Synthesis and thermal and photoluminescence properties of liquid crystalline polyacetylenes containing 4-alkanyloxyphenyl trans-4-alkylcyclohexanoate side groups

Article abstract of DOI:10.1021/ma0107962

Two series of polyacetylenes containing 4-alkanyloxyphenyl trans-4-n-alkylcyclohexanoate were synthesized using [Rh(nbd)Cl]₂, MoCl₅, and WCl₆ as initiators. Polymers 1P-3P, which contain no flexible spacer, show no mesomorphic properties due to the rigid poly(phenylacetylene) backbone. Polymers 4P-9P, which contain three or four methylene units in their spacers, exhibit both S_A and S_c phases. X-ray diffraction diagrams reveal that the liquid crystalline polyacetylene display an interdigitated bilayer structure for the smectic phases. Polymers 4P-9P are photoluminescent, and their emission wavelengths are about 500 nm. The photoluminescence intensity increased dramatically when a liquid crystalline polyacetylene was blended with poly(methyl methacrylate).

Full text of DOI:10.1021/ma0107962

1180
Macromolecules 2002, 35, 1180-1189
Synthesis and Thermal and Photoluminescence Properties of Liquid
Crystalline Polyacetylenes Containing 4-Alkanyloxyphenyl
trans-4-Alkylcyclohexanoate Side Groups
Ch in g-Hu a Tin g, J iu n -Ta i Ch en , a n d Ch a in -Sh u Hsu *
Department of Applied Chemistry, National Chiao Tung University, Hsinchu, Taiwan, 30050, R.O.C.
Received May 7, 2001
ABSTRACT: Two series of polyacetylenes containing 4-alkanyloxyphenyl trans-4-n-alkylcyclohexanoate
were synthesized using [Rh(nbd)Cl]2, MoCl5, and WCl6 as initiators. Polymers 1P -3P , which contain no
flexible spacer, show no mesomorphic properties due to the rigid poly(phenylacetylene) backbone. Polymers
4P -9P , which contain three or four methylene units in their spacers, exhibit both SA and Sc phases.
X-ray diffraction diagrams reveal that the liquid crystalline polyacetylenes display an interdigitated bilayer
structure for the smectic phases. Polymers 4P -9P are photoluminescent, and their emission wavelengths
are about 500 nm. The photoluminescence intensity increased dramatically when a liquid crystalline
polyacetylene was blended with poly(methyl methacrylate).
In tr od u ction
mesomorphic properties of the obtained polymers is
discussed. In addition, we report their photoluminescent
properties.
Side-chain liquid crystalline polymers (SCLCPs) are
of both theoretical and practical interest because they
combine the anisotropic properties of liquid crystals with
the polymeric properties and have potential application
in the fields of nonlinear optics, optical data storage,
piezoelectric transducer, and gas or liquid chromatog-
raphy stationary phase.1 So far, most SCLCPs have
been based on flexible polymer backbones, such as
polyacrylate, polymethacrylate, polysiloxane, and poly-
oxirane. Only a few rigid polymer backbones, such as
polynorborane2-14 and polyacetylene15-19 have been
used for the synthesis of SCLCPs because a rigid
polymer backbone is believed to play a destructive role
in the mesophase formation. Polyacetylene is the sim-
plest class of conducting polymer. In 1993, Shirakawa
et al. reported the first side-chain liquid crystalline
polyacetylene prepared by Ziegler-Natta and meth-
athesis initiators.15 To form a liquid crystalline phase
for the conducting polymer is the primary goal for
introducing a mesogenic moiety as a substituted group
in polyacetylene. The Japanese group successfully aligned
the polymers with a magnetic field and found that the
electrical conductivity of an aligned LC polyacetylene
increased about 2 orders of magnitude in the direction
parallel to the magnetic field.16 The limitation on the
synthesis of liquid crystalline polyacetylene is mainly
due to the polar functional groups, which may poison
the transition-metal initiators. Recently, Tang et al.
developed a variety of functionality-tolerant and air and
moisture-stable initiator systems and synthesized sev-
eral series of side-chain LC polyacetylenes containing
mesogens with ether, ester, amide and cyano func-
tionalities.20-24 Some of the side-chain LC polyacety-
lenes exhibit photoluminescent properties.25-27 In our
previous publications,28,29 we reported several series of
LC polysiloxanes and polyoxetanes containing 4-alka-
nyloxyphenyl trans-4-alkylcyclohexanoate side groups.
These polymers reveal polymorphism of mesophases. In
this study, polyacetylene was used as the polymer
backbone. The effect of the polymer backbone on the
Exp er im en ta l Section
Ma ter ia ls. trans-4-n-Propylcyclohexanoic acid (99%), trans-
4-n-butylcyclohexanoic acid (99%), and trans-4-n-pentylcyclo-
hexanoic acid (99%) were obtained from Tokyo Chemical Inc.
and were used as received. (Chloronorbornadiene)rhodium(I)
dimer ([Rh(nbd)Cl]2, 99%) was purchased from Strem. 4-Iodo-
phenol (99%), 4-dichlorobis(triphenylphosphine)palladium(II)
(PdCl2(PPh3)2, 99.99%), copper(I) iodide (98%), triphenylphos-
phine (99%), 2-methyl-3-butyn-2-ol (98%), hydroquinone (99%),
and 5-chloro-l-pentyne (98%) were purchased from Aldrich and
used without further purification. Tetrahydrofuran (THF) was
dried over sodium, and dioxane, triethylamine (Et3N), and
N,N-dimethylformamide (DMF) were dried over calcium hy-
dride and then distilled under nitrogen.
Tech n iqu es. 1H NMR spectra (300 MHz) were recorded on
a Varian VXR-300 spectrometer. Thermal transitions and
thermodynamic parameters were determined by using a
Perkin-Elmer Pyris 1 differential scanning calorimeter equipped
with a liquid nitrogen cooling accessory. Heating and cooling
rates were 10 °C/min. A Carl-Zeiss Axiphot optical polarized
microscope equipped with a Mettler FP82 hot stage and a FP80
central processor was used to observe the thermal transitions
and to analyze the anisotropic textures. Gel permeation
chromatography (GPC) was run on a Waters 510 LC instru-
ment equipped with a 410 differential refractometer, a UV
detector and a set of PL gel columns of 102, 5 × 102, 103, and
104 Å. The oven temperature was set at 40 °C. THF was used
as eluent and the flow rate was 1 mL/min. The UV-visible
spectra and photoluminescence of polymers were obtained from
a Shimadzu UV-1601 spectrophotometer and an RF-5301PC
spectrofluorophotometer, respectively.
Syn th esis of Mon om er s 1M)9M. The synthesis of cyclo-
hexane containing monomers 1M-9M is outlined in Scheme
1.
4-(3-Hyd r oxy-3-m eth yl-1-bu tyn yl)p h en ol (1). 4-Iodophe-
nol (5.00 g, 22.7 mmol), PdCl2(PPh3)2 (0.16 g, 0.23 mmol), CuI
(0.22 g, 1.1 mmol), and PPh3 (0.42 g, 1.6 mmol) were dissolved
in triethylamine (150 mL), and the mixture was stirred under
nitrogen. After all the catalysts were dissolved, 2-methyl-3-
butyn-2-ol (2.30 g, 27.3 mmol) was added. The resulting
solution was reacted at 70 °C for 15 h. After triethylamine
was removed under reduced pressure, the product was ex-
tracted with diethyl ether. The crude product was isolated by
* Corresponding author.
10.1021/ma0107962 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/12/2002

Macromolecules, Vol. 35, No. 4, 2002
Liquid Crystalline Polyacetylenes 1181
Sch em e 1. Syn th esis of Cycloh exa n e-Ba sed
Sch em e 2. Syn th esis of Sid e-Ch a in LC P olya cetylen e
Mon om er s 1M-9M
1P -9P
evaporating the solvent and purified by column chromatog-
raphy (silica gel, ethyl acetate:n-hexane ) 1:3 as eluent) and
further recrystallized from methanol to yield 5.0 g (77%) of
1
yellow crystals, mp ) 123 °C. H NMR (CDCl3, δ, ppm): 1.59
(s, 6H, -C(CH3)2-), 6.76 and 7.26 (dd, 4H, aromatic protons).
Hydroquinone (10.8 g, 97.5 mmol), NaOH (4.88 g, 122 mmol)
and a catalytic amount of KI were dissolved in ethanol (150
mL) under gentle heating and stirring. Then 5-chloro-1-
pentyne (5.00 g, 48.8 mmol) was added. The resulting mixture
was heated to reflux temperature and stirred for 24 h. The
solution was then poured into 350 mL of water and acidified
with 20 mL of 37% HCl aqueous solution. The crude product
which was isolated by filtration, was recrystallized from 96%
methanol to yield 5.9 g (70%) yellow crystals, mp ) 51.3 °C.
1H NMR (CDCl3, δ, ppm): 1.95 (s, 1H, -CtCH), 1.96 (m, 2H,
-CH2-), 2.38 (m, 2H, -CH2-Ct), 3.99 (t, 2H, -OCH2-), 6.76
(dd, 4H, aromatic protons).
4-(1-Eth yn yl)p h en ol (2). 4-(3-Hydroxy-3-methyl-1-buty-
nyl)phenol (3.00 g, 17.0 mmol) and KOH (2.10 g, 37.5 mmol)
were dissolved in dioxane (100 mL). The mixture was reacted
at 120 °C for 12 h. After the reaction system was cooled to
room temperature, 6 N HCl aqueous solution (100 mL) was
added. The mixture was extracted with ethyl ether. The crude
product was isolated by evaporating the solvent and purified
by column chromatography (silica gel, ethyl acetate:n-hexane
) 1:1 as eluent) to yield 2.0 g (99%) of yellow crystals. 1H NMR
(CDCl3, δ, ppm): 2.98 (s, 1H, -CtCH), 6.76 and 7.36 (dd, 4H,
aromatic protons).
4-(4-P en tyn yloxy)p h en ol (3) a n d 4-(5-Hexyn yloxy)-
p h en ol (4). Both compounds were prepared by etherification
of 5-chloro-1-pentyne or 6-chloro-1-hexyne with hydroquinone.
The synthesis of compound 3 is described below.
Compound 4: yield 73%, mp ) 74.5 °C. 1H NMR (CDCl3, δ,
ppm): 1.70-1.85 (m, 4H, -(CH2)2-), 1.94 (s, 1H, -CtCH),
2.25 (m, 2H, -CH2-Ct), 3.90 (t, 2H, -OCH2-), 6.75 (dd, 4H,
aromatic protons).

1182 Ting et al.
Macromolecules, Vol. 35, No. 4, 2002
Ta ble 1. Ch a r a cter iza tion of Mon om er s 1M-9M
4-(1-E t h yn yl)p h en yl tr a n s-4-n -a lk ylcycloh exa n oa t e
(1M)3M, n ) 3)5), 4-(4-P en tyn yloxy)p h en yl tr a n s-4-n -
a lk ylcycloh exa n oa te (4M)6M, n ) 3)5) a n d 4-(5-Hexy-
n yloxy)p h en yl tr a n s-4-n -a lk ylcycloh exa n oa te (7M)9M,
n ) 3)5). Monomers 1M-9M were synthesized by the same
method. The preparation of monomer 2M is described below.

Macromolecules, Vol. 35, No. 4, 2002
Liquid Crystalline Polyacetylenes 1183
Ta ble 2. Th er m a l Tr a n sition s a n d Th er m od yn a m ic P a r a m eter s of Mon om er s 1M-9M
phase transitions, °C
(corresponding enthalpy changes, kcal/mol):b
heating/cooling
monomer
ma
na
1M
2M
3M
4M
5M
6M
7M
8M
9M
0
0
0
3
3
3
4
4
4
3
4
5
3
4
5
3
4
5
K 79.2(4.78) I/I 63.9(0.08) N 32.1(4.42) K
K 62.9(5.48) I/I 53.8(0.08) N 31.1(5.01) K
K 63.4(7.37) N 87.5(0.14) I/I 86.6(0.11) N 46.1(7.27) K
K 66.2(5.04) I/I56.3(0.05) N 48.3(4.08) K
K 57.9(5.94) I/I 55.5(0.14) N 25.1(5.90) K
K 66.2(4.70) N 71.3(0.12) I/I 70.8(0.15) N 44.3(4.95) K
K 50.8(6.21) I/I 19.6(0.07) N 3.9(5.15) K
K 48.3(8.52) I/I 23.4(0.08) N 2.5(7.00) K
K 44.3(5.96) I/I 35(0.10) N 16.6(5.46) K
a
b
According to Scheme 1. K ) crystalline, N ) nematic, and I ) isotropic. Data taken from the second heating and first cooling scan.
Ta ble 4. P olym er iza tion of 4-(4-P en tyn yloxy)p h en yl
tr a n s-4-n -P en tylcycloh exa n oa te (6M)a
reacn
temp yield
b
no.
initiator
MoCl5
MoCl5/Ph4Sn dioxane
MoCl5/Ph4Sn toluene
solvent
dioxane
(°C) (%) Mw × 104 b Mw/Mn
1
2
3
4
5
6
7
8
9
80
80
80
80
rtd
80
rt
58
70
65
47
59
70
39
75
74
41
0.1
0.3
1.5
3.5
11.0
9.9
1.0
2.0
0.9
2.6
1.2
1.9
1.2
1.8
1.3
1.6
1.2
1.2
1.4
6.6
WCl6
dioxane
WCl6/Ph4Sn dioxane
WCl6/Ph4Sn dioxane
WCl6/Ph4Sn toluene
WCl6/Ph4Sn toluene
80
[Rh(nbd)Cl]2 DMF/Et3N ) 4:1c rt
10 [Rh(nbd)Cl]2 DMF/Et3N ) 4:1c 80
a
Carried out under nitrogen for 24 h. [M]0 ) 0.2 M, [[Rh(nb-
d)Cl]2] ) 10 mM, [WCl6] ) [MoCl5] ) [Ph4Sn] ) 20 mM,
F igu r e 1. Structural configurations of LC acetylene deriva-
tives with different spacer lengths: (A) three methylene units,
and (B) four methylene units.
b
[Rh(nbd)Cl]2 ) 10 mM, and nbd ) 2,5-norborndiene. Determined
d
by GPC in THF relative to polystyrene. c Volume ratio. rt ) room
temperature.
Ta ble 3. P olym er iza tion of 4-(1-Eth yn yl)p h en yl
tr a n s-4-n -Bu tylcycloh exa n oa te (2M)a
Ta ble 5. P olym er iza tion of Mon om er s 1M-9Ma
reacn
reacn
temp yield
b
temp yield
no.
initiator
solvent
(°C) (%) Mw × 104 b Mw/Mn
b
M
initiator
solvent
(°C) (%) Mw × 104b Mw/Mn
1
2
3
4
5
6
7
8
9
MoCl5/Ph4Sn dioxane
MoCl5/Ph4Sn dioxane
WCl6/Ph4Sn dioxane
WCl6/Ph4Sn dioxane
[Rh(nbd)Cl]2 THF
rtd
80
rt
0
0
0
1M [Rh(nbd)Cl]2 DMF/Et3N ) 4:1c rt
2M [Rh(nbd)Cl]2 DMF/Et3N ) 4:1c rt
3M [Rh(nbd)Cl]2 DMF/Et3N ) 4:1c rt
78
87
72
52
55
70
60
59
70
8.3
11.2
9.6
4.4
3.6
3.7
1.6
1.9
1.6
1.4
1.4
1.6
80
rt
0
4M WCl6/Ph4Sn dioxane
5M WCl6/Ph4Sn dioxane
6M WCl6/Ph4Sn dioxane
7M WCl6/Ph4Sn dioxane
8M WCl6/Ph4Sn dioxane
9M WCl6/Ph4Sn dioxane
80
8.0
7.9
49
56
51
80
87
44
27.4
17.3
23.5
13.6
11.2
8.7
2.2
4.3
2.2
5.2
3.6
2.1
80
80
80
80
80
[Rh(nbd)Cl]2 THF/Et3N ) 4:1c rt
9.9
7.8
[Rh(nbd)Cl]2 THF/Et3N ) 4:1c 80
[Rh(nbd)Cl]2 DMF
rt
9.0
[Rh(nbd)Cl]2 DMF/Et3N ) 4:1c rt
14.5
10 [Rh(nbd)Cl]2 DMF/Et3N ) 4:1c 80
a
Polymerization takes place under nitrogen for 24 h. [M]0
)
a
Carried out under nitrogen for 24 h. [M]0 ) 0.2 M, [[Rh(nb-
d)Cl]2] ) 10 mM, [WCl6] [MoCl5] ) [Ph4Sn] ) 20 mM, nbd ) 2,5-
0.2 M, [[Rh(nbd)Cl]2] ) 10 mM, [WCl6] ) [MoCl5] ) [Ph4Sn] ) 20
mM, nbd ) 2,5-norborndiene, DMF ) dimethylformamide, Et3N
) triethylamine, and rt ) room temperature. M ) monomer.
b
norborndiene, and Ph4Sn ) tetraphenyltin. Determined by GPC
d
in THF relative to polystyrene. c Volume ratio. rt ) room tem-
b
Determined by GPC in THF relative to polystyrene. c Volume
perature.
ratio.
trans-4-n-Butylcyclohexanoic acid (1.00 g, 5.87 mmol), 4-(1-
ethynyl)phenol (1.00 g, 8.46 mmol) and N,N-dimethylaminopy-
ridine (DMAP, 0.10 g, 0.85 mmol) were dissolved in dry CH2Cl2
(60 mL) under N2 atmosphere. The solution was cooled to 0
°C, and dicyclohexyl carbodiimide (DCC, 2.10 g, 10.2 mmol)
in 20 mL of dry CH2Cl2 was added dropwise. The reaction
mixture was stirred at room temperature for 14 h and filtered.
The crude product which was isolated by evaporating the
solvent was purified by column chromatography (silica gel,
ethyl acetate:n-hexane ) 1:1 as eluent) and finally recrystal-
lized from methanol to yield 1.2 g (68%) of yellow crystals, mp
experimental procedure for the polymerization of monomer 6M
is given below.
Monomer 6M (0.29 g, 0.80 mmol) was added into a Schlenk
tube with a three-way stopcock on the sidearm. The tube was
evacuated under vacuum and then flushed with dried N2 three
times through sidearm. Dioxane (2 mL) was injected into the
tube through a septum to dissolve the monomer. The initiator
solution was prepared in another tube by dissolving WCl6 (31.7
mg, 0.08 mmol) and Ph4Sn (34.2 mg, 0.08 mmol) in 2 mL of
dried dioxane. Both tubes were aged at room temperature for
30 min. The monomer solution was then transfer to the
initiator solution using a hypodermic syringe. The reaction
mixture was stirred at 80 °C under N2 for 24 h. The solution
was then cooled to room temperature, diluted with 2 mL of
THF and added dropwise to 300 mL of methanol through a
cotton filter with stirring. The polymer was separated by
filtration, purified by several reprecipitations from THF into
methanol, and dried in a vacuum oven to yield 0.21 g (70%).
Mn ) 6.2 × 104, Mw ) 9.9 × 104.
1
) 62.9 °C. H NMR chemical shifts of monomers 1M-9M are
summarized in Table 1.
P olym er iza tion of Mon om er s 1M)9M. Scheme 2 out-
lines the polymerization of monomers 1M-9M. All polymer-
ization reactions and manipulations were carried out under
nitrogen using either an inert-atmosphere glovebox or Schlenk
techniques in a vacuum line system, except for the purification
of the polymers, which was done in an open atmosphere. An

1184 Ting et al.
Macromolecules, Vol. 35, No. 4, 2002
Both monomers 3M and 6M reveal an enantiotropic
nematic phase while all of the other monomers show
only a monotropic nematic phase. The melting points
decrease gradually as the spacer length (i.e., m value)
increases. Among the first series of monomers 1M-3M,
which contain no methylene spacer, their melting points
show an odd-even effect for the terminal alkyl group.
This means that the melting point first decreases and
then increases as the length of the terminal alkyl group
increases. The odd-even effect is also observed in the
second series of monomers 4M-6M, which contain three
methylene units in their spacers. Both monomers 3M
and 6M, which contain a pentyl terminal group, show
an enantiotropic nematic phase. This means that the
longer alkyl terminal group stabilizes the nematic
phase. However, in the third series of monomers 7M-
9M, which contain four methylene units in their spacers,
all three monomers present only a monotropic nematic
phase, and their melting points decrease as the length
of the terminal alkyl group increases. Molecular simula-
tion shows that the acetylene group of these monomers
tilts away from the mesogenic core (Figure 1), which
could be the reason for only a monotropic nematic phase.
F igu r e 2. IR spectra of monomer 2M, polymer 2P , monomer
6M, and polymer 6P .
P r ep a r a tion of Th in F ilm s for P olym er s 1P )9P a n d
Blen d s of 9P a n d P oly(m eth yl m eth a cr yla te). The blends
of polymer 9P and poly(methyl methacrylate) (PMMA) were
prepared by dissolution of both polymers in chlorobenzene.
Polymers 1P -9P and blends were spin-coated from chloroben-
zene onto the cleaned quartz substrates. The solution concen-
tration was 0.5 wt %. The spinning conditions were 1000 rpm/s
for 10 s and 2000 rpm/s for 30 s.
P olym er iza tion Rea ction . Mo-, W-, and Rh-based
compounds are the most widely used initiators for the
polymerization of acetylene-based and phenylacetylene-
based monomers.15-24 Tang et al. found that [Rh(nbd)-
Cl]2 can initiate polymerization of phenylacetylene-
based monomers with cyano, ester and ether functionali-
ties, but is not an effective initiator for the polymeri-
Resu lts a n d Discu ssion
Mesom or p h ism of Mon om er s 1M)9M. The ther-
mal behavior of all monomers is reported in Table 2.
F igu r e 3. 1H NMR spectra of monomer 2M and polymer 2P .
Ta ble 6. Th er m a l Tr a n sition s of Liqu id Cr ysta llin e P olya cetylen es 1P -9P
cis
content
(%)b
phase transitions, °C
(corresponding enthalpy changes, kcal/mol)c
heating/cooling
polymer
ma
na
1P
2P
3P
4P
5P
6P
7P
8P
9P
0
0
0
3
3
3
4
4
4
3
4
5
3
4
5
3
4
5
84
88
84
0
170 (isomerization)d
185 (isomerization)d
189 (isomerization)d
G 57 Sx 138(0.19) SA 165(0.85) I/I 161(0.81) SA 135(0.09) Sx
G 59 Sx 148(0.23) SA 181(0.91) I/I 175(0.79) SA 144(0.11) Sx
G 69 Sx 166(0.18) SA 194(0.82) I/I 189(0.77) SA 160(0.10) Sx
G 54 Sx 119(0.23) SA 148(0.81) I/I 144(0.80) SA 115(0.10) Sx
G 63 Sx 119(0.21) SA 151(0.79) I/I 147(0.76) SA 117(0.09) Sx
G 63 Sx 126(0.19) SA 176(0.70) I/I 174(0.66) SA 120(0.11) Sx
0
0
0
0
0
According to Scheme 1. Determined by 1H NMR spectra. c G ) glassy, S ) smectic, I ) isotropic. Data taken from the second heating
a
b
d
and first cooling scan. Data taken from the first heating scan.

Macromolecules, Vol. 35, No. 4, 2002
Liquid Crystalline Polyacetylenes 1185
F igu r e 4. 1H NMR spectra of monomer 6M and polymer 6P .
F igu r e 6. DSC thermograms of polymer 6P (10 °C/min): (A)
heating scan and (B) cooling scan.
F igu r e 5. DSC thermogram of polymer 1P (10 °C/min): the
first heating scan.
zation of acetylene-based monomers.23 They also found
that WCl6-Ph4Sn is the best initiating system for the
polymerization of acetylene-based monomers with polar
functionalities.21 In this study, monomers 1M-3M
belong to phenylacetylene-based monomers while mono-
mers 4M-9M are acetylene-based monomers. There-
fore, we chose WCl6, MoCl5, and [Rh(nbd)Cl]2 as initia-
tors and Ph4Sn as a co-initiator. Table 3 lists the
polymerization results of monomer 2M. Neither WCl6-
Ph4Sn nor MoCl5-Ph4Sn initiate the polymerization of
monomer 2M, while [Rh(nbd)Cl]2 gives polymers with
reasonably high yields and molecular weights in all
cases. The yields are around 50% when THF or a
mixture of THF and Et3N are used as solvent. When
the solvent is changed from THF to DMF, the yield at
polymerization conditions for monomer 2M use (i) [Rh-
(nbd)Cl]2 as initiator, (ii) a mixture of DMF and Et3N
(4:1, volume ratio) as solvent, and (iii) polymerization
at room temperature. Table 4 reports the polymerization
results of monomer 6M. MoCl5 alone in dioxane gives
only oligomers. When Ph4Sn is used as co-initiator, the
molecular weights increase slightly (Mw ) 3430). For
the same MoCl5-Ph4Sn initiating system, changing the
solvent from dioxane to toluene, increases the weight
average molecular weights to 14600. WCl6 alone in
dioxane gives polymers with reasonably high molecular
weight and moderate yield (47%). When Ph4Sn is used
as co-initiator, both the molecular weight and yield
increase. For the same WCl6-Ph4Sn initiating system,
the solvent changing from dioxane to toluene, increases
the yield, but largely decreases the average molecular
weight. As far as the polymerization temperature, better
polymerization results are obtained at 80 °C than at
room temperature.
room temperature is improved but the Mw value de-
creases. The addition of Et3N to DMF improves the yield
at room temperature slightly. When the polymerization
temperature is raised to 80 °C, the yield largely de-
creases for the DMF/Et3N solvent system. The best

1186 Ting et al.
Macromolecules, Vol. 35, No. 4, 2002
F igu r e 7. Polarizing optical micrographs displayed by polymer 6P : (A) focal-conic fan texture of the smectic A phase obtained
after cooling from the isotropic melt to 170 °C; (B) broken fan texture of the smectic phase obtained after cooling from the isotropic
melt to 100 °C.
Although [Rh(nbd)Cl]2 gives polymers with high yield
at room temperature, the weight average molecular
Str u ctu r e Ch a r a cter iza tion of P olym er s 1P )9P .
Figure 2 compares the IR spectra of monomer 2M and
6M with those of their corresponding polymers 2P and
6P . Both monomers absorb at about 3260 and 2100
cm-1, due respectively to the CtC stretching and tC-H
stretching vibrations. Both acetylene absorption bands
disappear in the spectra of the polymers. A new double
bond absorption at 3050 cm-1 appears in the polymer
spectra, confirming that acetylene triple bonds have
been converted to conjugated double bonds during the
polymerization.
weight is quite low (Mw ) 9030). When the polymeri-
zation temperature is raised to 80 °C, the molecular
weight increases slightly, but the yield decreases dra-
matically. The best polymerization conditions for mono-
mer 6M use (i) WCl6-Ph4Sn as initiating system, (ii)
dioxane as solvent, and (iii) polymerization at 80 °C.
Table 5 reports the polymerization results of mono-
mers 1M-9M. Monomers 1M-3M were polymerized by
[Rh(nbd)Cl]2 in a mixture of DMF and Et3N at room
temperature, while monomers 4M-9M were polymer-
ized by WCl6-Ph4Sn in dioxane at 80 °C. All polymer-
izations give polymers with reasonably high yields and
molecular weights.
1
Figure 3 shows the H NMR spectra of monomer 2M
and polymer 2P . In the spectrum of polymer 2P , the
acetylene proton peak at 3.03 ppm disappears and two
new broad peaks appear at 5.77 and 6.60 ppm. Simi-

Macromolecules, Vol. 35, No. 4, 2002
Liquid Crystalline Polyacetylenes 1187
F igu r e 10. UV-visible and photoluminescence spectra of
polymer 9P .
F igu r e 8. X-ray diffraction patterns of polymer 6P measured
after cooling from the isotropic melt to 175 and 90 °C.
F igu r e 11. Photoluminescence spectra of polymer 9P and its
mixtures with PMMA.
Ta ble 7. UV-Visible a n d P h otolu m in escen t Da ta of
P olym er s 1P -9P '
UV-visiblea
photoluminescenceb
polymer
λabs (nm)
λex (nm)
λem (nm)
1P
2P
3P
4P
5P
6P
7P
8P
9P
394
331
330
326
227
226
228
226
227
227
393
395
396
278
278
278
279
279
279
396
396
278
279
279
280
279
280
515
488
478
500
485
519
a
Code: λabs, absorptive wavelength in UV-visible spectrum.
ex, exciting wavelength used in the measurement of
b
Code:
λ
F igu r e 9. Smectic layer structures of (A) polyacetylene, and
(B) polyoxetane.
photoluminescence; λem, emissive wavelength in photolumines-
cence spectrum.
onesue and Percec.30,31 have reported that the cis-olefin
proton peak appears at 5.82 ppm and trans-olefin proton
peak appears at 6.85 ppm for poly(phenylacetylene).
Therefore, we can calculate the cis content of a polymer
by the following equation:
No peak is observed in this region for polymer 6P .
Therefore we conclude that the trans-olefin content of
polymer 6P is 100%. The same is true for polymers 4P ,
5P , and 7P -9P .
Mesom or p h ic P r op er t ies of P olym er s 1P )9P .
Table 6 summarizes the phase transitions of polymers
1P -9P . Polymers 1P -3P show no mesophase while
polymers 4P -9P reveal two enantiotropic smectic
phases. Figure 5 displays a representative DSC trace
of polymer 1P . It exhibits a cis-trans isomerization
peak at 170 °C. The cis-trans isomerization was
cis content (%) ) {A5.77/[(A5.77 + A6.60)/5]} × 100
The cis contents of polymers 1P , 2P , and 3P are 84,
88, and 84% respectively. Figure 4 presents the 1H NMR
spectra of monomer 6M and polymer 6P . In the spectra
of 6P , only a broad peak at 6.85 ppm is observed in the
region from 5 to 7 ppm. According to Tang et al.,21 the
chemical shift of cis-olefin proton appear in the region
5.88-6.26 ppm for mesogen-containing poly(1-alkynes).
1
checked by H NMR. After polymer 1P was annealed
at 250 °C for 10 min, the chemical shift of the cis-olefin
at 5.77 ppm disappeared and the chemical shift of the
trans-olefin was supposed to form at about 7.0 ppm.

1188 Ting et al.
Macromolecules, Vol. 35, No. 4, 2002
However, it overlapped with the aromatic protons.
Similar result was reported by Matsunami et al.32 Since
polymer 1P has a very rigid polyacetylene backbone and
contains no spacer, it is very difficult for the mesogenic
side groups to form a mesophase. Polymer 1P under-
went decomposition and other aromatization side reac-
tion at over 300 °C, which agrees with the thermo-
gravimetry measurement.
at all. However, polymers 4P -9P present two UV-vis
absorption peaks at about 226 and 279 nm and emit a
luminescence peak at 500 nm. Figure 10 shows the
representative UV-vis and photoluminescent spectra
of polymer 9P . The photoluminescent intensity of
polymer 9P is quite weak due to strong intermolecular
interactions which give rise to self-quenching. Never-
theless, when 9P was blended with PMMA, its photo-
luminescent intensity increases dramatically (Figure ll).
The photoluminescent intensity increases as the weight
percentage of PMMA increases from 25 to 50. The
results demonstrate that the addition of a nonemissive
polymer as diluent for the LC polyacetylene reduces its
interchain interaction and therefore enhances its pho-
toluminescent intensity.
Figure 6 presents representative DSC thermograms
of polymer 6P . It displays two smectic phases on both
heating and cooling scans. The mesophase identification
have been achieved by optical polarizing microscopic
observation and X-ray diffraction. Figure 7A shows a
focal-conic fan, and Figure 7B shows a broken fan
textures exhibited by polymer 6P . Figure 8 presents the
temperature-dependent X-ray diffraction diagrams ob-
tained from powder samples of 6P at 175 and 90 °C. At
175 °C, the diffraction diagram presents a diffuse
reflection at 5.21 Å, which corresponds to the lateral
spacing of two mesogenic side groups, and a sharp first-
order reflection at 39.03 Å and a second-order reflection
at 19.30 Å, which correspond to smectic layers. The
optical polarizing micrograph reveals a focal-conic fan
texture at this temperature range. Both results are
consistent with a smectic A structure. When the mea-
suring temperature is lowered from 175 to 90 °C, the d
spacing of the first-order reflection decreases from 39.03
to 38.58 Å. The optical polarizing micrograph (Figure
7B) shows a broken fan texture at this temperature
range. The results indicate the formation of a tilted
smectic phase, probably a smectic C phase. The detailed
structure of the smectic phase showed be further studied
by X-ray diffraction pattern of an aligned monodomain
sample.
Con clu sion
A new series of side-chain LC polyacetylenes contain-
ing 4-alkanyloxyphenyl trans-4-alkylcyclohexanoate side
groups were synthesized using simple metal halide
initiators. Polymers 1P -3P without flexible spacer,
show no mesomorphic behavior, while polymers 4P -
9P containing three or four methylene units in their
spacers, display both smectic A and smectic C phases.
Compared to the corresponding polyoxetanes, the LC
polyacetylenes have much higher glass transition and
isotropization temperatures. X-ray diffraction results
also demonstrate that the LC polyacetylenes tend to
form an interdigitated bilayer structure for the smectic
phases. Polymers 4P -9P have a weak photolumines-
cence emission at about 500 nm. After blending with
PMMA, their photoluminescence intensities increase
dramatically.
Ack n ow led gm en t. The authors are grateful to the
National Science Council of the Republic of China for
financial support of this work (NSC 89-2216-E009-023).
Com p a r ison of th e Effect of Rigid a n d F lexible
P olym er Ba ck bon es on Th eir Mesom or p h ic P r op -
er ties. Since polyacetylene is a very rigid polymer
backbone, we can investigate the effect of a rigid
polymer backbone on their mesomorphic properties. In
a previous publication,29 we reported a series of poly-
oxetanes containing the same 4-alkanyloxyphenyl trans-
4-alkylcyclohexanoate side groups. The polyacetylenes
synthesized in this study have much higher glass
transition and isotropization temperatures than the
corresponding polyoxetanes. The reason could be due
to the rigidity of the polyacetylene backbones. As far as
the mesomorphic behavior, polymers 4P -9P reveal
smectic A and smectic C phases, while the polyoxetanes
display a nematic phase and two smectic phases. The
LC polymers with rigid polymer backbones have a
tendency to form smectic phases. The d spacing of
smectic layer for LC polyacetylene is 39.03 Å at 175 °C.
According to molecular calculation, the layer spacing is
considerably greater than molecular length of the side
group in a LC polyacetylene. The LC polyacetylenes
obviously tend to form an interdigitated bilayer struc-
ture (Figure 9A). According to our previous report,29 the
smectic layer spacing of a LC polyoxetane is about 29.7
Å which is very close to the molecular length of its side
groups. Thus, the LC polyoxetane which contain a more
flexible backbone form only the single layer smectic
arrangement (Figure 9B).
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