1052
Chemistry Letters Vol.36, No.8 (2007)
Control of Phase-transition Behavior and Polymerization Capability
by Methyl Addition to Dialkyl Fumarates Bearing Cyanobiphenyl Mesogens
Nozomu Fujii,1 Tsuyoshi Michinobu,1;2 and Kiyotaka Shigeharaꢀ1;2
1Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
2Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184-8588
(Received June 11, 2007; CL-070631; E-mail: jun@cc.tuat.ac.jp)
(i)
Addition of methyl groups to dialkyl fumarates bearing two
Br CH2
O
CN
HO
X
O
Y
CH2
O
6
CN
6
cyanobiphenyl mesogens dramatically changed the phase-transi-
tion behavior and polymerization capability. No mesophase
appeared and radical polymerization did not proceed in the
absence of the branched methyl groups, whereas well-defined
liquid-crystal phases were observed in the ambient temperature
range and high molecular weight polymers were obtained in
the presence of the branched methyl groups. An increase in
the number of the attached methyl groups decreases the liquid
crystalline temperature width as well as the molecular weight
of the resulting polymers.
2a: X = H, Y = H
2b
2c
1
: X = CH3, Y = H
: X = CH3, Y = CH3
(ii)
Y
X
O
NC
O
CH2
O
O
6
O
X
O
Y
CH2
O
CN
6
O
3a: X = H, Y = H
: X = CH3, Y = H
: X = CH3, Y = CH3
3b
3c
Scheme 1. Synthesis of dialkyl fumarates 3a–3c. (i) NaH,
ethylene glycol or 2-(tetrahydro-2-pyranyloxy)-1-propanediol
or 2,3-butanediol, benzyltriethylammonium chloride, THF,
24 h; then pyridinium p-toluenesulfonate (for 2b). 2a: 44%,
2b: 53%, 2c: 67%. (ii) maleic anhydride, p-TsOH, C6H6,
80 ꢁC, 12 h; then, morpholine, C6H6, 80 ꢁC, 4 h. 3a: 69%, 3b:
71%, 3c: 70%.
Increasing attention has been paid to dimeric liquid crystal-
line compounds, in which two mesogenic groups are typically
linked through a flexible spacer, for understanding and tuning
of phase behavior.1 Symmetry breaking by means of variation
of flexible alkyl chain spacer lengths or selection of mesogenic
groups is known to affect the phase-transition temperatures
and the resulting mesophases. For example, the odd–even effect
of alkyl chain carbon numbers on the thermal and optical proper-
ties was clearly observed.2 Nonsymmetric dimers bearing differ-
ent mesogenic groups tended to display higher-order liquid-crys-
talline phases due to the facile formation of segmented layers.3
Recently, a new approach to control of phase behaviors of
liquid-crystalline compounds, that is, methyl addition to alkyl
moieties, was proposed and verified for an antiferroelectric
compound by Thisayukta and Samulski.4 After methyl additions,
an anticlinic smectic CA phase newly appeared below the smec-
tic A phase.
Following our work on potent functionalities of poly(dialkyl
fumarate)s revealed by the electronic properties of the
Langmuir–Blodgett ultrathin films,5 we are recently trying to
give liquid-crystalline properties to poly(dialkyl fumarate)s.
One of the most significant features of poly(dialkyl fumarate)s,
in addition to the high thermal stability originating from the rigid
backbone, is the doubled side-chain density with respect to com-
mon vinyl polymers. Since high molecular weight poly(dialkyl
fumarate)s are usually prepared by radical polymerization of
branched dialkyl fumarates in bulk,6 the fumarate monomers
bearing two mesogenic groups become liquid-crystalline dimers,
and the branched moiety can serve as a methyl addition part
controlling the phase behavior. Here, we report for the first time
the remarkable change in both the phase behavior and the
polymerization capability by simple addition of methyl groups
to the dialkyl fumarates.
droxybiphenyl and dibromohexane, ether synthesis with an
excess amount of the diols (or the monoprotected propanediol
followed by deprotection) furnished 2a–2c in 44–67% yield
(Scheme 1). Here, addition of ammonium salts was found to
significantly improve the reaction yield owing to the inhibition
of Naþ ion chelation with the alkoxide moieties.9 The subse-
quent reaction with maleic anhydride in the presence of p-tolue-
nesulfonic acid (p-TsOH) followed by cis–trans isomerization,
catalyzed by morpholine,10 afforded the dialkyl fumarates bear-
ing two cyanobiphenyl mesogens 3a–3c in a satisfactory yield.
The compounds were unambiguously characterized by 1H NMR
and FT-IR measurements (see Supporting Information).12
The fumarates 3a–3c were thermally stable. Decomposition
temperatures of 3a–3c determined by thermogravimetric analy-
sis exceed 270 ꢁC. The phase-transition temperatures and the
phases determined by the differential scanning calorimetry
(DSC) measurements and polarized optical microscopy (POM)
images, respectively, are summarized in Table 1. The fumarate
3a without branched methyl groups did not show any mesophas-
Table 1. Phase behaviors of dialkyl fumarates 3a–3c
Phase-transition behaviora
63.1
3a
3b
3c
Cr
G
Iso
Iso
Iso
104.8
22.4
25.1
22.3
24.4
ꢂ4:7
ꢂ2:5
ꢂ2:5
ꢂ0:7
34.4
35.9
24.2
26.4
SA
SA
N
N
G
Cyanobiphenyl group was employed as a mesogen for a
comparison to other reported dimeric liquid-crystalline com-
pounds, especially of fumarates.2,7,8 Starting from 40-(6-bromo-
hexyloxy)-4-cyanobiphenyl (1), prepared from 4-cyano-40-hy-
aTransition temperatures (ꢁC) and phases determined by DSC (at a
rate of 10 ꢁC minꢂ1 upon cooling: top, upon heating: bottom) and
POM. Iso: isotropic liquid; SA: smectic A; G: glass; Cr: crystal.
Copyright Ó 2007 The Chemical Society of Japan