Side-on main-chain liquid crystalline polymers prepared by acyclic diene metathesis polymerization and thiol-ene click step-growth polymerization

Article abstract of DOI:10.1002/pola.27105

In this manuscript, we design and synthesize a series of X-shaped mesogenic monomers, bearing two terminal-alkene tails. Starting from these X-shaped monomers, acyclic diene metathesis polymerization and thiol-ene polyaddition are applied for the first time to prepare two series of side-on main-chain liquid crystalline polymers (LCPs), respectively. The mesomorphic behaviors and structure-property relationships of these new polymers are studied in detail by a combination of ¹H NMR, GPC, TGA, DSC, POM, and XRD experiments. It turns out that the length of alkoxy terminal chains of the embedded mesogens and the length of the spacer connecting two adjacent mesogens on the polymer backbones markedly influence the mesomorphic properties. Furthermore, a side-on main-chain liquid crystalline elastomer fiber is prepared by crosslinking the LCP using a UV-sensitive bifunctional benzophenone crosslinker. In comparison with the pure polymer fiber's monodomain alignment, the crosslinked elastomer fiber however shows nonaligned polydomain structure, probably due to the order competitions between the mixed crosslinkers, mesogens, and polymer backbones. 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 1086-1098 A series of X-shaped mesogenic monomers bearing two polymerizable terminal-alkene tails are designed and synthesized. Starting from these X-shaped monomers, acyclic diene metathesis polymerization (ADMET) and thiol-ene click step-growth polymerization are applied to prepare two series of side-on main-chain liquid crystalline polymers, respectively. Copyright

Full text of DOI:10.1002/pola.27105

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Side-On Main-Chain Liquid Crystalline Polymers Prepared by Acyclic
Diene Metathesis Polymerization and Thiol-Ene Click Step-Growth
Polymerization
Hong Yang, You-Jing Lv, Bao-Ping Lin, Xue-Qin Zhang, Ying Sun, Ling-Xiang Guo
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, China
Correspondence to: H. Yang (E-mail: pkuyh9@gmail.com) or B.-P. Lin (E-mail: lbp@seu.edu.cn)
Received 27 October 2013; accepted 17 January 2014; published online 4 February 2014
DOI: 10.1002/pola.27105
ABSTRACT: In this manuscript, we design and synthesize
a
mesomorphic properties. Furthermore, a side-on main-chain liq-
uid crystalline elastomer fiber is prepared by crosslinking the
LCP using a UV-sensitive bifunctional benzophenone cross-
linker. In comparison with the pure polymer fiber’s monodo-
main alignment, the crosslinked elastomer fiber however shows
nonaligned polydomain structure, probably due to the order
competitions between the mixed crosslinkers, mesogens, and
series of X-shaped mesogenic monomers, bearing two terminal-
alkene tails. Starting from these X-shaped monomers, acyclic
diene metathesis polymerization and thiol-ene polyaddition are
applied for the first time to prepare two series of side-on main-
chain liquid crystalline polymers (LCPs), respectively. The meso-
morphic behaviors and structure-property relationships of these
1
new polymers are studied in detail by a combination of
H
polymer backbones. V
2014 Wiley Periodicals, Inc. J. Polym.
C
NMR, GPC, TGA, DSC, POM, and XRD experiments. It turns out
that the length of alkoxy terminal chains of the embedded
mesogens and the length of the spacer connecting two adjacent
mesogens on the polymer backbones markedly influence the
Sci., Part A: Polym. Chem. 2014, 52, 1086–1098
KEYWORDS: liquid crystalline polymer; metathesis; thiol-ene;
polycondensation; fiber
INTRODUCTION Liquid crystalline polymers (LCPs), possess-
ing both the polymeric mechanical properties and the aniso-
tropic liquid crystalline state, have attracted more and more
scientific attentions during the past few decades. There have
been intensive research works applying LCPs as high-
strength and high-modulus fibers, engineering plastics, gas
separation membranes, stationary phases, electro-optic or
nonlinear optic materials, optical data storage materials,
were very limited. As shown in Scheme 1, most of previously
reported side-on MCLCPs were prepared by solution-
polycondensation of the mesogenic hydroquinone derivative
1 with various alkane-a,x-dioic acid dichlorides, giving the
corresponding polymers 2 and 3 with flexible or semi-rigid
backbones, respectively. Another traditional method was to
perform Suzuki coupling or Stille coupling reactions of the
mesogenic 1,4-dibromoaryl derivative 4 to provide conju-
1
,2
28
29
etc. In the viewpoint of molecular engineering, LCPs can
gated side-on MCLCPs 5 and 6. Herein, we design and
synthesize a series of novel X-shaped mesogenic monomers
7, bearing two polymerizable terminal-alkene tails. Starting
from monomers 7, acyclic diene metathesis polymerization
3
,4
be generally divided into four categories
Figure 1: end-on and side-on main-chain liquid crystalline
as shown in
5
–8
polymers (MCLCPs)
chain liquid crystalline polymers (SCLCPs),
as well as end-on and side-on side-
9
–15
30,31
32–34
depending on
(ADMET)
for the first time to prepare two series of side-on MCLCPs 8
and respectively. The mesomorphic behaviors and
and thiol-ene click chemistry
are applied
the positions where the mesogenic units are either embed-
ded (main-chain) or grafted as side groups (side-chain) to
the polymer backbones, and either longitudinally (end-on) or
laterally (side-on) attached into the polymeric systems.
9
structure-property relationships of these new polymers are
presented and discussed in this manuscript.
Among them, side-on MCLCPs, also known as shish-kebab-
Besides synthetic improvements and mesomorphic character-
izations, another important motivation behind that we revisit
this side-on MCLCPs subject is attracted by their unique
structures of polymer backbones, which will never be
directed into the same molecular directions of embedded
1
6
17,18
type LCPs or X-shaped LCPs,
comparing with the other
three kinds of LCPs, have been much less studied in the past
1
9–27
twenty years.
One of the main reasons is that the
known synthetic protocols of preparing side-on MCLCPs
Additional Supporting Information may be found in the online version of this article.
2014 Wiley Periodicals, Inc.
V
C
1
086
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instrument (New Castle, DE) under nitrogen purge at a heat-
ꢀ
ing rate of 10 C/min. Thermogravimetric analysis (TGA)
was performed on a Perkin-Elmer TGA7. Fourier transform
infrared (FTIR) spectra were obtained with a Nicolet 5700
FTIR spectrometer (Thermo Electron Scientific Instruments
22
Corp). A UV lamp (20 mW cm , k 5 365 nm; LP-40A;
LUYOR Corporation) was used to irradiate the samples to
perform the photo-crosslinking reactions.
Polarized optical microscopy (POM) observations of the liq-
uid crystalline textures of the monomers, polymers and
fibers were performed on an Olympus BX53P microscope
with a Mettler PF82HT hot stage. The images were captured
using a Microvision MV-DC200 digital camera with Phenix
Phmias2008 Cs Ver2.2 software.
FIGURE 1 Schematic illustration of structural models of LCPs.
mesogens even under external aligning forces. In fact, this
character is quite fascinating since the molecular directions
of mesogens contained in all the other three kinds of LCPs
X-ray scattering experiments were performed with a high-
flux small angle X-ray scattering (SAXS) instrument (SAXSess,
Anton Paar) equipped with Kratky block-collimation system
and a temperature control unit (Anton Paar TCS300). At
each single steady temperature, both SAXS and wide angle X-
ray scattering (WAXS) were simultaneously recorded on an
imaging-plate (IP), which extended to high-angle range (the
(
end-on MCLCPs, end-on SCLCPs and side-on SCLCPs) can be
always easily aligned in the same polymer backbones
stretching directions under external forces. Undoubtly,
exploring side-on MCLCPs possessing this unique backbone
structure might bring some information about how to use
these LCPs as some novel engineering materials. With this
long-term goal, we herein would also like to report our ini-
tial efforts of preparing crosslinked fiber materials using
side-on MCLCPs.
2
1
q range covered by the IP was from 0.06 to 29 nm
,
q 5 4psinh/k, where the wavelength k is 0.1542 nm of Cu-Ka
radiation and 2h is the scattering angle) at 40 kV and 40 mA
for 30 min.
Synthesis of Intermediate 12
EXPERIMENTAL
Typical procedure: to prepare 4-heptyloxy-benzoic acid
methyl ester: 4-Hydroxylbenzoic acid methyl ester (5.00 g,
32.9 mmol), KOH (2.12 g, 37.8 mmol, 1.15 equiv.), benzene
(20 mL), and DMF (20 mL) were added into a 250 mL
round-bottom flask. The reaction mixture was heated to 50
Materials and Instrumentation
Hoveyda-Grubbs second generation catalyst, potassium trime-
thylsilanolate were purchased from Aldrich. 2,5-Dihydroxyter-
ephthalic acid, C H2n1 1Br (n 5 7–12), tetrabutylammonium
n
ꢀ
bromide, 4-bromo-but-1-ene, oxalyl chloride, azodiisobutyroni-
trile (AIBN) were purchased from Aladdin (Shanghai). Liquid
crystal cells were purchase from Instec. Dichloromethane, trie-
thylamine, chlorobenzene, and DMF were distilled from CaH2
under nitrogen. THF was distilled from sodium-benzophenone
ketyl under nitrogen. Other chemical reagents were used
without further purification. All nonaqueous reactions were
conducted in oven-dried glassware, under a dry nitrogen
atmosphere. All flash chromatography were performed using
Macherey-Nagel MN Kieselgel 60 (0.063–1.2 mm). A Sigma
C, 1-bromo-heptane (6.55 mL, 36.2 mmol, 1.1 equiv.) was
then slowly added into the above flask via a syringe. The
reaction solution was refluxed for 6 h and then cooled to r.t.
After filtering off inorganic salts, the resulting solution was
concentrated by rotary evaporation to give a pale yellow
crude solid, which was redissolved in 100 mL diethylether
and washed by 10% sodium bicarbonate aqueous solution
(75 mL, twice), water (75 mL, twice), then dried over
sodium sulfate. The organic solution was concentrated by
rotary evaporation to give the desired product (6.84 g, Yield:
1
2K 15 centrifuge was used for isolation of polymers after
88%) as a white solid. H NMR (500 MHz, CDCl
3
, d): 8.99 (d,
precipitation.
J 5 8.1 Hz, 2H, Ar H), 6.92 (d, J 5 8.1 Hz, 2H, Ar H), 4.02 (s,
2
H, AOACH A), 3.90 (s, 3H, AOACH ), 1.81 (s, 2H, CH ),
2
3
2
1
All H NMR spectra were obtained using a Bruker HW500
1.37 (m, 8H, CH ), 0.90 (s, 3H, CH ).
2
3
MHz spectrometer (AVANCE AV-500) and recorded in CDCl3
(
internal reference 7.26 ppm). Gel permeation chromatogra-
Synthesis of Intermediate 13
phy (GPC) was performed on an HP 1100 high pressure liq-
uid chromatography, equipped with an HP 1047A refractive
index detector and a Plgel MIXED-C 300-7.5 mm column
Typical procedure: to prepare 4-heptyloxy-benzoic acid: 4-
heptyloxy-benzoic acid methyl ester (5.00 g, 20.0 mmol),
KOH (1.23 g, 79.4 mmol, 1.10 equiv.), water (30 mL), and
ethanol (30 mL) were added into a 250 mL round-bottom
(
packed with 5 lm particles). The column packing allowed
ꢀ
the separation of polymers over a wide molecular weight
range of 200–3,000,000. THF was used as the eluent at a
flask. The reaction solution was refluxed at 100 C for 6 h
and then cooled to r.t. The resulting solution was neutralized
by adding 3N HCl until pH 5 1. The appearing solid was fil-
tered and washed by ethanol and water. The crude solid was
recrystallized from methanol to give the desired product
ꢀ
low flow rate of 1 mL/min at 35 C. Polystyrene standards
were used as the references. Differential scanning calorime-
try (DSC) spectra were recorded on a TA Instruments Q20
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SCHEME 1 Representative synthetic protocols of side-on main-chain LCPs reported previously and herein this manuscript.
1
(
3.87 g, Yield: 82%) as a white solid. H NMR (500 MHz,
THF at ambient temperature under nitrogen. The reaction
mixture was stirred for 2 h at room temperature. The white
solid was filtered, washed with ether, and dried under vac-
uum to provide 3.20 g intermediate, which was immediately
added into a 50 mL pressure tube. Tetrabutylammonium
bromide (722 mg, 0.22 mmol, 0.20 equiv.) and 4-bromo-but-
CDCl , d): 8.06 (d, J 5 8.0 Hz, 2H, Ar H), 6.94 (d, J 5 8.0 Hz,
3
2
H, Ar H), 4.04 (m, 2H, AOACH A), 1.82 (s, 2H, CH ), 1.38
2 2
(m, 8H, CH ), 0.90 (s, 3H, CH ).
2 3
Synthesis of 2,5-Dihydroxy-terephthalic Acid
Dibut-3-enyl Ester (Intermediate 15)
,5-Dihydroxyterephthalic acid (2.31 g, 11.4 mmol) was
1-ene (4.70 g, 33.6 mmol, 3.0 equiv.) were then added into
ꢀ
2
this pressure tube. This neat reaction was heated at 60
C
added in one portion to a stirred slurry of potassium trime-
thylsilanolate (3.26 g, 22.8 mmol, 2.00 equiv.) in 100 mL dry
for 12 h and then diluted with 100 mL ethylacetate. After fil-
tering off the solids, the solution was then concentrated to
1
088
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SCHEME 2 Synthesis of X-shaped monomers and side-on main-chain LCPs.
give a crude solid, which was directly subjected to purifica-
tion by column chromatography (30:1 petroleum ether - eth-
CH Cl and then concentrated by rotary evaporation to give
a yellow crude solid, which was recrystallized from ethanol
2
2
ylacetate) to provide desire compound 15 (2.65 g, 76%) as a
to provide the desired compound 7-7 (600 mg, 49.5%) as a
1
1
yellow-green solid. H NMR (500 MHz, CDCl , d): 10.10 (s,
white solid. H NMR (500 MHz, CDCl ) d 8.16 (d, J 5 8.8 Hz,
3
3
2H, OH), 7.46 (s, 2H, Ar H), 5.82 (m, 2H, C@CHA), 5.16 (m,
4H, Ar H), 7.87 (s, 2H, Ar H), 6.99 (d, J 5 8.8 Hz, 4H, Ar H),
4
H, C@CH ), 4.41 (t, J 5 6.6 Hz, 4H, AOACH ), 2.54 (m, 4H,
5.66 (m, 2H, C@CHA), 5.01 (m, 4H, C@CH ), 4.22 (t, J 5 6.6
2
2
2
CH2).
Hz, 4H, AOACH ), 4.06 (t, J 5 6.6 Hz, 4H, AOACH ), 2.27
2
2
(
m, 4H, CH ), 1.80 (m, 4H, CH ), 1.39 (m, 20H, CH ), 0.91 (t,
2 2 2
Synthesis of Monomers 7-m
Typical procedure: to prepare 7-7: Oxalyl chloride (1.78 mL,
0.4 mmol, 12.5 equiv.) was added into a solution of 4-n-
heptylbenzoic acid (0.96 g, 4.08 mmol, 2.50 equiv.) in 20 mL
CH Cl . One drop of DMF was added into this reaction mix-
J 5 6.0 Hz, 6H, CH3).
2
Synthesis of Polymers 8-m
Typical procedure: to prepare 8-7: Monomer 7-7 (500 mg,
0.67 mmol), Hoveyda-Grubbs second generation catalyst (17
mg, 4 mol %) and dry chlorobenzene (2.5 mL) were added
into a 25 mL Schlenk-type flask. The reaction mixture was
2
2
ture, which was then allowed to stir for 1 h at room temper-
ature. The reaction mixture was concentrated by using
rotovap first and then a high vacuum oil pump to completely
dry. The residue was redissolved in 20 mL CH Cl . Triethyl-
ꢀ
stirred at 40 C for 24 h. Ethylene gas was removed by
applying vacuum continuously. The reaction mixture was
then poured into methanol (50 mL) containing 1N HCl (5%
v/v) to precipitate the polymer. The resulting brownish poly-
mer was further purified by dissolving in CH Cl , reprecipi-
tating from methanol several times, and drying in vacuo,
which gave the desired polymer 8-7 (225 mg, Yield: 45%) as
2
2
amine (2 mL) and compound 15 (500 mg, 1.63 mmol, 1.00
equiv.) were added into this resulting solution. The reaction
mixture was allowed to stir for 24 h at room temperature.
The resulting solution was neutralized by adding 3N HCl
until pH 5 7. The organic solution was extracted by 200 mL
2
2
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1
a brownish solid. H NMR (500 MHz, CDCl , d): 8.12 (d,
3
J 5 8.7 Hz, Ar H), 7.82 (s, Ar H), 6.94 (d, J 5 7.5 Hz, Ar H),
5.28 (m, AHC@CHA), 4.11 (m, AOACH2), 4.01 (m,
AOACH ), 2.13 (m, CH ), 1.80 (m, CH ), 1.36 (m, CH ), 0.89
2
2
2
2
(m, CH3).
Synthesis of Polymers 9-m
Typical procedure: to prepare 9-7: Monomer 7-7 (500 mg,
.67 mmol), 1,2-ethanedithiol (63 mg, 0.65 mmol), AIBN (2.7
mg, 2.5 mol %), and dry toluene (0.5 mL) were added into a
5 mL Schlenk-type flask. The flask was degassed and
0
2
exchanged with nitrogen via three freeze-thaw cycles. The
ꢀ
reaction mixture was heated at 60 C for 12 h and then
poured into methanol (50 mL) to precipitate the polymer.
The resulting polymer was further purified by dissolving in
CH Cl , reprecipitating from methanol several times, and
2
2
1
drying in vacuo, which gave the desired polymer 9-7 (336
FIGURE 2 H NMR spectra of monomer 7–8, ADMET polymer
-8, and thiol-ene polymer 9-8.
1
8
mg, Yield: 60%) as a milky solid. H NMR (500 MHz, CDCl ,
3
d): 8.15 (d, J 5 7.8 Hz, Ar H), 7.87 (s, Ar H), 6.99 (d, J 5 8.7
Hz, Ar H), 4.19 (m, AOACH ), 4.05 (m, AOACH ), 2.70 (m,
2
2
ASACH
2
), 2.40 (m, CH
2
), 1.83 (m, CH
2
), 1.38 (m, CH
2
), 0.91
silicone oil to prevent fibers to stick on the glass. The micro-
ꢀ
(m, CH3).
scope slide was placed on a hot stage and heated to 30 C,
ꢀ
which is in the liquid crystalline phase. At 30 C, the fibers
were irradiated by using a UV-lamp for 120 min. The
average diameter of the fiber prepared this way was about
Preparation of Crosslinked LCP Fibers
ADMET polymer 8–12 (50 mg, 0.057 mmol monomer units,
0 mol %), and the bifunctional benzophenone crosslinker,
4-[6-(4-Benzoyl-phenoxy)-hexyloxy]-phenyl}-phenyl-metha-
80–100 mm.
9
{
3
5
none (3.0 mg, 0.006 mmol, 10 mol %) were added into a 5
mL vial. The mixture was fully dissolved in 1 mL dry CH Cl ,
RESULTS AND DISCUSSION
2
2
the solvent was then removed in vacuo. The vial was placed
on a hot stage and heated to 80 C. When the mixture
became viscous, the fibers were drawn by dipping the tip of
a capillary pipette and pulling the mixture with it as quickly
as possible. The fibers were then placed on a microscope
slide, which has been surface-treated with several drops of
Synthesis and Characterization of the Polymers
Inspired by the molecular structure of a tetrafunctional
mesogenic thiol-ene crosslinker reported by Keller,
designed a series of novel X-shaped monomers 7-m (m 5 7,
8, 9, 10, 11, 12, represent different alkyl tail lengths). As
shown in Scheme 2, 2,5-dihydroxyterephthalic acid was first
ꢀ
3
6
we
TABLE 1 Molecular Weights and Thermal Properties of the Polymers
a
b
b
ꢀ
c
ꢀ
d
Polymer
[M]/[I]
M
n
(g/mol)
M
w
(g/mol)
M /M
w n
T
g
( C)
T ( C)
d
8
8
8
8
8
8
9
9
9
9
9
9
-7
25:1
25:1
25:1
25:1
25:1
25:1
40:1
40:1
40:1
40:1
40:1
40:1
8850
7590
6560
9700
10,150
10,890
4060
4330
4790
5250
6400
7280
17,260
14,660
10,440
17,270
20,510
17,640
7850
1.95
1.93
1.59
1.78
2.02
1.62
1.93
2.11
2.46
1.78
1.93
1.34
8
9
5
7
5
5
275
262
247
299
292
280
235
248
248
254
246
279
-8
-9
-10
-11
-12
-7
28
-8
9150
217
28
-9
11,790
9350
-10
-11
-12
23
12,370
9720
214
e
n/d
a
d
[
M]/[I] is the molar ratio of monomers and catalyst.
The temperature at which 5% weight loss of the sample from TGA-
b
ꢀ
Molecular weight measurements were analyzed by GPC based on cali-
DSC under nitrogen atmosphere at a heating rate of 10 C/min.
e
bration using polystyrene standards.
c
n/d: not detected.
ꢀ
T was determined from DSC first cooling scan at a rate of 10 C/min.
g
1
090
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SCHEME 3 Thiol-ene polymerization of monomer 7-7 and a variety of dithiols.
treated with potassium trimethylsilanolate to form its bispo-
tassium salt, which was heated with 4-bromo-but-1-ene
under neat conditions (no solvent) to give intermediate 15.
Meanwhile, 4-n-alkylbenzoic acids 13 were first transformed
to their corresponding acyl chlorides, which subsequently
reacted with intermediate 15 to provide the desired X-
shaped monomers 7-m. Comparing with traditional meso-
genic hydroquinone derivative monomers (Compound 1,
Scheme 1), which had to be prepared through long synthetic
routes including oxidation, substitution, and reduction reac-
Thiol-ene click chemistry, as a highly efficient radical-
initiated addition reaction, has been widely adopted to syn-
thesize various polymeric materials under benign condi-
3
2–34,44–50
tions.
several examples using thiol-ene chemistry to prepare end-
In LCP research field, there have also been
5
1–53
54–58
on MCLCPs
and SCLCPs.
Following the traditional
synthetic protocol of using AIBN as the radical initiator and
ꢀ
heating at 60 C in toluene, we performed this step-growth
and addition/chain polymerization of diolefins 7-m and 1,2-
1
ethanedithiol. H NMR spectra and the corresponding peak
1
6–27
tions,
these new X-shaped monomers are oxidative inert,
assignments of representative monomer 7–8 and polymer 9-
8 are shown in Figure 2. In comparison, the terminal olefin
protons of 7–8 originally located at 5.0–5.7 ppm disap-
highly air-stable and much easier to access.
Recently, Meier and coworkers have applied both ADMET
and thiol-ene polyaddition methods to investigate the two
peared, as well as the methylene protons of SACH A newly
2
appeared at 2.5–3.0 ppm, which implied that the desired
thiol-ene polymer 9-8 has been successfully prepared.
3
7
different polymerization behaviors of butadiene ditelomers.
Herein, we would also like to apply these two step-growth
polycondensations of diolefin-monomers to synthesize two
series of novel side-on MCLCPs 8-m and 9-m (herein, m 5 7,
As observed by GPC showed in Table 1, both ADMET poly-
merization and thiol-ene polymerization provided low molec-
ular weight short polymers with broad polydispersities.
These data are very consistent to the H NMR spectra of
polymers 8-m listed in the supporting information, present-
ing low degrees of polymerization (n 5 10–20), according to
8, 9, 10, 11, 12, represent different alkyl tail lengths) as
1
shown in Scheme 2. However, our interests are not focusing
on studying the polymerization behaviors, but trying to
understand the molecular structure-mesomorphic property
relationships of these two categories of polymers, which are
slightly different from each other on the backbone structures
only.
TABLE 2 Molecular Weights of Thiol-ene Polymers
C
[M]
M
n
M
w
Previous works using metal alkylidene-catalyzed ADMET,
demonstrated that end-on MCLCPs with excellent mesoge-
a
b
b
Polymer
(mol/L)
(g/mol)
(g/mol)
w n
M /M
c
3
8–43
9-7
1.3
2.0
2.0
2.0
2.0
2.0
4060
7850
1.93
1.84
1.70
1.79
1.57
1.64
nicity could be readily obtained.
Encouraged by these
c
9
-7
6020
11,040
11,390
12,430
17,610
17,790
pioneering works, we chose Hoveyda-Grubbs second gener-
ation catalyst (25:1 substrate/catalyst molar ratio), in com-
bination with chlorobenzene solvent and vacuum condition
d
9-7
6710
d
d
d
9-7-2
6950
(
Scheme 2) to polymerize X-shaped monomers. As shown in
9
9
-7-3
-7-4
11,200
10,850
Figure 2, when monomer 7–8 was polymerized, the original
terminal olefin protons located at ꢁ5.0 and ꢁ5.7 ppm,
became acyclic olefin protons so that the resonance signals
shifted to ꢁ5.2 ppm. Furthermore, calculating the integra-
tion ratio of acyclic olefin protons and the leftover terminal
olefin protons, could roughly measure the degree of
polymerization.
a
C[M] is the initial concentrations of monomers. The molar ratio of
monomers and catalyst is always 40:1.
b
Molecular weight measurements were analyzed by GPC based on cali-
bration using polystyrene standards.
c
The reaction solvent is toluene.
The reaction solvent is DMF.
d
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FIGURE 3 DSC curves of ADMET polymers 8-m and thiol-ene polymers 9-m during (A, C) the first cooling scan and (B, D) the sec-
ꢀ
ond heating scan at a rate of 10 C/min under nitrogen atmosphere.
olefin end group analysis. As to thiol-ene polymers, the end
groups of the corresponding polymers could be either mer-
capto group or unsaturated carbon-carbon double bond, and
the distributions of these two functional groups are highly
investigated the effects of monomer concentrations, solvents
and dithiols. As shown in Scheme 3 and Table 2, the monomer
concentrations (C[M]) markedly influenced the degree of poly-
merization. When the initial monomer concentration was raised
to 2.0 mol/L (entry 2, Table 2), the molecular weight of the cor-
responding polymer obviously increased. We also tried DMF
instead of toluene, as the reaction solvent (entry 3, Table 2), the
molecular weight of the corresponding polymer slightly
increased. With this optimal reaction condition of using a 2.0
mol/L initial monomer concentration and DMF solvent, a variety
of dithiols (1,2-ethanedithiol, 1,4-butanedithiol, 1,6-hexanedi-
thiol and 1,4-dimercaptomethylbenzene) have been examined
to prepare the corresponding thiol-ene polymers 9-7, 9-7-2, 9-
1
unpredictable. In another word, H NMR spectra of polymers
9-m could not give any reliable information about the degree
of polymerization since the end groups of polymers could
not be defined. Thus, only GPC data could bring us a conclu-
sion that these thiol-ene polymers are shorter oligomers
comparing with the ADMET polymers.
To optimize the thiol-ene polymerization reaction conditions to
prepare relatively higher molecular weight polymers, we further
1
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a
TABLE 3 Mesomorphic Properties of the Monomers and Polymers
Phase
ADMET
Polymer
Phase
Thiol-ene
Polymer
Phase
ꢀ
ꢀ
ꢀ
Monomer
Transitions ( C)
Transitions ( C)
Transitions ( C)
7
7
7
7
7
7
-7
K 88 I
I 47 K
K 80 I
I 38 K
K 96 I
I 45 K
K 85 I
I 24 K
K 84 I
I 36 K
K 78 I
I 34 K
8-7
G 19 N 62 I
9-7
G 4 I
I 58 N 8 G
I 28 G
G 0 I
-8
8-8
G 14 N 71 I
9-8
I 67 N 9 G
I 217 G
G 6 I
-9
8–9
G 11 SmC 68 I
I 64 SmC 5 G
G 16 SmC 75 I
I 71 SmC 7 G
G 17 SmC 77 I
I 72 SmC 5 G
G 13 SmC 75 I
I 71 SmC 5 G
9-9
I 28 G
G 9 I
-10
-11
-12
8–10
8–11
8–12
9–10
9–11
9–12
I 23 G
G 5 I
I 214 G
X 13 I
I 10 X
a
Determined by POM and X-ray diffraction. K 5 crystalline, N 5 nematic, SmC 5 smectic C, I 5 isotropic, G 5 glass phase, X 5 undefined phase. First
line obtained on heating, second line obtained on cooling.
7
-3, and 9-7-4 respectively. Comparing with 1,2-ethanedithiol
thermal transitions and phase properties of these monomers
and two series of polymers are listed in Table 3. All the X-
shaped monomers had no liquid crystalline phases and the
melting points showed an obvious odd-even effect in zigzag
and 1,4-butanedithiol (entry 3 and 4, Table 2), 1,6-hexanedithiol
and 1,4-dimercaptomethylbenzene (entry 5 and 6, Table 2)
seemed to be better reaction substrates for thiol-ene polycon-
densation. Our hypothesis is that a relatively long spacer con-
necting two terminal mercapto groups might stabilize the
possibly existing biradical intermediate (S-R-S ) in the polyaddi-
tion reaction. Nonetheless, even though we optimized the syn-
thetic technique and improved a little bit the degrees of
polymerization, both ADMET polymers and thiol-ene polymers
were still low molecular weight short polymers.
5
9,60
manner,
which were further confirmed by DSC experi-
ments (Figure S1, Supporting Information). In agreement with
DSC results (Fig. 3), all ADMET side-on MCLCPs had one sin-
gle enantiotropic liquid crystalline phase from room tempera-
ture up to their clearing points, while all thiol-ene polymers
oppositely showed no optical birefringence above room tem-
perature under POM observations. The only difference of the
molecular structures between ADMET polymers and thiol-ene
polymers is two obvious different lengths of the spacers con-
necting two adjacent mesogens on the main-chain polymer
backbones. As to ADMET polymers, the spacers contain 8 car-
bon atoms and 2 oxygen atoms, which are estimated as
.
.
Phase Transitions and Mesomorphic Properties of the
Polymers
The phase behaviors of all the polymers were investigated
by DSC, POM, SAXS, and WAXS. The thermal properties of
ADMET polymers 8-m and thiol-ene polymers 9-m are pre-
sented in Table 1. The temperatures at 5% weight loss under
ꢁ
1.30 nm by Dreiding models. In comparison, the spacer
lengths of thiol-ene polymers are probably around 2.14 nm
containing 12 carbon atoms, 2 oxygen atoms and 2 more sul-
fur atoms. Apparently, the spacer length might be too long so
that the intermolecular interactions of side-on embedded
mesogens are dramatically weakened and the corresponding
thiol-ene polymers 9-m could not be able to achieve liquid
crystalline orders above room temperature.
ꢀ
nitrogen (Td’s) were all above 230 C, indicating that all the
polymers had good thermal stabilities. Comparing with poly-
mer series 9-m, polymer series 8-m started to thermal-
decompose at slightly higher temperatures.
As shown in Figure 3, DSC experiments of these new poly-
mers all showed obvious glass transitions below room tem-
perature during the first cooling and subsequent heating
scans. The glass transition temperatures (T ’s) of ADMET
polymers were about 10–20 C higher than the T
ene polymers, which might be contributed to the more flexi-
ble main-chain backbones of polymer series 9-m. Besides the
glass transitions, apparent first-order phase transition proc-
esses of ADMET polymers were observed during the cooling
and heating circles. Interestingly, thiol-ene polymers however
The representative birefringent images of ADMET polymers
8-m are presented in Figure 4. Due to polymers’ high viscos-
ities, all the samples showed very complicated textures with
few-micron-sized tiny domains all over the observation fields
[Fig. 4(A–C)], which are difficult to be defined as any specific
mesophases. To further identify the phase structures, we
tried to make the sample films as thinner as possible, mean-
while align the samples and expected to observe some char-
acteristic textures. For example, the samples of 8–10 and 8–
g
ꢀ
g
’s of thiol-
g
did not show any other phase transitions above T ’s.
1
1 between two glass slides were mechanically sheared at
ꢀ
The mesomorphic behaviors of X-shaped monomers 7-m and
polymers 8-m, 9-m were further investigated by POM. The
50 C and annealed for several hours. Through these proc-
esses, although the micron-sized multi-domains became
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ꢀ
ꢀ
FIGURE 4 Polarized optical microscope images of (A) 8-7 maintained at 50 C, (B) 8-8 maintained at 50 C, (C) 8–9 maintained at
ꢀ
ꢀ
ꢀ
ꢀ
5
0 C, (D) 8–10 maintained at 50 C, (E) 8–11 maintained at 50 C, and (F) 8–12 maintained at 50 C.
ꢀ
FIGURE 5 One-dimensional WAXS patterns of polymers 8-m obtained at 50 C during (A) the heating and (B) cooling processes.
1
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TABLE 4 Layer Information of Side-on MCLCPs
To elucidate the mesomorphic phases of these side-on
MCLCPs exhibiting birefringent textures, simultaneous SAXS
and WAXS experiments were performed to identify the corre-
sponding phase structures. As illustrated in Figure 5, 8-7 and
21 a
)
b
c
0 d
Polymer
q (nm
d (nm)
l (nm)
h ( )
8
8
8
8
-9
2.10
1.96
1.91
1.72
2.99
3.21
3.29
3.65
3.81
4.10
4.28
4.57
38.3
38.4
39.7
37.0
8-8 both show two diffuse diffraction peaks at low angles
-10
-11
-12
and high angles, which demonstrate that there were no layer
structures inside these two samples and verify the existences
of nematic phases. On the contrary, when the n-alkoxy tails of
these side-on MCLCPs lengthen (m5 9 2 12), an obvious dif-
fraction peak appears at low angles and becomes sharper and
sharper, which confirm the existence of layer structures. The
corresponding lamellar thicknesses (d) of 8-m are listed on
Table 4. Comparing with the fully extended lengths (l) of the
laterally attached mesogens, as estimated by Dreiding models,
these novel side-on MCLCPs might have tilted SmC phases
a
b
c
Determined by SAXS.
Calculated by the equation d 5 2p/q.
Estimated by Dreiding models.
d
The tilt angle between molecular director and layer normal.
larger, the POM images [Fig. 4(D,E)] with plenty of compli-
cated disclination lines were still too complicated to be char-
acterized. Some birefringent areas of 8–11 sample [Fig.
ꢀ
and the tilt angles (h) are all perfectly around 38 according
to the equation h 5 cos (d/l).
21
4
(D)] appeared as schlieren-like texture which might be
6
1
nematic schlieren or smectic C schlieren. We also tried to
fill the sample of 8–12 into a 4 lm thick antiparallel
surface-rubbed liquid crystal cell. However, as shown in Fig-
ure 4(F), the surface anchoring effect was still not strong
enough to align such viscous polymer materials and obvi-
ously, the phase structure determinations could not be real-
ized by pure POM technique.
Crosslinked LCP Fibers
One major obstacle of using these novel LCPs as some opti-
cal materials, is that the glass transition temperatures (T ’s)
of these side-on MCLCPs are usually below room tempera-
ture. To prevent the materials melting at room temperature,
it is very important to find a way to get stabilized polymeric
g
FIGURE 6 Schematic illustration of preparation of crosslinked side-on main-chain LCPs.
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FIGURE 7 Photomicrographs of a pure 8–12 polymer thin fiber (A–C) and thick fiber (D–F), as well as an 8–12/crosslinker mixture fiber
before UV-illumination (G–I) and after UV-illumination (J–L) between polarizer and analyzer. These fiber were pulled from isotropic melts
ꢀ
ꢀ
ꢀ
ꢀ
at 80 C. The angles between fiber long axis and polarizer are (A, D, G, J) 45 , (B, E, H, K) 20 , (C, F, I, L) 0 , respectively.
3
5,42,43,58,66–68
networks by crosslinking these LCPs. Our design is illus-
trated in Figure 6, side-on MCLCPs are firstly mixed with
our recently reported bifunctional benzophenone crosslinker,
unidirection.
Following this protocol, we pre-
pared one thin pure 8–12 polymer fiber (around 40 mm in
diameter), another thicker pure 8–12 polymer fiber (around
90 mm in diameter), and one 8–12/crosslinker mixture fiber.
By rotating the fiber samples between polarizer and ana-
lyzer, the transmissions of two pure 8–12 polymer fibers
both maximized from near-extinction [Fig. 7(A–F)], when the
{
4-[6-(4-benzoyl-phenoxy)-hexyloxy]-phenyl}-phenyl-metha-
3
5
none, the sample mixtures are then aligned by some exter-
nal forces (pulling fibers, shearing, etc.). Finally, UV-
illumination initiates the photoreaction chemistry of benzo-
phenone units, which can attach to CAH r-bond in a H-
ꢀ
fiber axis changed to 45 from originally parallel to one
6
2–65
abstraction mechanism
so that a polymeric network
polarizer. There are several possibilities of molecular orienta-
tions based on this monodomain alignment phenomenon.
One possibility is that the laterally attached mesogens are
parallel or perpendicular to the fiber axis while the polymer
backbone is tilted. Another more likely possibility is that the
polymer backbone is parallel to the fiber axis while the side-
on mesogens are randomly tilted with a constant tilt angle of
would form by crosslinking neighboring aliphatic chains of
mesogens. The detailed preparation procedure of crosslinked
LCP fibers is described in the Experimental section.
Pulling fibers from isotropic-melt polymers is traditionally a
very efficient method to align liquid crystalline mesogens in
1
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ꢀ
around 38 in each layer so that the average molecular
director of mesogens is parallel to the polymer backbone,
potentially existing helical director structure is highly desirable
for photonic bandgap fibers and nonlinear optic materials. This
will be the subject of a future publication.
6
9–72
which might be very similar to de Vries SmA character.
On the contrary, 8–12/crosslinker mixture fiber showed
almost same transmissions when rotation angle of the sam-
ple between the fiber long axis and the polarizer kept chang-
ACKNOWLEDGMENTS
ꢀ
ꢀ
ing from 45 to 0 [Fig. 7(G–I)], which meant that the
mesogens formed a nonaligned polydomain structure. After
UV-illumination, the corresponding crosslinked MCLCP fiber
presented as a polydomain elastomer [Fig. 7(J–L)] as well,
although the birefringence changed. We also prepared a
crosslinked 8–12 polymer fiber with a fiber drawing temper-
This research was supported in part by National Natural Sci-
ence Foundation of China (Grant No. 21374016, 21002012),
Jiangsu Provincial Natural Science Foundation of China (Grant
No. BK2011588, BY2011153), and the Fundamental Research
Funds for the Central Universities. The authors would like to
gratefully thank Prof. Dong-Zhong Chen (Nanjing University)
for his help with XRD experiment measurements.
ꢀ
ature of 60 C which was below the clearing point (Figure
S4, Supporting Information) and a crosslinked 8-7 polymer
fiber (Figure S5, Supporting Information), these two fibers
both presented the similar POM behaviors. Furthermore, we
heated these samples from room temperature to the iso-
tropic phase and investigated the lengths of the fibers as a
function of temperature to measure the thermal actuation
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