Chiral polyisocyanates from an azomonomer with a very high chiral induction

Article abstract of DOI:10.1021/ma010491n

The synthesis of new chiral polyisocyanates is described. For this purpose a new chiral azobenzene containing monomer with the chiral center in α-position to the isocyanate group was synthesized. The anionic copolymerization was carried out in THF as solvent with potassium cyanide that was complexed by 18-crown-6 as initiator. This allowed a better control of the reaction and thereby the synthesis of polyisocyanates with a fairly low polydispersity. The copolymers show an extremely high transfer of chirality from the chiral side groups to the helical backbone in dilute solution. Copolymers with only 1.6 mol % of chiral side groups show nearly the full optical rotation and exist predominantly in the P-helical conformation. It was found that the isomerization of the azobenzene groups hardly has any influence on the conformation of the helix. Lyotropic liquid crystalline phases were formed from the polymers and 1,2-dichlorobenzene. It could be shown that even polymers with a very low content of chiral side groups (1.6 mol %) form cholesteric phases with a selective reflection in the visible range. The pitch of the cholesteric phase and thereby the wavelength of the selective reflection bands could be reversibly altered by the isomerization of the azobenzene groups.

Full text of DOI:10.1021/ma010491n

Macromolecules 2002, 35, 185-192
185
Chiral Polyisocyanates from an Azomonomer with a Very High Chiral
Induction
Ra lf Mr u k a n d Ru d olf Zen tel*
Institute of Organic Chemistry, University of Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
Received March 21, 2001; Revised Manuscript Received October 9, 2001
ABSTRACT: The synthesis of new chiral polyisocyanates is described. For this purpose a new chiral
azobenzene containing monomer with the chiral center in R-position to the isocyanate group was
synthesized. The anionic copolymerization was carried out in THF as solvent with potassium cyanide
that was complexed by 18-crown-6 as initiator. This allowed a better control of the reaction and thereby
the synthesis of polyisocyanates with a fairly low polydispersity. The copolymers show an extremely high
transfer of chirality from the chiral side groups to the helical backbone in dilute solution. Copolymers
with only 1.6 mol % of chiral side groups show nearly the full optical rotation and exist predominantly
in the P-helical conformation. It was found that the isomerization of the azobenzene groups hardly has
any influence on the conformation of the helix. Lyotropic liquid crystalline phases were formed from the
polymers and 1,2-dichlorobenzene. It could be shown that even polymers with a very low content of chiral
side groups (1.6 mol %) form cholesteric phases with a selective reflection in the visible range. The pitch
of the cholesteric phase and thereby the wavelength of the selective reflection bands could be reversibly
altered by the isomerization of the azobenzene groups.
In tr od u ction
Polyisocyanates with chiral side groups form lyotropic
cholesteric phases18,19 at higher concentrations (about
40 wt %). It was found that even small amounts of chiral
groups possess a large influence on the formation of the
cholesteric phase.20 The characterization of these cho-
lesteric phases requires polymers with a narrow mo-
lecular weight distribution to minimize the occurrence
of defects in the orientation. In addition to a low
polydispersity, a fairly low molecular weight is favorable
as a low viscosity improves the formation and orienta-
tion of the cholesteric phase.
Here we report the synthesis of a new chiral azomono-
mer with an extremely high chiral induction and its
polymerization to chiral copolymers, which follow the
“sergeants-and-soldiers” principle. Due to a modification
of the polymerization conditions the polydispersity could
be kept fairly low. Finally, lyotropic cholesteric phases
were prepared, the pitch of which can be switched by
photoisomerization.
Helical polymers1 (e.g., DNA, polypeptides) are fre-
quently found in nature. They have attracted attention
due to their chiral structure and the resulting chiro-
optical properties.2-4 The helical structure of natural
helical polymers is stabilized by hydrogen bonds and
can be altered by temperature or pH changes.
In addition to these well-known natural helical poly-
mers, several classes of synthetic helical polymers have
been prepared.5-9 One class of these synthetic helical
polymers are poly(alkyl isocyanates) that can be pre-
pared by cyanide initiated anionic polymerization of
alkyl isocyanates in DMF at low temperatures.10,11 It
was shown that poly(alkyl isocyanates) possess a helical
conformation due to steric reasons which exists in
solution as well as in the crystalline state. The exclusive
use of achiral isocyanate monomers leads to a racemic
mixture of M- and P-helical sequences in the polymer.
The ratio between M- and P-helices can be shifted by
the use of chiral comonomers. Due to the high cooper-
ativity within the polymer chains only small amounts
of chiral side groups in copolymers are necessary to
favor one twist sense of the helices. The ability of the
chiral side groups to force the achiral groups into a
certain conformation and to determine the twist sense
of the helix is called the “sergeants-and-soldiers”
effect.12-14 The extent of this effect depends on the chiral
induction of the monomer.
The conformation of the helices in a “sergeants-and-
soldiers” copolymer can be additionally influenced by
the isomerization of chiral side chains. The use of
photoisomerizable side groups (e.g., azobenzene groups)
may allow a photoinduced switching of the helix be-
tween two conformations.15-17 The influence of the
photoisomerization of azobenzene containing side groups
on the conformation of the helix has been shown for
several polymers by CD spectroscopy. In addition to the
chiral induction of the monomer, the distance between
the isomerizable group and the main chain is an
important factor in this influence.
Resu lts a n d Discu ssion
Syn th esis of a New Ch ir a l Mon om er . The chiral
center of the chiral side groups that were previously
incorporated in photochromic polyisocyanates had a
distance of at least two σ-bonds from the main chain.17
By lowering this distance to one σ-bond, the chiral
induction of the side groups should increase dramati-
cally. This is confirmed by the work of Green et al. who
reported very high values of specific rotation for poly-
(2-deuteriohexyl isocyanate) whose chirality only con-
sisted in an isotope effect.21 However, the synthesis of
polyisocyanates with a chiral center with a higher chiral
induction in R-position to the main chain implies that
the incorporation of a secondary isocyanate would be
necessary. It is known that secondary isocyanates do
not homopolymerize but the copolymerization of cyclo-
hexyl isocyanate with butyl isocyanate has been re-
ported in the literature.22 According to this report, it
should be possible to synthesize photochromic polyiso-
cyanates with chiral centers in R-position to the main
chain.
10.1021/ma010491n CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/30/2001

186 Mruk and Zentel
Sch em e 1. Syn th esis of Mon om er 6
Macromolecules, Vol. 35, No. 1, 2002
Sch em e 2. Used Mon om er s
nium catalysts.18,29 As these titanium catalysts are
deactivated in the presence of azobenzene groups,30 this
method is completely unsuitable for the synthesis of
photochromic polyisocyanates. Therefore an alternative
method for the synthesis of azobenzene group containing
polyisocyanates with a low polydispersity must be found.
One reason for the poor controllability of the classical
anionic polymerization is the fact that the polymer is
insoluble in the solvent so that it readily precipitates
after formation. The use of a better solvent (e.g., toluene
or THF) should improve the controllability of the
reaction. Another problem is the solubility of the initia-
tor. As alkali cyanides are insoluble in most organic
solvents the initiator has to be modified as well. Fol-
lowing this idea, Okamoto et al. polymerized HIC and
DMHIC in toluene with a solution of sodium cyanide
in DMF as initiator.31 Our approach to this problem is
the solubilization of cyanide in organic solvents by the
use of crown ethers. Crown ethers form stable cryptates
with alkali-metal ions and are able to solubilize alkali-
metal salts in organic solvents.
The polymerization reactions were carried out in THF
with a solution of 18-crown-6 and potassium cyanide in
THF as initiator at a temperature of -60 °C. In contrast
to the classical polymerization the precipitation of the
polymer did not occur until 5 min after the injection of
the initiator. The chiral monomer 6 was copolymerized
with HIC and DMHIC to form co- and terpolymers. The
structure of the resulting polymers is shown in Scheme
3.
The new isocyanate monomer 6 was prepared in a
five-step synthesis (Scheme 1) starting with alanine
methyl ester hydrochloride 1. After the BOC-protection
of the amino group23 the resulting ester was reduced
by sodium boranate/lithium chloride24 to give the BOC-
protected amino alcohol 3. In the next step the alcohol
3 was coupled with 4-hydroxyazobenzene in a Mit-
sunobu reaction.25 For the cleavage of the BOC protec-
tion group the resulting ether 4 was treated with
trifluoroacetic acid and afterward with hydrochloric acid
to give the hydrochloride 5. In the last step the
hydrochloride 5 was transformed into the isocyanate 6
by reaction with trichloromethyl chloroformate26,27 in
dioxane.
The syntheses of the comonomers hexyl isocyanate
(HIC, 7) and (R)-2,6-dimethylheptyl isocyanate
(DMHIC, 8) were carried out as described in the
literature.17,21,28 All used monomers are listed in Scheme
2.
Ch a r a cter iza tion of th e P olym er s. The amount of
the azo-containing monomer 6 in the resulting polymer
was determined by UV spectroscopy, whereas the ratio
between DMHIC and HIC was determined by NMR
spectroscopy. The ratio of the monomers in the reaction
batch and in the polymer is listed in Table 1.
It can be seen that in most cases the fraction of azo
monomer incorporated into the polymer was larger than
in the reaction batch. This does not match the expecta-
tions because of the steric demand of this monomer. One
possible explanation for this is that monomer 6 prefer-
ably coordinates to the potassium counterion.
Syn th esis of th e P olym er s. The conventional an-
ionic polymerization of isocyanates is carried out in
DMF with sodium cyanide as initiator at temperatures
lower than -40 °C.10,11 This method leads to polymers
with high polydispersity and high molecular weights.
As the formation of cholesteric phases from the resulting
polymers should be examined a fairly low polydispersity
and a fairly low molecular weight is required. An
alternative polymerization method that leads to poly-
isocyanates with low polydispersities and low molecular
weights is the coordination polymerization with tita-
The polydispersities and the molecular weights of the
polymers were determined by GPC with light scattering
detection. The results of the GPC analysis are shown
in Table 2.

Macromolecules, Vol. 35, No. 1, 2002
Ta ble 1. Ch a r a cter iza tion of P olym er s P 1-P 11 (Sch em e 3)a
ø(DMHIC) ø(monomer 6)
Chiral Polyisocyanates from an Azomonomer 187
ø(HIC)
specific rotation
polym
m
p
m
p
m
p
M (g/mol)
[R]D20(0.1 deg‚cm2‚g-1
)
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
P 9
P 10
P 11
0.995
0.990
0.984
0.983
0.974
0.971
0.962
0.854
0.860
0.870
0.869
0.995
0.984
0.988
0.970
0.970
0.965
0.953
0.928
0.927
0.921
0.905
0.005
0.010
0.016
0.017
0.026
0.029
0.038
0.005
0.016
0.012
0.030
0.030
0.035
0.047
128.0
129.7
129.0
131.9
131.7
132.6
134.5
130.2
132.2
133.8
136.6
+234
+416
+416
+442
+448
+442
+476
-348
+204
+344
+448
0.146
0.126
0.110
0.103
0.072
0.055
0.050
0.046
0.013
0.020
0.029
0.017
0.029
0.049
a
ø(HIC): mole fraction of HIC; ø(DMHIC): mole fraction of DMHIC ø(monomer 6): mole fraction of monomer 6 (Scheme 2). Key: m
) monomer ratio in the reaction batch; p ) ratio of comonomers in the polymer; M ) average molecular weight of repeating units.
Sch em e 3. Str u ctu r e of P olym er s
F igu r e 1. Specific rotation of the copolymers as function of
the fraction of chiral side chains.
It can be seen that the polydispersity of all polymers
is lower than 2 and the molecular weights of all
polymers are fairly low when a high ratio of initiator is
used. These results show that the new polymerization
method leads to much narrower molecular weight
distributions and lower molecular weights than the
conventional precipitation polymerization.18 The coor-
dination polymerization leads to even lower polydisper-
sities than the new method,18,29 but it is not applicable
to the synthesis of azobenzene containing polyisocyan-
ates. Therefore the modified anionic polymerization
represents the best way for the synthesis of low poly-
dispersity azobenzene containing polyisocyanates known
so far.
The influence of the chiral side chains on the twist
sense of the helices was examined by measuring the
specific rotations of the polymers in dilute solution
(Table 1 and Figure 1). It can be observed that even
polymer P 3 with a molar fraction of chiral side groups
of only 1.2 mol % shows a specific rotation of over 400°.
At higher fractions of chiral side groups, the specific
rotation hardly increases with the concentration of the
chiral groups but approaches a final value. The com-
parison of the azo-containing polymers with copolymers
of HIC and DMHIC (Figure 2 and Scheme 4) shows that
the final values of the specific rotation are quite similar.
The difference between these polymers is the concentra-
tion of chiral monomers that is necessary to receive
specific rotations that are close to the final value. In
the case of the new azo-containing polymers 1.2 mol %
F igu r e 2. Specific rotation of several copolymers in depen-
dence of the fraction of chiral side chains.
Ta ble 2. Resu lts of GP C An a lysis
polymer
Mw (g/mol)
Mn (g/mol)
Mw/Mn
P 1
P 2
P 3
P 4
P 5
P 6
P 7
P 8
P 9
P 10
P 11
112 600
133 400
197 300
140 000
243 000
288 500
268 100
235 500
238 200
282 300
230 900
64 300
68 900
107 500
80 100
156 800
173 100
170 500
172 800
148 000
161 000
134 600
1.75
1.94
1.84
1.75
1.55
1.67
1.57
1.36
1.61
1.75
1.72
of chiral monomer is a value sufficient to reach a value
of specific rotation that is higher than 80% of the final
value. In contrast to that, about 10 mol % of DMHIC
must be incorporated in a copolymer with HIC to reach
similar values of specific rotation.28 Other chiral azo
monomer containing polymers (Scheme 4) with a dis-
tance between the chiral center and the main chain that
is larger than six σ-bonds do not reach comparably high

188 Mruk and Zentel
Macromolecules, Vol. 35, No. 1, 2002
Sch em e 4. Str u ctu r e of F or m er ly Syn th esized P olyisocya n a te Cop olym er s
values of specific rotation even when the fraction of
chiral monomers is as high as 20 mol %12,15,16 (Figure
2).
less than 10% of the initial absorbance at the maximum
of the π-π* transition of the trans form is left after
irradiation, it can be concluded that at least 90% of the
azobenzene groups have been isomerized into the cis
form.
The influence of the photochemical isomerization on
the chirality of the system was examinated by ORD
spectroscopy. The ORD spectra of several polymers in
solution were measured before and after irradiation
(Figure 4).
The high values of specific rotation in the polymers
are caused by a transfer of chirality from the chiral
monomers to the main chain according to the “sergeants-
and-soldiers” experiment. The occurrence of the plateau
of specific rotation above a certain concentration of
chiral monomers can be explained by the fact that
almost all helices exist in one helix twist sense above
this point. Therefore a further increase of the concen-
tration of chiral monomers hardly has any further
influence on the conformation of the helices. Only 1.2
mol % of monomer 6 is sufficient to reach the plateau
of specific rotation. By that monomer 6 by far shows
the strongest transfer of chirality according to the
“sergeants-and-soldiers” experiment that is known so
far.
The influence of the photoisomerization of the azoben-
zene side groups on the properties of the polymers was
examinated by UV, ORD, and CD spectroscopy.
The photochemical trans f cis isomerization of the
azobenzene group can be easily observed by UV spec-
troscopy.32 The UV spectra of polymer P 7 before and
after irradiation at a wavelength of 365 nm are shown
in Figure 3. The most obvious sign of the isomerization
is the shift of the π-π* transition to shorter wave-
lengths and the loss of intensity of this transition. As
The optical rotation of the polymers increases during
isomerization. The difference between the specific rota-
tion of the cis and the trans form increases with shorter
wavelengths. However, in comparison to other sys-
tems,15,16 the influence of the photoisomerization on the
specific rotation is rather small. Due to the extremely
high chiral induction that is caused by the presence of
the chiral group in R-position to the main chain, the
helix is strongly forced into one conformation. This
conformation is so stable that it is hardly influenced by
the photoisomerization of the side chains.
The influence of the photoisomerization on the con-
formation of the main chain can also be observed by CD
spectroscopy. With this method a direct observation in
the absorption band of the main chain chromophore at
about 250 nm is possible. CD spectra of all monomer 6
containing polymers were measured in this area. As the
appearance of all CD spectra is quite similar with
exception of the absolute values of the molar ellipticity
only the spectrum of polymer P 7 is shown in Figure 5.
CD spectra in the range of the main chain absorption
show that all polymers possess very high molar ellip-
ticities. This result confirms the results of the ORD
spectroscopy and is another proof for the transfer of
chirality according to the “sergeants-and-soldiers” ex-
periment. As the maximal ellipticities of all polymers
show positive values it can be concluded that in all cases
the majority of helices exists in P-conformation. The
dependence of the maximal molar ellipticity on the mole
fraction of the chiral polymer in the bipolymers P 1-
P 7 is shown in Figure 6. Again it can be observed that
the increase of the maximal molar ellipticity levels off
F igu r e 3. UV spectra before and after irradiation.

Macromolecules, Vol. 35, No. 1, 2002
Chiral Polyisocyanates from an Azomonomer 189
F igu r e 7. CD spectra of P 7 between 300 and 500 nm.
F igu r e 8. UV spectrum of LC phase 400-800 nm.
is compatible with the results of the ORD spectroscopy
and confirms the high stability of the helix conforma-
tion. Therefore, the CD spectra are another proof for
the fact that the helix twist sense cannot be significantly
altered by photochemical isomerization.
F igu r e 4. ORD spectra of several polymers before and after
irradiation.
In addition to the CD spectra in the range of the main
chain absorption the CD spectra of polymer P 7 was
measured in the area of the side chain absorption before
and after irradiation (Figure 7). It can be observed
thatsin contrast to the range of the main chain
absorptionsthe CD spectra of the side chain region of
the cis and the trans isomer significantly differ from
each other. As the side chain chromophore changes
during the isomerization, this observation corresponds
to the expectations. The molar ellipticity in the range
of the side chain absorption is much lower than the
ellipticity in the area of the main chain absorption. This
can be explained by the fact that the main chain
chromophore is chiral itself whereas the side chain
chromophore only is positioned in a chiral environment.
F igu r e 5. CD spectra of polymer P 7.
Lyotr op ic Ch loster ic P h a ses. To examine the
formation of lyotropic liquid crystalline phases several
polymers were mixed with varying amounts of 1,2-
dichlorobenzene. It was found that mixtures with
polymer concentrations of more than 40 wt % form
liquid crystalline phases. Some of these phases reflect
light in the visible range. For further examination, films
of polymer P 2 were prepared from the lyotropic phase
and examinated by UV spectroscopy (Figure 8).
F igu r e 6. Dependence of molar ellipticity on the mole fraction
of monomer 6.
The UV spectrum in Figure 8 shows that, in addition
to the n f π* absorption band of the azobenzene
chromophore at 450 nm, a selective reflection band at
590 nm is found. This can be explained by the formation
of a cholesteric phase. The wavelength of the selective
reflection band λ relates to the helical pitch p of the
cholesteric liquid crystalline phase by λ ) np (n being
the refractive index, about 1.5 in our system).
with increasing amount of monomer 6. According to that
almost all helices in polymer P 7 with a fraction of 4.7
mol % of chiral side groups are in P-conformation.
Therefore a further increase of the fraction of chiral side
groups would hardly influence the equilibrium between
the helix twist senses.
Another result of the CD spectroscopy is the fact that
the photoisomerization of the side groups almost has
no influence on the conformation of the helix. This result
The influence of the photoisomerization on the posi-
tion and the intensity of this band can be examined by

190 Mruk and Zentel
Macromolecules, Vol. 35, No. 1, 2002
caused by the slow evaporation of the solvent. This
change of concentration leads to a change of the dimen-
sion of helical pitch and by that to a shift of the selective
reflection band.
The dependence of the position of the selective reflec-
tion band on the conformation of the azo group implies
that the side groups must have an influence on the
height of the helical pitch in the cholesteric phase.
However, as the fraction of the chiral azo monomer in
the examined polymer is as low as 1.6 mol %, it can be
concluded that the pitch of the cholesteric phase is
mostly caused by the helix itself and only minor changes
are caused by the side groups.
Con clu sion
F igu r e 9. UV spectra of LC phase during the isomerization.
A new photoisomerizable chiral isocyanate monomer
branched in R-position was synthesized. Despite of its
steric hindrance this monomer could be used success-
fully for the formation of bipolymers and terpolymers
with HIC and DMHIC. Thereby a photoisomerizable
monomer with an isocyanate group attached to a
secondary carbon atom was incorporated into a poly-
isocyanate copolymer for the first time.
The polymerization was performed in THF and initi-
ated by a solution of potassium cyanide complexed with
18-crown-6. Due to this variation of the conditions of
the anionic polymerization relatively narrow molecular
weight distributions and rather low molecular weights
could be obtained. The polydispersity of the resulting
polymers is low enough to allow the formation of
lyotropic liquid crystalline phases without a priori
fractionation. As azobenzene groups containing poly-
isocyanates are not polymerizable by coordination po-
lymerization with titanium catalysts the modified an-
ionic polymerization seems to be the best way to prepare
azo-containing polyisocyanates with a narrow molecular
weight distribution that is known so far.
F igu r e 10. Shift of the selective reflection-band during
irradiation.
irradiation of the film at a wavelength of 365 nm to
carry out the isomerization from the trans to the cis
isomer and with visible light to reisomerize the azoben-
zene groups. The process of isomerization and reisomer-
ization was carried out four times. After each irradia-
tion, the film was allowed to reorientate for 20 min. The
resulting UV spectra are shown in Figure 9. For reasons
of lucidity, only the spectra of the first two irradiation
cycles are shown.
The UV spectra show that the photoisomerization
causes a shift of the selective reflection band to shorter
wavelengths. In addition to that an increase of intensity
after the irradiation can be observed. The shift of the
helical pitch and the increase of intensity can be
reversed by reisomerization. The absorption bands
possess the same intensity after the first and the second
reisomerization and a similar position of the maximum.
The difference between the spectrum before the first
irradiation and the spectra after the reisomerization
processes can be explained by the fact that in the case
of the first spectrum almost all azobenzene chromophors
exist as trans isomers whereas the reisomerization only
leads to the photochemical equilibrium where a signifi-
cant part of the chromophors still possess cis configu-
ration. The shift of the maximum of the selective
reflection bands during all irradiation processes is
shown in Figure 10.
The new polymers show extremely high specific
rotations at a very low content of chiral side groups.
Therefore the polymers act as an excellent example for
the transfer of chirality between the side groups and
the helix according to the “sergeants-and-soldiers”
experiment. The extremely high chiral induction of the
azo monomer (the highest known so far) leads to an
enormous stability of the preferred helical conformation.
Therefore the photoisomerization of the side chains does
hardly influence the conformation of the helix. A pho-
tochemically induced switching of the helix conforma-
tion is not possible for this reason.
It could be shown that these polymers are able to form
cholesteric lyotropic liquid crystalline phases with selec-
tive reflection in the visible range even when the molar
fraction of chiral side groups is as small as 1.6 mol %.
This result supports the thesis that the pitch of the
cholesteric phase is mostly a result of the chirality of
the helix and not of the chirality of the side groups.
Otherwise it would be hard to explain, how a helical
pitch as short as 500 nm could result from 1 mol % of
chiral side groups. The position and the intensity of this
band can be reversibly switched by isomerization of the
azobenzene chromophore.
The reversibility of the shift of the maximum can also
be observed in Figure 9. In addition to the shift caused
by irradiation it is obvious that an additional shift takes
place that leads to a slight increase of the wavelength
of the maximum for the cis and the trans case. This shift
is probably due to the change of concentration that is
Exp er im en ta l P a r t
Ma ter ia ls. Commercial chemicals were used without fur-
ther purification. Solvents were dried according to literature
processes.

Macromolecules, Vol. 35, No. 1, 2002
Chiral Polyisocyanates from an Azomonomer 191
In str u m en ta tion . Gel permeation chromatography (GPC)
at room temperature was used to determine molecular weights
and molecular weight distributions, Mw/Mn of polymer samples
with a J asco PU-980 pump, standard column pore sizes 103
and 104 Å, J asco RI-930 detector, J asco UV-975 detector, and
Viscothek T60A dual detector. For the determination of the
molecular weight, the results of the light scattering detection
were used. IR spectroscopy was done on a Nicolet Protege 460
IR spectrometer with a Specac Golden Gate single reflection
ATR-unit. NMR spectra were measured on a Bruker 400 MHz
NMR spectrometer. Mass spectroscopy was done on a Varian
MAT 311 A mass spectrometer. UV/vis spectroscopy was
performed on a Shimadzu UV-2102 PC. ORD spectroscopy and
determination of the specific rotations were done on a Perkin-
Elmer 241 MC polarimeter. CD spectroscopy was performed
on a J asco J -600 spectropolarimeter. For irradiation of the
polymer solutions before the measurement of the UV spectra
an Amco UV lamp with an integrated filter centered at 365
nm was used. The irradiations before the measurement of the
ORD and CD spectra of polymer solutions and before the
measurement of the UV spectra of the liquid crystalline films
were performed with a Schott Sun-box. Melting points of air
stable compounds were obtained by measurement with an
Electrothermal IA 9100. The melting point of isocyanate 6 was
obtained with a Perkin-Elmer DSC 7.
carbonyl-L-alaninol, 7.93 g (40 mmol) of 4-hydroxyazobenzene,
and 10.5 g (40 mmol) of triphenylphosphane were dissolved
in 100 mL of anhydrous diethyl ether under nitrogen atmo-
sphere. The solution was cooled to 0 °C. Then a solution of 8.5
mL (8.9 g/40 mmol) of diisopropyl azodicarboxylate (90%
purity) in 30 mL of anhydrous diethyl ether was added
dropwise over 45 min. After that the mixture was stirred for
further 2 h at 0 °C and for 20 h at room temperature. The
precipitated triphenylphosphane oxide was separated by filtra-
tion. A 50 mL aliquot of diethyl ether was added to the filtrate.
After that the organic phase was washed with 40 mL of 2 N
aqueous sodium hydroxide three times and with 40 mL of
water once and dried over magnesium sulfate. The solvent was
evaporated in a vacuum. The crude product was purified by
flash chromatography with a 4:1 mixture of petrol ether and
ethyl acetate as eluent. 7.70 g (21.7 mmol/54%) of an orange-
colored solid with a melting point of 123-124 °C was obtained.
1H NMR (CDCl3), δ [ppm]: 7.89-7.95 (m, 4H, arom), 7.43-
7.53 (m, 3H, arom), 7.01-7.04 (m, 2 H, arom), 4.79 (bs, 1H,
NH), 4.13 (m, 1H, CH), 4.03 (d, 2H, R-CH2-OR), 1.48 (s, 9H,
CCH3), 1.33 (d, 3H, CH3, 3J ) 7.1 Hz). 13C NMR (CDCl3), δ
[ppm]: 161.2 (1C, arom), 155.2 (1C, RHN-C(O)-OR), 152.7
(1C, arom), 147.2 (1C, arom), 130.4 (1C, arom), 129.0 (2C,
arom), 124.8 (2C, arom), 122.6 (2C, arom), 114.8 (2C, arom),
79.6 (1C, O-CMe3), 71.3 (1C, R-CH2-OR), 45.9 (1C, R-CH-
(NHR)-R), 28.4 (3C, C(CH3)3), 17.8 (1C, CH3). IR (ATR), υ˜
[cm-1]: 3375, 2985, 2940, 1680, 1600, 1510, 1450, 1235, 1165,
1065, 1025, 840. Mass, m/z (intensity): 44 (52%, CO2), 57
(100%, CMe3+), 77 (75%, C6H5+), 102 (C(O)OCMe3+ + H), 121
(50%, HO-C6H5-N2+), 198 (59%, HO-C6H4-NdN-C6H5+),
282 (10%, M+ - HOCMe3), 355 (16%, M+).
Syn th esis of th e m on om er 4-((2,S)-isocyan atopr opoxy)-
a zoben zen e. N-ter t-Bu tyloxyca r bon yl-L-a la n in e Meth yl
Ester . A 11.63 g (83.4 mmol) sample of L-alanine methyl ester
hydrochloride was suspended in 160 mL of chloroform. A
solution of 6.98 g (82.8 mmol) of sodium hydrogen carbonate
in 120 mL of water, 16.6 g of sodium chloride and a solution
of 18.36 g (84.0 mmol) of di-tert-butyl carbonate in 80 mL of
chloroform were added to the mixture. The mixture was
refluxed for 3 h. After cooling, the phases were separated and
the aqueous phase was extracted twice with 40 mL of
chloroform. The organic phases were unified and dried over
magnesium sulfate. After that the solvent was evaporated in
a vacuum. For purification, the crude product was distilled in
a vacuum. Thus, 16.33 g (80.4 mmol/97%) of a colorless liquid
4-((2,S)-Am in op r op oxy)a zoben zen e Hyd r och lor id e. A
4.0 g (11.3 mmol) sample of 4-((2,S)-(tert-butyloxycarbonyl-
amino)propoxy)azobenzene was dissolved in 70 mL of dichlo-
romethane. Then 8 mL of trifluoroacetic acid was added under
stirring. Thereby the former orange yellow color of the solution
turned into deep red. The reaction flask was closed with a
bubble counter. The formation of a gas could be observed at
the beginning of the reaction. The solution was stirred for 41/2
h. After that it was poured on 200 mL of 2 N hydrochloric acid.
The hydrochloride precipitated as an orange yellow solid. After
cooling to 4 °C the solid was filtered and recrystallized from a
1:1 mixture of ethanol and ethyl acetate. 2.01 g (6.9 mmol/
61%) of a yellow solid that decomposes over 190 °C was
obtained. 1H NMR (DMSO-d6), δ [ppm]: 8.42 (bs, 3H, RNH3+),
7.80-7.89 (m, 4H, arom), 7.46-7.56 (m, 3H, arom), 7.14-7.18
(m, 2H, arom), 4.11-4.25 (m, 2H, R-CH2-OR), 3.58 (m, 1H,
1
with a boiling point of 66 °C at 0.015 mbar were obtained. H
NMR (CDCl3), δ [ppm]: 5.13 (bs, 1H, NH), 4.23 (m, 1H, CH),
3
3.67 (s, 3H, OCH3), 1.37 (s, 9H, C(CH3)3), 1.31 (d, 3H, CH3, J
) 6.4 Hz). 13C NMR (CDCl3), δ [ppm]: 173.6 (1C, R-C(O)-
OR), 155.0 (1C, RHN-C(O)-OR), 84.9 (1C, O-CMe3), 52.0
(1C, R-CH(NHR)-R), 49.0 (1C, OCH3), 28.1 (3C, C(CH3)3),
18.3 (1C, CH3). IR (film), υ˜ [cm-1]: 3370, 2980, 1715, 1455.
N-ter t-Bu tyloxycar bon yl-L-alan in ol. A 10.6 g (52.2 mmol)
sample of N-tert-butyloxycarbonyl-L-alanine methyl ester was
dissolved in 85 mL of THF p.a. The flask was flooded with
nitrogen. After that 4.4 g (104 mmol) of anhydrous lithium
chloride, 4.1 g (108 mmol) of sodium boranate, and 150 mL of
anhydrous ethanol were added. The mixture was stirred under
nitrogen at room temperature. After about 15 min, the reaction
temperature increased under vehement foaming. The stirring
was continued for about 16 h. After that the solution was
acidified with 10% aqueous citric acid until a pH of 4 was
reached. The solvent was evaporated until about 50 mL of
liquid was left. Now 50 mL of water was added. The mixture
was extracted with 100 mL of dichloromethane four times. The
unified organic phases were dried over magnesium sulfate and
3
CH), 1.31 (d, 3H, CH3, J ) 7.1 Hz). 13C NMR (DMSO-d6), δ
[ppm]: 160.6 (1C, arom), 152.0 (1C, arom), 146.5 (1C, arom),
130.9 (1C, arom), 129.4 (2C, arom), 124.5 (2C, arom), 122.2
(2C, arom), 115.4 (2C, arom), 69.0 (1C, R-CH2-OR), 46.0 (1C,
R-CH(NH3+)-R), 15.0 (1C, CH3). IR (ATR), υ˜ [cm-1]: 2860,
1600, 1500, 1240, 1145, 1040, 840. Mass, m/z (intensity): 36
(66%, H35Cl), 38 (17%, H37Cl), 44 (100%, H2N-CH+-CH3), 58
(89%, CH3CH(NH2)CH2+), 77 (67%, C6H5+), 121 (23%, HO-
C6H4-N2+), 198 (15%, HO-C6H4-NdN-C6H5+), 212 (10%,
C6H5-NdN-C6H4-O-CH2 + H), 255 (21%, M+ - HCl).
+
4-((2,S)-Isocya n a top r op oxy)a zoben zen e. An 800 mg (2.7
mmol) sample of 4-((2,S)-aminopropoxy)azobenzene hydrochlo-
ride was suspended in 30 mL of anhydrous dioxane under
nitrogen. Then, 1.8 mL (3.0 g/15.2 mmol) of trichloromethyl
chloroformate was added. The mixture was heated to 70 °C.
After about 1 h, the former suspension had transformed into
a clear solution. The mixture was stirred for another 16 h at
50 °C. After that the volatile components were evaporated in
a vacuum. The crude product was fractionated in a Kugelrohr
(Bu¨chi GKR 51). Then 590 mg (2.1 mmol/78%) of an orange
red solid with a melting point of 71-72 °C (DSC) was obtained.
the solvent was evaporated in
a vacuum. The obtained
colorless oil was cooled to 4 °C and slowly began to form a
crystalline solid. The crude product was recrystallized from a
mixture of diethyl ether and hexane to yield 8.31 g (47.4 mmol/
1
91%) of a colorless solid with a melting point of 53-55 °C. H
NMR (CDCl3), δ [ppm]: 4.78 (bs, 1H, NH), 3.73 (m, 1H, CH),
2
3
3.60 (dd, 1H, CH2, J ) 10.7 Hz, J ) 4.1 Hz), 3.48 (dd, 1H,
CH2, 2J ) 10.7 Hz, 3J ) 6.1 Hz), 1.43 (s, 9H, C(CH3)3), 1.13 (d,
3
3H, CH3, J ) 6.6 Hz). 13C NMR (CDCl3), δ [ppm]: 156.3 (1C,
Specific rotation: [R]D ) +19.2 [0.1‚deg‚cm2‚g-1]. 1H NMR
20
RHN-C(O)-OR), 79.6 (1C, O-CMe3), 67.0 (1C, R-CH2OH),
48.6 (1C, R-CH(NHR)-R), 28.3 (3C, C(CH3)3), 17.3 (1C, CH3).
IR (ATR), υ˜[cm-1]: 3455, 3350, 2985, 1675, 1530, 1160, 1060,
1025.
4-((2,S)-(ter t-Bu t yloxyca r b on yla m in o)p r op oxy)a zo-
ben zen e. A 7.01 g (40 mmol) sample of N-tert-butyloxy-
(CDCl3), δ [ppm]: 7.90-7.97 (m, 4H, arom), 7.44-7.55 (m, 3H,
arom), 7.03-7.07 (m 2H, arom), 3.96-4.08 (m, 3H, R-CH2-
OR + R-CH(NCO)-R), 1.41 (d, 3H, CH3, 3J ) 6.1 Hz). 13C
NMR (CDCl3), δ [ppm]: 160.5 (1C, arom), 152.7 (1C, arom),
147.5 (1C, arom), 130.5 (1C, arom), 129.0 (2C, arom), 125.1
(1C, R-NdCdO), 124.8 (2C, arom), 122.6 (2C, arom), 114.9

192 Mruk and Zentel
Macromolecules, Vol. 35, No. 1, 2002
(2) Vogl, O.; J aycox, G. D. Polymer 1987, 28, 2179-2182.
(3) Pu, L. Acta Polym. 1997, 48, 116-141.
(4) Okamoto, Y.; Nakano, T. Chem. Rev. 1994, 94, 349-372.
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Ch., III. Macromolecules 1988, 21, 1839.
(2C, arom), 71.9 (1C, R-CH2-OR), 50.4 (1C, CH), 19.3 (1C,
CH3). IR (film), υ˜ [cm-1]: 2940, 2235, 1600, 1500, 1235, 1145,
1040, 845, 765, 685. Mass, m/z (intensity): 41 (71%, CH2d
CH-CH2+), 56 (86%, H2CdN+dCdO), 77 (79%, C6H5+), 84
(58%, Me2CdN+dCdO), 176 (100%, M+-C6H5-N2), 204 (32%,
M+-C6H5), 281 (96%, M+)
(7) Kollmar, Ch.; Hoffmann, R. J . Am. Chem. Soc. 1990, 112,
8230.
P olym er iza tion . HIC and DMHIC were distilled a few
days before use. Monomer 6 was used not later than 3 days
after purification. A varying amount of the solid monomer 6
was placed in a 10 mL flask. After that the flask was closed
with a septum, evacuated, and filled with dry nitrogen. The
liquid monomers (about 800 mg of HIC and in some cases 100-
150 mg of DMHIC) and 4 mL of anhydrous THF were added
through syringes. After the solid monomer was dissolved
completely, the mixture was cooled to -60 °C. The solution
was stirred at -60 °C for 15 min. After that 2.0 mL of the
initiating solution was added. About 5 min after the injection
of the initiator, the formation of a gel could be observed. Then,
45 min after the injection of the initiator, the polymerization
was stopped by injection of 2 mL cold methanol. The mixture
was poured into 80 mL of methanol. The liquid was separated
by filtration. The polymer was solved in 30-50 mL of THF
and precipitated after the addition of 100-150 mL of methanol.
For the initiating solution, 65.1 mg (1 mmol) of potassium
cyanide and 264.3 mg (1 mmol) of 18-crown-6 were put into a
Schlenk flask. The flask was closed with a septum, evacuated,
and filled with dry nitrogen. After that, 20 mL of anhydrous
THF was added through a syringe. The mixture was shaken
several times and allowed to stand for several hours. As the
potassium cyanide did not dissolve completely, only the
solution was used as initiator while the undissolved solid was
placed at the bottom of the flask.
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F or m a tion of Lyotr op ic P h a ses. For the formation of
lyotropic phases, 30-40 mg of the polymer and the corre-
sponding amount of 1,2-dichlorobenzene (40-80 mg) were
allowed to stand in an Eppendorf micro test tube for 24 h.
Afterward, the mixture was cropped out on a microscope slide.
A cover slide was put on the film and was fixed with tape.
The film was allowed to orientate for 90 min, after that the
UV measurements were performed.
(22) Aharoni, S. M. Macromolecules 1979, 12, 94-103.
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Sci. U.S.A. 1972, 69, 730-732.
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J . Org. Chem. 1987, 52, 1252-1255.
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41, 2070-2071.
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439.
Ack n ow led gm en t. Financial support by the DFG
is gratefully acknowledged.We also thank Prof. Wulff’s
group in Du¨sseldorf for the access to the CD spectrom-
eter.
(30) Maxein, G. Personal communication.
(31) Okamoto, Y.; Nagamura, Y.; Hatada, K.; Khatri, C.; Green,
M. M. Macromolecules 1992, 25, 5536-5538.
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Refer en ces a n d Notes
(1) Lehninger, A. L. Prinzipien der Biochemie; Walter de
Gruyter: Berlin, 1987; p 168 ff.
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