Stereocontrol of optically active polymer by asymmetric anionic polymerization of 7-cyano-7-ethoxycarbonyl-1,4-benzoquinone methide

Article abstract of DOI:10.1021/ma100866n

Asymmetric anionic polymerization of a prochiral quinone methide monomer, 7-cyano-7-ethoxycarbonyl-1,4-benzoquinone methide (1), was examined using two chiral anionic initiators, lithium 4-isopropylphenoxide (ⁱPrPhOLi)/(- )-sparteine((-)-Sp) and ⁱPrPhOLi/(S)-(-)-2,2′- isopropylidenebis(4-phenyl-2-oxazoline) ((-)-PhBox) initiators, in a mixture solution of dichloromethane/toluene (30/70 in vol %), and optical activities of the resulting polymers and oligomers (1-mer and 2-mer) were investigated in detail. On asymmetric anionic polymerization of 1 with ⁱPrPhOLi/(-)- Sp initiator, stereoselectivity was quite low on both initiation and propagation reactions, while in the case of ⁱPrPhOLi/(-)-PhBox initiator, stereoselectivity was quite low on the initiation reaction, but the propagation reaction proceeded stereoselectively, resulting in an optically active polymer of 1 (poly(1)) with a large positive specific rotation.

Full text of DOI:10.1021/ma100866n

6962 Macromolecules 2010, 43, 6962–6967
DOI: 10.1021/ma100866n
Stereocontrol of Optically Active Polymer by Asymmetric Anionic
Polymerization of 7-Cyano-7-ethoxycarbonyl-1,4-benzoquinone Methide
Syoko Iizuka, Noboru Nakagaki, Takahiro Uno, Masataka Kubo, and Takahito Itoh*
Division of Chemistry for Materials, Graduate School of Engineering, Mie University,
1577 Kurima Machiya-cho, Tsu, Mie 514-807, Japan
Received April 19, 2010; Revised Manuscript Received July 24, 2010
ABSTRACT: Asymmetric anionic polymerization of a prochiral quinone methide monomer, 7-cyano-7-
ethoxycarbonyl-1,4-benzoquinone methide (1), was examined using two chiral anionic initiators, lithium 4-iso-
i
propylphenoxide (iPrPhOLi)/(-)-sparteine((-)-Sp) and PrPhOLi/(S)-(-)-2,20-isopropylidenebis(4-phenyl-2-
oxazoline) ((-)-PhBox) initiators, in a mixture solution of dichloromethane/toluene (30/70 in vol %), and optical
activities of the resulting polymers and oligomers (1-mer and 2-mer) were investigated in detail. On asymmetric
anionic polymerization of 1 with iPrPhOLi/(-)-Sp initiator, stereoselectivity was quite low on both initiation and
propagation reactions, while in the case of iPrPhOLi/(-)-PhBox initiator, stereoselectivity was quite low on the
initiation reaction, but the propagation reaction proceeded stereoselectively, resulting in an optically active polymer
of 1 (poly(1)) with a large positive specific rotation.
Introduction
atom and exocabonyl oxygen and stable aromatic ring formation
might take place to yield polymers with characteristic main-chain
Naturally occurring polymers such as proteins, polysaccharides,
nucleic acids, and so on have many optically active centers and
functional groups, and some of them show the characteristic func-
tionalities such as molecular recognition ability and catalytic acti-
vity because of controlled higher-order structure. Until now, an
enormous number of functional polymers have been synthesized
and have contributed to the development of our society. However,
synthetic polymers with the same functionalities as naturally
occurring polymers have not been reported yet. To construct the
functional polymers that will be as effective as naturally occurring
polymers, for example, synthesis of optically active polymers, is one
of the most interest topics and of importance. Asymmetric poly-
merization, classified in asymmetric synthesis polymerization (asy-
mmetric chirogenic polymerization), helix-sense-selective polymeri-
zation, and enantiomer-selective polymerization, is one of the
promising methods to introduce the chirality into the polymer
chain and to synthesize optically active polymers. There are a large
number of reports about the asymmetric polymerizations based on
vinyl monomers, diene monomers, cyclic olefin monomers, alde-
hyde monomers, isocyanate monomers, and so on.1
Unsubstituted 1,4-benzoquinone methide (QM) is an unstable
compound at room temperature, and it reacted spontaneously to
produce a dimer and/or oligomers.2 However, substitution of hy-
drogen atoms on an exomethylene carbon of the QM with electron-
accepting substituents and/or electron-donating ones reduces their
reactivities, leading to isolable monomers as crystals at room tem-
perature: for example, 7,7-dicyano-1,4-benzoquinone methide,3 7-
(alkoxycarbonyl)-7-cyano-1,4-benzoquinone methides,4 7,7-bis-
(alkoxycarbonyl)-1,4-benzoquinone methides,5 7,7-diphenyl-1,4-
benzoquinone methide,6 7-cyano-7-phenyl-1,4-benzoquinone me-
thide,7 4-(10,30-dithiolan-20-ylidene)-2,5-cyclohexanedien-1-one,8
and 2,6-dimethyl-7-phenyl-1,4-benzoquinone methide.9 We have
investigated their solution and solid-state polymerization beha-
viors.4,5,7-14 In the polymerizations of many QMs, carbon-oxygen
bond formation between the substituted exomethylene carbon
structures, poly(oxy-1,4-phenylene-substituted methylene)s. Here,
when the polymerizations of prochiral monomers, QMs with two
different substituents on the exomethylene carbon, take place, stereo-
centers could be generated in main chains through the poly-
merization process. If the stereocenters generated by the stereospecific
addition of the growing species to the exomethylene carbon of QM
would have either R or S configuration in excess by the asymmetric
synthesis polymerization, a novel optically active polyether, com-
posed of an asymmetric carbon having two different substituents
with a p-phenylene unit, should be formed (Scheme 1).
In the previous communication, to obtain an optically active
polymer having configurational chirality in the main chain, we
carried out asymmetric anionic polymerization of 7-cyano-7-ethoxy-
carbonyl-1,4-benzoquinone methide (1) with various chiral anionic
initiators and also investigated the effects of the chiral ligand and
solvent polarity on the specific rotations of the resulting polymers.15
In this work, we investigated asymmetric anionic polymeriza-
tions of 1 with two chiral anionic initiators, lithium 4-isopropylphe-
i
noxide (iPrPhOLi)/(-)-sparteine((-)-Sp) and PrPhOLi/(S)-(-)-
2,20-isopropylidenebis(4-phenyl-2-oxazoline) ((-)-PhBox) initia-
tors, and the stereostructures of oligomers such as 1-mer and
2-mer obtained by oligomerizations with both initiators in detail,
and then the stereocontrol in the formation of the optically active
polymer of 1 (poly(1)) was discussed.
Experimental Section
Measurements. Melting points were measured with a Yanaco
MP-S3 micro melting point apparatus. Infrared (IR) spectra
were recorded on a JASCO IR-700 spectrometer. 1H and13C NMR
spectra were measured with a JEOL JNM-EX270 (270 MHz for
1H) spectrometer in chloroform-d (CDCl3) with tetramethylsilane
as an internal standard. The specific rotation was obtained with a
JASCO P-1030 polarimeter. The molecular weights of polymers
were determined by gel permeation chromatography (GPC) on a
TOSOH CCPE chromatograph equipped with a JASCO RI-930
refractive index detector and two TOSOH TSKgel MultiporeHXL
-
M columns using tetrahydrofuran (THF) as an eluent at a flow rate
of 1.0 mL/min and polystyrene standards for calibration at room
*To whom correspondence should be addressed: e-mail itoh@chem.
mie-u.ac.jp; Tel þ81-59-231-9410; Fax þ81-59-231-9410.
pubs.acs.org/Macromolecules
Published on Web 08/04/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 17, 2010 6963
Scheme 1. Preparation of Optically Active Polymer by Polymerization
of Prochiral Monomer
Table 1. Asymmetric Anionic Polymerizationsa of 1 with the
iPrPhOLi/(-)-Sp and iPrPhOLi/(-)-PhBox Initiators at Various
[1]/[Initiator] Feed Ratios
d
[1]/
[initiator]
time/ yieldb/
Mwc/ [R]435
Mn (deg)
c
run
ligand
(-)-Sp
h
%
Mn
1
2
3
4
5
6
7
8
10
20
30
50
10
20
30
50
24
48
48
48
48
48
96
76
77
80
54
59
30
38
11
2400 1.12
1700 1.77
5700 2.15
6500 2.20
2100 1.28 þ90.4
1600 1.26 þ91.8
2600 1.31 þ58.4
1800 1.25 þ55.8
-5.9
-2.3
-4.4
-3.4
(-)-PhBox
temperature. The relationship of molecular weight with specific
rotation of polymer was examined by GPC on a Shodex System-21
equipped with a Shodex UV-41 detector and a JASCO OR-990
polarimetric detector using two columns, Shodex KF-803 and KF-
806 L, connected in series (eluent: THF; temperature: 40 ꢀC). Opti-
cal resolution of oligomers were performed on a JASCO PU-1580
chromatograph equipped with a UV (JASCO MD-910) and circu-
lar dichroism (JASCO CD-1595) detector using chiral column,
Daicel Chiralpak AD, at room temperature. A mixture solution of
hexane/ethanol (90/10 in vol %) was used as an eluent at a flow rate
of 0.5 mL/min.
120
a Conditions: [1] = 0.23 mol/L; solvent: dichloromethane/toluene =
30/70 (in vol %); temperature: -78 ꢀC. b Hexane-insoluble part.
c Determined by GPC as polystyrene standard. d In chloroform.
63.9 (CH2), 33.3 (CH), 23.9 (CH3), 21.0 (CH3), 13.6 (CH3). Anal.
Calcd for C22H23NO5: C, 69.28%; H, 6.08%; N, 3.67%; O,
20.97%. Found: C, 68.98%; H, 6.18%; N, 3.63%.
2-mer: 100.3 mg (17.2% yield); mp 90.5-92.0 ꢀC. IR (NaCl,
cm-1): νC-H 2970, νCN 2324, νCdO 1770, νC-O 1200. H NMR
1
Materials. 7-Cyano-7-ethoxycarbonyl-1,4-benzoquinone me-
thide (1) was synthesized according to the method reported pre-
viously.15 Toluene was purified in the usual manner and distilled
over sodium metal. THF and dichloromethane were distilled over
sodium metal and calcium hydride, respectively. 4-Isopropylphe-
nol (Tokyo Kasei Kogyo) was recrystallized from hexane. (-)-
Sparteine ((-)-Sp) (Tokyo Kasei Kogyo) was dried over calcium
hydride overnight and then distilled under reduced pressure. (S)-
(-)-2,20-Isopropylidenebis(4-phenyl-2-oxazoline) ((-)-PhBox)
(Aldrich) was used without further purification.
Asymmetric Anionic Polymerization. Asymmetric anionic
polymerization was carried out in a glass ampule equipped with a
three-way stopcock. A given amount of 1 was placed in the ampule,
dried under reduced pressure, and then filled with nitrogen. Into it
was added a mixture solution of dry dichloromethane/toluene (30/
70 in vol %) by a syringe, and the resulting solution was cooled to
-78 ꢀC. The polymerization was initiated by adding the initiator
solution, which was prepared by mixing lithium 4-isopropylphen-
oxide (iPrPhOLi) (1.0 equiv) and a chiral ligand (1.1 equiv) such as
(-)-Sp and (-)-PhBox in dry toluene at room temperature just
before use, and the reaction mixture was stirred at -78 ꢀC for a
given time. The polymerization was terminated by adding an excess
amount of dry acetic anhydride. The resulting solution was poured
into a large excess amount of hexane, and the deposited polymer
was collected by centrifugation and dried in vacuo.
(CDCl3, ppm): δ 7.81 (d, J = 8.91 Hz, 2H), 7.76 (d, J = 8.58 Hz,
2H), 7.23 (d, J = 8.90 Hz, 2H), 7.23 (d, J = 8.90 Hz, 2H), 7.15 (d,
J = 8.91 Hz, 2H), 6.99 (d, J = 8.58 Hz, 2H), 4.25 (q, J = 7.26 Hz,
4H), 2.87 (sept, J = 6.93 Hz, 1H), 2.33 (s, 3H), 1.29-1.17 (t, J =
7.26 Hz, 6H), 1.21 (d, J = 6.60 Hz, 6H). 13C NMR (CDCl3, ppm):
δ168.9 (CdO), 164.7 (CdO), 164.0 (CdO), 155.6 (Ar, quaternary),
152.3 (Ar, quaternary), 144.5 (Ar, quaternary), 130.0 (Ar,
quaternary), 128.8 (Ar, quaternary), 127.7 (Ar, CH), 127.5 (Ar,
CH), 127.4 (Ar, CH), 122.4 (Ar, CH), 122.3 (Ar, CH), 117.9 (Ar,
CH), 115.5 (CN), 115.0 (CN), 78.9 (æCÆ, quaternary), 78.8 (æCÆ,
quaternary), 64.3 (CH2), 63.9 (CH2), 33.3 (CH), 24.0 (CH3), 21.1
(CH3), 13.7 (CH3). Anal. Calcd for C33H32N2O8: C, 67.80%; H, 5.52%;
N, 4.79%; O, 21.89%. Found: C, 67.16%; H, 5.51%; N, 4.74%.
i
With PrPhOLi/(-)-PhBox Initiator. 1-mer: 55.6 mg (9.5%
yield). 2-mer: 48.5 mg (8.3% yield). Spectral data were the same
as those for the case with iPrPhOLi/(-)-Sp initiator.
Results and Discussion
i
Asymmetric Anionic Polymerization with PrPhOLi/(-)-
i
Sp and PrPhOLi/(-)-PhBox Initiators. In the previous
communication, we carried out the asymmetric anionic
polymerization of 1 with various chiral anionic initiators and
investigated the effects of the chiral ligand and solvent polarity
on the specific rotations of the resulting polymers. The polym-
erization of 1 using the iPrPhOLi/(-)-Sp initiator in a mixture
solution of dichloromethane/toluene (30/70 in vol %) at -78 ꢀC
afforded an optically active polymer of 1 (poly(1)) with a
maximum negative specific rotation ([R]435 = -5.9ꢀ), and the
Oligomerization and Isolation of 1-mer and 2-mer. Asymmetric
anionic oligomerizations of 1 with iPrPhOLi/(-)-Sp and iPrPhOLi/
(-)-PhBox initiators were carried out at the monomer/initiator
ratio of 2 in a mixture solution of dichloromethane/toluene (30/70 in
vol %) at -78 ꢀC for 12 h. The oligomerization was terminated by
adding an excess amount of dry acetic anhydride. The reaction mix-
ture was poured into 10 mL of chloroform, and the resulting solu-
tion was washed with water, 1 N hydrochloric acid, saturated
sodium bicarbonate aqueous solution, and saturated sodium chloride
aqueous solution and then dried over anhydrous magnesium sulfate.
The filtrate was concentrated and passed through a silica gel column
by using hexane/diethyl ether (2/1) as an eluent. The first elution
band and the second one were collected and placed under reduced
pressure to give 1-mer and 2-mer as white solids, respectively.
With iPrPhOLi/(-)-Sp Initiator. 1-mer: 114.8 mg (19.7% yield);
mp 61.5-62.5 ꢀC. IR (KBr, cm-1): νCH 2970, νCN 2310, νCdO 1766,
i
polymerization of 1 using the PrPhOLi/(-)-PhBox initiator
under the same condition afforded an optically active poly(1)
with a large positive specific rotation ([R]435 = þ90.4ꢀ).15
To investigate the relationship of the molecular weight with
the specific rotation of poly(1), asymmetric anionic polymer-
i
i
izations with the PrPhOLi/(-)-Sp and PrPhOLi/(-)-PhBox
initiators were carried out at various [monomer]/[initiator] feed
ratios. The results are summarized in Table 1.
Polymers were obtained as molecular weights in the range
1700-6500 at various [1]/[initiator] feed ratios for the case of
the iPrPhOLi/(-)-Sp initiator and in the range 1600-2600 for
thecaseoftheiPrPhOLi/(-)-PhBox initiator, respectively. In the
polymerization at a constant monomer concentration of
0.23 mol/L, specific rotation of the polymer obtained with iPr-
PhOLi/(-)-Sp initiator has a almost constant value of around
[R]435 =-4ꢀ regardless of the molecular weights, suggesting that
the stereocontrol is conducted in the same degree through whole
polymer. On the other hand, the specific rotations of polymers
obtained with the iPrPhOLi/(-)-PhBox initiator increased with
ν
C-O 1200. 1H NMR (CDCl3, ppm): δ 7.82 (d, J = 8.58 Hz, 2H),
7.21 (d, J = 8.91 Hz, 2H), 7.16 (d, J = 8.91 Hz, 2H), 7.00 (d, J =
8.58 Hz, 2H), 4.26 (q, J = 7.26 Hz, 2H), 2.87 (sept, J = 6.93 Hz,
1H), 2.32 (s, 3H), 1.22 (d, J = 6.93 Hz, 6H), 1.21 (t, J = 7.26 Hz,
3H). 13C NMR (CDCl3, ppm): δ 168.9 (CdO), 164.7 (CdO), 152.3
(Ar, quaternary), 152.0 (Ar, quaternary), 144.5 (Ar, quaternary),
130.9 (Ar, CH), 127.4 (Ar, quatrenary), 127.4 (Ar, CH), 122.2
(Ar, CH), 118.0 (Ar, CH), 115.5 (CN), 79.0 (æCÆ, quatrenary),

6964 Macromolecules, Vol. 43, No. 17, 2010
Iizuka et al.
Table 2. Asymmetric Anionic Oligomerizationsa of 1 with
iPrPhOLi/(-)-Sp and iPrPhOLi/(-)-PhBox Initiators
run
1
ligand
(-)-Sp
[1]/[initiator] time/h
yield/%
[R]435b (deg)
2
2
12
12
1-mer: 19.7
2-mer: 17.2
1-mer: 9.5
2-mer: 8.2
-2.5
-3.1
-1.5
þ20.2
2
(-)-PhBox
a Conditions: [1] = 0.23 mol/L, solvent: dichloromethane/toluene =
30/70 (in vol %); temperature: -78 ꢀC. b In chloroform.
Figure 1. GPC charts of (a) poly(1) obtained with the iPrPhOLi/(-)-Sp
initiator (Table 1, run 2) and (b) poly(1) obtained with the iPrPhOLi/(-)-
PhBox initiator (Table 1, run 6).
an increase in the [1]/[iPrPhOLi/(-)-PhBox] feed ratios, reached
a maximum value at the [1]/[iPrPhOLi/(-)-PhBox] ratio of 20
(Table 1, run no. 6), and then decreased. Similar behavior was
observed in an asymmetric anionic polymerization of 2,6-di-
methyl-7-phenyl-1,4-benzoquinone methide with fluorenyllithium
(FlLi)/(-)-Sp initiator.16 In the previous work, we proposed that
the aggregation state composed of the several propagating anions
were changed by the initiator concentration, and the optical
rotation of obtained polymers may be mainly governed by the
aggregation state. It is, therefore, considered that this behavior
Figure 2. HPLC chromatograms of optical resolution of the 1-mer
obtained with the iPrPhOLi/(-)-Sp initiator (column: Daicel Chiralpak
AD, eluent: hexane/ethanol = 90/10 (in vol %), flow rate: 0.5 mL/min).
The top chromatogram was measured by CD detector (254 nm) and
bottom by UV detector (254 nm).
i
observed in the case of the PrPhOLi/(-)-PhBox initiator is
ascribed to the initiator concentration.
Figures 1a,b show the GPC curves of poly(1)s obtained by
polymerization, stereoselectivity in an initiation reaction is defi-
nitely different from that in a propagation reaction. To obtain
further information on stereoselectivity in the 1-mer and 2-mer,
optical resolutions of them were conducted with high-pressure
liquid chromatography (HPLC) analysis on the chiral column
using hexane/ethanol (90/10 in vol %) as an eluent. The chro-
matogram of the optical resolution of the 1-mer obtained with
the iPrPhOLi/(-)-Sp initiator is shown in Figure 2, where top
and bottom chromatograms were monitored by CD and UV
detectors, respectively.
In Figure 2, the 1-mer with a negative CD sign was first
eluted and followed by the 1-mer with a positive CD one,
indicating that both components are enantiomers. From the
peak area obtained on the UV chromatogram in Figure 2, a
ratio of the first-eluted component (1-mer with a negative CD
sign)/the second-eluted one (1-mer with a positive CD sign) in
the enantiomers is determined to be 47/53 in mol %. The
absolute configuration of the chiral carbon in the 1-mer has not
been determined yet. Here, assuming that the first-eluted
component (1-mer with a negative CD sign) has a chiral carbon
of a R-configuration and the second-eluted one (1-mer with a
positive CD sign) does a chiral carbon of a S-configuration,
1-mer with the S-configurational chirality is formed in a slight
excess amount than 1-mer with the R-configurational chirality,
and the enantiomeric excess (ee) is calculated to be 6.0% ee(S).
This indicates that, as shown in Scheme 2, addition of a lithium
4-isopropylphenoxide anion coordinated with a (-)-Sp ligand
to the monomer 1 takes place in Re-face (front side) attack/Si-
face (back side) attack ratio of 47%/53%, leading to enantio-
mer in a R-configurational 1-mer/S-configurational 1-mer ratio
of 47/53 in mol %. However, as shown in a low ee value,
stereoselectivity in the initiation reaction is quite low.
asymmetric anionic polymerization with the iPrPhOLi/(-)-
i
Sp initiator (Table 1, run 2) and with the PrPhOLi/(-)-
PhBox initiator (run 6), respectively, monitored with ultra-
violet (UV) (bottom chromatogram) and polarimetric (PM)
detectors (top chromatogram).
The PM detector demonstrated a negative peak for the
poly(1) obtained with iPrPhOLi/(-)-Sp initiator and a positive
peak for the iPrPhOLi/(-)-PhBox initiator, and also both peak
patterns are quite similar to corresponding UV chromatograms,
which show unimodal GPC curves. These results indicate that
the optical rotations of poly(1) obtained with both initiator
systems do not depend upon the molecular weights, and the
configurations of all asymmetric carbons in the polymer chain
are controlled in almost the same degree. In other words, this
means that addition reactions of a propagating anion to the
monomer must take place with the same stereoselectivity in
every propagating step.
Stereoselectivity in the Initiation and Propagation Steps.
Okamoto and co-workers reported the asymmetric oligomer-
ization and chromatographic analyses of oligomers to obtain
information on the stereochemical mechanism of asymmetric
polymerization of triphenylmethyl methacrylate.17 To investi-
gate a degree of stereocontrol in the formation of an optically
active poly(1), we also carried out the asymmetric anionic oligo-
merization of 1 with the iPrPhOLi/(-)-Sp initiator and with the
iPrPhOLi/(-)-PhBox initiator at the [1]/[initiator] ratio of 2 for
12 h, and isolated 1-mer and 2-mer, which correspond to the
products formed at the initial stage in the polymerization, were
characterized. The results are summarized in Table 2.
i
The 1-mer and 2-mer obtained with the PrPhOLi/(-)-Sp
Next, to obtain information about the stereoselectivity in the
2-mer, optical resolution of the 2-mer was conducted with
HPLC analysis like the 1-mer. The chromatograms of the optical
initiator showed a small negative specific rotation similar to that
of corresponding polymer. On the other hand, the products
obtained with the iPrPhOLi/(-)-PhBox initiator showed a very
small negative specific rotation for the 1-mer and a large positive
one for 2-mer, indicating that, in the asymmetric anionic
i
resolution of the 2-mer obtained with the PrPhOLi/(-)-Sp
initiator are shown in Figure 3, where four diastereomers are
separated completely.

Article
From the peak area obtained on the UV chromatogram in
Figure 3, the ratio of the first-eluted component with a
positive CD sign/the second-eluted one with a negative
CD sign/the third-eluted one with a positive CD sign/the
fourth-eluted one with a negative CD sign is determined to be
Macromolecules, Vol. 43, No. 17, 2010 6965
25/22/25/28 in mol %. Each peak in Figure 3 was assigned as
follows. Addition of enantiomers in the 1-mer to monomer 1
might form four diastereomers in the 2-mer as shown in
Chart 1.
On the basis of the result of the optical resolution of the
1-mer, the total amount of the 2-mer with (R,R)- and (R,S)-
configurations derived from the R-configurational 1-mer should
be 47 mol %, and the total amount of the 2-mer with (S,R)- and
(S,S)-configurations derived from the S-configurational 1-mer
should be 53 mol %. In the UV chromatogram of the 2-mer in
Figure 3, the total amount of the first-eluted (25 mol %) and the
second-eluted (22 mol %) components is 47 mol % and also the
total amount of the second-eluted (22 mol %) and the third-
eluted (25 mol %) components is 47 mol %. Therefore, either a
combination of the first-eluted and the second-eluted compo-
nents or a combination of the second-eluted and the third-eluted
components might be assigned to the 2-mer with (R,R)- and (R,
S)-configurations. The CD spectra of each peak for the 2-mer
are shown in Figure 4, where the first-eluted component and the
fourth-eluted one are mirror images of each other and also the
second-eluted component and the third-eluted one are, indicat-
ing that each combination is enantiomers.
Therefore, a combination of the first-eluted and the second-
eluted components might be assigned to the 2-mer with (R,R)-
and (R,S)-configurations. On the assumption in optical reso-
lution of the 1-mer that the 1-mer with a R-configuration is
first eluted and followed by the 1-mer with a S-configuration,
the 2-mer with a (R,R)-configuration might be eluted faster
than that with a (R,S)-configuration. From these findings, the
Figure 3. HPLC chromatograms of optical resolution of the 2-mer
obtained with the iPrPhOLi/(-)-Sp initiator (column: Daicel Chiralpak
AD, eluent: hexane/ethanol = 90/10 (in vol %), flow rate: 0.5 mL/min).
The top chromatogram was measured by CD detector (254 nm) and
bottom by UV detector (254 nm).
Scheme 2. Stereoselectivity in the Addition Reaction of the
iPrPhOLi/(-)-Sp Initiator to Monomer 1 in the Initiation Step
Figure 4. CD spectra of the 2-mer obtained with the iPrPhOLi/(-)-Sp
initiator.
Chart 1. Diastereomers of the 2-mer

6966 Macromolecules, Vol. 43, No. 17, 2010
Iizuka et al.
Figure 5. HPLC chromatograms of optical resolution of the 1-mer ob-
tained with the iPrPhOLi/(-)-PhBox initiator (column: Daicel Chiralpak
AD, eluent: hexane/ethanol = 90/10 (in vol %), flow rate: 0.5 mL/min).
The top chromatogram was measured by CD detector (254 nm) and
bottom by UV detector (254 nm).
Figure 6. HPLC chromatograms of optical resolution of the 2-mer
obtained with the iPrPhOLi/(-)-PhBox initiator (column: Daicel Chiralpak
AD, eluent: hexane/ethanol = 90/10 (in vol %), flow rate: 0.5 mL/min). The
top chromatogram was measured by CD detector (254 nm) and bottom by
UV detector (254 nm).
Chart 2. Stereoselectivity on Initiation Reaction and Propagation
Reaction with the iPrPhOLi/(-)-Sp Initiator
Chart 3. Stereoselectivity on Initiation and Propagation Reaction with
the iPrPhOLi/(-)-PhBox Initiator
first-eluted, the second-eluted, the third-eluted, and the fourth-
eluted components in the chromatograms in Figure 3 could
be assigned reasonably in turn to the 2-mers with a (R,R)-
configuration, a (R,S)-one, a (S,R)-one, anda (S,S)-one, respec-
tively. On the basis of this assignment, the 2-mer with a (R,R)-
configuration (25 mol %) is formed in a slight excess amount
than that with a (R,S)-configuration (22 mol %) from a 1-mer
with a R-configuration, and the diastereomeric excess (de) is
calculated to be 6% de(R), and also 2-mer with a (S,S)-
configuration (28 mol %) is formed in a slight excess amount
than that with a (S,R)-configuration (25 mol %) from the 1-mer
with a S-configuration, and the de is calculated to be 6% de(S),
respectively. This indicates that setereoselectivity on the propa-
gation reaction from 1-mer to 2-mer by addition reaction of a
1-mer anion to a monomer is quite low. It is assumed that the
propagation reaction to 3-mer, 4-mer, 5-mer, and oligomer, and
polymer might proceed in same degree of stereoselectivity like as
the propagation reaction to the 2-mer. Stereoselectivity on the
initiation and propagation reactions in the polymerization of 1
with the iPrPhOLi/(-)-Sp initiator is summarized in Chart 2.
In the case of the iPrPhOLi/(-)-Sp initiator, the stereoselec-
tivity is quite low on both initiation reaction and propagation
reaction; that is, stereocenters generated in the resulting poly-
mer are not highly stereocontrolled.
initiator. Next, optical resolution of the 2-mer was conducted
with HPLC analysis, and the chromatograms are shown in
Figure 6.
Each component is assigned reasonably in turn to the 2-mer
with a (R,R)-configuration, a (R,S)-one, a (S,R)-one, and a (S,
S)-one as shown in Chart 1, and the ratio of the first-eluted
component/the second-eluted component/the third-eluted com-
ponent/the fourth-eluted component is determined to be 32/15/
35/18 in mol %. The 2-mers with a (R,R)-configuration (32 mol
%) and with a (S,R)-configuration (35 mol %) are produced in
almost twice amount higher than the corresponding ones with a
(R,S)-configuration (15 mol %) and with a (S,S)-configuration
(18 mol %), respectively, and also the diastereomeric excess (de)
is calculated to be 36% de(R) for the 1-mer with a R-configuration
and 32% de(R) is for the 1-mer with a S-configuration, respec-
tively. In the case with the iPrPhOLi/(-)-PhBox initiator, addi-
tion reaction of the 1-mer anion to a monomer might take place
stereoselectively regardless of the configurational chirality in a
1-mer anion to form a 2-mer with an excessive R-configura-
tionalchiralcarbon(32mol%þ 35 mol % = 67 mol %) in com-
parison with the S-configurational chiral carbon (15 mol % þ 18
mol % = 33 mol %). Probably, the propagation reaction to
3-mer, 4-mer, 5-mer, and oligomer, and polymer is considered to
proceed in same degree (R-configuration/S-configuration = 67/
33 in mol %) of stereoselectivity. Stereoselectivity on the initia-
tion and propagation reactions in the polymerization of 1 with
the iPrPhOLi/(-)-PhBox initiator is summarized in Chart 3.
i
In the case with the PrPhOLi/(-)-PhBox initiator, optical
resolution of isolated 1-mer was conducted with HPLC analysis
as the case with the iPrPhOLi/(-)-Sp initiator. The chromato-
grams of the optical resolution are shown in Figure 5, where the
1-mer with a R-configuration/the 1-mer with a S-configuration
ratio is determined to be 48/52 in mol %.
i
The 1-mer with the S-configurational chirality is formed in a
slight excess amount than that with the R-configurational chira-
lity, and the enantiomeric excess (ee) iscalculatedtobe4%ee(S).
Low ee value indicates that the stereoselectivity on the initia-
tion reaction is quite low as the case with the iPrPhOLi/(-)-Sp
In the case of the PrPhOLi/(-)-PhBox initiator, stereo-
selectivity is quite low on the initiation reaction, but high
stereoselectivity is observed in the propagation reaction. Such
high stereoselectivity on the propagation reaction results in the
poly(1) with a large positive specific rotation value (þ91.8ꢀ).

Article
Macromolecules, Vol. 43, No. 17, 2010 6967
Conclusions
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Asymmetric anionic polymerization of a prochiral quinone
methide monomer, 7-cyano-7-ethoxycarbonyl-1,4-benzoquinone
methide (1), was examined using two chiral anionic initiators,
iPrPhOLi/(-)-Sp and iPrPhOLi/(-)-PhBox initiators, in a mixture
solution of dichloromethane/toluene (30/70 in vol %), and optical
activity of the polymers obtained and oligomers (1-mer and 2-mer)
were investigated in detail. The iPrPhOLi/(-)-Sp initiator produced
the poly(1) with a small negative specific rotation value, and the
iPrPhOLi/(-)-PhBox initiator produced the poly(1) with a large
positive specific rotation value. From the stereostructures of the
1-mer and 2-mer obtained by the oligomerization of 1with both initi-
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iPrPhOLi/(-)-Sp initiator are quite low on both initiation and
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i
propagation reactions, while in the case of PrPhOLi/(-)-PhBox
2000, 33, 269–277.
initiator, stereoselectivity on the initiation reaction is quite low, but
the propagation reaction proceeded stereoselectively to form a
poly(1) with a large positive specific rotation; that is, a large positive
specific rotation value observed in the poly(1) is ascribed to the bias
in the absolute configuration of the chiral carbon in the polymer
main chain.
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Acknowledgment. This work was supported by Grant-in-Aid
for Scientific Research (No. 18350062 and No. 22550110) from
the Ministry of Education, Culture, Sports, Science and Technology,
Japan. The authors acknowledge Prof. Yoshio Okamoto, Nagoya
University, for GPC analysis using polarimetric detector and optical
resolution of oligomers.
molecules 2004, 37, 1251–1256.
(14) Kawaguchi, S.; Mitsuda, Y.; Uno, T.; Kubo, M.; Itoh, T. Kobunnshi
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