4494 Macromolecules 1999, 32, 4494-4496

Synthesis of a New Side-Chain Type Liquid Scheme 1. Synthesis and Chemical Structure of the
Crystal Polymer Poly[dicyclohexyl Monomer and the Polymer
vinylterephthalate]

Dong Zhang, Yuxiang Liu, Xinhua Wan, and

Qi-Feng Zhou*
Department of Polymer Science & Engineering, College of
Chemistry, Peking University, Beijing 100871, China

Received February 1, 1999

Revised Manuscript Received April 16, 1999
Introduction. The properties and the chemical struc-
ture dependence of side-chain liquid crystal polymers
have been extensively studied in recent years because
of their potential applications in nonlinear optics, side groups laterally attached to the main chain through
information storage, and display devices.1,2 A side-chain no decoupling spacers.
liquid crystal polymer often comprises three basic Synthesis. The synthetic route of the monomer and
components: a polymer backbone, a mesogenic unit, and polymer is shown in Scheme 1.
a flexible spacer that bridges the mesogenic unit and Vinylterephthalic acid (0.77 g, 4.0 mmol) and triph-
the polymer backbone. Its properties may be affected enylphosphine (2.2 g, 8.4 mmol) were dissolved in about
by varying the chemical structures of these components. 10 mLof dried pyridine to obtain solution A. Dried
Empirical rules have been developed for systematically cyclohexanol (0.84 g, 8.4 mmol) and hexachloroethane
producing side-chain liquid crystalline polymers.3,4 Ac- (2.1 g, 8.8 mmol) were dissolved in 10 mL of dried
cording to Finkelmann et al.,5 if mesogenic groups are pyridine to obtain solution B. Solution B was then
directly attached to the polymer main chain, thermal dropped into solution A slowly. The mixture was stirred
motions of the polymer segments and mesogenic groups at 60 °C for 5 h. After cooling to room temperature, the
are directly coupled. When the temperature is above the mixture was poured into dilute HCl and extracted with
Tg, the polymer tends to adopt statistical chain confor- chloroform three times. The organic layer was dried over
mations that hinder the anisotropic orientation of the anhydrous sodium sulfate and chloroform was removed
mesogenic groups. Therefore, flexible spacers should be by vacuum distillation. The residue was purified using
inserted between the backbone and mesogenic units to column chromatography on silica gel with chloroform
decouple their interactions. In such conditions, the as eluant to obtain a colorless sticky liquid, 0.91 g (yield
mesogenic side chains can be anisotropically ordered in 64%). The structure of the monomer was proven by mass
the liquid crystal state even though the polymer main spectroscopy (VG-ZAB-HS instrument), 1H NMR spec-
chains tend to adopt the statistical random coil confor- trometry (Bruker ARX 400), and IR (Vector 22 FT-IR)
mations. This idea of Finkelmann has been proven by as follows: mass spectrum 356(parent), 274(base), 257,-
many experiments and becomes a useful guide for 193,192,175; 1H NMR 1.30-1.96, m, 20H, 2× (5-CH2-
molecular design of side-chain liquid crystalline poly- ); 5.01-5.05, m, 2H, 2× (-OCH-); 5.39-5.77, 2d, 2H(d
mers. On the other hand, the mesogen-jacketed liquid CH2); 7.36-7.50, q, 1H (sCHd); 7.88-8.24, m, 3H
crystalline polymers (MJLCPs) form a different class of (phenyl); IR(cm-1) 1244 (C-O-C), 1717 (CdO), 2860
liquid crystalline polymers, not described by Finkel- (CH2, νas), 2936 (CH2, νs).
mann model.6 In MJLCPs, the rodlike mesogenic units The polymerization of the monomer was carried out
are connected at their gravity center (or a nearby in THF solution at 60 °C using 0.3% mol equiv AIBN
position) to the main chain through no or only very short as initiator. The polymer was precipitated in and
spacers. It has been proven that in this case the washed with methanol. The molecular weight of the
introduction of the flexible spacers to decouple the polymer was determined by a Waters 201 GPC instru-
interactions of the main chain and the mesogenic side ment with polystyrene as standard. Found: Mn ) 2.7
groups is not necessary for the polymer to form a liquid × 104, Mw/Mn ) 2.4.
crystalline phase.6-8 However, the rigid mesogenic units Results and Discussion. The monomer was an oily
are believed to be essential for these two classes of the liquid at room temperature, and no mesophase was
side-chain type polymers to form a liquid crystal phase, detected by means of polarizing optical microscope
although in some cases the mesogenic group itself (POM). Its differential scanning calorimetry (DSC) curve
cannot form a stable liquid crystal phase,9 and in few showed also only one melting endotherm at -30 °C.
others the low molar mass liquid crystals can be flexible The monomer was easily polymerized to a moderately
molecules.10 Here we report the synthesis of a new high molecular weight polymer by way of convenient
polymer poly[di(cyclohexyl) vinylterephthalate] in which radical polymerization. The WAXD of powder samples
neither flexible spacers nor conventional rigid mesogenic showed that the polymer was amorphous at room
units are included. We found that this polymer is temperature. The lack of any crystallinity of this and
amorphous below Tg but forms a liquid crystal phase other homologous MJLCPs6 indicates that the bulky
above Tg. To our knowledge, it is the first liquid side groups hinder the crystallization of these polymers
crystalline polymer that consists of nonrigid mesogenic as what the phenyl group does for atactic polystyrene.
Since the TGA analysis (at a heating rate of 20 °C/min
* To whom all correspondence should be addressed. in nitrogen atmosphere) had showed a 1% weight loss
10.1021/ma9901386 CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/11/1999
Macromolecules, Vol. 32, No. 13, 1999 Communications to the Editor 4495

instead be assigned to nematic if the arguments of Xu

et al.13 and Ober et al.14 were applied. Following these
workers, because of the mesogen-jacketed structure, the
polymer chain can be seen as a thick rod so that the
sharp diffraction at lower angles may be attributed to
the spacing between the rods rather than the thickness
of the smectic layers. Because we lack the detailed
information of the arrangement of the side groups and
their orientation relative to polymer main chain, we are
not in the position to declare with any certainty the type
of the mesophase of this new polymer.
Since the TGA analysis has showed a 1% weight loss
at 293 °C, the slow extinction of birefringence of this
polymer observed on POM at higher temperatures must
be caused by thermal decomposition, and the clearing
Figure 1. Powder X-ray diffraction pattern of the polymer point should be higher than 290 °C if no chemical
after treated at 210 °C for 30 min in argon. decomposition had taken place. In other words, the
mesophase of this new polymer is thermally very stable.
at 293 °C, our DSC experiments were run so as not to
exceed this temperature. The DSC heating curve showed It is interesting to ask why this polymer forms so
no endothermic peak but only one glass transition at stable a liquid crystal phase. Generally, a rodlike
99 °C which is 20 °C degrees lower than the Tg of its mesogenic unit often consists of two or more 1,4-
homologous polymer poly[bis(p-methoxyphenyl) vinyl- phenylene rings with conjugated interconnecting link-
terephthalate).11 These two polymers have the same ages and two terminal groups. The resulting mesogenic
carbon-carbon main-chain but different side groups. It unit should have an anisotropic linear or planar shape
is bis(p-methoxyphenyl) terephthalate for the one with with enhanced anisotropic polarizability and rigidity.
higher Tg but di(cyclohexyl) terephthalate for the poly- However in our new polymer, the side groups are di-
mer of this report. Because, for the carbon-carbon chain (cyclohexyl) terephthalate, formed by only one phe-
polymers, the polymer with higher steric hindrance nylene ring but two cyclohexyl rings. The two cyclohexyl
parameter would have higher Tg and less chain segment rings are in principle quite flexible and cannot conjugate
mobility,12 we see that the chain of poly[bis(p-methoxy- with the phenylene ring to which they attach through
phenyl) vinylterephthalate) is more sterically hindered ester groups. Furthermore, there is no electron-donor
by the side groups than the chain of the title polymer or electron-acceptor substituent used in the ends of the
which has less rigid side groups. side groups, whereas these polar substituents are neces-
When observed under a polarizing optical microscope, sary for low molar mass liquid crystals even if no
the polymer softened at about 140 °C and formed a cyclohexane ring is used.15 Thus, the degree of conjuga-
liquid crystal phase which showed increased birefrin- tion and the polarity and polarizability of the side group
gence with time. The birefringence remained strong di(cyclohexyl) terephthalate are all low and not favor-
above this temperature until at about 300 °C when able for stabilizing a mesophase. This point may be
extinction slowly occurred. However, no typical textures demonstrated by comparison of the two monomers, di-
of this birefringent phase could be used to identify the (cyclohexyl) vinylterephthalate and bis(4-methoxyphe-
mesophase. Therefore, a Rigaku Dmax-2400 diffracto- nyl) vinylterephthalate. The former has a very low
meter (with a nickel-filtered CuKR, 40 kV voltage, 30mA melting point at -30 °C and forms no liquid crystal
electric current, a scanning rate of 5°}/min, and a phase above melting; the latter melts at 109 °C and
scanning scope 1.5-40°) was used to take its X-ray forms a stable liquid crystal phase after melting.11 On
diffraction. The polymer was heated at 210 °C (signifi- the other hand, because the main chain of the polymer
cantly higher than its Tg of 99 °C but still much lower is nothing more than a substituted polyethylene chain,
than its Td of 293 °C) for 30 min in argon atmosphere we cannot find the answer from the viewpoint of the
so that the liquid crystal phase was formed. It was then polymer backbone either. The answer must then lie in
quenched to room temperature with ice water to obtain the unique architecture of this mesogen-jacketed type
a glassy liquid crystalline phase. After this treatment liquid crystalline polymer. In each repeating -CHCH2-
the sample was used for the X-ray diffraction study. The unit of poly(dicyclohexyl vinylterephthalate) there is a
diffraction pattern is shown in Figure 1. A diffused side group di(cyclohexyl) terephthalate which is at-
scattering halo at angles around 2θ ) 16.54° and a tached, in the side-on mode, to polymer backbone
sharp diffraction peak at 2θ ) 5.32° can be seen in the without a spacer. Thus, the population of the side groups
curve. This result may be interpreted by the presence around the main chain will be very high and accordingly
of a smectic-type structure. Thus the diffraction around the free volume for the bulky side groups will be small.
16.54° is attributed to the spatial correlation between The inversion between the axial and equatorial chair
di(cyclohexyl) terephthalate side groups, while the sharp conformers of cyclohexyl ring is thus dramatically
diffraction at 5.32° (corresponding to a distance of 1.66 hindered by the steric interactions. To increase the
nm) is attributed to the layer thickness. Since the packing efficiency and to reduce the repulsive force
observed thickness is shorter than the fully extended among the side groups as much as possible, the equato-
length of 1.73 nm of the side groups, the side groups rial conformers will be strongly favored and the side
must be arranged in the layer so that the long axis tilts groups tend to extend fully and align orderly. Therefore,
away from the layer normal. The tilt angle is calculated the originally flexible and potentially linear shaped di-
to be 20.3° since its cosine is 1.66/1.73. Considering the (cyclohexyl) terephthalate groups are converted to rigid
size of the tilt angle a smectic-C phase might be and linear shaped mesogenic units, resulting in the
assigned to this phase. However, the phase could formation of a liquid crystal phase. In other words, the
4496 Communications to the Editor Macromolecules, Vol. 32, No. 13, 1999

spatial interaction of the side groups plays very impor- (3) Percec, V.; Pugh, C. In Side Chain Liquid Crystal Polymers;
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Acknowledgment. This work was supported by the vinylterephthalic acid. Macromolecules, submitted for pub-
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