Synthesis and properties of reactive liquid crystal monomers

Article abstract of DOI:10.1080/15421406.2013.766943

A new series of nematic liquid crystal (LC) monomers containing double bonds in the side chain were designed and synthesized via the Steglich esterification reaction. Length of the side groups varied from 1 to 2 methylene units. The molecular structures o

Full text of DOI:10.1080/15421406.2013.766943

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Synthesis and Properties of Reactive
Liquid Crystal Monomers
Jianqiang Xie a , Guangming Zhu a , Yusheng Tang a & Zengping
Zhang b
a Department of Applied Chemistry , Northwestern Polytechnical
University , Xi’an , Shaanxi , P. R. China
b Key Laboratory for Special Area Highway Engineering of Ministry of
Education , Chang’an University , Xian , Shaanxi , P. R. China
Published online: 22 Apr 2013.
To cite this article: Jianqiang Xie , Guangming Zhu , Yusheng Tang & Zengping Zhang (2013): Synthesis
and Properties of Reactive Liquid Crystal Monomers, Molecular Crystals and Liquid Crystals, 575:1,
40-48
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Mol. Cryst. Liq. Cryst., Vol. 575: pp. 40–48, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.766943
Synthesis and Properties of Reactive Liquid
Crystal Monomers
JIANQIANG XIE,1,∗ GUANGMING ZHU,1 YUSHENG TANG,1
AND ZENGPING ZHANG2
1Department of Applied Chemistry, Northwestern Polytechnical University,
Xi’an, Shaanxi, P. R. China
2Key Laboratory for Special Area Highway Engineering of Ministry of
Education, Chang’an University, Xian, Shaanxi, P. R. China
A new series of nematic liquid crystal (LC) monomers containing double bonds in the
side chain were designed and synthesized via the Steglich esterification reaction. Length
of the side groups varied from 1 to 2 methylene units. The molecular structures of the
intermediates and the LC monomers were confirmed by Fourier transform infrared spec-
trum, elemental analysis, and nuclear magnetic resonance spectroscopy. The thermal
phase behavior of the LC monomers was investigated by differential scanning calorime-
try and polar optical microscopy coupled with hot stage. All of these LC monomers
showed only one nematic mesophase at room temperature during the cooling process.
Keywords Liquid crystal; nematic; reactive liquid crystal; synthesis
1. Introduction
The use of liquid crystals (LCs) in active-matrix display devices has stimulated the devel-
opment of synthetic approaches. Researchers have designed a number of functional LCs
[1–10]. LCs with C C are very important because of their low viscosity, low melting
point, high clearing points, and good low-temperature stability. In addition, the reactivity
of double bond in LCs is also very important. For example, it can be oxidized into epoxy
groups to prepare LC adhesive or triggered to self-polymerize to produce LC polymers.
Double bond can also be connected to the polymethylhydrosiloxane to form side-chain LC
polymers through the hydrosilylation reaction [11–17].
The position of the double bond is usually in the main chain. A few studies have been
conducted on LC monomer containing a double bond in the side chain. Patrick Keller
et al. prepared an LC elastomer using the LC monomer containing a double bond in the
side chain. The elastomer displayed a maximum reversible deformation amplitude of up
to 40% [18,19]. Due to the location of double bond in LC, the thermal motions of the
main chain have less interference in the organization of mesogenic units than it would have
if the attachment was at one of the ends [20–24]. In this article, a new series of reactive
LC monomers containing a double bond in the side chain were designed and synthesized.
∗Address correspondence to Guangming Zhu, Department of Applied Chemistry, Northwestern
Polytechnical University, Xi’an 710129, Shaanxi, P. R. China. E-mail: gmzhu@nwpu.edu.cn
40

Reactive Liquid Crystal Monomers
41
These monomers show liquid crystalline properties at room temperature. The influence of
the side-chain length and main-chain groups on the phase transition behavior is discussed.
Our approach has important implications for the synthesis of novel side-chain LC polymer.
2. Experimental Details
2.1. Instruments
Infrared (IR) spectra were recorded on a Fourier transform infrared (FT-IR) Bruker TEN-
SOR27 spectrometer by using KBr pellets. The structures of the products were characterized
by 1H (400 MHz) and 13C (400 MHz) nuclear magnetic resonance (NMR; Bruker Avance
400 MHz spectrometer) in CDCl3 with tetramethylsilane as the internal standard. The
purity of the products was evaluated by high-performance liquid chromatography (Shi-
madzu LC-10A). The liquid crystalline textures were observed by an Olympus BX51 polar
optical microscope (POM) equipped with a Linkam THMS600 hot stage. The transforma-
tion temperature and associated enthalpy change were determined by differential scanning
calorimetry (DSC; DSC 204 F1 Phoenix of NETZSCH) at a scanning rate of 10◦C·min−1
.
2.2. Synthesis and Characterization of Monomers
Allyl bromide and n-octyl bromide were purchased from Acros Organics and used with fur-
ther purification. The purification methods of allyl bromide and n-octyl bromide are as fol-
lows. They were washed with 5% sodium bicarbonate solution three times and subsequently
with distilled water three times and dried with anhydrous magnesium sulfate overnight. Fi-
nally, the purified allyl bromide and n-octyl bromide were obtained by filtration and vacuum
distillation. All the other solvents and reagents were AR or CP grade and used without fur-
ther purification. N,Nꢀ-dicyclohexylcarbodiimide (DCC) and 4-pyrrolidinopyridine (4-PP)
were purchased from J&K. 4-(trans-4-Propylcyclohexyl)phenol were obtained from Shiji-
azhuang Huarui Scientific and Technological Co. Ltd. The procedure of preparing a series
of LC monomers is depicted in Scheme 1.
Compound ethyl 2,4-dihydroxybenzoate was synthesized as previously described in
literature [25]. The synthesis conditions of the other compounds were experimentally
determined.
Synthesis of Intermediates and Products.
Synthesis of Ethyl 2,4-Dihydroxybenzoate (II). 2,4-Dihydroxybenzoic acid (8.00 g,
51.90 mmol) was dissolved in 40 ml of ethanol (31.56 g, 686.08 mmol). The solution was
heated to 60◦C under stirring condition. Concentrated sulfuric acid (3 ml) was dropwise
added to the reaction vessel in 30 min. Then the reaction mixture was refluxed for 12 h.
After cooling, the pH of the solution was adjusted to 5–6 with the aqueous solution of
sodium bicarbonate. After extracted with ether and distilled under reduced pressure, white
crystals (5.24 g, 55.4%) were obtained (FT-IR [KBr, cm−1]: 3500, 2960, 2930, 2870, 2850,
1650, 1450, 1280, 1150, 1086, 830, 794, 764, 718.)
Synthesis of Ethyl 2-Hydroxy-4-(octyloxy)benzoate (III). Ethyl 2,4-dihydroxybenzoate
(8.00 g, 43.92 mmol) was dissolved in 40 ml of acetone. Potassium carbonate (20 g,
144.73 mmol) was added to the mixture and the mixture was heated to reflux under a
nitrogen atmosphere. Acetone solution (32 ml) of n-octyl bromide (9.64 g, 50 mmol) was

42
J. Xie et al.
OH
COOH
HO
I
C2H5OH
H2SO4
OH
II
III
HO
COO
K2CO3 C8H17Br
Acetone
OH
IX
O
COO
C8H17
Br
K2CO3
CH2
n
Acetone
CH2
n
n
O
n=1
n=2
IV
V
O
COO
C8H17
H2O
KOH
Methanol
CH2
O
HO
O
COOH
HO
C8H17
VIII
n=1
n=2
VI
VII
IX
4-PP
DCC
4-PP DCC
ether
Ether
CH2
O
CH2
n
O
O
COO
C8H17
O
COO
C8H17
XII
n=1
n=2
X
XI
Scheme 1.
dropwise added to the reaction vessel in 2 h and then refluxed for 15 h. After cooled to room
temperature, the mixture was filtered and was washed with 3 × 5 ml of tetrahydrofuran. The
filtrate was concentrated to produce yellow crystals. This crude product was recrystallized
by using methanol and yielded ethyl 2-hydroxy-4-octyloxybenzoate (β, 12.91 g, 69.1%).

Reactive Liquid Crystal Monomers
43
FT-IR (KBr, cm−1): 3210, 3080, 2931, 2860, 2840, 1650, 1575, 1498, 1472, 1400,
1267, 1190, 1150, 1024, 832, 780, 755, 728, 697.
1H NMR (400 MHz, CDCl3) δ 11.09 (s, 1H), 7.80–7.73 (m, 1H), 6.49–6.39 (m, 2H),
4.39 (q, J = 7.1 Hz, 2H), 3.99 (t, J = 6.6 Hz, 2H), 1.87–1.73 (m, 2H), 1.53–1.20 (m, 13H),
0.91 (t, J = 6.9 Hz, 3H).
13C NMR (400 MHz, CDCl3) δ 14.02, 14.70, 22.94, 26.49, 28.71, 28.96, 29.06, 31.65,
61.28, 69.64, 103.64, 106.48, 108.35, 133.26, 163.20, 166.01, 170.51.
Synthesis of Ethyl 2-(allyloxy)-4-(octyloxy)benzoate (IV). Compound III (8.0 g,
27.20 mmol) was dissolved in 40 ml of acetone. Potassium carbonate (18 g, 130.26 mmol)
was added to the mixture. The solution was heated to reflux under a nitrogen atmosphere.
Acetone solution (16 ml) of allyl bromide (3.63 g, 30 mmol) was dropwise added to the
reaction vessel in 2 h and refluxed for 10 h. After cooled to room temperature, the mixture
was filtered and the cake was washed with 3 × 5 ml of tetrahydrofuran. The filtrate was
concentrated to give yellow liquid. The crude product was washed with methanol/water
(1:1) to give yellow liquid compound IV (9.09 g, 90.1%).
FT-IR (KBr, cm−1): 3080, 2931, 2850, 1740, 1610, 1501, 1250, 1120, 1000, 930, 834,
756.
1H NMR (400 MHz, CDCl3) δ 7.84 (d, J = 8.5 Hz, 1H), 6.46 (dd, J = 8.5, 1.9 Hz,
2H), 6.19–5.83 (ddt, J = 17.2, 10.5, 3.0 Hz, 1H), 5.54 (dd, J = 17.2, 1.6 Hz, 1H), 5.29 (dd,
J = 10.6, 1.4 Hz, 1H), 4.57 (dd, J = 3.0, 1.7 Hz, 2H), 4.31 (q, J = 7.1 Hz, 2H), 3.95 (t, J
= 6.5 Hz, 2H), 1.92–1.57 (m, 2H), 1.54–1.04 (m, 13H), 0.88 (t, J = 6.8 Hz, 3H).
13C NMR (400 MHz, CDCl3) δ 14.10, 14.36, 22.66, 25.98, 29.11, 29.34, 31.80, 60.34,
68.18, 69.35, 100.58, 105.41, 112.82, 117.35, 132.64,133.77,160.21, 163.59.
The procedures of preparing ethyl 2-(but-3-enyloxy)-4-(octyloxy)benzoate (V) were
similar to the procedure of compound IV. The final crude product was purified by column
chromatography (silica gel, methylene chloride/petroleum ether = 12:7) to yield compound
V (yield: 45.1%).
FT-IR (KBr, cm−1): 3032, 2930, 2854, 1750, 1600, 1505, 1250, 1105, 101, 930, 835,
758.
1H NMR (400 MHz, CDCl3) δ 7.81 (t, J = 6.7 Hz, 1H), 6.47 (d, J = 2.2 Hz, 1H), 6.46
(dd, J = 8.6, 2.2 Hz, 1H), 6.44 (d, J = 2.1 Hz, 1H), 5.17 (dd, J = 17.2, 1.6 Hz, 1H), 5.09
(dd, J = 10.2, 0.9 Hz, 1H), 4.37–4.23 (m, 2H), 4.03 (t, J = 6.7 Hz, 2H), 3.95 (t, J = 6.5 Hz,
2H), 2.59 (m, 2H), 1.83–1.68 (m, 2H), 1.49–1.38 (m, 2H), 1.38–1.18 (m, 13H), 0.93–0.81
(m, 3H).
13C NMR (400 MHz, CDCl3) δ 14.11, 14.37, 22.65, 25.97, 29.11, 29.21, 29.23, 29.32,
31.79, 33.58,53.76, 60.39, 65.80, 68.16, 68.24, 69.47, 100.25, 105.29, 112.68, 117.06,
133.72, 134.37,160.45, 163.58, 165.89, 210.78.
Synthesis of 2-(Allyloxy)-4-(octyloxy)benzoic acid (VI). A solution of compound IV (8.0 g,
23.95 mmol), methanol (40 ml), and sodium hydroxide (4 g, 100 mmol) was heated to
reflux for 8 hours while stirring. Then the methanol was removed under reduced pres-
sure and 100 ml of deionized water was added. After cooling, the solution was adjusted
to pH 1–2 with concentrated HCl. The precipitate was filtered and recrystallized from
methanol/petroleum ether (1:4) to give white crystals of compound VI (7.33 g, 92.3%).
FT-IR (KBr, cm−1): 3470, 2920, 2850, 2560, 1680, 1600, 1508, 1462, 1280, 1200,
1150, 1088, 937, 849, 753, 722, 671, 652.
1H NMR (400 MHz, CDCl3) δ 10.66 (s, 1H), 8.15 (d, J = 8.8 Hz, 1H), 6.66 (dd, J =
8.8, 1.9 Hz, 1H), 6.54 (d, J = 1.8 Hz, 1H), 6.19–6.05 (m, 1H), 5.49 (dd, J = 5.8 Hz, 2H),

44
J. Xie et al.
4.78 (d, J = 5.6 Hz, 2H), 4.03 (t, J = 6.5 Hz, 2H), 1.90–1.75 (m, 2H), 1.47 (d, J = 7.8 Hz,
2H), 1.42–1.20 (m, 8H), 0.91 (t, J = 6.6 Hz, 3H).
13C NMR (400 MHz, CDCl3) δ 14.14, 22.79, 25.87, 29.04, 29.23, 29.31, 31.74,
68.62,70.74, 100.13, 107.36, 110.35, 120.68, 130.83, 135.58, 158.51, 164.61,165.25.
The procedures for the preparation of 2-(but-3-enyloxy)-4-(octyloxy)benzoic acid (VII,
a yield of 91.5%) were similar to the procedure of compound VI.
FT-IR (KBr, cm−1): 3430, 2930, 2852, 2560, 1650, 1600, 1506, 1452, 1250, 1200,
1150, 1088, 937, 849, 753, 722, 668, 654.
1H NMR (400 MHz, CDCl3) δ 10.88–10.38 (m, 1H), 8.14 (d, J = 8.8 Hz, 1H), 6.65
(dd, J = 8.8, 2.3 Hz, 1H), 6.53 (d, J = 2.2 Hz, 1H), 5.88 (d, J = 6.9 Hz, 1H), 5.31 (d, J =
1.5 Hz, 1H), 5.29–5.21 (m, 1H), 4.28 (t, J = 6.2 Hz, 2H), 4.03 (t, J = 6.5 Hz, 2H), 2.69 (d,
J = 6.6 Hz, 2H), 1.88–1.75 (m, 2H), 1.48 (s, 2H), 1.42–1.23 (m, 8H), 0.91 (t, J = 6.9 Hz,
3H).
13C NMR (400 MHz, CDCl3) δ 14.15, 22.65, 25.96, 29.05, 29.23, 29.31, 31.80, 33.40,
68.59, 68.62, 99.67, 107.08, 110.10, 119.09, 132.94, 135.57, 158.67, 164.58, 165.34.
Synthesis of 4-(4-Propylcyclohexyl)phenyl 2-(allyloxy)-4-(octyloxy)benzoate (X). A solu-
tion of compound VI (8.0 g, 26.14 mmol), 4-(trans-4-propylcyclohexyl)phenol (6.0 g,
27.5 mmol), DCC (7.0 g, 33.4 mmol), 4-PP (0.3 g, 2.03 mmol), and ether (60 ml) was
heated to reflux for 30 h under stirring. After cooling, the precipitate was filtered and
washed with ether. The filtrate was merged and concentrated to give crude product. The
final crude product was purified with column chromatography (silica gel, methylene chlo-
ride/petroleum ether = 12:7) to yield compound X (10.61 g, 80.2%).
FT-IR (KBr, cm−1): 3015, 2938, 2850, 1720, 1600, 1580, 1515, 1459, 1380, 1250,
1200, 1127, 1010, 870, 837, 801, 700, 663.
1H NMR (400 MHz, CDCl3) δ 8.07 (d, J = 8.7 Hz, 1H), 7.25 (d, J = 8.5 Hz, 2H),
7.16–7.08 (m, 2H), 6.61–6.47 (m, 2H), 6.09 (ddt, J = 17.2, 10.5, 4.7 Hz, 1H), 5.59 (dq,
J = 17.2, 1.6 Hz, 1H), 5.30 (dd, J = 10.6, 1.5 Hz, 1H), 4.64 (dt, J = 4.6, 1.6 Hz, 2H),
4.03 (t, J = 6.6 Hz, 2H), 2.65–2.31 (m, 1H), 1.86 (m, 6H), 1.64–1.18 (m, 17H), 1.15–0.98
(m, 2H), 0.92 (m, 6H).
13C NMR (400 MHz, CDCl3) δ 14.16,14.45, 20.07, 22.71, 25.99, 29.12, 29.36, 33.58,
36.99, 39.73, 44.12, 68.35, 69.40, 100.66, 105.59, 111.48, 117.56, 121.62, 127.64, 132.38,
134.47,145.07, 149.01, 160.99, 164.08, 164.28.
Elemental analysis: C33H46O4 requires C78.26, H 9.09; found: C 78.12, H 9.14.
The procedures for the preparation of 4-(4-propylcyclohexyl)phenyl 2-(but-3-
enyloxy)-4-(octyloxy)benzoate (XI) and 4-(2-allyloxy-4-octyloxy benzoate)-4ꢀ-propylbi-
phenyl (XII) were similar to the procedure of compound X.
4-(4-Propylcyclohexyl)phenyl 2-(but-3-enyloxy)-4-(octyloxy)benzoate (XI). Yield: 76.8%.
FT-IR (KBr, cm−1): 3025, 2924, 2860, 1735, 1690, 1601, 1510, 1456, 1360, 1250,
1200, 1150, 1008, 868, 835, 806, 705, 660.
1H NMR (400 MHz, CDCl3) δ 8.03 (d, J = 8.7 Hz, 1H), 7.25 (d, J = 8.4 Hz, 2H),
7.11 (d, J = 8.5 Hz, 2H), 6.52 (d, J = 2.0 Hz, 2H), 6.09–5.87 (m, 1H), 5.23–5.13 (m,
1H), 5.13–5.05 (m, 1H), 4.10 (t, J = 6.7 Hz, 2H), 4.03 (s, 2H), 2.62 (d, J = 6.7 Hz, 1H),
2.54–2.39 (m, 2H), 1.90 (m, 4H), 1.63–1.18 (m, 19H), 1.14–1.02 (m, 2H), 0.90 (m, 6H)
13C NMR (400 MHz, CDCl3) δ 14.10, 14.42, 20.04, 22.66, 25.99, 29.12, 29.22, 29.33,
31.80, 33.50, 34.43, 37.02, 39.73, 44.11, 68.34, 68.45, 100.41, 105.54, 117.16, 121.49,
121.56, 127.64, 131.04, 134.37, 142.23, 144.99, 158.92, 161.29, 164.29.
Elemental analysis: C34H48O4 requires C78.46, H 9.23; found: C 78.41, H 9.30.

Reactive Liquid Crystal Monomers
45
4-(2-Allyloxy-4-octyloxybenzoate)-4ꢀ-propylbiphenyl (XII). Yield: 82.1%.
FT-IR (KBr, cm−1): 3020, 2920, 2857, 1728, 1690, 1601, 1510, 1456, 1360, 1250,
1200, 1150, 1008, 868, 835, 806, 705, 660.
1H NMR (400 MHz, CDCl3) δ 8.07 (d, J = 8.7 Hz, 1H), 7.54–7.63 (d, J = 8.5 Hz,
2H), 7.52 (d, J = 7.5 Hz, 2H), 7.22–7.33 (m, 4H), 6.44–6.57 (m, 2H), 6.07 (ddt, J = 16.1,
9.8, 6.1 Hz, 1H), 5.58 (dq, J = 16.1, 1.6 Hz, 1H), 5.28 (dd, J = 9.8, 1.5 Hz, 1H), 4.64 (dt,
J = 6.1, 1.6 Hz, 2H), 4.03 (t, J = 6.2 Hz, 2H), 2.62 (t, J = 7.8 Hz, 2H), 1.74–1.85 (m, 2H),
1.67–1.72 (m, 2H), 1.41–1.52 (m, 2H), 1.2–1.40 (m, 8H), 0.94–1.03 (m, 3H), 0.84–0.92
(m, 3H).
13C NMR (400 MHz, CDCl3) δ 12.98, 14.01, 22.93, 24.48, 26.49, 28.71, 28.96, 29.06,
31.64, 38.11, 69.64, 71.17, 104.65, 107.03, 113.63,117.52, 122.06, 127.25, 127.99, 130.21,
133.98, 134.46, 134.74, 139.07, 141.84, 153.11, 162.31, 165.05, 167.58.
Elemental analysis: C32H46O4 requires C79.2, H 8.0; found: C 79.14, H 8.02.
3. Results and Discussion
These new reactive LC monomers were synthesized as shown in Scheme 1. The purities
of these compounds and the reaction time were verified by thin layer chromatography
(TLC). From the FT-IR, compound X at 3085 cm−1 might be a result of the existence of
the double bond group, because it might be –C C–H stretching bands. The characteristic
absorption of the target compound X at 1720 cm−1 can attributed to the stretching bands of
the carbonyl group (C O). The peak at 2938 cm−1 corresponds to the methyl group and
the peak at 2850 cm−1 might be a result of the existence of methylene group. Compared
with compound X, the characteristic absorptions of the target compounds XI and XII could
be find in the FT-IP spectra (3025, 2924, 2860, and 1735 cm−1 for the target compound XI;
3020, 2920, 2857, and 1728 cm−1 for the target compound XII).
In the 1H NMR (400 MHz, CDCl3) of compound X, the chemical shifts at δ 6.09 (ddt, J
= 17.2, 10.5, 4.7 Hz–CH C), δ 5.59 (dq, J = 17.2, 1.6 Hz, –C CH2), and δ 5.30 ppm (dd,
J = 10.6, 1.5 Hz,–C CH2) verify the double bond in the molecular structure of compound
X. In the 13C NMR (400 MHz, CDCl3) of compound X, the chemical shifts at δ 164.28 ppm
are attributed to the carbonyl carbon. Compared with compound X, similar chemical shifts
could be found in the 1H NMR and 13C NMR of compounds XI and XII.
Based on the above analysis, we confirm that the reactive LC monomers (compounds
X, XI, and XII) have been successfully synthesized.
The mesomorphic properties of the target products (compounds X, XI, and XII) were
investigated with a POM equipped with a heating stage and differential scanning calorime-
ter. The results (during the first heating–cooling run) of the target products (compounds X,
XI, and XII) are shown in Fig. 1.
Figure 1shows that thetarget products displaytwoexothermicpeaks duringthe cooling,
which should be attributed to phase transition (as confirmed by the POM images in Fig. 2).
From Fig. 2, it can be seen that the final products show optical texture characteristics of
the nematic phase. However, compounds X and XI display two endothermic peaks during
the heating and they do not show the optical texture characteristics of LC phase. The
colors of compounds X and XI changed from white to yellow at the first endothermic peak
temperature, suggesting that their first endothermic peaks may be the crystal-to-crystal
transition peaks induced by the conformational reorganization of molecules into a more
stable crystalline configuration. The target products melt at the second endothermic peaks.
Therefore, the second endothermic peaks correspond to the melting points of the target
products. The phase transition temperatures of compounds X, XI, and XII are shown in
Table 1.

46
J. Xie et al.
compound XII
55.8
Cooling
12.1
Heating
Cooling
70.2
compound XI
compound X
0.95
36.8
Heating
Cooling
31.9
55.2
11.3
Heating
38.21
62.77
69.02
-20
0
20
40
60
80 100 120 140 160 180 200 220
Temperature (°C)
Figure 1. DSC curves of the target product (compounds X, XI, and XII) during the first heating-
cooling run.
Figure 1 shows that the phase-transition temperature of compound XI shifts to a
temperature lower than that of compound X. This is due to the two methylene groups in
the side chain of the compound XI. The flexibility of compound XI is higher than that
of compound X, especially for the side-chain flexibility. So molecules of compound XI
may close more easily to form the orientation to exhibit a monotropic nematic mesophase.
Therefore a slight change of the side chain has big effect on liquid crystalline behavior.
Compared with compound X, compound XII has more phenyl groups and little changes
were found in the LC interval. But it can still be seen that the increased rigidity of the main
chain makes the LC interval shift to a higher temperature region. So a slight change of the
polarity of the main chain has little effect on liquid crystalline behavior.
From the above analysis, the flexibility of side chains affects more than that of the
main chain on liquid crystalline behavior.
However, the target products (compounds X, XI, and XII) do not show LC phase during
the heating process, which can be attributed to the steric hindrance that makes it difficult
for the molecules to be active during the heating process. The steric hindrance is easy to
overcome during the cooling process, since the molecules move more easily in the liquid
Table 1. Phase transition temperatures of compounds X, XI, and XII. Enthalpy values in
J·g−1 are given in parentheses
Compound
Phase transition temperatures on cooling
Melting point
X
XI
XII
Cr 11.3 (5.31) N 55.2 (1.72) I
Cr 0.95 (9.40) N 36.8 (1.65) I
Cr 12.1 (6.85) N 55.8 (1.37) I
69.02 (44.18)
62.77 (58.72)
70.20 (38.15)

Reactive Liquid Crystal Monomers
47
Figure 2. POM images of compounds X, XI, and XII on cooling. (a) Polarizing optical textures
of compound X at 24.7◦C. (b) Polarizing optical textures of compound XI at 7.8◦C. (c) Polarizing
optical textures of compound XII at 35.4◦C.
state than in solid state. Therefore, the steric effect can be overcome by molecular motion
and polymer molecules arrange to form the LC phase.
4. Conclusions
A series of novel ester LC monomers with double bonds in the side chains were successfully
synthesized and their structures were characterized by elemental analysis, NMR, and FT-IR
spectrum. The effect of the side-chain length on the phase transition behavior was stud-
ied. The LC range of 4-(4-propylcyclohexyl)phenyl 2-(but-3-enyloxy)-4-octyloxybenzoate

48
J. Xie et al.
shifts to lower temperatures region in comparison with 4-(4-propylcyclohexyl)phenyl 2-
allyloxy-4-octyloxybenzoate. When the phase transition takes place, the change in the side
chains is more significant than that in the main chains. Our novel ester LC monomers with
double bonds in the side chains are important for extending the applications of LCs.
Acknowledgments
This work is financially supported by the Basic Research Foundation of Northwestern
Polytechnical University (JC201047). We would like to thank Dr. Baojian Liu for his
assistance in obtaining some of the NMR spectra and Prof. Xiaodong Fan for the analysis
of DSC and FT-IR.
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