Synthesis, characterization and conducting properties of sulfonated poly and co-polymer of cholesteryl 4-pentenoate with 1-hexene

Article abstract of DOI:10.1080/17415993.2012.753602

This manuscript reports the synthesis of the liquid crystalline (LC) monomer cholesteryl 4-pentenoate and its sulfone and co-polysulfone with 1-hexene. The polymers were characterized by IR, ¹H-NMR, differential scanning calorimetry and thermogravimetric analysis techniques. The LC phases were observed under a hot stage fitted with a polarizing optical microscope. The elemental analysis for the sulfone group was done by SEM (energy dispersive X-ray analysis) analysis. The amount of elemental sulfur present in polysulfone and co-polysulfone is 4.95 weight (%) and 5.71weight (%), respectively. The molecular weights were determined by Gel Permeation Chromatography, using tetrahydrofuran as the solvent. The number average molecular weight of the polysulfone is 5950 g/mol. The molecular weights of the co-polysulfones were found to be higher than the polysulfone. From I-V characteristics, the breaking voltage for the polymers was computed to be around 5 V. The conductivity of the polysulfone and co-polysulfones increases with increasing temperature.

Full text of DOI:10.1080/17415993.2012.753602

Journal of Sulfur Chemistry, 2013
Vol. 34, No. 5, 450–457, http://dx.doi.org/10.1080/17415993.2012.753602
Synthesis, characterization and conducting properties of
sulfonated poly and co-polymer of cholesteryl 4-pentenoate with
1-hexene
Samiul Hoquea, Narendra Nath Dassa, Krishna Gopal Bhattcharyyab and Neelotpal Sen Sarmaa*
aPhysical Sciences Division, Polymer Section, IASST, Paschim Boragaon, Guwahati 781 035, Assam, India;
bDepartment of Chemistry, Gauhati University, Guwahati 781014, Assam, India
(Received 31 August 2012; final version received 25 November 2012)
This manuscript reports the synthesis of the liquid crystalline (LC) monomer cholesteryl 4-pentenoate
and its sulfone and co-polysulfone with 1-hexene. The polymers were characterized by IR, 1H-NMR,
differential scanning calorimetry and thermogravimetric analysis techniques. The LC phases were observed
under a hot stage fitted with a polarizing optical microscope. The elemental analysis for the sulfone group
was done by SEM (energy dispersive X-ray analysis) analysis. The amount of elemental sulfur present
in polysulfone and co-polysulfone is 4.95 weight (%) and 5.71weight (%), respectively. The molecular
weights were determined by Gel Permeation Chromatography, using tetrahydrofuran as the solvent. The
number average molecular weight of the polysulfone is 5950 g/mol. The molecular weights of the co-
polysulfones were found to be higher than the polysulfone. From I–V characteristics, the breaking voltage
for the polymers was computed to be around 5V. The conductivity of the polysulfone and co-polysulfones
increases with increasing temperature.
Keywords: co-polymerization; co-polysufone; differential scanning calorimetry; liquid-crystalline poly-
mers; conducting properties
1. Introduction
Polymer liquid crystals are the materials which show both the properties of polymers and liquid
crystals. So they can show the mesophase characteristic of ordinary liquid crystals and at the same
time retain the valuable properties of polymers. Liquid crystalline (LC) polymers such as styrene
cross-linked polymer synthesized by high-temperature polycondensation reactions, chiral LC
*Corresponding author. Emails: neelot@sify.com, neelot@iasst.gov.in
© 2013 Taylor & Francis

Journal of Sulfur Chemistry 451
poly(ester/β-sulfoxide) synthesized by asymmetric oxidation of prochiral nematic poly(ester/β-
sulfide), thermotropic LC polymers, involving 4ꢀ-cyano-4-biphenylyl mesogenic groups, and
polymer films prepared by bulk polymerization LC monomers, etc., have been investigated by
different workers (1–7).
It is known that cholesteryl, cholestanyl, and sitosteryl esters exhibit LC properties. Fawcett
(8) in the year 1991 synthesized both cholesteryl 10-undecenoate sulfones and cholestanyl
10-undecenoate. Dass et al. (9) and Dass (10) synthesized a number of poly(1-olefin sulfones) by
free radical reactions in solvent mixtures of chloroform and sulfur dioxide with eicos-l-ene,
hexadec-l-ene, and 4-pentenoic acid. These esters and their sulfones showed LC properties.
Osada and Blumstein (11) have synthesized the co-polymer of cholesteryl methacrylate with alkyl
methacrylate and identified its LC properties. Filip et al. (12) also studied the properties of differ-
ent LC co-polymers, using cholesteryl moiety as a mesogenic group. However, co-polysulfones
having cholesteryl 4-pentenoic acid with n-hexene have not been previously explored.
Observation of conductivity behavior of these LC polymers is another interesting aspect of
their application in the modern world. Shu Hsu (13) synthesized different types of side chain LC
polymers and observed their conductivity behavior.Yoshimoto and Hanna (14) fabricated a novel
organic semiconductor composite consisting of an LC semiconductor, that is, 6-(4ꢀ-octylphenyl)-
2-dodecyloxynaphthalene (8-PNP-O12), and a cross-linked polymer derived from 1, 6-hexanediol
diacrylate by photo-polymerization and studied its charge carrier transport properties.
In the present work, the monomer cholesteryl 4-pentenoate (CHPE) is synthesized and
polymerized/co-polymerized with 1-hexene at different ratios in the presence of liquid SO2.
Liquid SO2 acts as a solvent and also facilitates the sulfonation of the monomer which in turn
increases its conductivity (15–18). Polysulfones inserted into the polymer chain also impart phys-
ical strength and hydrolytic stability to the system (19–21). SO2 is produced in situ and liquefied
at −60◦C using a high vacuum system and liquid nitrogen.
Figure 1. IR spectra of CHPE, PCHPES and co-polysulfone of cholesteryl 4-pentenoate and 1-hexene (PCHPESH21).

452 S. Hoque et al.
2. Results and discussion
2.1. FTIR spectra
The FTIR spectra shows that the peak for the ester group at around 1720 cm−1 is present in the
monomer CHPE and also in polysulfone PCHPES and co-polysulfone PCHPESH. But the peak
−1
=
for the C C bond at around 1639 cm is absent after polymerization. The IR-spectra of the
polysulfone and co-polysulfone display strong asymmetric and symmetric vibration bands for
the sulfone group near 1311 and 1126 cm−1, respectively (Figure 1). This is indicative of the
formation of polysulfone and co-polysulfones with 1-hexene.
2.2. 1H-NMR spectra
1
=
The H-NMR signals for the CH and OC–H protons of the mesogenic residue of these polymers
=
appear at δ 5.4 and δ 4.7 ppm, respectively (Figure 2(b) and (c)), but the other signals for H2C
=
and CH protons found in the ester CHPE are absent. A new signal is found at around 3.4 ppm
for the proton of the main chain, containing the sulfone group (Figure 2(b) and (c)). This indicates
the formation of polysulfone and co-polysulfones.
2.3. Elemental analysis by SEM (EDX)
The elemental analysis by the SEM (energy dispersive X-ray analysis) method indicates the
presence of sulfur in the polymers, confirming the formation of polysulfone and co-polysulfones.
2.4. Molecular weight determination
¯
¯
The number average molecular weight (Mn) of the polysulfone PCHPES is 5950 g/mol and Mw is
6720 g/mol with polydispersity 1.13, but the molecular weight of the co-polysulfones are found
higher than the polysulfone (Table 1).
Figure 2. 1H-NMR of (a) CHPE, (b) PCHPES and (c) PCHPESH11.

Journal of Sulfur Chemistry 453
Table 1. Number average molecular weight (Mn), weight average molecular weight (Mw), polydispersity and LC ranges
of polysulfone and copolysulfones.
Monomer
feed ratio
−3
¯
Mn × 10
Polymer
Mw × 10−3
¯
Polydispersity
LC range (◦C)
PCHPES
–
5.95
6.49
34.51
79.46
6.47
6.72
6.62
80.84
92.40
8.44
1.13
1.02
2.34
1.16
1.30
125–143
91–140
139–187
91–134
105–136
PCHPESH21
PCHPESH11
PCHPESH12
PCHPESH23
2:1
1:1
1:2
2:3
2.5. Thermal study
The isotropic to cholesteric phase transition temperature of the polymers can be observed under
a hot stage microscope and photographed with the attached camera. The LC ranges for the
polysulfone and co-polysulfones are also given in Table 1.
The differential scanning calorimetry (DSC) heating curves of polysulfone and co-polysulfones
(Figure 3) display phase transitions which are quite similar. The peak seems to consist of two
distinct phase transitions. The phase transition at the higher temperature is for the isotropic melt,
as observed under the polarizing microscope. Moreover, a slow increase of the specific heat
is observed just before the phase transition at lower temperature for the polysulfone and co-
polysulfones. It seems possible that this increase results from the occurrence of a glass transition
as reported (22).
The polymer and the co-polymers are stable up to about 220◦C, but beyond that temperature
they decompose as confirmed by DSC and thermogravimetric analysis (TGA) (Figure 4) analysis.
The materials decompose completely around 400◦C.
2.6. Conductivity study
The I–V characteristics of the film of about 25 μm thickness show that the breaking voltage for
polysulfone is very close to 5V and that for co-polysulfone PCHPESH11 it is slightly higher than
7V. After this breaking voltage, there is a sudden rise of about 50 mA current when the voltage
Figure 3. DSC data of polysulfone and co-polysulfones.

454 S. Hoque et al.
Figure 4. TGA thermograms of polysulfone and co-polysulfones.
Figure 5. AC conductivity measurements of CHPE and its polysulfone and co-polysulfones

Journal of Sulfur Chemistry 455
is raised further. The breaking voltage decreases with rising temperature for both the polysulfone
and co-polysuylfone. Both the polysulfone and co-polysulfones show ohmic behavior before and
after the breaking voltage.
The bulk resistances of the monomer CHPE, polysulfone and co-polysulfons are evaluated by
the complex impedance-admittance plots recorded from 30◦C to 100◦C, using a HIOKI 3532-50
LCR Hi-TESTER frequency response analyzer as done previously (23). The plots are recorded in
the frequency range from 42 Hz to 30 kHz, keeping the signal amplitude of 20 mV. The geometry
of the cell for the measurement of conductivity is SS | polymer pellet | SS, where the SS plate (SS
stands for Stainless Steel) is used as the electrode. It is observed that the conductivity of CHPE is
almost constant with the rise in temperature (Figure 5). The conductivities of polysulfone and co-
polysulfones are increased gradually with the rise in temperature (Figure 5) except PCHPESH11.
This can be explained in the following way:
SO2 is a good Lewis acid, therefore it can attract electrons from the adjacent C–C bond due
to the inductive effect. The unfavorable accumulation of charge density around the S atom of the
SO2 group causes a back donation influenced by the subtle external changes such as temperature.
Due to this “nephlauxetic effect”, there is an expansion of the electron cloud which ultimately
facilitates the electron transfer and thus the conductivity of the material when there is a change
of temperature in the surroundings. A mechanism of conduction is proposed and shown below:
3. Conclusion
The polymerization of the mesogenic monomer CHPE with and without 1-hexene can be ini-
tiated by t-butyl hydroperoxide, following a simple free-radical mechanism of polymerization
and co-polymerization. When SO2 is passed into the reaction mixture in chloroform, the free
radical HOSO2• and t-butyl-O-SO2• are produced at low temperature (24). It is assumed that
the homopolymerization of CHPE and co-polymerization with 1-hexene is initiated by t-butyl
hydroperoxide at −60◦C, following the reaction scheme shown below.

456 S. Hoque et al.
It is observed that both PCHPES and PCHPESH show good LC behavior in its melting range.
The DCbreakdown voltage of co-polysulfones is higher than polysulfone. The co-polysulfones are
more stable than the polysulfone. The AC conductivity of the polysulfone and co-polysulfones
increases with the increasing temperature. Polysulfones with cholesteric units have not been
extensively explored and, therefore, a new kind of material has been synthesized which can be
used for making a thermal switch, thermistor, etc.
4. Experimental
4.1. Materials
Cholesterol (Merck), 4-pentenoic acid (Merck), ethyl methylketone (Merck), chloroform (Ran-
baxy), methanol (Ranbaxy), AIBN (Merck) were used without further purification. 1-hexene
(Merck) was purified as reported in the literature (25).
4.1.1. Preparation of 4-pentenoylchloride
The 4-pentenoyl chloride was prepared by distillation of 4-pentenoic acid and benzoyl chloride
(in 1:2 mole ratios) with 0.05 g of hydroquinone at its boiling point in a 25 cm long distillation
column. The receiver contains hydroquinone and it was kept inside an ice bath. Redistillation
gave 68–72% of 4-pentenoyl chloride.
4.1.2. Preparation of CHPE
One mole of both cholesterol and 4-pentenoyl chloride were dissolved in ethyl methyl ketone
(MEK) and then stirred for 5–6 h inside a round bottom flask in the presence of triethyl amine
and a small amount of hydroquinone in an ice bath. The reaction mixture was then filtered and
washed several times with distilled water. The organic layer was separated and distilled to remove
MEK. The viscous organic liquid on cooling gave CHPE. CHPE was recrystallized and finally
purified by column chromatography.
The yield of the product was 76.40% and its LC range was 67–108◦C.
4.1.3. Preparation of polysulfone
The ester CHPE was dissolved in chloroform and transferred into a vacuum tube. A mixture
of t-butylhydroperoxide with methanol (1:1 mole ratio) was then added to it. The vacuum tube
was kept inside a 1.5 l liquid N2 canister below −60◦C (i.e. below the boiling point of SO2).
The reaction tube was then evacuated by a high vacuum pump by the freeze thaw technique to
remove atmospheric oxygen and to make the tube vacuum. SO2 gas prepared by the interaction of
Na2SO3 and H2SO4 was introduced into the reaction tube. Finally, the tube was sealed and after
sealing the mixture was kept at low temperature for 2–3 more hours.
When the reaction was complete, the reaction mixture was poured into acidified methanol.
The dirty white colored polysulfone was then recrystalized from hot methanol containing traces
of chloroform. Finally, the residue was washed several times with MEK.
4.1.4. Preparation of co-polysulfones
The mesogenic ester CHPE with 1-hexene at different mole ratios were dissolved in chloroform
and the polymerization was done as before. The co-polysulfones were isolated and recrystallized

Journal of Sulfur Chemistry 457
from hot methanol containing traces of chloroform. Finally, the co-polymers were washed several
times with MEK and dried over a desiccator.
4.2. Physical measurement
The proton NMR spectra were recorded by JMM-ECS 400 MHz NMR in chloroform D.
The molecular weights were measured in THF with a three column GPC system
(WATERS). The IR spectra were recorded using Bruker Vector22 system in the range
400–4000 cm−1. The LC range was detected by the phase change with the help of Mettler Toledo
F90 hot stage fitted with polarizing optical microscope and camera. The elemental analysis was
carried out using the SEM–EDX method (Leo 1430VP). The DSC and TGA results were obtained
from Perkin Elmer DSC 6000 and TGA 4000 by heating the samples at a heating rate 5◦C and
10◦C per minute, respectively. The I–V characteristics were carried out by the two-probe method.
The AC conductivity was measured by HIOKI 3532-50 LCR HiTESTER.
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