with M.

Full text of DOI:10.1246/cl.130267

doi:10.1246/cl.130267
Published on the web May 30, 2013
915
Synthesis and Characterization of Novel Liquid Crystalline Polystyrene
Bai Wang,1,2 Hong Wei Ma,1,2 Yan Shai Wang,1,2 and Yang Li*1,2
State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering,
1
School of Chemical Engineering, Dalian University of Technology, Dalian 116024, P. R. China
Liaoning Key Laboratory of Polymer Science and Engineering, School of Chemical Engineering,
Dalian University of Technology, Dalian 116024, P. R. China
2
(
Received March 27, 2013; CL-130267; E-mail: liyang@dlut.edu.cn)
The paper introduces a simple method to synthesize novel
liquid crystalline polystyrene (LC-PS) containing a mesogenic
unit at a side chain by hydrosilylation through poly[(4-
vinylphenyl)dimethylsilanol] (PVPDMS) and liquid crystal
monomer cholesteryl 4-[4-(undec-10-enoyloxy)benzoyloxy]-
phenyladipate (M). The LC-PS had high purity, high molecular
weight, narrow distribution, good thermal stability, and typical
thermotropic liquid crystallinity (SA phase), as shown by
(VPDMS) can be directly polymerized by anionic initiator as a
silyl-hydride functional monomer without the tedious protec-
tion­deprotection chemistries. Therefore, the target product can
be obtained by using the hydrosilylation reaction of the side
chain of preformed anionically polymerized functional main
chain and liquid crystal monomer with the ethenyl-terminal
group.
The synthesis of liquid crystal monomer cholesteryl 4-[4-
(undec-10-enoyloxy)benzoyloxy]phenyladipate (M) was pre-
1
H NMR analyses, size exclusion chromatography (SEC),
thermogravimetry analysis (TGA), differential scanning calo-
rimetry (DSC), polarizing optical microscopy (POM), and X-ray
1
sented in ref 13. Yield: 86%. H NMR (CDCl3): ¤ 0.67­2.61
[m, 67H, ­(CH2)8­, ­(CH2)4­, and cholesteryl-H], 4.62 (m, 1H,
diffraction (XRD). The glass-transition temperature (T ) of LC-
­CH­ in cholesteryl), 5.02 (m, 2H, ­CH =CH­), 5.37 (d, 1H,
g
2
PS was much lower than that of PVPDMS, while the clearing
point (Ti) of LC-PS significantly increased compared with M.
>C=CH­ in cholesteryl), 5.80 (m, 1H, CH2=CH­), 7.13­8.23
(m, 8H, Ar­H). HRMS calcd for C57H80O8 893.2436, found:
+
8
93.2383 ([M] , MALDI-TOF-MS).
The synthesis of VPDMS was presented in ref 16. A typical
In previous years, considerable attention has been focused
on side-chain liquid crystalline polymers because of their
application potential that combined the characteristics of macro-
molecules, such as processability and glassy behavior, with the
anionic polymerization reaction was performed in a Schlenk
flask at 50 °C with s-BuLi as the initiator in cyclohexane used
VPDMS as monomer. The reaction mixture was quenched with
degassed 2-propanol after 4 h. The homopolymer was precipi-
tated by adding a large amount of ethanol and freeze-dried.
1
,2
electrooptical properties of low-molar-mass mesogens. Gen-
erally speaking, there are three essential structural units for side-
chain liquid crystalline polymers: the mesogenic group, which is
attached as a pendant to the main chain via the spacer, the
flexible spacer, and the polymeric backbone. Polystyrene is a
candidate of main chain with several obvious advantages: high
modulus, tough resins, transparent grade to rubber modified, and
blends with outstanding impact resistance and mechanical
1
Yield: 98%. H NMR (CDCl ): ¤ 0.31 [d, 6H, Si(CH ) ], 1.36­
3
3 2
1.81 [m, 3H, (­CH2­CH­)n], 4.40 (p, 1H, Si-H), 6.21­7.26 (m,
4H, Ar­H).
0.5 g of
M (0.56 mmol) and 0.089 g of PVPDMS
(0.55 mmol) were dissolved in 25 mL of toluene, in which 2
drops of Karstedt’s catalyst were added (Scheme 1). The mixture
was allowed to proceed for 48 h at 60 °C and was then stirred
with Quadra Pure TU overnight to remove the catalyst. The
resultant colorless solution was filtered, and the crade product
was precipitated by pouring it into anhydrous methanol. The
solid precipitate was purified by extractions with ethyl alcohol
for 72 h to remove the excess M. The product was dried in
vacuum oven to give LC-PS as a white powder (0.54 g). Yield:
3
properties, and low refractive index. Therefore, liquid crystal-
line polystyrene (LC-PS) is of considerable interest for
applications such as information storage, nonlinear optics, and
liquid crystal display.4 The existing LC-PS is currently
,5
6
­8
prepared either from mesogenic monomers
or by grafting
The former is
LC moieties onto preformed polymers.4
,5,9­12
1
usually a cumbersome and high-cost solution process that often
results in poor yields, while the molecular weight of product is
uncontrollable. For the latter, quantitative conversion is difficult
to achieve, and the polydispersity of product is wide because of
not only the incomplete reaction but also the wide molecular-
weight distribution of parent polymers at the root.
93%. H NMR (CDCl ): ¤ 0.18 [d, 6H, Si(CH ) ], 0.67­2.58 [m,
3
3 2
74H, (­CH2­CH­)n, ­(CH2)10­, ­(CH2)4­, and cholesteryl-H],
4.63 (m, 1H, ­CH­ in cholesteryl), 5.36 (d, 1H, >C=CH­ in
cholesteryl), 6.31­8.16 (m, 12H, Ar­H).
CH2 CH
CH2 CH
n
n
Our interest has been in the design of a kind of LC-PS with
high molecular weight and narrow distribution by a compara-
tively simple method. Hydrosilylation with the advantages of
mild reaction conditions, highly efficient, quantitative reaction,
and high purity of the product has been widely used in the
synthesis of side-chain liquid crystalline polymers,13 and the
living anionic polymerization was proved to be most successful
in controlling and resulting in a narrow molar mass distribu-
(1)Mg
2)SiH(CH ) Cl
(1)s-BuLi
(2)CH OH
M
Pt
(
3
2
3
SiH
Cl
SiH
Liquid Crystalline side-chain
:
M: CH2=CH(CH2)8COO-
COO
2 4
OOC(CH ) COO
1
4
15
tion. In our previous study, (4-vinyiphenyl)dimethylsilanol
Scheme 1. Synthetic route of LC-PS.
Chem. Lett. 2013, 42, 915­917
© 2013 The Chemical Society of Japan
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9
16
Figure 1. 1H NMR spectra of M, LC-PS, and PVPDMS.
Figure 3. The second heating and the first cooling DSC curves
of PVPDMS, M, and LC-PS.
(
a)
(b)
Figure 4. Representative POM of the textures (magnification:
©200): (a) texture on heating to 162 °C for LC-PS; (b) texture on
cooling to 120 °C for LC-PS.
Figure 2. SEC curves of LC-PS and PVPDMS.
Table 1. SEC results of PVPDMS and LC-PS
Table 2. DSC results of LC-PS and M
Mn /©104
a
Mn /©104
b
Mw/Mn
a
Tg/°C
Tm/°C
Ti/°C
¦T /°C
PVPDMS
LC-PS
3.87
21.8
4.00
21.3
1.07
1.09
b
PVPDMS
LC-PS
M
96.2
35.8
®
®
®
91.0
®
209.0
180.6
®
173.2
89.6
a
Determined by SEC analysis (polystyrene standards). The Mn
of PVPDMS was calculated from monomer/initiator ratio, and
that of LC-PS was calculated from M/Si­H ratio.
a
¦T = Ti ¹ Tg (Tm).
in Table 2. The pictures of POM shows both M and LC-PS
exhibited a typical liquid crystal texture in the LC phase. The
three endothermic peaks on the DSC curve of M represented a
The 1H NMR spectra of compounds synthesized in this
work are shown in Figure 1. Compared with spectra of
PVPDMS, the peak at 4.4 ppm in spectra of LC-PS completely
disappeared. It showed that the Si­H group had 100%
participated in the hydrosilylation reaction, and compared with
spectra of M, the peaks at 5.02 and 5.80, which represent the
terminal vinyl group of M, are not observed in that of LC-PS.
This means that the purified LC-PS was obtained without
unreacted liquid crystal monomer M mixed.
melting transition (T ) at 91 °C, a smectic-to-cholesteric phase
m
transition at 137.4 °C, and a cholesteric-to-isotropic phase
transition at 180.6 °C. The DSC curves of LC-PS showed a
glass transition and LC to isotropic phase transition. All the
phase transitions of M and LC-PS were reversible, and their
corresponding peaks could be found at the cooling cycle. The
transition temperatures noted with DSC were consistent with
those observed with POM. This indicated that both M and LC-
PS were thermotropic round-trip liquid crystals. The mesophase
temperature range (¦T) of LC-PS was higher 83.6 °C than that
of M, which showed that the polymerization could anchor the
LC phase and that it expanded the mesophase temperature range
considerably. Tg of LC-PS was significantly reduced compared
with that of PVPDMS. A possible reason is that the mass of
alkyl chain in M grafted on PVPDMS diluted the rigid structure
of main chain, decreasing Tg of target product.
Molecular weights and molecular weight distributions of
PVPDMS and LC-PS were monitored by SEC (Figure 2 and
Table 1). The curve of LC-PS markedly shifted to the left and
Mn of LC-PS increased considerably comparing with that of
PVPDMS, which showed a large amount of M was grafted to
the polymer main chain. Compared with that of PVPDMS, the
M /M of LC-PS changed little, from 1.07 to 1.09. This
w
n
indicated that there was no distinct effect on the molecular
weight distribution of polymer by hydrosilylation and that the
number of M grafted on every main chain was approximately
equal. That is, a high-molecular-weight target product was
obtained, while keeping narrow molecular-weight distribution.
The mesomorphism of M and LC-PS was investigated with
DSC and POM (Figure 3 and Figure 4). Results are summarized
The thermo-decomposing temperature of PVPDMS is
276.4 °C, and that of LC-PS is 303.5 °C (Figure 5; 2% weight
loss), which exhibits good thermal stability. It is demonstrated
that the rigid group of M grafted on PVPDMS improved the
thermostability of target product.
Chem. Lett. 2013, 42, 915­917
© 2013 The Chemical Society of Japan
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9
17
(
SA phase). Tg of LC-PS was much lower than that of PVPDMS,
and its ¦T was noticeably widened compared with liquid
crystal monomer M. This showed that LC-PS holds promise
to influence a broad-temperature-range of applications as a
material.
This work was sponsored by the National Natural Science
Foundation of China (Nos. 21034001 and 21174021).
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© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/