Smectic aggregates of sheet-like side-chain liquid crystalline polyacetylenes directly formed during solution polymerization

Article abstract of DOI:10.1021/ma100619h

We have successfully synthesized two side-chain liquid crystalline polyacetylenes (denoted as P3-5 and P2-5) with the mesogenic units based on a "phenyl-ester-phenyl" motif directly linked to the semirigid polyacetylene backbone in a terminal-on mode without flexible spacer. During the polymerization catalyzed by Rh(nbd)[B(C₆H₅)₄] in toluene, the resultant polymers (or oligomers) automatically precipitate out of the polymerization solution as red solids, which cannot be dissolved in common organic solvents. Wide-angle X-ray diffraction results confirm that the polymer precipitates possess a smectic (or sanidic) structure, suggesting that the molecules without flexible spacers are sheet-like and can stack parallel to each other during polymerization. We in situ monitored the solution polymerization and the aggregation processes using UV-visible and ¹H NMR spectroscopy methods. The experimental results indicate that after reaching a critical molecular weight the P3-5 and P2-5 molecules self-aggregate in solution and that the smectic aggregation proceeds during the solution polymerization. An apparent first-order polymerization kinetics can be obtained after the polymerization and aggregation slowed down. The resultant polymers are cis-rich with the main-chain absorption at ～455 nm. This implies that a considerably long effective conjugation length is achieved in the sheet-like cis-rich polyacetylene derivatives.

Full text of DOI:10.1021/ma100619h

6014 Macromolecules 2010, 43, 6014–6023
DOI: 10.1021/ma100619h
Smectic Aggregates of Sheet-Like Side-Chain Liquid Crystalline
Polyacetylenes Directly Formed During Solution Polymerization
Lu-Mei Liu,† Kai-Peng Liu,† Yu-Ping Dong,‡ Er-Qiang Chen,*,† and Ben Zhong Tang*,§
†Beijing National Laboratory for Molecular Sciences, Department of Polymer Science and Engineering and the
Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and
Molecular Engineering, Peking University, Beijing100871, China, ‡College of Material Science and
Engineering, Beijing Institute of Technology, Beijing 100871, China, and §Department of Chemistry,
The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong, China
Received March 22, 2010; Revised Manuscript Received June 7, 2010
ABSTRACT: We have successfully synthesized two side-chain liquid crystalline polyacetylenes (denoted as
P3-5 and P2-5) with the mesogenic units based on a “phenyl-ester-phenyl” motif directly linked to the
semirigid polyacetylene backbone in a terminal-on mode without flexible spacer. During the polymerization
catalyzed by Rh(nbd)[B(C6H5)4] in toluene, the resultant polymers (or oligomers) automatically precipitate
out of the polymerization solution as red solids, which cannot be dissolved in common organic solvents.
Wide-angle X-ray diffraction results confirm that the polymer precipitates possess a smectic (or sanidic)
structure, suggesting that the molecules without flexible spacers are sheet-like and can stack parallel to each
other during polymerization. We in situ monitored the solution polymerization and the aggregation processes
using UV-visible and 1H NMR spectroscopy methods. The experimental results indicate that after reaching
a critical molecular weight the P3-5 and P2-5 molecules self-aggregate in solution and that the smectic
aggregation proceeds during the solution polymerization. An apparent first-order polymerization kinetics
can be obtained after the polymerization and aggregation slowed down. The resultant polymers are cis-rich
with the main-chain absorption at ∼455 nm. This implies that a considerably long effective conjugation
length is achieved in the sheet-like cis-rich polyacetylene derivatives.
Introduction
of poly(n-{[(40-heptoxy-4-biphenylyl)oxy]carbonyl}-1-alkyne)s
can cause the LC phase to change from mixed mono- and bilayer
In the past few decades many researchers have endeavored to
develop liquid crystalline (LC) polyacetylenes.1-6 From a view-
point of technological applications, introducing side-chains with
mesogenic moieties to the conjugated polyacetylene main-chain
may result in new types of functional polymers which combine
the different molecular electronic and optical properties.6 In
general, the synthetic strategy of side-chain LC polyacetylenes
(SCLCPAs) also follows the concept of “flexible spacer”,7 which
is widely used in the synthesis of conventional side-chain LC
polymers with flexible main-chains. To effectively decouple the
dynamics of the fairly rigid polyacetylene main-chain and the
mesogenic side-chains, flexible spacer with a reasonably long
length in SCLCPA plays an important role in the LC phase
formation. Le Moigne et al. have reported for the first time a
SCLCPA with the bulky cholesteryl mesogenic group placed at
the end of the flexible spacers.8 Another early example is a group
of side-chain LC poly(1-pentyne)s synthesized by Akagi and
Shirakawa, in which the polyene main-chain and the phenylcy-
clohexylyl unit are bridged through an ether functional group.9,10
After those early works, many studies on the subject of SCLCPAs
have been published.11-14 A body of experimental results have
confirmed that the flexible spacer is crucial with respect to the LC
behaviors of SCLCPAs, particularly considering that the rigid
polyacetylene backbone might somewhat hinder the LC phase
formation.3 For example, Tang et al. have systematically studied
the LC properties of SCLCPAs with different flexible spacer
length.15,16 They have found that an increase in the spacer length
smectic structures to a homogeneous monolayer smectic structure.15
For a series of poly(n-{[(40-cyano-4-biphenylyl)oxy]carbonyl}-1-
alkyne)s, while the polymer with a spacer of two methylene units
just forms a nematic phase, increasing the number of the methylene
units to 8 results in smectic phase.16 Therefore, one may conclude
that the longer the flexible spacer is, the better the mesogenic
order the SCLCPA will have. In this context, the LC phases of
SCLCPAs are largely dependent on the LC properties of the
mesogenic groups on the side-chains, but the contribution of
conjugated backbone to LC phase formation has been paid little
attention.
However, the polyacetylene backbone itself in fact can be part of
the building block of the LC phase structure, considering its
semirigid and anisotropic nature. It is well-known that the main-
chain rigid rods bearing aromatic backbones can display nematic
phase.17 When flexible side-chains are introduced to the rigid rod,
two limiting conformations of the individual chain molecules can
be envisioned.18 If the side-chains wrap the rigid backbone in a
more or less uniform manner, the chain molecule is cylindrical and
can pack into columnar LC phases. On the other hand, if the side-
chains and the backbone share approximately a same plane, the
molecular shape looks sheet- or board-like. According to Wendorff
et al., the board-like chain molecules can form various sanidic LC
phases.18 In both the limiting cases, the rigid backbone dominates
the shape persistency, which is essential to the LC phase formation.
In this regard, we believe that further evaluating the role of the
polyacetylene backbone playing in LC phases is worth doing.
Here, we still focus on SCLCPAs containing typical rod-like
mesogenic moieties. Of particular interest is to ask: when the
*To whom correspondence should be addressed. E-mail: (E.-Q.C.)
eqchen@pku.edu.cn; (B.Z.T.) tangbenz@ust.hk.
pubs.acs.org/Macromolecules
Published on Web 06/28/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 14, 2010 6015
Chart 1. Chemical Structures of P3-5 and P2-5
Scheme 1. Synthetic Routes to the Monomers M2-5 and M3-5
spacer is gradually shortened until its absence, or in other words,
when the dynamics of the polyacetylene backbone and mesogenic
side groups becomes hard to be decoupled, what will the LC
phase behavior and the molecular shape of the SCLCPA be?
Costa et al. have investigated the influence of short spacer on the
phase formation of SCLCPA.19-21 They have found that the
polymers with only a short flexible spacer may exhibit enantio-
tropic smectic phase behavior, even though the monomers do not
show any LC properties. Moreover, the polymers with a three
methylene spacer exhibit a double layer tilted smectic structure,
while those with only a single methylene inserted between the
backbone and mesogens can have an interdigitated smectic struc-
ture. According to these results, it is very possible that the LC
phases are constructed by the backbone and side-chains in a
cooperative manner. Recently, we have reported that a trans-rich
poly(5-{[(40-heptoxy-4-biphenylyl)carbonyl]oxy}-1-pentyne) with
the spacer containing three methylene units exhibits a highly
ordered smectic phase with a frustrated structure of molecular
packing.22 The building block of the LC phase is actually the
whole chain molecule with a sheet-like (or ribbon-like) shape, of
which the side-chains are parallel but extend to opposite direc-
tions on both sides of the backbone. Furthermore, we have found
that the sheet-like molecules can self-assemble into monolayer
lamellae in dilute solutions with the solvents selective for the side-
chain tails.23 In this case, the backbone and mesogens are tightly
coupled together, which shall be attributed to the rather short
spacer of three methylene units.
So far the results published in literatures suggest that the side-
chain polyacetylene with the mesogenic moieties terminally
attached to the backbone without flexible spacer cannot render
any LC structure.24-27 However, from a simple geometric point
of view, it is still possible that directly linking the rod- or lath-like
mesogenic groups to the polyaceylene backbone with highly regular
sterostructure (either cis or trans) can result in a planar struc-
ture,24,25 giving sheet-like polyacetylene derivatives which can
pack parallel to form smectic LC structure or sanidic structure.18
Such sheet-like polyacetylene molecule is of interest, because the
unique molecular architecture may lead to a greatly improved
effective conjugation, and thus may offer better electronic and
optical properties for the polyacetylenes amenable for advanced
applications.10,28
Our wide-angle-X-ray diffraction (WAXD) data indicate that the
sheet-like molecules can be considered as the building block of
smectic (or sanidic) structure. Similar to that observed in other
systems,29 the aggregation during polymerization resulted in
quite uniform nanosized polymer particles. On the basis of in
situ monitoring of the polymerization processes by UV-vis and
1H NMR techniques, which have been seldom used in kinetic
study of acetylene polymerization,30 we found that the ordered
packing of smectic structure occurred just after the polymer
molecules had reached a “critical molecular weight”. The cis-rich
polymers showed a main-chain absorption at ∼455 nm, indica-
tive of a considerably long effective conjugation length. Further-
more, an apparent first-order polymerization kinetics was
observed after the polymerization and aggregation had slowed
down during the solution polymerization process.
Experimental Section
Materials. 2-Methyl-3-butyn-2-ol (Alfa Aesar), bis(triphenyl-
phosphine)palladium(II) chloride (Acros), copper(I) iodide
(Acros), 4-dimethylamino-pyridine (Acros), (bicyclo[2.2.1]hepta-
2,5-diene)chlororhodium(I) dimer ([Rh(nbd)Cl]2, Aldrich), and
2,5-norbornadiene (Alfa Aesar) were all purchased and used
without further purification. The solvents were treated as fol-
lows: tetrahydrofuran (THF), triethylamine (Et3N), and toluene
were dried over calcium hydride, potassium hydroxide, and
sodium under nitrogen, respectively. Other chemicals were
obtained from Beijing Chemical Co. and were used as received.
Synthesis of Monomers. The synthetic route to the monomers
M3-5 and M2-5 is shown in Scheme 1.
(a). Synthesis of 4-Ethynylbenzoic Acid. The reaction proce-
dures are similar to that reported elsewhere.31 To a 250 mL
three-necked flask were added methyl 4-bromobenzoate (25 g,
116 mmol), 2-methyl-3-butyn-2-ol (15 mL, 154 mmol), Ph3P
(0.3 g, 1.15 mmol), CuI (87 mg, 0.46 mmol) in 100 mL of dry
Et3N and 40 mL of dry pyridine. PdCl2(Ph3P)2 (80 mg, 0.12 mmol)
was then added to the above mixture under nitrogen. After
refluxing for 2.5 h at ca. 88 °C, the mixture was cooled to room
temperature and was filtered to remove the insoluble triethyla-
mine hydrobromide. The salt was washed with triethylamine
and ethyl ether until the ether washings were clear. The com-
bined filtrates were reduced to dryness under reduced pressure,
and the crude product was washed with water. The residue was
dried under vacuum to yield 22 g of methyl 4-(3-hydroxy-3-
methylbut-1-ynyl) benzoate. The above solid was dissolved in
300 mL of refluxing solution of n-butanol with 25 g of KOH.
After refluxing for 10 min, the mixture was cooled in an ice-bath.
After filtration, the collected residue was acidified to pH 2 by
HCl. The mixture was extracted with ethyl acetate. After
removing the solvent, the residue was dried under vacuum to
In this paper, we intend to construct sheet-like polyacteylenes
with high shape persistency by directly attaching the mesogenic
units to the semirigid backbone in a terminal-on mode without
flexible spacer. Two poly(phenylacetylene) (PPA) derivatives
(denoted as P3-5 and P2-5) were synthesized and characterized
carefully, whose chemical structures are shown in Chart 1. Without
any spacer, the portion of “backbone þ mesogenic cores” shall be
rather rigid if the main-chain stereoregularity is adequately high.
We found that the solution polymerization using Rh(nbd)-
[B(C6H5)4] (nbd =2,5-norbornadiene) catalyst directly afforded
bright red polymer aggregates with smectic (or sanidic) structure.

6016 Macromolecules, Vol. 43, No. 14, 2010
Liu et al.
yield 12 g of 4-ethynylbenzoic acid as yellow solids (yield 82%).
1HNMR (400MHz, CDCl3), δ(ppm):8.07 (d, 2H, Ar-Horthoto
COOH), 7.59 (d, 2H, Ar-H meta to COOH), 3.27 (s, 1H, ꢀCH).
(b). Synthesis of 4-[(4-Hydroxyphenyl)oxycarbonyl]phenylac-
etylene. Potassium 4-ethynylbenzoate (2 g) was added to dry
THF with two drops of dimethylformamide (DMF). The mix-
ture was cooled with an ice bath, to which 2 mL of SOCl2 was
added dropwise. The mixture was then refluxed for 1.5 h.
Afterward, the solution was concentrated by a rotary evapora-
tor to give an orange solid of 4-ethynylbenzoyl chloride. Hydro-
quinone (2.5 g) was dissolved in dry THF with a small amount of
pyridine, to which a THF solution of 4-ethynylbenzoyl chloride
was added under stirring via a dropping funnel with a pressure-
equalization arm. The reaction mixture was refluxed for 8 h and
then the solvent was removed by a rotary evaporator. The crude
product was purified by column chromatography using petro-
leum ether/ethyl acetate (3/1, v/v) as the eluent. After recrystal-
lization from ethanol, 4-[(4-hydroxyphenyl)oxycarbonyl]phen-
ylacetylene was obtained as white solids (1.9 g, yield 65%). 1H
NMR (400 MHz, CDCl3), δ (ppm): 8.17-8.14 (d, 2H, Ar-H
ortho to COO), 7.64-7.61 (d, 2H, Ar-H ortho to CtCH),
7.09-7.06 (d, 2H, Ar-H ortho to OOC), 6.88-6.85 (d, 2H,
Ar-H ortho to OH), 4.85 (s, 1H, OH), 3.28 (s, 1H, tCH).
(c). Synthesis of 4-(Pentyloxy)benzoic Acid. 4-Hydroxyben-
zoic acid (14 g, 0.1 mol) and KI (0.1 g) were dissolved in 50 mL of
ethanol, to which 12 g (0.21 mol) of KOH in 10 mL of water was
added dropwise with stirring. After being cooled, the reaction
mixture was titrated dropwise with 0.1 mol of 1-bromopentane.
After refluxing for 24 h, the reaction mixture was poured into a
large amount of water, followed by acidification to pH 3-4 by
HCl. The white precipitate was filtered, washed with water and
recrystallized in ethanol. 4-(Pentyloxy)benzoic acid was ob-
1H, tCH), 1.85-1.82 (m, 2H, OCH2CH2), 1.47-1.40 (m, 4H,
CH2CH2CH3), 0.97-0.94 (t, 3H, CH3). 13C NMR (100 MHz,
CDCl3), δ (ppm): 164.79, 164.38 (COO), 163.63 (aromatic
carbon linked with OC5H11), 148.67, 148.12 (aromatic carbon
linked with OOC), 132.30 (aromatic carbon ortho to HCtC),
132.27 (aromatic carbon ortho to COO and meta to OC5H11),
130.05 (aromatic carbon linked with COO and para to HCtC),
129.40 (aromatic carbon ortho to COO and meta to HCtC),
127.54 (aromatic carbon linked with HCtC), 122.77, 122.47
(aromatic carbon ortho to OOC), 121.35 (aromatic carbon
linked with COO and para to OC5H11), 114.34 (aromatic carbon
ortho to OC5H11), 82.68 (HCtC-), 80.57 (HCtC-), 68.34
(OCH2), 28.79 (OCH2CH2CH2CH2CH3), 28.13 (OCH2CH2-
CH2CH2CH3), 22.42 (OCH2CH2CH2CH2CH3), 13.99 (CH3).
MS: m/z = 428 (Mþ). Anal. Calcd for C27H24O5: C, 75.68; H,
5.65; Found: C, 75.43; H, 5.64.
(f). Synthesis of 4-(Pentyloxy)phenyl 4-Ethynylbenzoate (M2-5).
Monomer M2-5 was synthesized by esterification of 4-ethynylben-
zoic acid with 4-(pentyloxy)phenol. The experimental procedures
were similar to those for the synthesis of M3-5 described above.
Monomer M2-5 was obtained as white solid in a yield of 60%.
1H NMR (400 MHz, CDCl3), δ (ppm): 8.15 (d, 2H, Ar-H
meta to CtCH), 7.61 (d, 2H, Ar-H ortho to CtCH), 7.10 (d,
2H, Ar-H meta to OCH2), 6.93 (d, 2H, Ar-H ortho to OCH2),
3.96 (t, 2H, OCH2), 3.27 (s, ꢀCH), 1.79 (m, 2H, OCH2CH2),
1.47-1.38 (m, 4H, CH2CH2CH3), 0.94 (t, 3H, CH3). 13C NMR
(100 MHz, CDCl3), δ (ppm): 164.79 (COO), 157 (aromatic
carbon linked with OC5H11), 144.13 (aromatic carbon para to
OC5H11), 132.17 (aromatic carbon ortho to CtCH), 129.94
(aromatic carbon meta to CtCH), 129.70 (aromatic carbon
para to CtCH), 127.32 (aromatic carbon linked with CtCH),
122.25 (aromatic carbon meta to OC12H25), 115.13 (aromatic
carbon ortho to OC12H25), 82.72 (HCꢀC-), 80.41 (HCꢀC-),
68.43 (OCH2), 29.67 (OCH2CH2), 29.08 (CH2CH2CH3), 22.42
(CH2CH3), 13.97 (CH3). MS: m/z = 308 (Mþ). Anal. Calcd for
C20H20O3: C, 77.92; H, 6.49; Found: C, 77.96; H, 6.33.
1
tained as white solids. H NMR (400 MHz, CDCl3), δ (ppm):
12.62 (s, 1H, COOH), 7.90-7.86 (d, 2H, Ar-H ortho to COOH),
7.04-7.00 (d, 2H, Ar-H ortho to OCH2), 4.05-4.03 (t, 2H,
OCH2), 1.85-1.81 (m, 2H, OCH2CH2), 1.47-1.41 (m, 4H,
CH2CH2CH3), 0.97-0.93 (t, 3H, CH3).
(g). Synthesis of Rh(nbd)[B(C6H5)4] Catalyst. Synthesis of this
catalyst was carried out according to the method reported by
(d). Synthesis of 4-(Pentyloxy)phenol. The compound was
prepared according to the literature method with some modifi-
cation.32 Hydroquinone (11 g, 0.1 mol) was dissolved in 100 mL
of dimethyl sulfoxide (DMSO), to which 16.8 g (0.3 mol) of
powdered KOH was added under stirring. Within 0.5 h, 0.1 mol
of 1-bromopentane was added to the above mixture under
stirring via a dropping funnel with a pressure-equalization
arm. The reaction mixture was kept in 40 °C water bath with
vigorous stirring for 3 h and the course of reaction was followed
by TLC. When completed, the reaction mixture was poured into
400 mL cold water followed by acidification to pH 1. The crude
product was filtered out and washed with cold water. After
purified by column chromatography using dichloromethane
(DCM) as the eluent, 4-(pentyloxy)phenol was obtained as
white solid in a yield of ca. 20%. 1H NMR (400 MHz, CDCl3),
δ (ppm): 6.80-6.73 (m, 4H, Ar-H), 4.53 (s, 1H, OH), 3.90 (t,
2H, OCH2), 1.76 (m, 2H, OCH2CH2), 1.43-1.36 (m, 4H,
CH2CH2CH3), 0.92 (t, 3H, CH3).
Schrock et al.33 RhCl3 3H2O (250 mg) was dissolved in water
3
(1 mL), and Na[B(C6H5)4] (1 g) in methanol (20 mL) was added.
2,5-Norbornadiene (nbd; 1 mL) was then added and the solution
was allowed to stand until crystals were deposited. The yellow
crystalline products were filtered and air-dried. The desired
complex was obtained in a yield of ca. 70%. Anal. Calcd for
C31H28RhB: C, 72.40; H, 5.49; Found: C, 70.14; H, 5.41.
Polymerization. Using Rh(nbd)[B(C6H5)4] as catalyst and dry
toluene as solvent, a typical procedure for the polymerization of
M3-5 is as follows: Into a polymerization tube was added 7 ꢁ
10-3 mmol (3 mg) of monomer M3-5, and then 3 mL of dry
toluene with 0.2 mg of Rh(nbd)[B(C6H5)4] was put into the tube.
After 24 h reaction at room temperature, the red suspension of
polymer product was directly filtered out. After being washed
three times with acetone, the product was dried under vacuum to
a constant weight. The yield was over 80%.
Instruments and Measurements. Elemental analysis was per-
formed with an Elementar Vario EL instrument. Mass spectra
were recorded on a Finnigan-MAT ZAB-HS spectrometer. FT-
IR and Raman measurements were carried out using a Magna-
IR750 (Nicolet) FT-IR spectrometer and an IFS-106 (Bruker)
Raman spectrometer, respectively. Differential scanning calo-
rimetry (DSC, PerkinElmer Pyris I with a mechanical refriger-
ator) was utilized to study the phase transitions of the monomers
and the polymers. Thermogravimetric analysis (TGA) was per-
formed with a TA SDT 2960 instrument at a heating rate of
10 °C/min in nitrogen or air atmosphere. Gel permeation chro-
matography (GPC) was carried out on a Waters 515 GPC
instrument using THF as an eluent at 35 °C. The GPC calibra-
tion curve was obtained with linear polystyrene standards.
UV-vis absorption and turbidity measurements were perfor-
med with a Lambda 35 (Perkin-Elmer) UV-vis spectrometer.
(e). Synthesis of 4-[4-(Pentyloxy)benzoyloxy]phenyl 4-Ethy-
nylbenzoate (M3-5). First, 0.5 g (2.1 mmol) of 4-[(4-hydroxy-
phenyl)oxycarbonyl]phenylacetylene, 1.0 g (4.8 mmol) of 4-
(pentyloxy)benzoic acid, 0.15 g (1.3 mmol) of DMAP, 0.23 g
(1.2 mmol) of TsOH were dissolved in 100 mL of dry DCM, to
which the DCM solution of 1.5 g (7.3 mmol) of DCC was added
dropwise with stirring. The reaction mixture was stirred for 24 h
at room temperature. After the solvent was removed, the crude
product was purified by a silica gel column using petroleum
ether/DCM (1:1 by volume) as eluent. M3-5 was obtained as
white solid in a yield of ca. 60%.
1H NMR (400 MHz, CDCl3), δ (ppm): 8.18-8.13 (t, 4H,
Ar-H meta to CtCH or OCH2), 7.64-7.61 (d, 2H, Ar-H
ortho to CtCH), 7.26 (s, 4H, Ar-H ortho to OOC), 6.99-6.97
(d,2H, Ar-H ortho to OCH2), 4.07-4.03 (t, 2H, OCH2), 3.28 (s,

Article
Macromolecules, Vol. 43, No. 14, 2010 6017
We followed the change of UV-vis absorption during polym-
erization. The polymerization was carried out in a 1 cm colori-
metric container in open air at room temperature. The absorp-
tion from 350 to 750 nm was recorded continuously right after
adding Rh(nbd)[B(C6H5)4] to the toluene solution. The turbidity
was measured at a wavelength of 700 nm, and the solvent of dry
toluene was used as the reference.
1H NMR (400 MHz) spectra were recorded on a Bruker
ARX400 spectrometer at room temperature using deuterated
chloroform (CDCl3) as the solvent and tetramethylsilane (TMS)
as the internal standard. To monitor the polymerization process
by 1H NMR, the in situ measurement was carried out on a
Mercury Plus (Varian) 300 MHz spectrometer. In an NMR
tube, M3-5 was dissolved in toluene-d8 (∼10 mg/mL) with trace
of DCM as an internal standard. The spectra were recorded
sequentially immediately after the addition of Rh(nbd)[B(C6H5)4].
To identify the LC structure of the polymer particles, WAXD
experiments were performed with a high-flux small-angle X-ray
scattering instrument (SAXSess, Anton Paar) equipped with
Kratky block-collimation system and a Philips PW3830 sealed-
tube X-ray generator (Cu KR). The diffraction patterns were
recorded on an imaging-plate which extends to the high-angle
range (up to 2θ of 42°). The diffraction peak positions were
calibrated with silver behenate.
After the polymerizations completed, the morphology of
polymer particles was examined using scanning electron micro-
scopy (SEM; JEOL JSM-6700F). The SEM sample was pre-
pared by placing a drop of the polymerization suspension on a
clear silica surface with the excess solution blotted by a filter
paper. After drying under ambient conditions and later in
vacuum, the SEM sample was sputtered with a gold layer.
Computer Modeling. Single molecules of P3-5 and P2-5 con-
taining 10 repeating units with a perfect cis-transoidal structure
of the polyacetylene backbone was constructed by Materials
Studio 5.0 (Accelrys). We further assumed that the alkyl tails on
side chains were in all trans conformation. The energy of this
molecule was minimized using Smart Minimizer incorporated in
the Discover Molecular Simulation Program.
Figure 1. FT-IR spectra of the monomers and the corresponding
polymers. (a) M3-5 and P3-5; (b) M2-5 and P2-5.
Results and Discussion
During the polymerization, both the colorless solutions of M3-5
and M2-5 turned to bright red and then became more and more
turbid with lengthening reaction time, implying that the resultant
polymers P3-5 and P2-5 formed aggregates. The red aggregates
could not dissolve in common organic solvents such as n-octane,
cyclohexane, toluene, CHCl3, THF, and DMF.
We attempted to investigate the molecular shape and LC
properties of polyacetylene derivatives with the mesogenic units
directly linked to the polyacetylene backbones without flexible
spacer. In order to easily tune the length and thus the aspect ratio
of the mesogenic core (the ratio of the length to diameter of the
rod-like mesogens) on the side-chains, we designed the side-chain
structure to be “phenyl-ester-phenyl” type. The synthetic
routes to the monomers M3-5 and M2-5 are shown in Scheme 1,
where M denotes monomer, numbers of 3 and 2 refer to the
number of benzene rings in the side chain, and number of 5 is the
number of carbon atoms of the tail. Both the monomers were
synthesized by esterification of the substituted benzoic acid with
the corresponding substituted phenol, whose chemical structures
were confirmed by 1H NMR, 13C NMR, mass spectroscopy and
elemental analysis (see the Experimental Section for details). M3-
5 and M2-5 are white crystals at low temperatures. Upon heating
from 20 to 160 °C, only single meting process of crystals could be
detected for M3-5 and M2-5, with the melting temperature of 141
and 81 °C, respectively. The monomers can be readily dissolved in
common organic solvents such as CHCl3, THF, and toluene. The
resultant solutions are transparent and colorless.
Since the polymer aggregates could not be dissolved in com-
mon organic solvents, the molecular characterization became
somewhat difficult. Because of the poor solubility, we could not
get the molecular weight (MW) information by such routine
methods as GPC and laser light scattering, while the matrix-
assisted laser desorption/ionization and time-of-flight mass spec-
troscopy (MALDI-TOF) method did not work either. However,
as shown in Figure 1, the FT-IR results are still adequate to
support that the polymerizations indeed give the desired poly-
mers (or at least the oligomers). In the spectra of monomers, the
tC-H vibration peaks at about 3300 cm-1 is clearly observed.
Disappearance of this peak in the spectra of the polymerization
products indicates that the alkynyl group CtCH has been
consumed. We checked the thermal stability of the polymers.
The TGA results (Figure 2) show that the temperatures for 5%
weight loss of the samples are around 340 °C in nitrogen atmo-
sphere. The thermal stability of the polymers is considerably
improved in comparison with PPA which degrades at about
270 °C.38 This implies that the backbones are protected by the
large side-chains, which prevented the polymer from decom-
posing at low temperatures.39
To polymerize the monomers under mild conditions, we choose
organorhodium complexes as the catalyst, which are robust with
high tolerance to polar moiety in acetylene monomer and polar
solvent (even in water34 or air35). In the Rh catalysts family, the
zwitterionic Rh complex of Rh(nbd)[B(C6H5)4] shows excellent
activity even without any cocatalyst.36,37 For this reason, we chose
it as the catalyst to carry out the polymerization in toluene.
We used Raman technique to study the stereostructure of the
polymer backbones. The Raman spectra reveal that both of P3-5

6018 Macromolecules, Vol. 43, No. 14, 2010
Liu et al.
Figure 4. WAXD powder patterns of P3-5 and P2-5 solid particles
obtained directly from solution polymerization.
Figure 2. TGA results of P3-5 and P2-5 measured under nitrogen
atmosphere.
vector (q = 4π(sin θ)/λ) ratio of 1:2:3 can be identified in the
low-angle region, while in the high-angle region only an amor-
phous halo centered at a q value of ∼14 nm-1 is observed. This
result indicates that the smectic structure is developed within the
solid particles right after they precipitated out from the poly-
merization solution. Compared to P2-5 with a shorter side-chain,
P3-5 gives the diffractions appearing at lower angles, indicating
that the smectic layer periodicity is side-chain length dependent.
Our DSC experiments could not detect any first-order transition
or glass transition of P3-5 and P2-5 from room temperature to
250 °C, implying that the polymers should be rigid and their LC
transition temperatures could be higher than 250 °C (Since the cis-
rich samples may degrade upon thermo treatment, we did not heat
the samples to higher temperatures).
In our previous studies,22,23 we have identified that the
molecule of poly(5-{[(40-alkoxy-4-biphenylyl)carbonyl]oxy}-1-
pentyne) with a relatively short spacer of three methylene units
is sheet-like, which can stack parallel together to form a highly
ordered smectic phase or monolayer lamellae in dilute solutions.
For P3-5 and P2-5 without spacer, we consider that the LC
structures observed shall be also constructed by the packing of
whole sheet-like molecules, and the smectic (or sanidic) layer
periodicity is determined by the width of the molecular sheet. It
has been suggested that the cis-PPA derivatives without spacers
can have the side-chains attached to the cis-transoidal backbone
with a “herringbone” configuration for both head-to-tail and
head-to-head arrangements.24 Taking P3-5 as an example, Figure 5
depicts our molecular simulation result after energy minimiza-
tion, which indeed shows a sheet-like conformation. Considering
the angle of 60° between the polyacetylene backbone and the side-
chain (see the top view in Figure 5), the width of the molecular
sheet (L) should be 2l sin 60°, where l is the side-chain length with
the alkyl tails in a fully extended conformation. As shown in
Table 1, the d-spacings of the first-order diffractions (d1’s) of P3-5
and P2-5 are almost identical to the calculated values of L.
Considering that the mesogenic side-chains are tilted 30° away
from the layer normal, one may assign the LC structure to be
smectic C (SC). Here, it is also worth mentioning that we in fact
can take the whole sheet-like molecules as the building block of
the LC structure. Therefore, we consider that the LC structure
can belong to a sanidic structure with a one-dimensional long-
range order along the layer normal (i.e., ΣO phase suggested by
Wendorff et al.).18
Figure 3. Raman spectrarecorded from the red precipitates ofP3-5 and
P2-5.
and P2-5 possess cis-polyene structure (Figure 3). The peaks at
1537, 1340 and 962 cm-1 can be assigned to the stretching CdC
band in cis-polyacetylene (although it is partially overlapped with
that of the phenyl ring), the stretching vibration of cis C-C bond
coupled with the single bond connecting the main chain and
phenyl ring, and the C-H deformation band of the cis main-
chain, respectively.40-42 On the other hand, the scattering peaks
corresponding to the trans form, which shall appear at around
1480 and 1190 cm-1 42,43
are scarcely observed. Therefore, we
,
conclude the red precipitates are cis-rich polyacetylenes. This
result is consistent with that usually found in the monosub-
stituted polyacetylenes obtained from Rh-catalyzed polymeri-
zation, of which the main-chain is stereoregularly cis-trans-
oidal.30,35
We speculate that the insolubility of the polymers is related to
the internal ordered structure of the precipitates, which can be
detected by WAXD method. To prepare the WAXD samples, the
insoluble precipitates were carefully washed with toluene to
remove the residual monomers and catalysts. Therefore, the
diffractions of the samples at room temperature shall solely be
given by the polymer particles which were not subjected to any
further thermal treatment. As depicted in Figure 4, the WAXD
patterns of P3-5 and P2-5 clearly feature a smectic (or sanidic)
structure. The diffractions uptothe third-order with the scattering
Figure 6 schematically depicts the molecular sheet and its
smecitc (or sanidic) packing of P3-5 and P2-5. Owing to the termi-
nal attachment of the mesogens without flexible spacers, the side-
chains and backbone are tightly coupled together. Therefore, the

Article
Macromolecules, Vol. 43, No. 14, 2010 6019
Figure 7. SEM image of the P3-5 solid particles obtained after the
polymerization completed.
samples were prepared after the polymerization in toluene solu-
tion had completed. Because the trace residue monomers and
catalysts can be neglected, the morphology observed should
mainly come from the polymers. Figure 7 shows a SEM image
of P3-5 particles. In some areas the larger aggregates are
obviously the clusters of the small particles which adhered to
each other. The isolated particles look relatively uniform,29 with
an average lateral size of ∼300 nm. The polygonal and/or near
rectangular shapes shall be largely associated with the smectic
structure. For the rectangular shaped particles, the aspect ratio of
the length to width can reach a value of ∼3. However, since the
aggregation occurred during polymerization, the polymeric mo-
lecules were hard to adjust their packing in the solid particles.
Therefore, the smectic structure is less perfect. On the basis of the
WAXD profiles of P3-5 and P2-5 (Figure 4), the apparent
correlation length along the smectic layer normal was estimated
to be ∼15 nm using the Scherrer equation, which is much smaller
than the particle size observed under SEM.
Figure 5. Illustrations of the sheet-like molecule of P3-5 after energy
minimization: (a) top view; (b) side view. The carbon atoms on main
chain are shown by pink color.
Table 1. Relationship between the Smectic Periodicity and the
Molecular Width
We were interested in knowing when and how the smectic
aggregates of P3-5 and P2-5 were formed in solution. It is
reasonable to assume that the aggregation mechanism is very
similar to that of crystallization during solution polymerization
of crystalline polymers.44 One may imagine that at the beginning
of polymerization the resultant oligomers with relatively good
solubility are dissolved in solution, so at that time the solution is
transparent macroscopically. Once the oligomers further grow to
a “critical MW”, the aggregation occurs. Afterward, the newly
formed polymer (or oligomer) molecules may either assemble by
themselves to form new aggregates, or join the existing solid
particle leading to the particle growth in size. According to the
morphology images showing that the polymer particles with large
and relatively uniform size (see Figure 6), the latter case should be
more possible. It is worth mentioning that the polymerization of
M3-5 and M2-5 in toluene catalyzed by Rh(nbd)[B(C6H5)4]
proceeded considerably slowly at room temperature. Therefore,
we could follow the polymerization process using various tech-
niques, and thus the relationship between polymerization and
aggregation can be studied in great detail.
polymer
q1a/nm-1
d1a/nm
Lb/nm
d1/L
LC structure
C (or ΣO)
P3-5
P2-5
1.48
1.82
4.24
3.45
4.15
3.10
1.02
1.11
S
SC (or ΣO)
a The scattering vector (q1) and the corresponding d-spacing (d1) of
the first-order diffraction. b Molecular width L obtained by molecular
simulation with an assumption of all trans conformation for the methyl-
ene units.
Using UV-vis technique, we traced the polymerization pro-
cess of M3-5 in toluene with a low initial monomer concentration
([M]0 = 0.23 mmol/L) and a molar ratio of monomer/Rh(nbd)-
[B(C6H5)4] of 25:1. In the spectra shown in Figure 8a, the
absorption band in the range from 400 to 550 nm is attributed
to π-π* electronic transition of the polyene backbone. This band
appears rapidly and becomes a pronounced peak as the polym-
erization proceeds, whose position shifts to longer wavelength.
The plot of the peak position of polyene backbone absorption vs
polymerization time (t) illustrates that the red shift takes place
very fast in the first 25 min and then gradually leaves off at ∼455
nm after 60 min (Figure 8b). The bathochromic shift by ca. 25 nm
of the peak position indicates a significant increase of the average
conjugation length of P3-5, which may also partially reflect the
increase of averageMW. Notethat itis seldom to obtain a cis-rich
PPA with the main-chain absorption at such a long wavelength.
Figure 6. Schematic illustration of the smectic (or sanidic) structure of
P3-5 and P2-5. Within each smectic layer, the polymer molecules are
sheet-like and stack parallel to each other with the backbones located at
the layer center.
molecule bears a rather rigid inner part including the planar cis-
backbone and mesogenic cores on both sides of the backbone. In
each smectic layer of the molecular stacking, the conjugated
backbones are located at the center portion. We presume that
the intermolecular interaction (including π-π stacking, dipole-
dipole interaction, etc.) is so strong that the smectic aggregates of
P3-5 and P2-5 cannot be dissolved in common organic solvents.
The morphology of the polymer particles directly precipitated
out form the solutions was examined using SEM. The SEM

6020 Macromolecules, Vol. 43, No. 14, 2010
Liu et al.
Figure 9. GPC curves of the soluble components obtained during the
polymerization of M3-5 in toluene at different polymerization time.
The [M]0 was 10 mg/mL and the molar ratio of monomer/Rh(nbd)-
[B(C6H5)4] was 25:1.
a set of GPC curves of the soluble compounds obtained at various
t’s. The peak with a retention time of ca. 28.5 min corresponds to
the monomer. As the polymerization gradually consumes the
monomer, this peak becomes smaller with increasing t. Interest-
ingly, after 10 min of polymerization, a minor peak located at a
retention time of ca. 25 min can be observed, which shall be
related tothe soluble P3-5with an apparent number-average MW
(Mn) of 2900 g/mol. This peak remains at the same position
disregarding increasing t, indicating that the P3-5 with Mn over
2900 g/mol would precipitate out. Comparison between the
apparent Mn’s of the two GPC peaks gives that the degree of
polymerization of the soluble P3-5 is approximately 5. However,
we are not able to determine the exact “critical MW” at this
moment.
Figure 8. (a) In situ UV-vis detection of P3-5 polymerization at an
initial monomer concentration ([M]0) of 0.1 mg/mL. (b) Peak position
of main-chain absorption and the value of “absorption” at 700 nm as a
function of reaction time t. The absorption data were recorded con-
tinuously and repeatedly from 750 to 350 nm.
We monitored the whole polymerization process with different
[M]0’s using turbidity measurement. As mentioned above, we
used the value of “absorption” at 700 nm as the solution
turbidity. Figure 10a describes the changes in solution turbidity
during the M3-5 polymerizations with a fixed monomer/Rh-
(nbd)[B(C6H5)4] ratio of 25:1 and four [M]0’s. Obviously, after an
induction period the solution turbidity increases rapidly and then
gradually levels off. An increase in [M]0 leads to a shorter
induction period and a steeper turbidity increase, which can be
ascribed to the faster polymerization rate and more polymer
molecules produced in solution. For polymerization of M2-5, a
similar behavior of the turbidity change with t was observed.
However, the aggregation ability of P2-5 is lower than that of P3-
5. Figure 10b shows the comparison of turbidities as functions of t
between M3-5 and M2-5, with a same [M]0 of 2.3 mmol/L (i.e.,
1 mg/mL for M3-5) and a monomer/Rh(nbd)[B(C6H5)4] ratio of
25:1. It is found that shortening the length of mesogenic core can
delay and damp the turbidity increase. We suggest that when
more phenyl rings are incorporated into the molecules, the inter-
action (particularly π-π interactions between the side-chains)
will be enhanced, resulting in a stronger tendency of aggregation
of the sheet-like molecules in solutions.
We consider that the rigid sheet-like shape of the P3-5 (also P2-5)
molecules shall be rather persistent in solution and therefore,
increasing MW will lead to the increase of conjugation length.
Moreover, parallel packing of the molecules in the smectic
structure may make the molecules more sheet-like.
In Figure 8a, we also observe the increase of “absorption” at
700 nm. As this wavelength is far away from the peak position of
polyene backbone absorption at ∼455 nm, we consider that the
“absorption increment” at 700 nm should mainly arise from the
scattering of the P3-5 particles formed during polymerization,
which can thus be taken as a measurement of the solution
turbidity. The value of “absorption” at 700 nm changing with t
is also shown in Figure 8b, wherein a sigmoidal curve is observed
with its largest slope located at t of ∼25 min. The comparison
between the two curves in Figure 8b reveals that at the beginning
the overall aggregation of P3-5 molecules lags behind the increase
of P3-5 conjugation length. This result infers that there is a
“critical MW” for P3-5 aggregation.
To confirm the existence of the “critical MW”, we used PPh3 as
a terminator to stop the polymerization at various stages,36 and
tried to detect the trace amount of the residual soluble polymer
(oligomer) in the solution via GPC. Taking M3-5 as an example,
the polymerization was carried out in toluene with a [M]0 of 10
mg/mL and a molar ratio of monomer/Rh(nbd)[B(C6H5)4] of
25:1. At different t of polymerization, we transferred 0.2 mL of
solution into a vial containing 5 mg of PPh3 by filtration. After
toluene in the vial was removed, the residual material was
redissolved in THF for the GPC measurement. Figure 9 shows
1
We also monitored the polymerization using H NMR tech-
nique to further investigate the relationship between polymeri-
1
zation and aggregation. Figure 11a shows a set of H NMR
spectra of M3-5 polymerization in toluene-d8 recorded at 25 °C
with an [M]0 of 23 mmol/L (10 mg/mL) and a monomer/Rh-
(nbd)[B(C6H5)4] molar ratio of 25:1. The resonance signal at δ
2.78 is assigned to the alkyne hydrogen (tCH) of the monomer,

Article
Macromolecules, Vol. 43, No. 14, 2010 6021
1
Figure 11. In-situ H NMR monitoring of P3-5 polymerization with
DCM as the internal standard in toluene-d8. The initial monomer
concentrations ([M]0) is 10 mg/mL. (a) Set of 1H NMR spectra recorded
at various t’s. (b) Monomer conversion {1 - [M]/[M]0} and log{ [M]/
[M]0} asfunctions oft. The value of[M]/[M]0 was measuredfrom the 1H
NMR spectra.
Figure 10. (a) In situ turbidity detection of the P3-5 polymerization
with different initial monomer concentrations ([M]0’s) at a wavelength
of 700 nm. (b) Turbidities as functions of t for P3-5 and P2-5 at a fixed
[M]0 of 2.3 mmol/L.
polymerization rate, i.e., the slope of the conversion curve
becomes smaller than that of the first stage. Note that the reaction
condition applied for the H MNR measurement is identical to
while that centered at δ 3.55 is attributed to the hydrogen of
-OCH2 group on the side-chains. At the beginning of polymeri-
zation (e.g., at t = 4 min), the originally well-resolved peaks of
-OCH2 becomes obviously broad. This may be attributed to that
the signal of the -OCH2 groups in the soluble polymer molecules
with reduced mobility is superimposed on that of the monomers.
When the monomer concentration iscontinuously decreased with
increasing t, the signal of -OCH2 turns to be an apparently single
broad one. We expect that the environment of -OCH2 shall be
very rigid when the P3-5 molecules are packed into the aggre-
gates, and the corresponding signal will be hardly detected.
However, -OCH2 signal can be found in a fairly long time of
polymerization (up to 60 min), reflecting that the newly formed
P3-5 with low MW is dissolved in the solution. This observation
suggests again that only when the P3-5 molecules reach a “critical
MW” or the solubility limit, they will pack into the aggregates
with smectic structure.
The concentration of the monomer [M] is proportional to the
integration of alkyne hydrogen (tCH) resonance peak at δ 2.78,
and therefore, the value of [M]/[M]0 is equal to the ratio of the
integrated area at a reaction time of t to that recorded at t = 0. In
Figure 11b, we plot the monomer conversion of {1 - [M]/[M]0}
measured from the 1H NMR spectra as a function of t. Appar-
ently, there are two stages of polymerization. The first stage is
ranged from t of ∼3 to ∼30 min. It is observed that after a short
induction time of ∼3 min, the monomer conversion increases
rapidly with increasing t, seemingly showing a linear behavior. At
t > ∼30 min (the second stage of polymerization), the overall
1
that for the turbidity measurement with [M]0 = 23 mmol/L (10
mg/mL) (see Figure 10a). We find that the induction time of
polymerization is rather close to the onset of turbidity increase
indexed as t0 in Figure 10a. Moreover, while the polymerization
enters the second stage, the turbidity in fact approaches the
plateau value. Although the turbidity and 1H NMR result were
measured from different experiments and thus the comparison
between them cannot be made strictly, the coincidences still
indicates that the aggregation indeed occurs at the early stage
of polymerization, and is closely related to the polymerization
process. Since the onset of turbidity increase locates at the very
beginning of polymerization, we suspect that once the polymeri-
zation takes place, the P3-5 molecules possess a MW distribution,
of which the largest ones form aggregates immediately. On the
other hand, we note that at t of ∼90 min the residual resonance of
the alkyne hydrogen is still detectable, implying that the monomers
are not fully exhausted. Therefore, during a long period of time, the
aggregation and polymerization occur simultaneously.
According to the literatures,30,45 an insertion mechanism is
generally accepted for the Rh-catalyzed acetylene polymeriza-
tion. Thus, a simple propagation reaction can be described as:
Mn- Rh þ M f Mn þ 1- Rh
where Mn-Rh andM denote the intermediate active propagation
chain and the monomer, respectively. Therefore, the propagation

6022 Macromolecules, Vol. 43, No. 14, 2010
Liu et al.
rate can be written as -d[M]/dt = k[M][Mn-Rh], where k is the
rate constant. The above rate formula can be rewritten as log{[M]0/
[M]} = k[Mn-Rh]t. Interestingly, we find that the plot of
log{[M]0/[M]} ∼ t presents a linear behavior for the second stage
of polymerization (i.e., t > 30 min), inferring that the concentra-
tion of active propagation chain [Mn-Rh] is nearly constant. On
the other hand, we observe that the slope of log{[M]0/[M]} ∼ t
curve for the first stage of polymerization is always smaller.
Assuming that the value of k remains constant during the whole
polymerization process, the smaller slope indicates that the
[Mn-Rh] is lower. In addition to the possibly unstable initiation
process of M3-5 polymerization with the catalyst of Rh(nbd)-
[B(C6H5)4], we presume that the rather fast aggregation occurred
in the first stage of polymerization greatly affects the value of
[Mn-Rh]. The Rh complex at the chain ends might either be
trappedin the solid particles after the chain packing or be released
to solution later on; however, we cannot determine which one of
the two possibilities is more likely on the basis of the polymer-
ization kinetics data we obtained.
After both the processes of aggregation and polymerization
slow down, namely, after the first stage, the polymerization seems
to enter a steady state, giving the apparent first-order polymeri-
zation kinetics, whose behavior is highly reminiscent of that
observed in traditional acetylene polymerization under catalyst
system of Ti(OBu)4-AlEt3.46 For the Ti(OBu)4-AlEt3 catalyzed
acetylene polymerization, the propagation mechanism is believed
to be cis-insertion of monomer, wherein the active site remaining
relatively constant is attributed to the presence of two types of
active sties. Although we are unable to deduce a similar explana-
tion for our polymerization system, we still consider that the first-
order polymerization process we observed deserves further in-
vestigation, which may be utilized to control the growth of
polymer products. Furthermore, when the resultant polymers
can form uniform nanoparticles directly during polymerization,29
we might take advantage of the first-order polymerization to
develop a technique to control the particle morphology.
For P3-5 and P2-5 molecules which can aggregate during
solution polymerization, we consider that the rigid sheet-like
molecular structure with the mesogenic side chain directly linked
to the polyacetylene backbone plays an important role. However,
according to the literatures, such arrangement of backbone and
mesogenic side-chain may not be certain to result in the sponta-
neous molecular assembly in solutions. Advincula et al. synthe-
sized a monomer of 4-(dodecyloxy)phenyl 4-ethynylbenzoate
(denoted as M2-12 below). Compared to M2-5, M2-12 just
possesses a longer alkyl tail with 12 carbon atoms.25 After poly-
merizing M2-12 under WCl6 in dry toluene, they have found that
the resultant polymer (denoted as P2-12) possesses a very good
solubility in common organic solvent. In our work, we also
synthesized P2-12 using Rh(nbd)[B(C6H5)4] as the catalyst, with
an attempt to improve the solubility of the polymer with
increasing the side-chain tail length. In contrast to that obtained
by Advincula et al., we found that the polymerization behavior of
M2-12 was similar to that of M2-5, and the red P2-12 was
insoluble. It seems that the detailed chain stereostructures affect
the polymer solubility. It is known that polymerization of acetyl-
ene monomer with W-based catalyst usually yields trans-rich
main chain, whistle the catalyst of rhodium complexes usually
gives cis-rich structure. However, considering that a perfect trans-
cisoidal main chain is actually rather planar, we suspect that that
the polymer backbone of the sample synthesized by Advincula
et al. might be stereorandom with a relatively lower trans-content,
namely, have more conformational defects.
polymers we studied. Hsu et al. have synthesized a monomer of
4-ethynylphenyl 4-pentylcyclohexanecarboxylate and polymeri-
zed it by [Rh(nbd)Cl]2.27 The resultant polymer has good
solubility but exhibits no LC structure. In comparison with
P2-5, it seems that introducing a cyclohexyl ring, which may
display either chair or boat conformation, to the side-chain
weakens the interaction between side-chains and thus disfavor
the LC ordering. Tang et al. have reported that polymerization of
2-(4-octoxyphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole using
[Rh(nbd)Cl]2 as the catalyst also gives a soluble polymer in
common organic solutions.47 Compared with M2-5, their mono-
mer changes the ester linkage in the side chain to the 1,3,4-
oxadiazole moiety which can also provide significant π-π inter-
action. At this stage, it is hard for us to conclude what chemical
structure of side-chain polyacetylene with mesogenic group
directly attached to the backbone will lead to sheet-like molecules
which can undergo solution LC ordering spontaneously. There-
fore, in order to increase the effective conjugation length of the
sheet-like polyacetylene and to obtain better LC ordering, a more
systematic study is required.
Summary
We have successfully synthesized two side-chain liquid-crystal-
line polyacetylenes (P3-5 and P2-5) with the mesogenic unit
directly linked to the semirigid backbone in a terminal-on mode.
Our experimental results show that during the polymerization
reactions, both P3-5 and P2-5 can directly self-aggregate into red
insoluble nanoparticles with fairly uniform size of ∼300 nm. The
cis-rich P3-5 and P2-5 are sheet-like. After reaching a “critical
MW” during polymerization, they may stack parallel to each
other with strong intermolecular interaction, forming smectic or
sanidic structure. P3-5 with larger slenderness ratio of the
mesogenic unit has stronger aggregation capability than P2-5.
During the polymerization process, the fast aggregation is well
correlated to the fast polymerization. Afterward, the polymeri-
zation reaches a steady state with a first-order kinetics. It is worth
noting that the effective conjugation length of the cis-rich P3-5
and P2-5 we obtained is considerably long. Therefore, the
molecular design described in this paper may provide a method
to directly produce high performance opto-electronic materials of
polyacetylene derivatives with improved properties.
Acknowledgment. This work was supported by the National
Natural Science Foundation of China (NNSFC Grants:
20129001, 20874082, 20774006, 20990232 and 20974028) and
the Research Grants Council of Hong Kong (602707).
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