New chiral thiols and related side chain liquid crystalline polymers

Article abstract of DOI:10.1080/15421400701205628

New series of chiral thiols with three aromatic rings and the N = N group in the molecular core have been synthesized and grafted on polybutadiene diols backbone. The resulting polymers possess the OH end groups for possible preparation of the liquid crys

Full text of DOI:10.1080/15421400701205628

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New Chiral Thiols and Related
Side Chain Liquid Crystalline
Polymers
Alexej Bubnov a , Miroslav Kašpar a , Zdena
Sedláková b & Michal Ilavský b c
a Institute of Physics, Academy of Sciences of
the Czech Republic, Na Slovance , Prague, Czech
Republic
b Institute of Macromolecular Chemistry, Academy
of Sciences of the Czech Republic , Prague, Czech
Republic
c Faculty of Mathematics and Physics, Charles
University , Prague, Czech Republic
Published online: 22 Sep 2010.
To cite this article: Alexej Bubnov , Miroslav Kašpar , Zdena Sedláková
& Michal Ilavský (2007) New Chiral Thiols and Related Side Chain Liquid
Crystalline Polymers, Molecular Crystals and Liquid Crystals, 465:1, 93-107, DOI:
10.1080/15421400701205628
To link to this article: http://dx.doi.org/10.1080/15421400701205628
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Mol. Cryst. Liq. Cryst., Vol. 465, pp. 93–107, 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400701205628
New Chiral Thiols and Related Side Chain Liquid
Crystalline Polymers
Alexej Bubnov
ꢀ
Miroslav Kaspar
Institute of Physics, Academy of Sciences of the Czech Republic,
Na Slovance, Prague, Czech Republic
ꢁ
ꢁ
Zdena Sedlakova
Institute of Macromolecular Chemistry, Academy of Sciences of the
Czech Republic, Prague, Czech Republic
Michal Ilavsky´
Institute of Macromolecular Chemistry, Academy of Sciences of the
Czech Republic, Prague, Czech Republic; Faculty of Mathematics and
Physics, Charles University, Prague, Czech Republic
New series of chiral thiols with three aromatic rings and the N ¼ N group in the
molecular core have been synthesized and grafted on polybutadiene diols back-
bone. The resulting polymers possess the OH end groups for possible preparation
of the liquid crystal ordered networks. Basic properties of the thiols and resulting
polybutadiene diols have been studied by polarizing microscopy and differential
scanning calorimetry. The resulting diol-based polymers show the liquid crystal-
line behaviour in a broad temperature range. The effect of the side chain structure
and density of grafting on the properties of polybutadiene diols has been studied
and discussed.
Keywords: chiral thiols; differential scanning calorimetry; diols; liquid crystal; polariz-
ing optical microscopy; polybutadiene
This work was supported by Grants No. IAA4112401 from the Grant Agency of the
Academy of Sciences of the Czech Republic; No. 202=05=0431 from the Czech Science
Foundation and ESF-COST Action D35 WG0013=05.
Address correspondence to Dr. Alexej Bubnov, Liquid Crystal Group, Department of
Dielectrics, Institute of Physics, Academy of Sciences of the Czech Republic, Na
Slovance 2, 18221 Prague, Czech Republic. E-mail: bubnov@fzu.cz
93

94
A. Bubnov et al.
INTRODUCTION
Thermotropic liquid crystalline (LC) polymers with alternating rigid
(mesogenic) and flexible (spacer) units in the side chain have attained
considerable attention in literature as they present a unique combi-
nation of the properties typical for monomeric liquid crystals with
those peculiar to polymers [1,2].
Since 1984 [3], the development of the side chain ferroelectric LC
polymers is an active research subject in the liquid crystal research.
Most of the work was devoted to studies on polyacrylates [4] and poly-
siloxanes [5–9]. In order to study the polar properties, LC polymers
need to be designed in such a way that mesogenic groups are sepa-
rated from the polymer backbone by a flexible spacer chains so that
the motion of the side chains is decoupled from that of the backbone
[10]. Tilted smectic phases are typically exhibited by LC polymers
where the length of the flexible spacer is greater than six methylene
units. It was also shown that an increase of the backbone flexibility
results in the preferential tendency for tilted smectic phases to be
exhibited [11]. The polymers with flexible backbones are responding
on the applied electric field much faster than those with more rigid
backbones [12].
In recent years, the azobenzene containing LC polymer systems
have received increasing attention because of their unique properties
that allow various applications triggered by light. A variety of LC azo
polymers has been synthesized in which the azobenzene moiety plays
roles as both mesogene and photosensitive chromophore [13–19].
Polybutadiene diols grafted with liquid crystalline side chains of dif-
ferent structure offer the design and synthesis of novel linear and
crosslinked materials, which might exhibit properties that respond
demands photonics applications. On the other hand, the network for-
mation based on well-defined polyols and polyisocyanates leads to con-
trolled chain construction with well predictable final network
structure. In that case, it is possible to determine the effect of cross-
linking in the LC-state on development, final structure and physical
properties of such systems. Possible crosslinking in magnetic or elec-
tric fields allows to prepare macroscopically oriented structure and
to determine the effect of orientation on physical properties.
A very promising type of flexible backbone (but not well studied
until now) can be based on dienes such as polybutadiene. In this case,
the double bond exists in each 1.4 or 1.2 monomeric unit and these
double bonds can be used for further chemical reactions so that vari-
ous LC monomers can be grafted on polybutadiene backbone. The
grafting can be proceeded through the terminal thiol group attached

Chiral Thiols and Related Side Chain LC Polymers
95
to the end of flexible spacer of the LC molecule. As the OH terminated
polybutadiene diols with various length (molecular weights within
500–20,000) are commercially available, their grafted analogues can
be used for preparation of linear or crosslinked LC liquid crystalline
polymers. Our preliminary results have shown that approximately
50 mole percent of double bonds of polybutadiene diols can be used
for grafting and concentration of the OH groups (functionality of diols)
is practically unaffected by grafting reaction [20,21]. High flexibility of
polybutadiene backbone together with the absence of the polar groups
(that could interact with mesogens) suggests that the LC ordering of
the side chain segments will not be affected by the backbone. Several
types of the chiral thiols with two aromatic rings have been synthe-
sized and grafted on polybutadiene diols backbone. Only few of the
resulting polymers possess the liquid crystalline behaviour and the
viscosity was high that makes impossible the advanced studies
[13,20,22]. In order to improve the mesomorphic and physical proper-
ties, longer hydrocarbon chain in the chiral part of the molecule should
be used.
In present work, the effect of structural changes in the mesogenic
=
thiols possessing three benzene rings and the photosensitive N N
group in the molecular core has been investigated with respect to
the mesomorphic properties of the resulting polymers. Several types
of rod-like liquid crystalline molecules with thiol group attached to
flexible spacer end and with the chiral group, based on (S)–2–methyl-
butanol or (S)–2–methyl–2–alkyloxyethanol have been synthesized.
The effect of the side chains structure, type of chiral group and density
of grafting (G) on the polybutadiene diols has been studied. General
chemical formulae and corresponding abbreviations for the side chain
units (monomers) and respective polymers are presented in Table 1.
TABLE 1 Chemical Formula of the Side Chain Unit with Three Phenyl Rings
=
Molecular Core Incorporating the N N group
Monomer
MK16
MK15
R
CH3CH2CꢀH(CH3)CH2
C6H13
Polymer
K16
K15
MK12
C9H19
K12
MK13
C12H25
K13

96
A. Bubnov et al.
Synthesis
The whole synthetic procedure for the polymers with side chain units
possessing three phenyl rings and the azo group in the core is divided
in several parts, namely, (i) preparation of the chiral chains for the
thiols; (ii) synthesis of the chiral thiol units; (iii) preparation of the
butadiene polymers with side chain thiol units. The structures of
intermediates and final products were characterised by 1H–NMR spec-
troscopy using a 200 MHz Varian NMR spectrometer and solutions in
CDCl3 with tetramethylsilane as an internal standard.
Chiral Chains for Thiols
General procedure for synthesis of the chiral chains is presented in
Scheme 1. The ethyl-(S)-lactate (500 mmol) was alkylated by corre-
sponding alkyl iodide (300 mmol) in diethylether in the presence of sil-
ver (1) oxide (300 mmol). The reaction was carried out under vigorous
stirring for 48 hours. The solid residue was removed by filtration;
diethylether was evaporated and resulting liquid was distilled in
vacuum. The product (1) was added slowly drop wise to an ether sus-
pension of excess lithiumaluminium hydride (LiAlH4). After that the
SCHEME 1 General procedure for synthesis of the chiral chains (R is C6H13
–
(n-hexyl); C9H19 – (n-nonyl); C12H25 – (n-dodecyl); and CH3CH2CꢀH(CH3)
CH2 – ((S)-2-methylbutyl)).

Chiral Thiols and Related Side Chain LC Polymers
97
mixture was stirred for 8 hours and the excess of LiAlH4 was removed
by addition of the ethyl acetate. Then, water was added dropwise
under ice cooling. The solid was separated by filtration and extracted
with ether. After drying by sodium sulphate, the ether was evaporated
in order to obtain (2) – the light yellow liquid. The tosylate (3) was pre-
pared by usual method with tosylchloride (TsCl) in dry pyridine at 0ꢁC.
1H-NMR data (200 MHz, CDCl3) for (1) with R ¼ (S)-2-methylbutyl:
4.15 q (2H, COOCH2); 3.95 q (1H, CꢀHꢂCOO); 3.10 m and 3.40 m (2H,
CH2O); 1.40 d (3H, CH3CꢀꢂCOO); 1.20–1.70 m (3H, CꢀHꢂCH2); 1.25 t
(3H, CH3CH2ꢂOCO); 1.40 d (3H, CH3ꢂCꢀꢂCOO); 0.90 t (3H, CH3).
1H-NMR data (200 MHz, CDC13) for (3) with R ¼ (S)-2-methylbutyl:
7.80 and 7.30 d þ d (4H, arom.); 3.90 (2H, SꢂOꢂCH2); 3.55 m (1H,
CꢀHꢂO); 3.20 d (2H, OꢂCH2); 2.40 s (3H, CH3Ar); 1.60 and 1.40 and
1.20 m þ m þ m (3H, CꢀHCH2); 1.10 d (3H, CH3CꢀꢂOꢂ); 0.80 t (3H,
CH3CH2).
Chiral Thiols Units
=
All three-ring side chain units incorporating the N N group were
synthesized according to the procedure presented in Scheme 2.
4-hydroxybenzoldehyde was alkylated by corresponding tosylate (3)
and potassium hydroxide in the ethanol=water solution at reflux for 10
days. The solvents were evaporated; the residue was poured into water
and extracted by dichloromethane. The organic extract was dried by
potassium hydroxide, filtered and evaporated. The residual liquid
was reduced by sodium borohydride in dioxane at 70ꢁC for 3 hours.
Then the reaction mixture was poured into a very dilute hydrochloride
acid (HCl) (1–2%) and extracted twice with dichloromethane. The
organic extract was washed with water and then evaporated. This
compound was heated at 80ꢁC in excess of concentrated hydrobromic
acid for 2 hours without further purification. The reaction mixture
was poured in a water=ice mixture, extracted with dichloromethane,
dried over calcium chloride (CaCl2) and evaporated in vacuum.
4-x-bromhexyloxy-40-hydroxyazobenzene (5) was obtained from
4-acetamidophenol using standard methods of alkylaltion, diazotation
and coupling with phenol. The crude product was crystallized from ethanol.
Intermediates (4) and (5) were reacted in acetonitrile in the pres-
ence of an excess of dry potassium carbonate at room temperature
for about 12 hours. The reacted mixture was poured into excess of
water; the precipitate was filtered off and washed with ethanol. The
product was heated at reflux for 10 hours with thiourea in dry ethanol.
The resulting thiouronium salt was crystallized from ethanol=
tetrahydrofuran mixture. The yellow product was hydrolyzed with
NaOH in ethanol=water mixture for 8 hours under argon atmosphere

98
A. Bubnov et al.
SCHEME 2 General procedure for synthesis of the three-rings side chain
units, where Rꢀ is the (S)-2-methyl-2-[(S)-2-methylbutyloxy]-ethyl (denoted
as MK16), (S)-2-methyl-2-hexyloxyethyl (denoted as MK15); (S)-2-methyl-
2-nonyloxyethyl (denoted as MK12) (S)-2-methyl-2-dodecyloxyethyl (denoted
as MK13).
under reflux. Then it was cooled down in refrigerator and the product
was collected by filtration under argon and dried in vacuum at 50ꢁC.
The sodium mercaptide was acidified by HCl and mercaptane was
isolated by extraction with dichloromethane; both operations were
carried out under the argon atmosphere.
1H-NMR data (200 MHz, CDCl3) for (4) with Rꢀ ¼ (S)-2-methyl-2-
[(S)-2-methylbutyloxy]-ethyl-: 7.25 d (2H, HAr ortho to CH2Br);

Chiral Thiols and Related Side Chain LC Polymers
99
6.87 d (2H, HAr ortho to ꢂO); 4.50 s (2H, CH2Br); 3.70–4.00 m (3H,
ꢂOCꢀHꢂCH2ꢂOꢂ); 3.35 m (2H, CꢀꢂCH2OCꢀ); 1.30–1.60 m (3H,
CH2ꢂCꢀHꢂCH2); 1.22 d (3H, CH3CꢀꢂOꢂ); 0.90 m (6H, 2xCH3).
1H-NMR data (200 MHz, CDC13) for (5): 7.82 dd (4H, HAr ortho to
=
ꢂN Nꢂ); 6.95 dd (4H, HAr, ortho to ꢂOꢂ); 4.05 t (2H, CH2ꢂOꢂ);
3.42 t (2H, CH2Br); 1.80 m (4H, CH2); 1.50 m (4H, CH2).
1H-NMR data (200 MHz, CDC13) for final product MK16: 7.87 d
=
(4H, HAr ortho to ꢂN ); 7.38 d (2H, HAr ortho to ꢂCH2); 6.90–7.10
d þ d þ d (6H; HAr ortho to ꢂOꢂ); 5.02 s (2H, ArꢂCH2); 4.00 t (4H,
CH2OAr); 3.70–3.90 m (1H, ꢂOꢂCꢀH); 3.35 m (2H, CH2ꢂOꢂCꢀ);
2.52 m (2H, CH2SH); 1.30–1.80 m (11H, CH2, CH); 1.25 d (3H,
CH3CꢀꢂO); 0.90 m (6H, 2xCH3).
1H-NMR data (200 MHz, CDCl3) for final product MK13: 7.86 d (4H,
=
HAr ortho to ꢂN ); 7.38 d (2H, HAr ortho to ꢂCH2); 6.90–7.10 dþdþd
(6H HAr ortho to ꢂOꢂ); 5.03 s (2H, ArꢂCH2); 4.00 m (4H, CH2OAr);
3.70–3.90 m (1H, ꢂOꢂCꢀH); 3.55 t (2H, CH2OCꢀ); 2.52 m (2H, CH2SH);
1.30–1.80 m (31H, CH2, CH3); 090 t (3H, CH3).
Polymers with Side Chain Thiol Units
General scheme of preparation of the polymers with chiral thiols
side chains is presented in Scheme 3. Side chain liquid crystalline
polymers had been prepared by the addition of a mesogenic thiol to
SCHEME 3 General procedure for preparation of the side chain polymers.

100
A. Bubnov et al.
double bonds of OH-terminated telechelic polybutadiene Krasol 3000
at 80ꢁC in the presence of azoisobutyronitril (AIBN) as initiator.
After dissolving, reaction mixtures consisting of thiol, HO-termi-
nated polybutadiene Krasol 3000, AIBN and toluene as solvent were
placed into glass ampoules. Then, the reaction mixture was bubbled
by nitrogen for 10 min. and ampoules were sealed. The addition pro-
ceeded at temperature 60ꢁC for 48 hours. Then, the reaction mixtures
were poured into an excess of methanol and precipitated products
were purified by reprecipitation from chloroform solutions to meth-
anol. As the 1H-NMR spectra are very similar for the synthesised poly-
mers, the representative one is presented below.
1H-NMR data (200 MHz, CDCl3) for (K12): 7.90 (4H, HAr ortho to
=
ꢂN Nꢂ); 7.38 (2H, HAr ortho to ꢂCH2); 6.90–7.10 dþdþd (6H, HAr
=
=
ortho to ꢂOꢂ); 5.40 m (1.8H, ꢂCH ); 5.00 m (3.1H, CH2 and CH2O);
4.00 m ꢀ(4H, CH20Ar); 3.70–3.90 m (1H, ꢂOꢂCꢀH); 3.55 t (2H,
=
CH2OC ); 2.55 m (4H, CH2ꢂSꢂ); 2.05 m (3.7H, CH2ꢂCH CH2); 1.10–
1.90 m (43H, CH2, CH3); 0.90 t (3H, CH3).
METHODS OF STUDY
For side chain units (monomers), sequence of phases and phase tran-
sition temperatures were determined on cooling from characteristic
textures and their changes observed on planar cells of different thick-
ness in the polarising microscope (NICON ECLIPSE E600POL). Due
to relatively high viscosity of the prepared polymers it was necessary
to use the shearing technique [23]. In that case, the sample is prepared
between two ITO coated glass plates provided with a special electrode
pattern by etching in order to have a well-defined overlapping area (in
present case the overlapping area is 16.8 mm2). The distance between
the glass plates is maintained by evaporated silicon-monoxide spacers
of 3 mm thickness. The orientation of the smectic layers is such that
they are mutually parallel with the layer normal parallel to the glass
plates. This orientation is achieved by shearing the plates relative to
each other in a specially constructed shear cell-holder. The LINKAM
LTS E350 heating stage with TMS 93 temperature programmer was
used for the temperature control, which enabled temperature stabilis-
ation within ꢃ0.1 K. The phase transition temperatures were checked
by differential scanning calorimetry (DSC - Pyris Diamond Perkin-
Elmer 7) on cooling=heating runs at rates of 5K minꢂ1 and 10K minꢂ1
for monomers and polymers, respectively. The samples of 4–10 mg
hermetically sealed in aluminium pans were placed in a nitrogen
atmosphere.

Chiral Thiols and Related Side Chain LC Polymers
101
EXPERIMENTAL RESULTS AND DISCUSSION
Sequence of phases and phase transition temperatures for the three
phenyl ring side chain thiol units and resulting polymers determined
on cooling for the isotropic phase obtained by observation in polarizing
microscope and checked by DSC are summarized in Table 2.
Due to active hydrophilic SH end chain of the studied side chain
units it was not possible to study the mesomorphic properties details.
Nevertheless, by means of texture observations in Polarizing Micro-
scope the liquid crystalline phases for the side chain units were
identified. Typical microphotographs of the textures corresponding
to the paraelectric SmA phase, ferroelectric SmCꢀ phase and low
temperature non-tilted smectic phase are shown on Figure l(a-d) for
the monomer MK12. This monomer is the only one that shows the
SmA phase on the transition from the isotropic phase. Monomers
MK16, MK15 and MK13 possess the chiral nematic phase (typical
texture is presented on Figure 2(a)) and a polar tilted SmXꢀ phase
(ferroelectric switching under low frequency electric field was
observed). In order to identify the type of this phase, further studies
are necessary.
Commercial OH-terminated telechelic polybutadiene Krasol 3000
that was used for polymers preparation possess the glass transition
temperature at ꢂ41ꢁC (detected by DSC). The prepared three phenyl
ring polymers with indicated density of grafting show the meso-
morphic behaviour. A variety of highly ordered smectic phases
(denoted as SmX or=and LCX) and some crystal modifications (denoted
as CrX) are detected by texture observation in polarizing microscope
(see Table 2). Microphotographs of the textures typical for the pre-
pared polymers are presented on Figure 2(b–d) for polymer K15–50
and on Figure 3 for polymer K13–30. These textures are very similar
to those presented in Ref. 22 for the non-tilted smectic phase that was
proved by X-ray diffraction on non-oriented samples.
On Figures 4–5, the representative DSC plots on cooling runs
are presented for the indicated side chain units and related
polymers. While compare the DSC curves for the monomers and
for the polymers, it can be mentioned that the peaks are broader
for the polymers. Relatively high viscosity of the polymers and the
phase coexistence region was detected for some of the polymeric
materials.
As one of the aims of the present paper was to check the influence of
the length of the chiral chain of the monomer on the density of graft-
ing, we regret to mention that no pronounced effect was found for
these series of new polymeric liquid crystalline compounds.

102

Chiral Thiols and Related Side Chain LC Polymers
103
FIGURE 1 Microphotographs of the textures for monomer MK12 obtained on
cooling: the isotropic SmA phase transition at 126.5ꢁC (a); fan-shaped texture
of the SmA phase at 120ꢁC (b); texture of the ferroelectric SmCꢀ phase with
disclination lines at 99ꢁC (c); the crystal CrX phase at 85ꢁC (d). The width of
the pictures is about 300 mm. (See COLOR PLATE IV)
SUMMARY AND CONCLUSION
Several types of new chiral thiols with three aromatic rings possessing
=
the N N group and differing in the length of the chiral chain have
been synthesized. Depending on the side chain length, the synthesized
monomers (thiols) possess the chiral nematic phase, the paraelectric
SmA phase, a polar tilted smectic phase (probably the ferroelectric
SmCꢀ phase) and the non-tilted low temperature smectic phase. The
phases were identified basically by the textural observations in polar-
izing microscope. Due to active hydrophilic SH end chain it was not
possible to study the properties in details.
Using the synthesized thiols, respective side chain polymers have
been prepared and studies by Differential Scanning Calorimetry and
Optical Polarizing Microscopy. The effect of structural changes in
the mesogenic thiols has been investigated with respect to the meso-
moiphic properties of the resulting polymers. The resulting polymers

104
A. Bubnov et al.
FIGURE 2 Microphotographs of thꢁe textures obtained on cooling for: oily
streak texture of the Nꢀphase at 121 C for monomer MK16 (a); the isotropic-
SmX phase transition at 133ꢁC for polymer K15–50 (b); texture of the low tem-
perature orthogonal SmX phase at 124ꢁC for polymer K15–50 (c); highly
ordered crystal phase at 64ꢁC for polymer K15–50 (d). The width of the pic-
tures is about 300 mm. (See COLOR PLATE V)
FIGURE 3 Microphotographs of the textures obtained on cooling for polymer
K13–30: texture of the low temperature orthogonal SmX phase at 119ꢁC (a)
and highly ordered crystal phase at 110ꢁC (b). The width of the pictures is
about 300 mm. (See COLOR PLATE VI)

Chiral Thiols and Related Side Chain LC Polymers
105
FIGURE 4 DSC plots on cooling runs for: monomer MK16 and polymers K16–
30 and K16–50 as indicated. Arrows indicate the pronounced changes in Cp.
FIGURE 5 DSC plots on cooling runs for: monomer MK15 and polymers K15–
30 and K15–50 as indicated. Arrows indicate the pronounced changes in Cp.

106
A. Bubnov et al.
possess the liquid crystalline phases, probably of the smectic type that
need to be confirmed by X-ray studies. Density of grafting effect on
mesomorphic behaviour has been checked. The mesomorphic behav-
iour of the studied polymers increases with density of grafting. The
main disadvantage of the studied polymers is a very high viscosity
that makes impossible detailed studies. Nevertheless, probably a
slight modification of the monomer’s structure can improve the proper-
ties of this class of materials and make it possible to use them for
applications. One of the possibilities is to increase the polarity of the
molecule by attaching additional chiral centre to the molecule.
The present paper presents mostly the synthetic work and the
preliminary studies of the mesomorphic properties of the newly
synthesized liquid crystalline side chain units and the related poly-
mers. Further studies (results of dielectric spectroscopy and electro-
optics, X-ray and photoferroelectric effect studies) of the synthesized
polymers are in progress now and will be presented elsewhere in order
to identify precisely type of the detected polymer’s mesophases and
their structural and electro-optic properties.
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