a synthesis of a new liquid crystal polyester...liquid crystals can be broadly classified into two...
TRANSCRIPT
A SYNTHESIS OF A NEW LIQUID CRYSTAL POLYESTER
A THESIS Presented to
The Academic Faculty
by
Takashi Namba
In Partial Fulfillment of the Requirements for the Degree
Master of Science in Polymers
Georgia Institute of Technology June 1992
I I
A SYNTHESIS OF A NEW LIQUID CRYSTAL POLYESTER
APPROVED:
Malcolm B. Polk, Chairman
Fred Cook
Satish Kumar
Date Approved by Chairman ^/^if^z
11
ACKNOWLEDGEMENTS
* I sincerely thank Dr. Malcolm B. Polk for his unending patience
and guidance during the course of this research. I also thank Dr. Fred
Cook and Dr. Satish Kumar for serving on my reading committee.
Secondly, I would like to show my special thanks to Nippon
Shokubai Co., Inc. for giving me an opportunity to study in the
Georgia Institute of Technology and supporting me during my
residence in the program for the M.S. in polymers.
I am also grateful to Mrs. Lynn Boyd, the director of the
Corporate Liaison Program, for helping me any time I faced
problems, even trivial ones.
Finally, I thank my wife, Yuka, and my son, Kazuyuki, for
supporting me physically and mentally. I believe I could not have
finished my master's work if they had not been with me.
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENT i i i
LIST OF TABLES v i
LIST OF FIGURES v i i
SUMIVIARY X
CHAPTER
I. INTRODUCTION AND BACKGROUND 1
History and Definition of Liquid Crystals Classes of Liquid Crystals Classes of Order Structures, Pathways, and Properties of LCP's Method of Mesophase Identification
Differential Scanning Calorimetry and Differential Thermal Analysis Miscibility Studies X-ray Diffraction Polarized Optical Microscopy
Objectives
II. EXPERIMENTAL DETAILS 32
Measurements and Instrumentation Chemicals Synthesis of Poly[oxy(2-acetyl-1,4-phenylene)-oxy-terephthaloyl-co-oxy(2-acetyl-1,4-phenylene)oxyisophthaloyl]
Asymmetric Reduction of Poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-1,4-phenylene)oxyisophthaloyl]
III. RESULTS AND DISCUSSION 3 9
General Comments on Synthetic Method Solubility Dilute Solution Viscosity Spectroscopic Analysis
Fourier Transform Infrared Spectroscopy Proton Nuclear Magnetic Resonance Spectroscopy Carbon-13 Nuclear Magnetic Resonance Spectroscopy Conclusions From Spectroscopic Analysis
Thermal Analyses Differential Scanning Calorimetry Thermogravimetric Analysis
IV. CONCLUSIONS AND RECOMMENDATIONS 5 7
APPENDIX 63
REFERENCES 66
V
LIST OF TABLES
Tabig PaqQ
1. Structures of Commercial Thermotropic Liquid Crystalline Polymers 3
2. Monomers Used in the Synthesis of Liquid Crystal Polyesters 1 4
3. Absorption Peaks and Assignments for the Polymer Before the Reduction 4 2
4. Absorption Peaks and Assignments for the Polymer After the Reduction 4 5
V
LIST OF FIGURES
Figure Page
1. The Viscosity Behavior of the PPTA-Sulfuric Acid System 5
2. Schematic Representation of the Different Types of Mesophases 8
3. Schematic Representation of the Structures of Liquid Crystal Polymers 1 0
4. Schematic Diagram of the Structure of a Main-chain Liquid Crystal Polymer 1 3
5. Methods to Lower Melting Point 1 5
6. Melting Point vs. Structure 1 7
7. Schemes for Syntheses of Side-chain Liquid Crystal Polymers 1 8
8. Examples of the Applications of Main-chain Liquid Crystal Polymers 2 0
9. Thermal Behavior of Liquid Crystal Polymers 2 3
10. X-ray Diffraction Patterns for Unoriented (a) Nematic and (b) Smectic Phases 27
11. Textures of Liquid Crystals in POM 2 9
12. Synthesis of Poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-1 ,4-phenylene)oxyisophthaloyl] 35
V
Figure Page
13. Asymmetric Reduction of Poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-1,4-phenylene)oxyisophthaloyl] 37
14. FTIR Spectrum of the Polymer Before the Reduction 41
15. FTIR Spectrum of the Polymer After the Reduction 4 4
16. Proton NMR Spectrum of the Polymer Before the Reduction 4 6
17. Proton NMR Spectrum of the Polymer After the Reduction 4 7
18. Carbon-13 NMR Spectrum of the Polymer Before the Reduction 4 9
19. Carbon-13 NMR Spectrum of the Polymer After the Reduction 50
20. Differential Scanning Calohmetry Thermogram of the Polymer Before the Reduction 5 2
21. Thermogravimetric Analysis Thermogram of the Polymer Before the Reduction 53
22. Differential Scanning Calorimetry Thermogram of the Polymer After the Reduction 55
23. Thermogravimetric Analysis Thermogram of the Polymer After the Reduction 56
24-1. Scheme for the Organic Synthetic Route 6 0
24-2. Scheme for the Organic Synthetic Route 61
IX
Figure Page
25. Proton NMR Spectrum of valinol 64
V'^' 26. Carbon-13 NMR Spectrum of valinol 6 5
SUMMARY
The importance of main-chain liquid crystal polymers(LCP's)
as structural materials is continuously increasing. Generally
speaking, there are currently two major interests in main-chain LCP
research. The first is the reduction of the cost in LCP manufacture,
and the second is the improvement of their mechanical or thermal
properties.
In order to improve the mechanical properties, LCP's should
have as highly rigid and linear a structure as possible. However, an
LCP which has a more rigid structure is more difficult to process
since it has a higher softening temperature. Thus, thermotropic
LCP's in the current market are usually co-polymerized with rigid
monomers (sometimes with less rigid monomers) in order to reduce
the rigidity or linearity of the structures and in order to depress the
processing temperature. Another drawback of current LCP's is their
poor transverse mechanical properties compared to their mechanical
properties along the molecular direction. This problem is the result
of their highly oriented structure in only one direction. Currently,
the solution to this problem is glass fiber filling. Glass fibers are
dispersed in LCP's randomly to relieve the anisotropy of LCP's.
However, these fibers also increase the density of composites, and
this would be a problem when they are used for an application in
which the density of the material is the main concern. Another
possible solution to the poor transverse properties is the change or
modification of the mesophase of LCP's from nematic/smectic to
cholesteric, and this may be accomplished by introducing a chiral
center in their structure.
In this thesis, a new main-chain LCP, poly[oxy(2-acetyl-1,4-
phenylene)oxyterephthaloyl-co-oxy(2-acetyl-1 ,4-phenylene)oxyiso-
phthaloyl] was synthesized by applying a method described by
Onwunaka. An asymmetric reduction was also performed on
poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-
1,4-phenylene)oxyisophthaloyl]. The structure of the polymer before
reduction was confirmed by nuclear magnetic resonance
spectroscopy (NMR) and Fourier transform infrared spectroscopy
(FTIR). The reduction of poly[oxy(2-acetyl-1,4-phenylene)oxytere-
phthaloyl-co-oxy(2-acetyl-1,4-phenylene)oxyiso-phthaloyl] was
successful since the disappearance of the keto carbonyl carbon was
confirmed by FTIR and carbon-13 NMR. However, the structure of the
polymer after reduction could not be determined by those
spectroscopic techniques because the polymer contained (s)-(+)-2-
amino-3-methyl-butan-1-ol as an impurity which was not
separated. The inherent viscosity of the polymer after reduction
was 0.11 dL/g which was much less than that of the polymer before
reduction (0.42 dL/g).
CHAPTER I
^^ INTRODUCTION
Historv and Definition of Liquid Crystals
Liquid crystal behavior was discovered by F. Reinitzer during
his study of cholesteryl benzoate in ISSSJ He noticed that this
material changes from the solid crystalline state to the turbid
anisotropic liquid at 149°C, and becomes a transparent isotropic
liquid at 179°C. Materials which have such a behavior as cholesteryl
benzoate were successively called "flowing crystals" or "crystalline
liquids" until O. Z. Lehmann et al. called them "liquid crystals."2
Almost fifty years after the discovery of low molar mass
liquid crystals, F. C. Bawden et al. reported the first polymeric
liquid crystal.3 They observed that a solution of tobacco mosaic
virus formed two phases, one of which was birefringent, above a
critical concentration. After this, other basic studies on polymeric
liquid crystals were done by A. Elliot et al.'^ and C. Robinson et al.^
in the 1950's. Even after those pioneering workers investigated
polymeric liquid crystals, it took twenty more years before
polymeric liquid crystals were recognized as important structural
materials.
In the 1970's, E. I. DuPont de Nemours and Co. developed and
commercial ized high-strength f ibers (Kevlar®) from po ly (p-
phenylene terephthalamide)(PPTA).6
" _ / =
- " - \ J n
This was the first commercialized LCP product, and it has led to
numerous developments in both new materials and applications, as
well as in the underlying science. Although Kevlar® has
extraordinarily high strength, it can be processed from only special
solutions and is not melt processible because its melting point is so
high that it degrades before melting.
Following the invention of Kevlar®, the first melt processible
LCP was reported by W. J. Jackson et al .^ , and commercialized
immediately by Dart Industries and other U. S. and European
companies. Table 1 shows structures of common thermotropic LCP's
commercially available with the brand name and the suppliers.
The liquid crystalline state has been defined in several
different ways. In the chemical dictionary, it is defined as an
intermediate or mesomorphic ("middle" phase) state between solid
and liquid.8 From this definition, the liquid crystalline phase is
sometimes referred to as a mesophase. Blumstein states that the
Table 1. Structures of Commercial Themotropic Liquid Crystalline Polymers
1. X7G (Eastman Kodak), Novaculate (Mitsubishi Kasei), Rodmn (Unitika), Idemitsu LCP (Idemitsu Petro Chem.)
V / C-O—CHsCHsO-
n \ //
-im
2. Xydar (Amoco), Econol (Sumitomo Chemical)
- - 0 \ / ^ /r\ //
0 ^
H 3. Vectra (Hoechst-Celanese, Polyplastics)
" ^ / °<^i
liquid crystalline state is a state of matter which simultaneously
has certain properties of a liquid and certain properties of a
crystal.9 Still another definition is available in which the liquid
crystalline state is described as a melt or solution which shows
some type of anisotropic order in the fluid state. This final
definition is probably most important because it clearly
distinguishes the liquid crystalline state either from both isotropic
melts and solutions and crystals which have three dimensional
order.
Classes of Liquid Crystals
Liquid crystals can be broadly classified into two groups:
lyotropic and thermotropic^'IO"''"^
Lyotropic liquid crystals are materials which show liquid
crystallinity in solutions above the critical concentration and in a
particular temperature range, and most LCP's investigated in early
researches like PPTA are classified as lyotropic liquid crystals. Fig.
1 shows the schematic relationships between the viscosity of the
solution and the concentration or temperature in the PPTA-sulfuric
acid solution system. The viscosity increases rapidly until a
critical concentration is reached. At the critical concentration, the
number of molecules in the solution is so high that the molecules
begin to align in order to remain in the solution and the lyotropic
V) o u
>
Concentration
Temperature
Fig. 1. The Viscosity Behavior of PPTA-Sulfuric Acid System
mesophase is formed. Above the critical concentration, the
viscosity decreases sharply by the formation of the lyotropic
mesophase up to a particular concentration. In this region, the
solution is a mixture of the isotropic liquid phase and the lyotropic
mesophase. As the concentration increases further, the solution
becomes fully lyotropic, and the viscosity increases again.
Lyotropic LCP's do not usually show LC behavior in bulk, primarily
because their melting points are generally so high that they degrade
before they melt.
Thermotropic liquid crystals are materials which show liquid
crystallinity when they melt. Thermotropic LC's are classified into
two subclasses, enantiotropic and monotropic. In enantiotropic LC's,
the LC state is thermodynamically stable and is observed both upon
heating from a solid phase and upon supercooling from an isotropic
liquid phase to below the clearing temperature. In monotropic LC's,
on the other hand, the LC state is metastable with respect to the
solid, and is observed only upon supercooling.
Classes of Order
The liquid crystalline state is characterized by long range
orientationally ordered mesogens which are responsible for the
liquid crystallinity of molecules. LC phases are classified into four
classes by their degree of order. The schematic images of these
four phases are shown in Fig. 2.^^ The ovals In the figure represent
the mesogens.
The first class of LC phases is called the smectic state. This
is the most ordered state of all of the four phases. In this state, the
mesogens are arranged in ordered (Fig. 2(a)) or unordered (Fig. 2(a'))
layers, and their centers of gravity are mobile in two dimensions (in
the smectic plane). Smectic phases are further classified with
respect both to the order of the arrangement of the mesogens in each
smectic plane and to the angle, called the tilt angle, made between
the long axes of the mesogens and the plane. For example, smectic A
(SA) is characterized by no order in mesogen arrangement in the
smectic plane and 0° in the tilt angle, and smectic C (Sc) is
characterized by no order in mesogen arrangement in the plane like
SA and non 0° in the tilt angle. In the case of low molar mass LC's,
11 smectic states from smectic A (SA) to smectic K(SK) have been
discovered''3 ^ but in the case of LCP's, only a few smectic states
have been observed.
Normal SA SB SE
Tilted SC S I—SF —SJ—SG—SH—SK
The order of the phases increases from left to right and lines
indicate which transitions can occur upon heating or cooling.
a'
y IV W Y
Fig. 2. Schematic Representation of the Different
Types of Mesophases. Smectic with ordered
(a) and unordered (a') arrangement of the molecules
in layers; b) nematic; c) cholesteric; and d)
discotic. " 2
The second phase, called the nematic state, is characterized by
the orientation of the long axes of mesogens along a certain
direction with an unordered arrangement of centers of gravity of
the mesogens (Fig. 2(b)). Generally speaking, the materials in the
nematic phase have lower viscosity than those in the smectic phase
since the nematic state allows for translational mobility of
constituent mesogens.
The third phase is called the cholesteric state. This phase is
formed by derivatives of cholesterol or optically active chiral
nematic LC's. The cholesteric state is characterized by the
mesogens assembled in layers, a similar arrangement to that in the
nematic phase, but each layer is rotated by a certain angle with
respect to the preceding layer so that some helical twisting of the
mesogens occurs on the whole, describing a helix with pitch p (Fig.
2(c)).
The last LC phase is called the discotic phase. This phase
consists of plate-like mesogens, instead of rod-like mesogens as in
the other three phases. The mesogens lie in the plane of the layers,
forming close hexagonal packings (Fig. 2(d)).
Structures. Pathways, and Properties of LCP's
Fig. 3 shows the schematic representation of the structures of
LCP's. The structures of LCP's are generally classified into three
10
1 • Main Chain LCP's
(a)
(b)
J ] ] I
] I I r (c)
(d)
2_Side_CliainJ_CE!s
3. 1. and 2. mmhinficl
s^—
Fig. 3. Schematic Representation of the Sturctures of LCP's
11
groups: main chain LCP's, side chain LCP's, and the type in which both
main chain and side chain are combined.
In main chain LCP's, mesogenic groups are incorporated into
the polymer backbone. Main chain LCP's are further classified into
four types. The first type is the all-rigid LCP, in which the polymer
backbone consists of only the mesogenic group. The second type is
the semi-rigid LCP's in which mesogenic groups are connected by
some soft segment like an alkylene group. The third type is
characterized by soft branches on the mesogenic groups. The last
type consists of mesogenic groups with more than two
functionalities. These mesogenic groups are connected to each other
with/without soft segments and have soft branches.
In side chain LCP's, the mesogen is hung like a pendant on the
polymer backbone through a short or long flexible spacer. The spacer
preserves the delicate interactions between pendant mesogens by
decoupling the main chain motion from that of the pendant group.
The last type of LCP has a structure which has both main chain
and side chain LCP features.
The factors required to make a polymer have LC behavior are
basically similar to those for low molecular weight LC's suggested
by Gray et al.14 ,
1. Mesogenic groups should have rod-like or disk-like
structures.
12
2. Mesogenic groups should have sufficient permanent dipoles
to stabilize the liquid crystalline state.
Fig. 4 shows the most common structures exhibiting LC
behavior and a variety of chemical structural units that are
available for the synthesis of main chain LCP's.15-16 Most main-
chain LCP's are synthesized through condensation polymerization,
and several kinds of monomers commonly used for thermotropic
LCP's are listed in Table 2.''^ Although the combinations of these
monomers give all rigid (all aromatic) liquid crystal polymers, they
are often not melt processible because of their extremely high
melting points.
In order to lower the melting point of LCP's, several
techniques to interrupt the crystalline order are available, as
summarized in Fig. 5.18
1. Copolymerization of several mesogenic monomers such
as p-hydroxy-benzoic acid (PHB) or 2-hydroxy-6-
naphthoic acid produces random copolymeric structures
with depressed melting points.
2. Use of monomers with bulky side groups, such as
phenylhydroquinone, prevents close packing in the
polymer crystals.
1 3
B Lateral substituent on aromatic
rings such as CI, Br, CH3,
OCH3, phenyl, n-alkyl
Various types 0 f end groups
-OR -R -COOR - C O R
-OOCR - O O C O / i CH coo;i - C N - C I - N O 7 - H - F - B r - J -R' - N = C = 0 - O i l -OR' -OCOR' - N H , -COOR' -CR=CR-COOR -SR - N H / ?
- N H C P f i - N K , - N = C = S
- 0 ( C H j ) „ O f t - O C F 3
D
A Various types of r i g i d core
and r i n g systems
• ^ 0 ^ - ^ ^ ^ {{ 34 K^ -Qi] ®-
^0^ X CI, Dr. \, on, OR, OOCR R is C„H,„.,
CHj. CN. NO,
-^Q:P—^^
- ^
N;=^ N - N
-<Q—0- -13-N - N
-<o>-oy OI J^ N-N
C Various types c >f b r i d g i n g groups
J^ ^C^^ i
0
- c = c -CH
H
r~ H 0 0
CM CU Cll ctr'^N-™-
-Hg— —N=C=N— - ( C H , ) , -
d Si^ N
CH, CH,
-NH(CH,),- -0(CH,)„O-
-COO(CH,)„OOC— —(CH,),COO —
Fig. 4. Schematic Diagram of the Structure of a Main
Chain Liquid Crystal Polymer 15-16
1 4
Table 2. Monomers Used in the Synthesis of Liquid Crystal
Polyesters ^^
Aromatic diol Aromatic dicarboxylic acid Hydroxyacid
MO-
X \
-OH
MO-
Y
-OH
O O
HOC—{(J\-COH
O O
HOC-/Q/-<^OH
H0--(j3Vc0H
X
0 11 COH
(X, Y - halogen or alkyl) (X » halogen, alkyl) (X - halogen, alkyl)
MO, O
oTo OH
<oc-/Q V-/O/-COM
H 0 - < 0 } - { 0 ) - 0 H o H
HOC
HO—( ( ) > - 0 H oTo
COM II
o O
MO.
^ ^ ^ ^ COH
HO-('Q\-CH=CHCOH
oc ^ Cj \-o^ (^ V-c oi!
(X = H, halogen, alkyl)
1 !>
1) Copolymerization
H0-< O >-C-OH HO
0 II C-OH
2) Bulky Side Group
3) Bent Comonomer
HoYoVoH
R=CH3. C I , H
0 0 1 I
»0^,,^^W H O C . ^ - \ COH HO-rOrCOH
Ol ' TOT ' lO HO-/c\o !2^°"
4) FTexible Spacer
4CH2)-n ; 40-CH2-CH2>„
CH
; -(O-Sih I ' CH
Fig. 5. Methods to Lower Melting Point ^^
16
3. Use of bent comonomers which contain the 1,3-
disubstituted phenylene structure, such as isophthalic
acid, (which are not inherently liquid crystalline
precursors) interrupts crystalline order.
4. Flexible spaces, such as alkylene groups, decrease
polymer rigidity. Polymers with the mesogenic groups in
the side chain are included in this class.
The effects of these melting point depressing technique are also
shown in Fig. 6.
Side chain LCP's are usually synthesized through radical
polymerization. There are two radical synthetic routes to introduce
mesogenic groups in a polymer.''^ Those routes are shown in Fig. 7.
In scheme I, the mesogenic group is attached to the monomer before
polymerization. In scheme II, on the other hand, the mesogenic
groups are attached after the backbone polymer is synthesized.
Scheme I is more common because it is easier to synthesize a side-
chain LCP and introduces mesogenic group more completely.
LCP's have several superior properties over conventional
plastics. Almost all features of LCP's are caused by the combination
of the polymer specific properties such as the ease of processing
and the anisotropic properties of the liquid crystalline state.'' ^
Positive and negative features of main chain LCP's are listed below.
I /
Bulky Side Group T (Melting) °C
u w
> 600
-c-ZoVc-o/oVo-
o
340
Bent Monomer
0 0 0
(o<o>o)(?<o>!|(E O O i l 0.5
T (Mel t ing) °C
400
°<o>!)("<o>')(2 0.5
" "oTo--' )('1ojk = 350
Flexible Group
0 0
- C - < ^ O ) ^ C - O H ( ^ O -
T (Melt ing) °C
> 400
r - c / o Vo-(CH^)^-o/o\c-o-/o\o- 210
CH.
Fig. 6. Melting Points vs. Structure 18
-^ - .J .— M.
ScbemeJ
18
Attaching
Mesogens
Polymerization
SchemeJl
Polymerize
Attaching mesogens
Fig. 7. Schemes for Side-chain Liquid Crystalline Polymers
19
Positive
(1) Exceptionally high mechanical properties (high
modulus, high strength, good wear resistance,
etc.)
(2) High dimensional stability (thermal expansion
coefficients are as low as 2 x 10-^ °C-'')
(3) High thermal stability
(4) High chemical stability
(5) Lower density compared with that of metals
Negative
(1) High cost
(2) Poor properties transverse to the machine
direction
(3) Weak weld lines
Fig. 8 shows some examples of the applications of main chain
LCP's .
The applications of side-chain LCP's are still under
investigation. However, the high mobility of mesogens in side-chain
LCP's gives quick response as fast as low molar mass LC's to
external stimuli, and side-chain LCP's have great potential as
optical memory storage, and holographic imaging, or nonlinear
optics.
20
Fig. 8. Examples of the Applications of Main-chain LCP's 1) connectors, coil bobbins, and gears; 2) coil bobbins; 3) speakers; 4) surface mount connectors; 5) multifilament yarns; 6) plastic storage containers
21
Methods of Mesophase Identification
There are several techniques for lower mass LC's to verify
that a mesophase exists and to classify it as nematic, cholesteric,
or smectic A-K. Noel lists some of these developments as the
following:20
(1) Differential scanning calorimetry (DSC) or differential
thermal analysis (DTA) to examine enthalpy changes.
(2) Miscibility studies to compare the behavior of unknown
liquid crystal phases with those of known origin.
(3) X-ray diffraction to study differences in molecular order.
(4) Polarized light microscopy to examine textures and
optical patterns.
In the case of LCP's, a combination of these methods is often
necessary to identify the mesophase types because of their broad
molecular weight distributions, high viscosities, and features as
mixtures of polycrystalline and amorphous material.21
An additional complication for polymer systems is that more
than one type of mesophase may be present, depending on the
22
temperature. Such materials, which are termed polymorphic,
complicate the identification procedure.22
Differential Scanning Calorimetrv (DSC) and Differential Thermal
Analysis fPTA)
DSC and DTA are popular and convenient methods for
determining the thermal behavior of materials. Transition
temperatures and transition enthalpies are easily measured by these
methods, and transition entropies can be conveniently calculated.
Fig. 9 shows a typical DSC scan of thermotropic LCP's with
descriptions of the transitions and interpretations of the structural
arrangements in the different phases.23 Below the glass transition
temperature (Tg), no long range molecular motion is allowed and the
material behaves like semicrystalline plastics. At Tg, the
amorphous region starts to undergo liquid-like motions and the slope
of the curve suddenly changes. Above Tg, the amorphous region
becomes liquid-like, but the crystalline region maintain the
structure. In some LCP's, the transition at TKI-K2 is observed
indicating a crystal lattice change. At the melting point (Tm), the
crystalline region undergoes melting and three-dimensional order is
lost. Above Tm, the system becomes a fluid, but continues to have a
loose, two-dimensional order which can be characterized as a
smectic phase (LCi). As the temperatureincreases, more order is
-^- INCREASING TEMPERATURE
MOLECULAR
ARRANGEMENT
TRANSITION:
FROM:
TO:
TKI K2 TM - ''"K2 LCI
Glass/Crystalline Crystalline-1 Cry3talline-2
Crystalline-1
TLC1-LC2 TIJC2-I
Liquid Crystal-l Liquid Crystalline Melt-2
Cry3talline-2 Liquid Crystal-l Liquid Crystal-2 Isotropic Melt (i.e., Smectic) (i.e., Nematic)
Fig. 9. Thermal Behavior of Liquid Crystal Polymers 23
24
increases, more order is lost and the other transitions to less
ordered liquid crystalline phases, i.e., the nematic state, can
sometimes be observed before the final transition temperature is
reached. At the last transition temperature, all liquid crystalline
order is lost, and the system finally becomes an isotropic fluid.
This temperature is often referred to as the clearing temperature.24
Since only two transitions, the glass transition at Tg and melting at
Tm, are observed in the case of conventional semicrystalline
plastics, the multiple transitions are clear evidence of the
existence of liquid crystallinity.
DSC and DTA not only reveal information about the transition
temperature, but sometimes make it possible to classify liquid
crystalline phases from their enthalpy data. Krigbaum states rough
guidelines about the isotropization enthalpies for nematics (0.30 -
0.85 kcal/mol of repeating unit) and smectics (1.5 - 5.0 kcal/mol of
repeating unit).'' ^
Miscibilitv Studies
Mutual miscibility is the method developed by Sackmann and
Demus according to the rule of selective miscibility.25 in this
method, a liquid crystal whose mesophase is unknown is mixed with
a reference liquid crystal whose mesophase is known. If the mixture
shows liquid crystallinity at any composition, the unknown and the
25
reference have the same type of the mesophase. However, the
converse is not necessarily true^O , and in this case, further
analysis by a different technique is required. Mutual miscibility has
been successfully applied to nematic26-27 ^ smectic A 2 8 and
smectic C^^ phases.
For the quick determination of mesophases by mutual
miscibility, the contact method can be conveniently used.29 In this
method, a small amount of the unknown liquid crystal in the
isotropic state is placed between a glass slide and a cover slip.
Then the reference liquid crystal is introduced from one end of the
cover slip by capillary flow. As the reference liquid crystal
diffuses into the unknown liquid crystal, a continuous composition
gradient is produced from one end to the other. After cooling, the
sample is examined under a polarizing microscope equipped with a
heating stage in order to observe the miscibility.
X-rav Diffraction
X-ray diffraction gives information about the arrangement and
mode of the packing of molecules and the types of order present in
the mesophase."13 X-ray diffraction studies can be performed using
either unoriented or oriented samples. Unoriented samples are
easier to prepare, but oriented samples are more informative for
structural analysis. Oriented liquid crystalline samples can be
26
prepared either by cooling them in a strong magnetic field from the
isotropic melt to the mesophase or by melting a single crystal or an
oriented fiber very carefully.
Fig. 10 shows X-ray diffraction patterns of the unoriented
nematic phase and the unoriented smectic phase.
A diffractogram of the unoriented nematic mesophase is
characterized by a weak, diffuse outer ring and a strong diffuse
inner ring.30 The outer ring and the inner ring indicate the average
distance between neighboring molecules and the length of the unit
cell, respectively. The diffuse rings indicate that only short range
positional order exists in the nematic phase. Cholesterics give very
similar diffraction patterns to these of the nematic phase. If the
nematic phase is oriented, splitting of those two rings is observed.
The diffractogram of unoriented smectic A or C mesophases is
characterized by one or more sharp inner reflections and a diffuse
outer ring. The sharper inner reflections indicate smectic layering
and the diffuse outer ring shows short range order within the layers.
In the case of oriented SA or Sc samples, the inner rings degenerate
to reflections which lie along the meridian, but the outer ring shows
different reflections between SA and Sc mesophases. For SA, the
outer reflection degenerates to a broad ark-like reflection lying on
the equator. For Sc, the outer reflection splits into doublets
positioned around the equator.
A diffractogram of the smectic B mesophase is characterized
by sharp inner and outer reflections. These sharp reflections are due
L /
(a)
(b)
Fig. 10. X-ray Diffraction Patterns for Unoriented (a)
Nematic and (b) Snnectic Phases 30
28
to the higher ordered mesophase characteristic of SB in which
smectic layers are no longer two dimensional fluids but seem to be
two dimensional solids.
The other smectic mesophases are less strongly characterized
with respect to their molecular structure.
Polarizing Optical Microscopy
Polarizing optical microscopy (POM) is a very useful technique
for a preliminary characterization of the mesophases. In POM, a
picture of a thin layer of a liquid crystal is observed by means of a
microscope usually in linearly polarized light. The features of the
picture, termed "texture" by Friede|31-32 ^ are caused by the
existence of different kinds of defects, and mesophases (nematic,
smectic, and cholesteric) are identified based on this postulate.
Several textures observed in POM are shown in Fig. 11.
Nematic LCP's generally show a threaded texture or a schlieren
texture. Semi-rigid and side-chain nematic LCP's usually show
similar textures to lower mass LC's, but not all aromatic LC
polyesters show these typical textures.
Smectic LCP's show several kinds of textures: schlieren, fan-
shaped batonnets, and mosaic, etc. The schlieren texture is observed
both in the smectic phase and in the nematic phase, but in the case
of nematic LCP's, the image can be brightened by applying shear to
29
30
the sample placed between two glass plates. This is caused by the
lower viscosity of nematic LCP's compared with that of smectic
LCP's.
Cholesteric LCP's also show several kinds of textures: fan-
shaped, planer, and Grandjean textures, etc. In order to identify a
mesophase which shows a fan-shaped texture, the sample is placed
between two glass plates and then one of the glass plates is shifted
after the fan-shaped texture is observed. The mesophase is
identified as smectic if the texture remains fan-shaped. It is
identified as cholesteric if the texture changes to a planar or
Grandjean texture.
ObiQCtivg?
There were two main objectives to this research. The first
one was to synthesize the nematic liquid crystalline copolymer,
poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-
1,4-phenylene)oxyisophthaloyl], by applying the procedure
established by Onwunaka^^ Since a homogeneous system is
preferred to accomplish the reduction, the polymer being reduced
should be soluble in a solvent which will not destroy the reducing
system. Unfortunately, the nematic liquid crystalline homopolymer,
poly[oxy(2-acetyl-1,4-phenylene)oxyterephthaloyl], synthesized by
Onwunaka was only partially soluble in solvents suitable for
31
reduction, e.g. 1,1,2,2-tetrachloroethane or chloroform. Thus, in
this research, a third monomer, isophthaloyi dichloride, was used to
make the polymer structure less linear, to loosen the packing of
molecules, and consequently to enhance the solubility of the polymer
in those solvents.
The second objective of this research was to produce a
cholesteric liquid crystalline polymer through asymmetric
reduction. It was believed that the introduction of optically active
groups would change the starting nematic liquid crystalline polymer
to a cholesteric one. In consideration of safety, cleanliness, and
high optical yield of the reducing system as discussed by Haley34^
the borane/aminoalcohol reducing system developed by Itsuno et
al.35 was adopted for this research.
The goal of this project is the synthesis of a cholesteric liquid
crystalline polymer in order to improve nematic liquid crystalline
polymers' poor transverse mechanical properties, which are the
result of the high molecular orientation of nematic liquid
crystalline polymers in only one direction. Since the molecular
direction of cholesteric liquid crystalline polymers is twisted plane
by plane, uniform mechanical properties in all directions would be
expected.
32
CHAPTER II
EXPERIMENTAL DETAILS
Measurements and Instrumentation
Intrinsic viscosities were measured at 30°C with an Ubbelohde
type viscometer in 2-chlorophenol.
Proton NMR spectra and carbon-13 NMR spectra were obtained
with a Varian XL-400 NMR. All of the NMR spectra were obtained on
polymer solutions in trifluoroacetic acid. Fourier Transform
infrared (FTIR) spectra were obtained with a Perkin Elmer 1600
Series FTIR spectrometer and all samples were prepared as
potassium bromide pellets.
Differential scanning calorimetry (DSC) was performed on a
Seiko Instruments DSC 220C under nitrogen atmosphere. All of the
DSC scans were baseline corrected. A temperature range of -50°C to
300°C was used in each scan with a scan rate of 20°C per minute.
Thermogravimetric analysis (TGA) was performed on a Perkin Elmer
TGA 7 Series under nitrogen atmosphere. A temperature range of
50°C to 500°C was used in each scan with a scan rate of 20°C per
minute.
33
Chemicals
TerephthaloyI chloride and isophthaloyi dichloride obtained
from Aldrich Chemical Company, Inc. were recrystallized from
hexane. 2',5'-Dihydroxyacetophenone obtained from Aldrich Chemical
Company, Inc. was also recrystallized from a 3:1 volume/volume
mixture of deionized water and ethanol. 1M-Borane/tetrahydrofuran
(THF) complex and (s)-(+)-2-amino-3-methyl-butan-1-ol {(s)-(+)-
valinol) obtained from Aldrich Chemical Company, Inc. were used as
obtained. 1,1,2,2-Tetrachloroethane (TCE) obtained from Aldrich
Chemical Company, Inc. was distilled and stored over molecular
sieves 4X. 1,2-Dichlorobenzene, HPLC grade, obtained from Aldrich
Chemical Company, Inc. was used as obtained. Chloroform obtained
from Fisher Scientific was dried over calcium chloride overnight
before use. THF obtained from Fisher Scientific was dried over
calcium hydride overnight and distilled before used. Pyridine
obtained from Fisher Scientific was distilled and stored over
molecular sieves 4X under nitrogen pressure. Other solvents, e.g.,
acetone, methanol, ethanol, hexane, and petroleum ether were used
with no further purification.
34
Synthesis of Poly[oxv(2-acetvl-1 •4-phenvlene)oxvterephthalovl-co-
oxy(2-acetyl-1 •4-phenvlene)oxyisophthaloyl]
The procedure for the polycondensation reaction was also
taken directly from the process developed by Onwunaka^S, A
schematic for this process can be found in Fig. 12.
2',5'-Dihydroxyacetophenone (4.565 g, 0.0300 mole) was
dissolved into a mixture of 35 mL of dry TCE and 8 mL of dry
pyridine in a 250 mL three necked flask equipped with a mechanical
stirrer, a condenser, a nitrogen inlet and sodium hydroxide moisture
trap. TerephthaloyI chloride (3.66 g, 0.0180 mole) and isophthaloyi
chloride (2.44 g, 0.0120 mole) were dissolved in 35 mL of dry TCE,
and the mixture was added dropwise to the solution of 2',5'-
dihydroxyacetophenone under a nitrogen atmosphere with stirring
for 10 min. An additional 35 mL of dry TCE was gradually added
during the polymerization in order to reduce the viscosity of the
system to facilitate stirring. The mixture was stirred for 12 hrs at
room temperature. Then, the mixture was poured into 300 mL
acetone and precipitated for 12 hrs. The precipitate was filtered,
washed several times with acetone, water, methanol, and petroleum
ether. It was dried in vacuo for 24 hrs at 110 °C. The weight of the
dry product was 7.60 g.
35
TCE, Pyridine
r.t. 12 hrs
Fig. 1 2. Synthesis of Poly[oxy(2-acetyl-1,4-phenylene)oxy-terephthaloyl-co-oxy(2-acetyl-l,4-phenylene)oxy-isophthaloyl]
36
Asymmetric Reduction of Polv[oxv(2-acetvl-1.4-phenylene)oxytere-
phthalovl-cQ-Qxv(2-acetvl-1.4-phenvlene)oxvisophthalovl]
The procedure for the asymmetric reduction was devised using
the model of Itsuno^S^ jn which he reduced low molecular weight
aromatic ketones. Fig. 13 gives a schematic depiction of the
proposed reduction.
The polymer (1.00 g) was dissolved in 50 mL of dry TCE and the
solution was allowed to stand for 24 hrs. The solution was heated
slightly to facilitate solubility. The polymer solution was then
poured into a 125 mL pressure equalizing funnel (A), and stored
under nitrogen atmosphere.
(s)-(+)-Valinol (2.064 g) was dissolved in 10 mL of THF and
then, this valinol/THF solution was charged to a 250 mL three
necked flask equipped with a nitrogen inlet with a sodium hydroxide
moisture trap, a thermometer, pressure equalizing funnel (A),
pressure equalizing funnel (B) with a septum inlet on the top of it, a
condenser, and a magnetic stirrer. IM-Borane/THF complex (40 mL,
40 mmol borane) was charged into funnel (B) under nitrogen pressure
and added dropwise to a stirred valinol solution at -50 °C during 20
min. The solution was gradually warmed to 30 °C and stirred for 10
hrs at that temperature to complete the borane/valinol complex
formation.
37
1. Borane/Valinol THF/TCE 40°C, 8 hrs
2. dil.-HCi
H-C*-OH I CHo
Fig. 13. Asymmetric Reduction of Poly[oxy(2-acetyl-l ,4-phenylene)oxyterephthaloyl-co-oxy(2-acetyl-1,4-
phenylene)oxyisophthaloyl]
38
After the reducing agent complex was formed, the polymer
solution was added dropwise over 10 min and the solution was
stirred for 8 hrs. The resulting mixture was decomposed by 50 mL
of 0.2M-hydrochloric acid over 10 min, and immediately neutralized
by 1M-sodium hydroxide solution. The mixture was poured into 300
mL of acetone, and stirred for 24 hrs. Since no precipitate was
observed, the mixture was directly subjected to rotary evaporation
to remove all solvents. The residue recovered from the mixture was
dried for 24 hrs in vacuo at 110°C. The resulting solid was
dispersed into 300 mL of water, and stirred for 30 min. The
dispersion was filtered, and the solid was washed with water
several times. Then, the solid was dried for 48 hrs In vacuo at
110°C, and 0.5 g of a pink powder was recovered.
39
CHAPTER III
RESULTS AND DISCUSSION
General Comments on Synthesis Method
In his process for the synthesis of Poly[oxy(2-acetyl-1,4-
phenylene)oxyterephthaloyl], Onwunaka devised three different
routes to form the polymer. In each pathway, hydrochloric acid (HCI)
was liberated as a polycondensation by-product. If the removal of
HCI was insufficient during the polymerization, a side-reaction
presented by Haley might occur. For this reason, the author chose
the third of Onwunaka's routes, in which pyridine was used as a HCI
scavenger. The fact that the polymer was synthesized, was
confirmed by FTIR, proton NMR and carbon-13 NMR analyses. The
results will be presented later.
Solubility
Before reduction, the polymer was soluble in 2-chlorophenol,
TCE, and trifluoroacetic acid. It was also partially soluble in
chloroform. It was not soluble in water, methanol, and acetone.
40
The reduced polymer was soluble in 2-chlorophenol, TCE, and
trifluoroacetic acid. It was partially soluble in acetone, methanol,
THF, and chloroform.
Dilute Solution Viscosity
The intrinsic viscosity of the reduced material was 0.11 dL/g
which was much less than that of the polymer before reduction (0.42
dL/g). This decrease of the inherent viscosity was probably caused
by hydrolysis of the ester linkage during reduction and purification.
Spectroscopic Analysis
FTIR
The FTIR spectrum of Poly[oxy(2-acetyl-1,4-phenylene)oxy-
terephthaloyl-co-oxy(2-acetyl-1 ,4-phenylene)oxyisophthaloyl] (the
polymer before reduction) is shown in Fig. 14, and the peak
assignments are given in Table 3. At 3457 cm•^ the polymer shows
a small, broad peak, which is probably a water impurity. The peak at
1738 cm-"" is indicative of the ester carbonyl stretch, while the
ketone carbonyl peak is seen at 1691 cm-''. The stretching
absorption for the ester C-0 is observed between 1014 cm-"" and
41
c o • M
o 13
TJ <U
(H Q)
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42
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43
1240 cm-"" as four strong, sharp peaks. The absorption peak at 717
cm-i is indicative of the aromatic =CH out-of plane bending.
The FTIR spectrum of the reduced polymer is shown in Fig. 15,
and the peak assignments are given in Table 4. When this spectrum
is compared with the spectrum of the starting material, there are
three major differences. The first difference is the disappearance
of the keto C=0 peak at 1691 cm-"", and the second is the increase of
intensity of the broad -OH peak at about 3400 cm-"". The third
difference is the disappearance of the -CHS peaks at 1410 cm-"" and
1358 cm-'' which probably shifted or lost their intensity by
reduction. These three differences imply that the reduction was
successful. However, further characterization by NMR was needed to
confirm this.
Proton NMR
Fig. 16 shows the proton NMR spectrum of the starting
material. The spectrum is characterized by a peak at 2.8 ppm, which
indicates the presence of the methyl protons of the acetyl group, and
aromatic peaks between 7.2 ppm and 9.4 ppm, which indicate the
presence of aromatic protons from the dioxyacetophenoic ring,
terephthaloyi ring, and isophthaloyi ring.
Fig. 17 shows the proton NMR spectrum of the material after
the reduction. Based on proton NMR theory, protons attached to alkyl
44
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47
X
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en CD 1
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48
carbons appear between 0 ppm and 3 ppm, alcoholic protons between
3 ppm and 6 ppm, and aromatics between 6.5 ppm and 9 ppm. Thus,
according to the structure of the material after the reduction, a
methyl doublet, a proton quartet from the proton attached to the
carbon which has the alcoholic hydroxyl group, an alcoholic proton
singlet, and aromatic peaks should be observed in each region, but
there are more peaks observed. By comparing this spectrum with the
proton NMR spectrum of valinol shown in Fig. 25, APPENDIX II, some
of the peaks could be assigned as peaks of valinol, but it is still hard
to assign all peaks completely. However, the disappearance of the
methyl peak at 2.9 ppm suggests a successful reduction as discussed
in FTIR spectral characterization.
Carbon-13 NMR
Fig. 18 shows the carbon-13 NMR spectrum of the starting
material. The spectrum is characterized by a peak at 30.5 ppm
which indicates a methyl carbon peak of the acetyl group, fifteen
aromatic carbon peaks between 122 ppm and 152 ppm, two carbonyl
carbon peaks at 169 ppm, and a keto carbon peak at 206 ppm.
Fig. 19 shows the carbon-13 NMR spectrum of the material
after the reduction. Like the proton NMR spectrum, this spectrum is
complicated, and more carbons are observed than those expected
from the reduced polymer's structure. By comparing this spectrum
with the carbon-13 NMR spectrum of valinol shown in Fig. 26,
4V
c o o ZJ
- D <D
a: Q)
JO. +-» Q) i _
o «4— Q)
CQ L -
(U
E o Q-0)
E k-+-» O 0) Q.
CO
a:
CO
c o
XI i _ CO
U
c»
D)
5U
CO
Q 1— t o
UJ
o z UJ 3 o • ^
Ld OvJ CM CO 05
1 < CNJ
< •* u c\j u . Q_ en _ j 1
1 - i .
OJ 3 " * Q., o 1—
i r •z. •<*- UJ
1 oo bJ > UJ
«— Q. 1— _J _1 CD X < O (-1 O UJ Q if) U-
CL D_ c
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• a (U
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CO
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c O L . OJ
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O)
d)
51
APPENDIX II, some of the peaks could be assigned as peaks of valinol,
but it is still hard to assign all of the peaks completely. However,
the disappearance of the keto carbonyl carbon peak at 206 ppm still
suggests a successful reduction.
Conclusion on Spectroscopic Analysis
Based on the characterization of FTIR and NMR spectra, it is
clear that the synthesis of the starting material was successful. It
is also clear that the reduction of the starting material was
successful because no keto carbonyl peaks were observed in both
FTIR and carbon-13 NMR spectra. However, the proton and carbon-13
NMR spectra of the reduced material were too complicated to assign
all peaks, although some of the additional peaks could be assigned as
the peaks of valinol. A possible explanation for the contamination of
the reduced material by valinol is the strong hydrogen bonding
between ester carbonyls in the polymer and alcohol or amino groups
in valinol.
Thgrmal Analy?g?
Fig. 20 and Fig. 21 show the DSC scan and the TGA thermogram
of the polymer before reduction, respectively. The DSC scan shows
52
DSC <Natne>
0Bl_4_p.2 <Date>
92/05/07 10: 43 a i r
-1500
<Sample> 0B1-4-R
5.600 iTig ( 5.600 mg)
<Reference>
<Comment> <Temp. program [C] [C/min] [min]> IK - 5 0 . 0 - 300.0 20.00 1.00
<Gas> 0.0 ml/min 0.0 ml/min
-2375
-3250
-4125
1.000 mg <Sampling> 1.0 sec
-5000 -60 30
qeorala tech school of chem enq
120 210 TEMP C (Heating)
300
Fig. 20. Differential Scanning Calorimetry Thermogram of the Polymer Before the Reduction
53
T6A File Name: obl42 Saddle Height: 15.424 rag Sat Hay 09 15: 54: 31 1992
Ob1-4-2
100.0
PERKIN-ELMER 7 Series Thermal Analysis System
95.0-,
= 90.0
85.0
BO.O -
75.0
70.0
65.0 -
50.0
nitrogen
200.0 250.0 300.0
Tempepatupe ('C)
1 r
350.0 400.0
Takashl
450.0
0.0 Bin NATS i: KO.o C/Bin
Fig. 21. Thermogravimetric Analysis Thermogram of the
polymer before the Reduction
54
endotherms at 140°C, 170°C, and 237°C. The endothernns at 140°C
and 170°C are the nnelting points of different size crystals. The
endotherm at 237°C probably represents the clearing temperature,
but this should be confirmed by POM. The base line shift above
260°C represents the polymer degradation because a significant
weight loss was observed above 250°C in the TGA thermogram (Fig.
21).
Fig. 22 and Fig. 23 show the DSC scan and the TGA thermogram
of the polymer after reduction, respectively. The DSC scan shows
several endotherms at 87°C, 163°C, 196°C, 205°C, and above 270°C.
The endotherm at 87°C probably represents the melting of valinol.
The endotherm at 163°C is the melting point (Tm). The endotherm at
205°C is the boiling point of valinol. No clearing temperature was
observed because of the significant baseline shift above 205°C
caused by the evolution of valinol and polymer degradation (Fig. 23).
Finally, the endotherms above 270°C probably represent the severe
degradation of the polymer.
55
DSC <Name>
0Bl_4_p.l <Date>
<Saniple> 0B1-4-R
5.600 mg ( 5.600 mg)
<Reference>
<Cofnment> <Teinp.program [C] [C/mlnl [min] > 1« - 5 0 . 0 - 300.0 20.00 1.00
<Gas> 0,0 ml/min 0.0 ml/min
92/05/07 10:10 air
1000
-1000-
-3000
u O
-5000
1.000 mg <Sampling> 1.0 sec
-yooo"--60 30
qeorqla tech school of chem enq 120 210
TEMP C (Heating) 300
Fig. 22. Differential Scanning Calorimetry Thermogram of the Polymer After the Reduction
56
TGA File Name: obl4r Sample Weight: 10.742 mg Frl May 08 15:24:06 1992
obl-4-r
PERKIN-ELMER 7 Series Thermal Analysis System
nitrogen
50.0 100.0 150.0 i>00.0 250.0 300.0 350.0 400.0
Temperature (*C) Takashi
450.0 500.0
0.0 sin lUTV ix ao.O C/Bln
Fig. 23. Thermogravimetric Analysis Thernnogram of Polymer After the Reduction
the
57
CHAPTER IV
CONCLUSIONS AND RECOMMENDATION
Conclusions
Poly [oxy(2-acety 1-1,4-phenylene)oxyterephthaloyl-co-oxy(2-
acetyl-1,4-phenylene)oxyisophthaloyl] was synthesized by
applying the procedure of Onwunaka. The synthesis was
confirmed through FTIR and NMR analyses. The intrinsic
viscosity of the polymer was 0.42 dL/g in 2-chlorophenol at
30°C.
The reduction of poly[oxy(2-acetyl-1,4-phenylene)oxytere-
phthaloyl-co-oxy(2-acetyl-1,4-phenylene)oxyisophthaloyl]
was successful since the disappearance of a keto carbonyl
carbon was confirmed through FTIR and NMR analyses.
However, some additional peaks which should not be observed
according to the structure of the reduced material were also
observed in proton and carbon-13 NMR spectra. Some of the
additional peaks could be assigned to the peaks from valinol by
comparing them with proton and carbon-13 NMR spectra of
valinol. The inherent viscosity of the polymer after reduction
58
was 0.11 dL/g, which was much less than that of the polymer
before reduction (0.45 dL/g).
59
Recommendations
The author strongly recommends the future attempts to use
the organic synthetic route indicated in Fig. 24-1 and 24-2. There
will be two major advantages to this method compared with the
polymeric route demonstrated in this thesis. Thq first advantage is
the ease of reduction. Since the reduction will be done at the stage
before polymerization, there will not be any problem dissolving the
material, and the optical yield of reduction, which cannot be
measured when the polymeric route is taken, will also be easily
confirmed. The second advantage will be the absence of any side
reaction caused by the insufficient removal of hydrochloric acid
'\ since the ketone in 2'.5'-dihydroxyacetophenone will have already
been reduced before polymerization. In addition to these advantages,
the protection of alcohol by the trimethylsilyl group etc. is also
preferable to prevent transesterification, which might occur when
the polymer is heated.
A disadvantage of the organic synthetic route is the number of
steps. In the case of the polymeric route , there are only two steps,
polymerization and reduction. The organic synthetic route requires
at least five steps.
In order to accomplish this organic synthetic route. It is
necessary to protect the phenolic hydroxyl group and alcohol
0 0
I. Protection of phenolic -OH
„o-Q-OH CHj
CH / C=0
VJ °^U-\!\J c=o
CH /
2. Asymmetr ic reduction
.CHo
vJ °^J-\AJ c=o
CH /
.CH,
vj °-^r\iyj . H
""< 'OH
Fig. 24-1 Scheme for the Organic Synthetic Route 1
3. Protect ion of the chiral alcoholic -OH
.CHo
v_/ °^Kr\Aj ^ " 3 OH
.CHn
\ // °^ /r\A // CH. . H
^"3 o-Si-CH3
4. Deprotection of phenolic -OH
.CH2
VJ °^^^\;XJ CH ^ ^ \ ' ^ " 3 -"3 0 -S I -CH3
CH3
HO V //
OH
. H
^ O—Si-CHo
CH3
Polymerizat ion w i t h Terephthaloyl Chloride
Fig. 24-2 Scheme for the Organic Synthetic Route 2
6 2
produced by reduction. Methods to protect alcoholic hydroxyl groups
are well established, and one of these routinely used methods should
be adopted. Several methods to protect phenolic hydroxyl groups are
also available, but most of these methods will not work for the
phenolic hydroxyl groups in 2',5'-dihydroxyacetophenone because of
the hydrogen bonding of one of the hydroxyl groups with the ketone
as shown below.
HO
According to current literature, only the protection by methyl ether
and benzyl ether have been successful, but the protection by methyl
ether will be technically more difficult than that by benzyl ether
because the system is heterogeneous and requires phase transfer
reagents. The protection by benzyl ether is a homogeneous reaction
and therefore easier.
• • • i » >
63
APPENDIX
The proton and carbon-13 NMR spectra of valinol are shown in Fig. 25
and Fig.26.
••ii l l l lfHlil lMik
64
X o H-Ui
u o z u cr bJ Ovl
en en
o z
< < UJ CNJ U . 01 rvi t— _J I D t D_ O I—
bJ ' - Ul > UJ a . I— _i _ j X < O n bJ Q t/> U.
6 5
a i / i
UJ
o 2:
^ UJ ff 3 0
- i UJ OJ 0 on CD
1 :lt UJ r\ l i f tn —1
rvJ 1 f- k
Z} • *
o a. 0 1— o 2; z UJ
(—( OJ UJ > UJ
_) D- H- _J _J
< X < 0 t-H
> UJ Q en u_
o c "(i >
M—
o E E •M
o
s. CO Q :
z m c o
XI k -OJ
U <£> OvJ
d)
6 6
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