Mesogen-jacketed liquid crystalline polymers with a polynorbornene main chain: Green synthesis and phase behaviors

Article abstract of DOI:10.1021/ma500606e

We synthesized a new series of MJLCPs with a polynorbornene main chain, PNbnPT (n = 8, 10, 12, 14, 16, 18, which is the number of carbons in the side-chain alkyl tails), through ring-opening metathesis polymerization and investigated their phase behaviors. The monomers were facilely synthesized in satisfactory yields through a key intermediate, N-(2,5-dicarboxylphenyl)-cis-5- norbornene-exo-2,3-dicarboximide. Differential scanning calorimetry, polarized light microscopy, and wide-angle X-ray diffraction results demonstrate that the phase behavior of PNbnPT is strongly dependent on the alkyl-tail length. Polymers with relatively short alkyl tails, PNb8PT and PNb10PT, are amorphous in the whole temperature range before degradation. When the alkyl-tail length increases, PNbnPTs (n = 12, 14, 16, 18) develop into the smectic A phase, and the degree of order increases with increasing alkyl-tail length. The synthetic pathway for MJLCPs with a polynorbornene main chain in this work is more robust and greener than those reported to the best of our knowledge. And MJLCPs with a polynorbornene main chain show quite different phase behaviors compared with those with a polyethylene backbone, and they may be used to create fascinating functional materials.

Full text of DOI:10.1021/ma500606e

Article
pubs.acs.org/Macromolecules
Mesogen-Jacketed Liquid Crystalline Polymers with a
Polynorbornene Main Chain: Green Synthesis and Phase Behaviors
Yu-Feng Zhu, Zhen-Yu Zhang, Qi-Kai Zhang, Ping-Ping Hou, De-Zhao Hao, Yang-Yang Qiao,
Zhihao Shen,* Xing-He Fan,* and Qi-Feng Zhou
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education,
College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China
ABSTRACT: We synthesized a new series of MJLCPs with a
polynorbornene main chain, PNbnPT (n = 8, 10, 12, 14, 16, 18,
which is the number of carbons in the side-chain alkyl tails), through
ring-opening metathesis polymerization and investigated their phase
behaviors. The monomers were facilely synthesized in satisfactory
yields through a key intermediate, N-(2,5-dicarboxylphenyl)-cis-5-
norbornene-exo-2,3-dicarboximide. Differential scanning calorimetry,
polarized light microscopy, and wide-angle X-ray diffraction results
demonstrate that the phase behavior of PNbnPT is strongly dependent on the alkyl-tail length. Polymers with relatively short
alkyl tails, PNb8PT and PNb10PT, are amorphous in the whole temperature range before degradation. When the alkyl-tail length
increases, PNbnPTs (n = 12, 14, 16, 18) develop into the smectic A phase, and the degree of order increases with increasing
alkyl-tail length. The synthetic pathway for MJLCPs with a polynorbornene main chain in this work is more robust and greener
than those reported to the best of our knowledge. And MJLCPs with a polynorbornene main chain show quite different phase
behaviors compared with those with a polyethylene backbone, and they may be used to create fascinating functional materials.
the LC phase formation, indicating the influence of the polymer
backbone on liquid crystallinity. The most studied MJLCPs are
those with a polystyrene main chain, which can be synthesized
by controlled radical polymerizations.18 A series of MJLCPs
with side-chain cores based on 2-vinylhydroquinone,19 2-vinyl-
1,4-phenylenediamine,8 2-vinylterephthalic acid (VTA),20−25
vinylbiphenyl,16,26−28 vinylterphenyl,29,30 and oxadiazole-con-
taining conjugated structure14,31 have been successfully
synthesized.18 By hydrosilylation of polymethylhydrosiloxane
with the styrenic monomers, MJLCPs with a polysiloxane main
chain were synthesized.32
Besides MJLCPs with flexible backbones, the jacketed
structure has also been applied to polymers with rigid
backbones, such as polyalkynes33,34 and polythiophenes.35,36
In addition, MJLCPs with a polynorbornene backbone and
laterally attached 2,5-bis[(4′-n-alkoxybenzoyl)oxy] mesogens in
side chains display enantiotropic nematic mesophases.37
Terminating the hydrocarbon substituents of the mesogen
with immiscible components, such as fluorocarbon segments or
oligodimethylsiloxane segments, can induce smectic layering in
these polymers with a polynorbornene backbone.38,39 Very
recently, Yang et al. reported a new series of MJLCPs with a
polynorbornene backbone.40 The polymers are liquid crystal-
line when the number of carbon atoms in the alkoxy terminal
chains of the side-chain mesogen is more than nine.
INTRODUCTION
■
The decoupling concept, in which flexible spacers are used to
decouple the motion of the ordered arrangement of side-chain
mesogens from that of the random-coiled main chain, is
undoubtedly a landmark for the development of side-chain
liquid crystalline polymers (LCPs).1 To hinder the rotational
motion around the long axis of the side-chain mesogen,
Finkelmann et al. synthesized a side-chain LCP with mesogens
laterally attached to the polymer backbone.2 Mesogen-jacketed
liquid crystalline polymer (MJLCP), proposed by Zhou et al. in
the late 1980s,3,4 is an LCP with bulky side groups (often
mesogens) laterally attached to the main chain through a short
spacer or a single covalent bond.5 Although MJLCPs have
molecular structures like side-chain LCPs, they exhibit
properties similar to those of main-chain LCPs, such as
forming banded textures by shearing in the liquid crystalline
(LC) state and having large persistence lengths in solutions,6−8
because the backbones of MJLCPs are forced to take the
extended-chain conformation owing to the steric effect of the
crowded side chains, the “jacketing” effect.9 Many LC phases of
MJLCPs have been reported, including columnar nematic
(ΦN),10−12 hexatic columnar nematic (ΦHN),11,13 hexagonal
columnar (ΦH),11,14 rectangular columnar,12,15 smectic A
(SmA),14,16 and smectic C17 phases, in which the MJLCP
chain behaves like a cylinder or tablet to form the LC phases.
With the development of the structural library of MJLCPs,
many efforts have been put on the molecular engineering of the
MJLCP main chains. The first reported series of MJLCPs are
based on polyacrylates and polymethacrylates.3,4 Compared to
polyacrylates, the polymethacrylate backbone tends to depress
Received: March 24, 2014
Revised: April 24, 2014
© XXXX American Chemical Society
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dimensional (1D) wide-angle X-ray diffraction (WAXD) experiments
were performed according to the procedures described previously.12,52
For 1D WAXD experiments, about 35 mg of the powder sample was
dissolved in tetrahydrofuran (THF, ∼ 1 mL) and cast into a film on a
copper substrate. Solvent was then allowed to evaporate at ambient
temperature for hours, and the sample was dried overnight at 35 °C in
vacuum before characterization. To avoid thermal degradation, the
sample chamber was charged with nitrogen during measurements.
Two-dimensional (2D) WAXD experiments were performed using a
Bruker D8Discover diffractometer with VANTEC 500 as a 2D
detector. Corundum was used for calibration of the reflection peak
positions. The diffraction patterns were recorded in the transmission
mode at ambient temperature using uniaxially oriented fiber samples.
Macroscopically oriented fiber samples of all polymers were drawn
with a pair of tweezers at 190−230 °C for 2D WAXD experiments. All
fiber samples were annealed at 100 °C overnight before character-
ization.
In the past two decades, Percec and co-workers have
intensively investigated and synthesized the other type of
jacketed polymers, namely dendron-jacketed polymers,41−43 by
a set of living polymerization methods, such as living cationic
polymerization,44,45 living polymerization of acetylenes,46 and
living metathesis polymerization.47 With the consideration of
the high tolerance of functional groups like fullerene, the
convenient synthesis by ring-opening metathesis polymer-
ization (ROMP),48,49 and the excellent properties of poly-
norbornenes,50,51 the exploration of MJLCPs with a poly-
norbornene main chain is of great interest. The reported
procedures for synthesizing MJLCPs with a polynorbornene
main chain are either too complicated or using heavy metal
catalysts, which is not environmentally friendly. In addition,
they are not robust enough to create various MJLCPs. Herein,
we designed and synthesized a new series of MJLCPs with a
polynorbornene main chain, PNbnPT (n = 8, 10, 12, 14, 16, 18,
which is the number of carbons in the side-chain alkyl tails,
Chart 1), via a green and robust pathway. The phase behaviors
Synthetic Procedures. The p-(n-alkoxyl)phenols (Cn-OH, n = 8,
10, 12, 14, 16, 18) were prepared as described in the literature.53 The
chemical structures and synthetic procedures of all the monomers and
polymers are illustrated in Scheme 1. The experimental details are
described as follows.
N-(2,5-Dicarboxylphenyl)-cis-5-norbornene-exo-2,3-dicarboxi-
mide (NbTA). cis-5-Norbornene-exo-2,3-dicarboxylic anhydride (2.00
g, 12.2 mmol) and glacial acetic acid (30.0 mL) were charged in a 100
mL three-necked flask. 2-Aminoterephthalic acid (2.21 g, 12.2 mmol)
was added stepwise to the reaction mixture at 120 °C in 30 min. Then
the reaction mixture was stirred for 12 h. After cooling to ambient
temperature, the mixture was poured into cold water (∼150 mL) and
vigorously stirred for 2 h. After filtration and drying under an infrared
Chart 1. Chemical Structure of PNbnPT
1
lamp, the product was obtained as a white powder. Yield: 80%. H
NMR (400 MHz, DMSO, δ, ppm): 13.53 (s, 2H), 7.77−8.11 (m, 3H),
6.37 (s, 2H), 3.19−3.26 (d, 2H), 2.85−2.89 (d, 2H), 1.36−2.02 (m,
2H). MS (HR-ESI): [M − H]−/z, Calcd 326.0670; Found 326.0662.
Anal. Calcd for C17H13O6N: C, 62.39; H, 4.00; N, 4.28. Found: C,
62.40; H, 4.22; N, 4.28.
Monomers (NbnPTs). The synthetic procedures for the monomers
with different alkyl-tail lengths are similar. The details are described
below with Nb8PT as an example.
of PNbnPTs are strongly dependent on the length of the alkyl
tail. PNbnPTs with long alkyl tails (n = 12, 14, 16, 18) develop
into the SmA phase, and those with short alkyl tails (n = 8, 10)
are amorphous.
C8-OH (1.36 g, 6.12 mmol), NbTA (1.00 g, 3.06 mmol), N,N′-
dicyclohexylcarbodiimide (DCC, 3.15 g, 15.3 mmol), 4-
(dimethylamino)pyridine (DMAP, 0.17 g, 1.53 mmol), and CH2Cl2
(50.0 mL) were loaded in a 100 mL round-bottomed flask and stirred
for 24 h at ambient temperature. The floating solid was filtered off, and
the solvent was evaporated. The crude product was purified by silica
gel column chromatography with CH2Cl2 and subsequent recrystal-
EXPERIMENTAL SECTION
■
Materials and Characterization Methods. Dichloromethane
(CH2Cl2) was pretreated by the Braun solvent purification system. cis-
5-Norbornene-exo-2,3-dicarboxylic anhydride (96%) and N,N-dime-
thylformamide (DMF, HPLC) were purchased from J&K Chemical.
Grubbs catalyst (second generation) was purchased from Sigma-
Aldrich. All other reagents were obtained from commercial sources
1
lization from ethanol. Yield of Nb8PT: 70%. H NMR (400 MHz,
1
and used as received unless otherwise noted. H NMR (400 MHz),
CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H), 6.85−6.99
(m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H), 2.87 (s, 2H),
1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m, 20H), 0.88 (t,
6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9, 176.4, 163.6,
163.2, 157.3, 143.9, 143.6, 138.0, 134.6, 132.8, 132.2, 131.5, 130.7,
13C NMR (100 MHz), high-resolution mass spectrometry (MS),
elemental analysis, gel permeation chromatographic (GPC) measure-
ments, thermogravimetric analysis (TGA), differential scanning
calorimetry (DSC), polarized light microscopy (PLM), and one-
Scheme 1. Synthetic Pathway of the Polymers (PNbnPTs)
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122.2, 115.2, 68.5, 68.4, 48.5, 45.6, 43.5, 31.6, 29.3, 25.7, 22.6, 14.0.
MS (HR-ESI): [M + H]+/z, Calcd 736.3849; Found, 736.3836. Anal.
Calcd for C45H53O8N: C, 73.44; H, 7.26; N, 1.90. Found: C, 73.17; H,
7.16; N, 2.12.
PNb10PT, PNb12PT, PNb14PT, PNb16PT, and PNb18PT were
similarly prepared starting from corresponding monomers as white
solids with yields of 93, 92, 94, 95, and 92%, respectively.
Nb10PT was prepared starting from C10-OH. Yield: 74%. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H),
6.85−6.99 (m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H),
2.87 (s, 2H), 1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m,
28H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9,
176.4, 163.6, 163.2, 157.3, 143.9, 143.6, 138.0, 134.6, 132.8, 132.2,
131.5, 130.7, 122.2, 115.2, 68.5, 68.4, 48.5, 45.6, 43.5, 31.9, 29.6, 29.4,
29.3, 26.1, 22.7, 14.1. MS (HR-ESI): [M + H]+/z, Calcd 814.4295;
Found, 814.4293. Anal. Calcd for C49H61O8N: C, 74.31; H, 7.76; N,
1.77. Found: C, 73.95; H, 7.63; N, 1.95.
RESULTS AND DISCUSSION
■
Synthesis of Monomers and Polymers. As shown in
Scheme 1, the monomers can be prepared in three very efficient
steps with commercially available starting materials. The key
intermediate for the synthesis of this kind of MJLCPs with a
polynorbornene main chain was NbTA, which can be facilely
synthesized by the imidization of cis-5-norbornene-exo-2,3-
dicarboxylic anhydride and 2-aminoterephthalic acid with a
yield of 80%. This key intermediate NbTA may open a new
avenue for the synthesis of MJLCPs containing a poly-
norbornene main chain, which is similar to VTA for the
synthesis of MJLCPs with a polyethylene backbone.5 Cn-OH
was synthesized by Williamson etherification of hydroquinone
and the corresponding 1-bromoalkane. Then Steglich ester-
ification of NbTA and Cn-OH gave the monomers NbnPTs in
satisfactory yields. The chemical structures of the monomers
were verified by the combination of 1H/13C NMR, high-
resolution MS, and elemental analysis.
Nb12PT was prepared starting from C12-OH. Yield: 80%. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H),
6.85−6.99 (m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H),
2.87 (s, 2H), 1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m,
36H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9,
176.6, 163.6, 163.0, 157.2, 143.7, 143.6, 138.0, 134.6, 132.8, 132.2,
131.5, 130.7, 122.2, 115.2, 68.4, 48.5, 45.6, 43.5, 31.9, 29.7, 29.6, 29.4,
26.0, 22.7, 14.1. MS (HR-ESI): [M + H]+/z, Calcd 848.5101; Found,
848.5075. Anal. Calcd for C53H69O8N: C, 75.06; H, 8.20; N, 1.65.
Found: C, 75.12; H, 7.95; N, 1.63.
Nb14PT was prepared starting from C14-OH. Yield: 72%. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H),
6.85−6.99 (m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H),
2.87 (s, 2H), 1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m,
44H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9,
176.6, 163.6, 163.0, 157.2, 143.7, 143.6, 138.0, 134.6, 132.8, 132.2,
131.5, 130.7, 122.2, 115.2, 68.4, 48.5, 45.6, 43.5, 31.9, 29.7, 29.6, 29.4,
26.0, 22.7, 14.1. MS (HR-ESI): [M + H]+/z, Calcd 904.5727; Found,
904.5724. Anal. Calcd for C57H77O8N: C, 75.71; H, 8.58; N, 1.55.
Found: C, 75.71; H, 8.64; N, 1.53.
High-molecular-weight (MW) polymers with relatively
narrow MW distributions were obtained through ROMP in
dichloromethane with Grubbs second-generation catalyst. The
molecular characteristics of the polymers are summarized in
Table 1. GPC results show that Mn’s of PNbnPTs are 3.5 ×
Table 1. Molecular Characteristics and Thermal Properties
of the Polymers
Ttransition (°C) (and
corresponding enthalpy change,
kJ/mol)
Nb16PT was prepared starting from C16-OH. Yield: 68%. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H),
6.85−6.99 (m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H),
2.87 (s, 2H), 1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m,
52H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9,
176.6, 163.6, 163.0, 157.2, 143.7, 143.6, 138.0, 134.6, 132.8, 132.2,
131.5, 130.7, 122.2, 115.2, 68.4, 48.5, 45.6, 43.5, 31.9, 29.7, 29.6, 29.4,
26.0, 22.7, 14.1. MS (HR-ESI): [M + H]+/z, Calcd 960.6353; Found,
960.6362. Anal. Calcd for C61H85O8N: C, 76.29; H, 8.92; N, 1.46.
Found: C, 76.31; H, 8.87; N, 1.37.
a
b
c
Mn
(×105 Da) PDI
Td
polymer
(°C)
PNb8PT
3.86
3.74
3.64
3.65
3.50
3.45
1.24
1.30
1.29
1.35
1.34
1.32
401
404
394
408
399
402
PNb10PT
PNb12PT
PNb14PT
PNb16PT
PNb18PT
1 (13.44), 143 (1.92)
25 (25.10), 147 (3.05)
a
b
Determined by GPC in THF using polystyrene standards. 5%
Nb18PT was prepared starting from C18-OH. Yield: 75%. 1H NMR
(400 MHz, CDCl3, δ, ppm): 8.00−8.50 (m, 3H), 7.00−7.18 (m, 4H),
6.85−6.99 (m, 4H), 6.32 (s, 2H), 3.88−4.04 (q, 4H), 3.40 (s, 2H),
2.87 (s, 2H), 1.73−1.85 (m, 4H), 1.53−1.71 (s, 2H), 1.17−1.53 (m,
60H), 0.88 (t, 6H). 13C NMR (100 MHz, CDCl3, δ, ppm): 176.9,
176.6, 163.6, 163.0, 157.2, 143.7, 143.6, 138.0, 134.6, 132.8, 132.2,
131.5, 130.7, 122.2, 115.2, 68.4, 48.5, 45.6, 43.5, 31.9, 29.7, 29.6, 29.4,
26.0, 22.7, 14.1. MS (HR-ESI): [M + NH4]+/z, Calcd 1033.7245;
Found, 1033.7263. Anal. Calcd for C65H93O8N: C, 76.81; H, 9.22; N,
1.38. Found: C, 76.81; H, 9.19; N, 1.28.
weight loss temperature evaluated by TGA under a nitrogen
c
atmosphere at a heating rate of 10 °C/min. Evaluated by DSC
during the second heating cycle at a rate of 20 °C/min.
105−3.9 × 105 Da, with polydispersity indexes (PDIs) of 1.24−
1.35. Figure 1 gives 1H NMR spectra of Nb16PT and
PNb16PT in CDCl3. The characteristic resonance of vinyl H
appearing at δ = 6.32 ppm completely disappears after
polymerization, and the resonance peaks of PNb16PT are
rather broad and consistent with the expected polymer
structure, indicating the successful polymerization as well.
Thermal Properties of PNbnPTs. TGA and DSC were
used to investigate the thermal properties of PNbnPTs. TGA
results show that all the polymers exhibit excellent thermal
stabilities with the 5% weight loss temperatures above 390 °C
in nitrogen atmosphere, as shown in Table 1.
Figure 2 shows the DSC traces of all polymers on the first
cooling and subsequent heating processes at a rate of 20 °C/
min under nitrogen. All the samples were heated to 280 °C first
to eliminate thermal history. Glass transitions occur at
100−120 °C with small changes in heat flow possibly due to
the rigidity of the polymer chains that renders quite slow glass-
transition kinetics. As shown in Figure 2, the DSC traces of the
Polymers (PNbnPTs). The synthetic procedures for the polymers
with different alkyl-tail lengths are similar. The details are described
below with PNb8PT as an example.
Nb8PT (200 mg) and Grubbs second-generation catalyst (4.61 mg)
were loaded in a dry Schlenk tube with a magnetic stirring bar. After
three pump−purge cycles with high purity nitrogen, CH2Cl2 (∼2 mL)
was injected to the mixture under vigorous stirring to initiate
polymerization. After the reaction mixture was stirred at ambient
temperature for 1 h, a few drops of vinyl ethyl ether were added to the
reaction mixture using a syringe. Then polymerization was stopped
after an additional hour. The viscous liquid was diluted with 5 mL of
CH2Cl2 and passed through a short alumina column to separate the
catalyst, and then the polymer was precipitated out in 100 mL of
methanol. By filtration and drying in vacuum at 35 °C for 24 h, the
target polymer PNb8PT was obtained as a white solid. Yield: 95%.
C
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indicating the enantiotropic phase behaviors of PNb16PT and
PNb18PT.
Phase Structures of PNbnPTs. Phase behaviors of
PNbnPTs were then examined by PLM observation of samples
cast from THF solutions and slowly dried at ambient
temperature. Properties of PNbnPTs with long alkyl tails (n
= 12, 14, 16, 18) are similar. For example, PNb18PT shows
strong birefringence at 80 °C, which implies a possible LC
phase, as shown in Figure 3a. However, these four samples only
Figure 1. 1H NMR spectra of PNb16PT (top) and Nb16PT (bottom)
in CDCl3.
Figure 3. PLM micrographs of PNb18PT (a) and PNb8PT (b) at 80
°C.
exhibit noncharacteristic textures. On the other hand, PNb8PT
(Figure 3b) and PNb10PT do not show any birefringence in
the whole temperature range even when the samples are
sheared, indicating that these two samples with relatively short
alkyl tails are amorphous.
Figure 2. DSC traces of PNbnPTs at a rate of 20 °C/min under a
nitrogen atmosphere.
Because DSC and PLM results do not provide enough
structural information on the polymers, variable-temperature
1D WAXD experiments were conducted to further identify the
phase structures of the polymers. In agreement with the PLM
results, two kinds of 1D WAXD patterns are observed for the
polymers, as shown in Figures 4 and 5 for PNb18PT and
PNb10PT, respectively, as examples. For PNb18PT, two
diffraction peaks with a scattering vector ratio of 1:2 are
observed in the low-angle region during the first heating
process, demonstrating a long-range ordered smectic phase.
The broad halo in the wide-angle region is characteristic of the
disordered packing of the side chains. After heating to higher
temperatures, the diffraction pattern remains the same until
isotropization at about 150 °C when the sharp diffraction peak
in the low-angle region becomes broad. The isotropization
temperature of 150 °C for PNb18PT from 1D WAXD results is
consistent with DSC results. Upon cooling, the two diffraction
peaks originating from the smectic phase develop again,
indicating the enantiotropic phase behavior of PNb18PT.
polymers are different depending on alkyl-tail length. Upon
heating, an endothermic peak is observed at the temperature
range of 140−150 °C during heating for PNb16PT and
PNb18PT. The enthalpy values of these transitions are
relatively small (1.92−3.05 kJ/mol), and no more transitions
are observed at even higher temperatures. These results suggest
that the transitions at the temperature range of 140−150 °C
may be related to the isotropization of the polymers from a
possible LC phase. The increasing enthalpy change with
increasing alkyl-tail length indicates that a more ordered
structure may be generated in polymers with longer alkyl tails.
For PNb16PT and PNb18PT, an additional peak is observed at
1 and 25 °C, respectively, during heating. This transition
corresponds to melting of the crystals formed by the long alkyl
tails. In the cooling cycles, one or two exothermic peaks appear,
consistent with the endothermic peaks during heating,
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Figure 5. 1D WAXD patterns of PNb10PT during the first heating (a)
Figure 4. 1D WAXD patterns of PNb18PT during the first heating (a)
and subsequent cooling (b) processes.
and subsequent cooling (b) processes.
Because the temperature was not low enough during cooling,
the crystallization process of the side-chain alkyl tails indicated
by DSC results was not observed. However, for PNb10PT, the
as-cast sample only gives two broad halos in the low- and wide-
angle regions. And the diffraction pattern remains almost
unchanged in the whole temperature range during heating and
cooling processes, indicating the amorphous nature of
PNb10PT.
PNb8PT and PNbnPT (n = 12, 14, 16) have very similar
diffraction patterns as PNb10PT and PNb18PT, respectively.
The 1D WAXD patterns of the as-cast PNbnPTs at ambient
temperature are summarized in Figure 6a. This set of diffraction
profiles demonstrates clearly that the phase behaviors of
PNbnPTs strongly depend on the alkyl-tail length. For the
diffraction in the low-angle region, namely the (100) diffraction
of the smectic phase, the intensity decreases, and the peak
becomes broader with decreasing alkyl-tail length until the
polymer becomes amorphous (for PNb10PT and PNb8PT).
And the (200) diffraction can hardly be observed when n is 14
or less. These results show that the longer alkyl tail can induce
the smecticity of the polymers. This inducing effect should stem
from the nanophase separation of the benzoate core and alkyl
tails in the side chains. Hence, our results show that increasing
the alkyl tails of the side-chain mesogens in these polymers is
also an effective way to induce smectic phases, in addition to
the incorporation of immiscible segments at the side-chain
terminals reported by Pugh et al.38,39,54 In comparison to the
amorphous polymers (n = 8, 10), the scattering halo in the
high-angle region, which represents the average lateral distance
between side chains, slightly shifts to lower angles for the LCPs
(n = 12, 14, 16, 18) with d-spacing values changing from 0.43 to
0.46 nm. This slight shift indicates that the formation of LC
phases may be accompanied by the conformational change of
the side chains.55 Figure 6b shows the plot of the d-spacing
values of the (100) peak for the LCPs (n = 12, 14, 16, 18) and
the low-angle halo of the amorphous polymers (n = 8, 10) in
Figure 6a against number of carbons in the alkyl tails. The d-
spacing values of the (100) peak for the LCPs (n = 12, 14, 16,
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To determine the alignment of the mesogens relative to the
polymer main chain in the smectic layer, 2D WAXD
experiments were then carried out. Figure 7a shows the 2D
Figure 7. 2D WAXD pattern of PNb18PT at ambient temperature
with the X-ray beam perpendicular to the fiber direction (a) and
azimuthal scan (b) of (a) in the low- and high-angle regions.
WAXD pattern of PNb18PT at ambient temperature. A pair of
diffraction arcs with a d-spacing value of 3.74 nm originating
from the layer structure are located on the equator. In addition,
a pair of diffused halos at 2θ ≈ 20° (d-spacing of ∼0.45 nm)
corresponding to the average distance between neighboring
mesogens are more or less concentrated on the meridian with
rather broad azimuthal distributions. As shown in Figure 7b, the
azimuthal scans of the diffraction arc and the diffused halo in
the low- and high-angle regions, respectively, have an offset of
90°, indicating an SmA phase of PNb18PT. The estimated
length of the benzoate core in the side chains of PNb18PT is
∼2 nm according to the literature,11 and the simulated length of
the C18 alkyl tail with the all-trans conformation is ∼2 nm.
Therefore, the side chain of PNb18PT should be about 6 nm.
However, the d-spacing values of the low-angle peak in the 1D
WAXD pattern at ambient temperature is 3.74 nm, which is
much smaller than the calculated side-chain length of about 6
nm. The inconsistency may be attributed to two reasons. First,
the side chains may not be in an ideally all-trans conformation.
Second, the alkyl tails may adopt an interdigitated packing.
Other LC PNbnPTs (n = 12, 14, 16) have similar 2D WAXD
patterns as PNb18PT. Therefore, on the basis of 1D and 2D
WAXD results, we can conclude that the phase behavior of
Figure 6. 1D WAXD patterns of the as-cast PNbnPTs at ambient
temperature (a) and d-spacing values of the (100) peak or the low-
angle halo in (a) as a function of the number of carbons in the alkyl
tails (b).
18) changes approximately linearly with n following an
equation of d = 2.48 + 0.07n. The intercept of 2.48 nm,
which indicates the diameter of the rigid core of the polymer, is
slightly larger than the estimated length of ∼2 nm for the
benzoate core in the side chain,11 possibly owing to the off-
center arrangement of the side chains relative to the rigid
polynorbornene backbone. In addition, the slope of 0.07 nm is
the projected length of two methylene units along the side-
chain axis in the SmA phase. The theoretical value can be
estimated to be ∼0.25 nm assuming an all-trans conformation.
The great inconsistency between these two values may suggest
a significant interdigitation between the polymer side chains in
the smectic layers. On the other hand, the d-spacing values of
the low-angle halo of the amorphous polymers (n = 8, 10)
deviate from the line, which can be immediately attributed to
their different states from the LC phases of the polymers with
longer alkyl tails.
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PNbnPT is greatly dependent on the alkyl-tail length. When n
is more than 10, the polymer is induced to develop an SmA
phase, and the degree of order increases with increasing alkyl-
tail length. The polymer is amorphous when n is no more than
10. Figure 8 depicts the molecular packing of PNbnPTs at
ambient temperature.
on the alkyl-tail length. PNb8PT and PNb10PT are amorphous
in the whole temperature range before degradation. When the
alkyl tail becomes longer, PNbnPTs (n = 12, 14, 16, 18)
develop into the SmA phase, and the LC phase becomes more
ordered with increasing alkyl-tail length. Compared with
polyethylene-based MJLCPs, MJLCPs with a polynorbornene
main chain exhibit different phase behaviors resulting from their
specific structural characteristics. Such MJLCPs with a very high
MW (Mn > 105 Da) may be fabricated into polymer films and
fibers to be used as structural materials. In addition,
controllable stereoregularity and high functional group
tolerance of polynorbornenes render these MJLCPs to be
potential functional materials, such as photovoltaic materials.
With the new development of organic metal catalysts and
ROMP reactions, polynorbornene-based MJLCPs with well-
defined composition, topology, stereoregularity, and function-
ality will be prepared to explore new functional materials with
fascinating ordered structures and properties.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: zshen@pku.edu.cn (Z.S.).
*E-mail: fanxh@pku.edu.cn (X.-H.F.).
Figure 8. Schematic drawing of the molecular packing of PNbnPTs at
ambient temperature.
Notes
The authors declare no competing financial interest.
Comparison of MJLCPs with Polynorbornene and
Polyethylene Main Chains. From a synthetic point of view,
this work describes a new method by introducing a key
intermediate, NbTA, for facilely synthesizing varieties of
MJLCPs with a polynorbornene main chain. Compared with
the reported pathways,37−40,47 the method in this work is more
robust and greener, with elimination of complicated synthetic
procedures and polluting heavy metal catalysts. From a
molecular point of view, polynorbornene is a more rigid main
chain than polyethylene because of the double bonds and five-
membered rings in the backbone. More flexible units are
needed in these polymers to balance the rigidity of the
backbone for the formation of LC phases. On the other hand,
the length of the repeating unit of polyethylene is ∼0.25 nm
compared with ∼0.5 nm for that of polynorbornene.24,56 In
other words, the mesogen density in a polynorbonene-based
MJLCP is only about one-half of that in a polyethylene-based
MJLCP. Therefore, longer alkyl tails are required in the side
chains for the polymers to become liquid crystalline. Compared
to MJLCPs with a polystyrene backbone, the entropy change of
the main chain and side chains in polynorbornenes cannot
compensate for the enthalpy increase at high temperatures.13,57
Hence, MJLCPs with a polynorbornene main chain have lower
isotropization temperatures than those of MJLCPs with a
polystyrene main chain and can go into the isotropic state
during heating. To prove this hypothesis, molecular engineering
of the side chains, such as introducing more bulky biphenyl or
terphenyl groups to the side chains, is currently ongoing in our
laboratory.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21134001, 21374002, and
20990232). The authors gratefully thank Dr. Wen-Ping Chen
at Donghua University for her help with 2D WAXD
measurements.
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