Synthesis and structure of poly(phenyl isocyanate)s bearing an optically active alkoxyl group

Article abstract of DOI:10.1002/1099-1395(200007)13:7<361::AID-POC255>3.0.CO;2-E

Novel optically active phenyl isocyanate derivatives (1-6) bearing an (R)-sec-butoxy, (S)-2-methylbutoxy or (S)-3,7-dimethyloctyloxy group at the meta or para position on the phenyl ring were prepared and polymerized with an anionic initiator in tetrahydrofuran (THF). The resulting polymers from 1, 2, 4 and 6 showed much greater specific rotation than that of the corresponding monomers and an intense circular dichroism (CD) band in the main-chain absorption region, indicating that these polymers have a predominantly one-handed helical conformation in solution. On the other hand, the polymers obtained from 3 and 5 showed a much smaller specific rotation than that of the above polymers at room temperature. The polymers from 2 and 5 showed a remarkable change in optical activity with change in temperature, and the specific rotation of the polymers changed from a positive to a negative value with decrease in temperature. The CD band of the polymers in the absorption region due to the main chain changed from a positive to a negative peak with a change in specific rotation. These results indicate that poly-2 and poly-5 undergo a thermally induced helix-helix transition in THF. The temperature for the helix-helix transition of poly-2 was independent of the degree of polymerization. Poly-2 exhibited a reversible helix-helix transition in chloroform and diethyl ether and also in THF, whereas in toluene and dichloromethane such a transition was not observed. Copyright

Full text of DOI:10.1002/1099-1395(200007)13:7<361::AID-POC255>3.0.CO;2-E

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 361–367
Synthesis and structure of poly(phenyl isocyanate)s
bearing an optically active alkoxyl group
Kyoko Hino, Katsuhiro Maeda and Yoshio Okamoto*
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan
Received 4 January; revised 22 February; accepted 23 February
ABSTRACT: Novel optically active phenyl isocyanate derivatives (1–6) bearing an (R)-sec-butoxy, (S)-2-
methylbutoxy or (S)-3,7-dimethyloctyloxy group at the meta or para position on the phenyl ring were prepared and
polymerized with an anionic initiator in tetrahydrofuran (THF). The resulting polymers from 1, 2, 4 and 6 showed
much greater specific rotation than that of the corresponding monomers and an intense circular dichroism (CD) band
in the main-chain absorption region, indicating that these polymers have a predominantly one-handed helical
conformation in solution. On the other hand, the polymers obtained from 3 and 5 showed a much smaller specific
rotation than that of the above polymers at room temperature. The polymers from 2 and 5 showed a remarkable
change in optical activity with change in temperature, and the specific rotation of the polymers changed from a
positive to a negative value with decrease in temperature. The CD band of the polymers in the absorption region due
to the main chain changed from a positive to a negative peak with a change in specific rotation. These results indicate
that poly-2 and poly-5 undergo a thermally induced helix–helix transition in THF. The temperature for the helix–helix
transition of poly-2 was independent of the degree of polymerization. Poly-2 exhibited a reversible helix–helix
transition in chloroform and diethyl ether and also in THF, whereas in toluene and dichloromethane such a transition
was not observed. Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: polyisocyanate; optically active polymer; aromatic isocyanate; helix; anionic polymerization; helix–
helix transition
INTRODUCTION
poly(phenyl isocyanate) derivatives is rigid enough to
have a helical conformation in solution as well as
poly(alkyl isocyanate)s.7 Especially, an optically active
poly(phenyl isocyanate) bearing an amide group, poly{3-
[((S)-(a-methylbenzyl)carbamoyl]phenyl isocyanate},
has an almost completely one-handed helical structure
even at room temperature and showed a chiral discrimi-
nation ability toward some racemates.8
Recently, we also reported that the poly(phenyl iso-
cyanate) bearing an optically active ester group, poly{3-
[(S)-sec-butoxycarbonyl]phenyl isocyanate} {poly[(S)-
3secBuOCPI]}, undergoes a reversible helix–helix tran-
sition in tetrahydrofuran (THF) with change in tempera-
ture. This thermally induced helix–helix transition was
specific for poly[(S)-3secBuOCPI] and was not observed
for analogous polymers bearing an optically active ester
group whose structure is slightly different from that of
poly[(S)-3secBuOCPI].9
Poly(alkyl isocyanate)s are known to have a helical
structure in solution and in the solid state.1 The helix in
solution is not rigid, but dynamic, with quickly moving
helix reversals in the main chain. Therefore, the
asymmetric synthesis of an optically active polymer
possessing a stable single-handed helical structure, which
has been observed for poly(triarylmethyl methacry-
late)s,2 polyisocyanide3 and polychloral,4 has not yet
been achieved. However, optically active polymers with
a prevailing one-handed helical structure can be obtained
by introducing a small amount of optically active
comonomer units into an achiral isocyanate sequence
through copolymerization5a–e or by initiating the poly-
merization with an optically active initiator5f because of
the long persistence length of the helix.
On the other hand, poly(aryl isocyanate)s, which have
an aryl group connected directly to the main chain, have
been postulated to exist as a random coil rather than a
helix in solution owing to the lack of stiffness in the main
chain.6 However, we found that the backbone of the
*Correspondence to: Y. Okamoto, Department of Applied Chemistry,
Graduate School of Engineering, Nagoya University, Furo-cho,
Chikusa-ku, Nagoya 464-8603, Japan.
E-mail: okamoto@apchem.nagoya-u.ac.jp
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 361–367

362
K. HINO, K. MAEDA AND Y. OKAMOTO
In the present study, novel optically active phenyl
distilled on to CaH2, followed by vacuum distillation on
to LiAlH4, and stored under nitrogen. This was distilled
again under high vacuum just before use for polymeriza-
tion. Piperidine (Wako) was dried over CaH2 and
distilled under reduced pressure. Lithium amide of
piperidine was prepared from piperidine in THF by
adding an equimolar amount of tert-butyllithium (1.7 M
in pentane) at room temperature.
Optically active phenyl isocyanate derivative 1 was
synthesized according to Scheme 1. Other optically
active phenyl isocyanate derivatives were also prepared
in an analogous method used for the synthesis of 1.
isocyanates bearing a chiral alkoxyl group at the meta or
para position, 3-[(R)-sec-butoxy]phenyl isocyanate (1),
3-[(S)-2-methylbutoxy]phenyl isocyanate (2), 3-[(S)-3,7-
dimethyloctyloxy]phenyl isocyanate (3), 4-[(R)-sec-
butoxy]phenyl isocyanate (4), 4-[(S)-2-methylbutoxy]-
phenyl isocyanate (5) and 4-[(S)-3,7-dimethyloctyloxy]-
phenyl isocyanate (6), were synthesized and polymerized
using an achiral anionic initiator. In order to clarify the
effect of the difference in the chiral groups of the side-
chain and the position of the asymmetric center on the
polymer main-chain conformation, chiroptical properties
of the obtained polymers and their temperature and
solvent dependences were investigated in detail.
3-[(R)-sec-Butoxy]benzoic acid. Treatment of (S)-sec-
butanol (13.9 g) and p-toluenesulfonyl chloride (77.8 g)
in dry pyridine at 0°C under a nitrogen atmosphere gave
(S)-sec-butyl p-toluenesulfonate (38.2 g). The William-
son ether synthesis using ethyl 3-hydroxybenzoate (27.4
g) and (S)-sec-butyl p-toluenesulfonate, followed by
purification with silica gel chromatography using diethyl
ether–hexane (1:10) as the eluent, provided ethyl 3-[(R)-
sec-butoxy]benzoate (20.4 g). This compound was
hydrolyzed with 2 M NaOH under reflux in ethanol to
give 3-[(R)-sec-butoxy]benzoic acid. Yield, 15.0 g
EXPERIMENTAL
Materials. (S)-(‡)-sec-Butyl alcohol (Aldrich), (S)-( )-
2-methyl-1-butanol (Kishida), ethyl 3-hydroxybenzoate
(TCI), ethyl 4-hydroxybenzoate (Aldrich), p-toluenesul-
fonyl chloride (TCI), ethyl chloroformate (Kishida) and
sodium azide (Kishida) were used as purchased. (S)-3,7-
Dimethyl-1-octanol was prepared from (S)-( )-citronel-
lol (Azmax) by hydrogenation with palladium on
activated carbon (Pd, 10%) in ethanol.
(41%). IR (neat, cm 1), 1691 (C
=O of carboxylic acid);
1H NMR (400 MHz, CDCl3), ꢀ 1.0 (t, 3H, CH3), 1.3 (d,
THF was dried over sodium benzophenone ketyl and
3H, CH3), 1.6–1.8 (m, 2H, CH2), 4.3–4.4 (m, 1H, CH),
Scheme 1. Synthesis of 1
Copyright 2000 John Wiley & Sons, Ltd.
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OPTICALLY ACTIVE POLY(PHENYL ISOCYANATE)S
363
7.1 (m, 1H, aromatic), 7.4 (t, 1H, aromatic), 7.6 (m, 1H,
aromatic), 7.7 (m, 1H, aromatic), 12.4 (s, 1H, COOH).
prepared from (S)-3,7-dimethyloctanol (12.1 g) accord-
ing to an analogous method used for 3-[(R)-sec-
butoxy]benzoic acid. Yield, 9.6 g (45%). IR (KBr, cm 1),
1
3-[(R)-sec-Butoxy]phenyl isocyanate (1). The modified
Curtius rearrangement was employed to convert 3-[(R)-
sec-butoxy]benzoic acid to the isocyanate 1.10 3-[(R)-
sec-Butoxy]benzoic acid (10.9 g) was dissolved in
acetone (200 ml) and the solution was cooled in an ice-
bath. To this solution were added Et3N (7.7 g) and then
ethyl chloroformate (9.2 g). After the reaction mixture
had been stirred for 1.5 h at 0°C, sodium azide (6.6 g) in
H2O (30 ml) was slowly added dropwise. The mixture
was stirred for 1.5 h at 0°C and poured into a mixture of
ice–water (300 ml) and toluene (200 ml). The organic
layer was separated, washed with ice-water and dried
over magnesium sulfate. After filtration, the filtrate was
heated under reflux for 2 h and then cooled to room
temperature. The solvent was evaporated in vacuo and
the residue was distilled under reduced pressure (b.p. 57–
1685 (C=
O of carboxylic acid); H NMR (400 MHz,
CDCl3), ꢀ 0.87 (d, 6H, 2CH3), 0.96 (d, 3H, CH3), 1.1–1.9
(m, 10H, 4CH2 and 2CH), 4.1 (m, 2H, OCH2), 7.2 (m,
1H, aromatic), 7.4 (t, 1H, aromatic), 7.6 (m, 1H,
aromatic), 7.7 (m, 1H, aromatic).
3-[(S)-3,7-Dimethyloctyloxy]phenylisocyanate(3). Com-
pound 3 was prepared from 3-[(S)-3,7-dimethyloctyl-
oxy]benzoic acid in the same manner as 1. Yield,
25
25
3.4 g (36%). [a]365
8.4°, [a]D
=C=
2.7° (c 1.3, THF).
IR (neat, cm 1), 2268 (N
O); H NMR (400 MHz,
1
CDCl3), ꢀ 0.87 (d, 6H, 2CH3), 0.93 (d, 3H, CH3), 1.1–1.9
(m, 10H, 4CH2 and 2CH), 3.9–4.0 (m, 2H, OCH2), 6.61
(t, 1H, aromatic), 6.66 (m, 1H, aromatic), 6.73 (m, 1H,
aromatic), 7.2 (t, 1H, aromatic). Analysis: calculated for
C17H25NO2: C, 74.14; H, 9.15; N, 5.09. Found: C, 73.99;
H, 9.39; N, 5.20%.
25
60°C/0.3 mmHg) to give 1 (7.8 g) in 73% yield, [a]365
95°, [a]D
25
33° (c 1.0, THF). The enantiomeric excess
of 1 was determined to be 80% by chromatographic
4-[(R)-sec-Butoxy]benzoic acid. This was prepared
analogously from (S)-sec-butanol (14.3 g) and ethyl 4-
enantioseparation on a chiral column (CHIRALCEL OD,
Daicel)11 using hexane–ethanol (99.8:0.2) as the eluent.
hydroxybenzoate (24.3 g). Yield, 10.5 g (28%). IR
1
1
IR (neat, cm 1), 2267 (N
=
C=
O); H NMR (400 MHz,
(KBr, cm 1), 1671 (C
=
O of carboxylic acid); H NMR
CDCl3), ꢀ 1.0 (t, 3H, CH3), 1.3 (d, 3H, CH3), 1.6–1.8
(m, 2H, CH2), 4.2–4.3 (m, 1H, CH), 6.6–6.7 (m, 2H,
aromatic), 6.7–6.8 (m, 1H, aromatic), 7.2 (t, 1H,
aromatic). Analysis: calculated for C11H13NO2: C,
69.09; H, 6.85; N, 7.32. Found: C, 69.09; H, 6.96; N,
7.56%.
(400 MHz, CDCl3), ꢀ 1.0 (t, 3H, CH3), 1.3 (d, 3H, CH3),
1.6–1.8 (m, 2H, CH2), 4.4 (m, 1H, CH), 6.9 (m, 2H,
aromatic), 8.0 (m, 2H, aromatic).
4-[(R)-sec-Butoxy]phenyl isocyanate (4). Compound 4
was prepared from 4-[(R)-sec-butoxy]benzoic acid in the
25
same manner as 1. Yield, 4.0 g (39%). [a]365
98°,
25
3-[(S)-2-Methylbutoxy]benzoic acid. This compound
was prepared from (S)-2-methyl-1-butanol (13.5 g)
according to an analogous method used for the synthesis
[a]D
(N
28° (c 1.0, THF). IR (neat, cm 1), 2275
O); H NMR (400 MHz, CDCl3), ꢀ 1.0 (t, 3H,
1
=
C=
CH3), 1.3 (d, 3H, CH3), 1.5–1.8 (m, 2H, CH2), 4.2 (m,
1H, CH), 6.8 (m, 2H, aromatic), 7.0 (m, 2H, aromatic).
Analysis: calculated for C11H13NO2: C, 69.09; H, 6.85;
N, 7.32. Found: C, 69.04; H, 6.99; N, 7.53%.
of 3-[(R)-sec-butoxy]benzoic acid. Yield, 9.9 g (31%). IR
1
(KBr, cm 1), 1685 (C
=
O of carboxylic acid); H NMR
(400 MHz, CDCl3), ꢀ 0.96 (t, 3H, CH3), 1.03 (d, 3H,
CH3), 1.2–1.3 (m, 1H, CH2), 1.5–1.6 (m, 1H, CH2), 1.8–
1.9 (m, 1H, CH), 3.8–3.9 (m, 2H, OCH2), 7.2 (m, 1H,
aromatic), 7.4 (t, 1H, aromatic), 7.6 (m, 1H, aromatic),
7.7 (m, 1H, aromatic).
4-[(S)-2-Methylbutoxy]benzoic acid. The acid was pre-
pared analogously from (S)-2-methyl-1-butanol (16.2 g)
and ethyl 4-hydroxybenzoate. Yield, 11.3 g (30%). IR
1
(KBr, cm 1), 1676 (C
=
O of carboxylic acid); H NMR
3-[(S)-2-Methylbutoxy]phenyl isocyanate (2). Com-
pound 2 was prepared from 3-[(S)-2-methylbutoxy]ben-
zoic acid in the same manner as used for the synthesis of
(400 MHz, CDCl3), ꢀ 0.96 (t, 3H, CH3), 1.03 (d, 3H,
CH3), 1.2–1.3 (m, 1H, CH2), 1.5–1.6 (m, 1H, CH2), 1.8–
1.9 (m, 1H, CH), 3.8–3.9 (m, 2H, OCH2), 6.9 (m, 2H,
aromatic), 8.1 (m, 2H, aromatic).
25
25
1. Yield, 5.6 g (58%). [a]365 ‡37°, [a]D ‡9.7° (c 1.0,
THF). IR (neat, cm 1), 2270 (N O); 1H NMR
=C=
(400 MHz, CDCl3), ꢀ 0.95 (t, 3H, CH3), 1.01 (d, 3H,
CH3), 1.2–1.3 (m, 1H, CH2), 1.5–1.6 (m, 1H, CH2), 1.8–
1.9 (m, 1H, CH), 3.7–3.8 (m, 2H, OCH2), 6.6 (t, 1H,
aromatic), 6.65 (m, 1H, aromatic), 6.7 (m, 1H, aromatic),
7.2 (t, 1H, aromatic). Analysis: calculated for
C12H15NO2: C, 70.22; H, 7.37; N, 6.82. Found: C,
70.15; H, 7.55; N, 6.98%.
4-[(S)-2-Methylbutoxy]phenyl isocyanate (5). Com-
pound 5 was prepared from 4-[(S)-2-methylbutoxy]ben-
zoic acid in the same manner as used for the synthesis of
25
25
1. Yield, 4.6 g (42%). [a]365 ‡39°, [a]D ‡13° (c 1.1,
THF). IR (neat, cm 1), 2276 (N O); 1H NMR
=C=
(400 MHz, CDCl3), ꢀ 0.94 (t, 3H, CH3), 1.0 (d, 3H,
CH3), 1.2–1.3 (m, 1H, CH2), 1.5–1.6 (m, 1H, CH2), 1.8–
1.9 (m, 1H, CH), 3.7–3.8 (m, 2H, OCH2), 6.8 (m, 2H,
aromatic), 7.0 (m, 2H, aromatic). Analysis: calculated for
3-[(S)-3,7-Dimethyloctyloxy]benzoic acid. The acid was
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 361–367

364
K. HINO, K. MAEDA AND Y. OKAMOTO
Table 1. Polymerization of optically active phenyl isocyanates (1±6) bearing an alkoxyl group with Li piperidide in THF at 98°Ca
25
365
25
D
c
c
d
d
Run
Monomer
Yield (%)b
Mn (Â104)
Mw/Mn
‰ꢁŠ
‰ꢁŠ
1
2
3
4
5
6
1
2
3
4
5
6
68
70
100
64
86
2.8
3.4
5.3
5.8
3.7
4.3
1.2
1.1
1.3
1.1
1.4
1.2
3129
‡1331
‡260
‡2079
20
648
‡283
‡55
‡424
2
93
‡542
‡113
a
Conditions: monomer 0.5 g, [monomer]/[initiator] = 50, THF 5 ml, time 4 h.
b MeOH-insoluble part.
c
Determined by SEC with a light-scattering detector.
d In THF.
C12H15NO2: C, 70.22; H, 7.37; N, 6.82. Found: C, 70.09;
H, 7.56; N, 7.05%.
calculated for C17H25NO2: C, 74.14; H, 9.15; N, 5.09.
Found: C, 73.91; H, 9.29; N, 5.36%.
Polymerization. Polymerization was carried out in a glass
ampule under a dry nitrogen atmosphere in THF at
98°C. The lithium amide of piperidine was used as an
initiator. The monomer and THF were placed in a glass
ampule and cooled to 98°C. The polymerization
reaction was initiated by adding the initiator solution
with a syringe. After 4 h, the reaction was terminated by
adding a 10-fold excess of HCl in methanol to the
initiator, then the polymer was precipitated in a large
amount of methanol. The polymer was collected by
centrifugation and dried in vacuo at room temperature.
4-[(S)-3,7-Dimethyloctyloxy]benzoic acid. This com-
pound was prepared from (S)-3,7-dimethyloctanol (20.0
g) and ethyl 4-hydroxybenzoate according to an analo-
gous method described above. Yield, 11.0 g (31%). IR
1
(KBr, cm 1), 1664 (C
=
O of carboxylic acid); H NMR
(400 MHz, CDCl3), ꢀ 0.87 (d, 6H, 2CH3), 0.95 (d, 3H,
CH3), 1.1–1.9 (m, 10H, 4CH2 and 2CH), 4.0–4.1 (m, 2H,
OCH2), 6.9 (m, 2H, aromatic), 8.0 (m, 2H, aromatic).
4-[(S)-3,7-Dimethyloctyloxy]phenyl isocyanate (6).
Compound 6 was prepared from 4-[(S)-3,7-dimethyloc-
tyloxy]benzoic acid in the same manner as 1. Yield, 4.5 g
Measurement. Optical rotation was measured on a Jasco
DIP-181 polarimeter. CD spectra were measured with a
Jasco J-720 spectrometer. 1H NMR spectra were taken on
25
25
(41%). [a]365
11° [a]D
3.1° (c 1.2, THF). IR
(neat, cm 1), 2276 (N
=C
=
O); 1H NMR (400 MHz,
1
CDCl3), ꢀ 0.87 (d, 6H, 2CH3), 0.93 (d, 3H, CH3), 1.1–1.8
(m, 10H, 4CH2 and 2CH), 3.9–4.0 (m, 2H, OCH2), 6.8
(m, 2H, aromatic), 7.0 (m, 2H, aromatic). Analysis:
a Varian Gemini-2000 (400 MHz for H) spectrometer
with tetramethylsilane (TMS) as the internal standard in
Figure 2. Temperature dependence of speci®c rotation of
poly-1 (&), poly-2 (*), poly-3 (~), poly-4 (&), poly-5 (*)
and poly-6 (~) in THF. The concentration of the polymers
Figure 1. CD spectra of poly(phenyl isocyanate)s bearing an
alkoxyl group in THF at ambient temperature (ca 23±25°C)
1
was 1.0 g dl
Copyright 2000 John Wiley & Sons, Ltd.
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OPTICALLY ACTIVE POLY(PHENYL ISOCYANATE)S
365
CDCl3 at ambient temperature or 60°C. IR spectra were
recorded on a Jasco FT/IR-620 spectrometer. The
molecular weight of the polymer was determined by
SEC measurement on a Shodex System-21 GPC system
equipped with a Shodex RI-71S detector and a Wyatt
Technology DAWN DSP-F multi-angle light-scattering
detector using THF as an eluent at 40°C. Two SEC
columns, Shodex KF-803 and KF-806L, were connected
in series. The determination of the enantiomeric excess
(%e.e.) of the chiral monomers was performed on a Jasco
PU-980 liquid chromatograph equipped with Jasco 970-
UV and Jasco OR-990 polarimetric detectors using a
chiral column (CHIRALCEL OD, Daicel)11 using
hexane–ethanol as the eluent.
RESULTS AND DISCUSSION
Figure 3. CD spectra of poly-2 (1.1 mg ml 1) at ambient
temperature (ca 23±25°C) (a) and 45°C (b) in THF
Table 1 shows the results of the polymerization of 1–6
with the lithium amide of piperidine as an initiator in
THF at 98°C. Among the polymers obtained with an
optically active group at the meta position of the aromatic
ring, poly-1 and poly-2 showed a much greater specific
rotation than that of the corresponding monomer, and the
then quickly returned back to the original positive value
when the sample solution was restored to 25°C. The CD
spectra of poly-2 in THF at 25°C and at 45°C are
shown in Fig. 3. At 45°C, the polymer showed an
intense negative peak in the absorption region due to the
main chain, and at 25°C a positive peak, and this CD
spectral change was accompanied by only minor changes
in the UV spectra. These results indicate that poly-2
undergoes a reversible helix–helix transition with change
in temperature. The specific rotation of poly-5 also
changed reversibly from a small negative to a positive
value with decrease in temperature, and the CD spectral
pattern was reversed along with the change in the specific
rotation. Poly-5 also appears to undergo a reversible
helix–helix transition in THF, although the change in the
optical activity of poly-5 was not as significant as that in
poly-2.
25
specific rotation of poly-1 ([a]D
648°) is the largest
one in the optically active polyisocyanates so far
reported, to the best of our knowledge. The CD spectra
of poly-1–6 (runs 1–6 in Table 1) in THF at room
temperature are shown in Fig. 1. Poly-1 and poly-2
showed a very intense CD band in the region of the main-
chain absorption (210–310 nm), indicating that these
polymers have a predominantly one-handed helical
conformation in solution. Poly-3 also showed a CD band
in the main-chain region, but its intensity was much
lower than those of poly-1 and poly-2. For the polymers
bearing an optically active group at the meta position on
the aromatic ring, the optical rotation decreased as the
asymmetric center on the side chain moves away from the
polymer main chain. On the other hand, the polymers
with an optically active group at the para position, poly-4
and poly-6, showed a much greater specific rotation and
CD band compared with the corresponding monomers,
whereas those of poly-5 were very small. The optical
activity of the para-substituted polymers may follow the
odd–even rule.
Figure 2 shows the temperature dependence of the
specific rotation of the polymers in THF. The absolute
values of the specific rotation of poly-1, poly-3, poly-4,
and poly-6 increased with decrease in temperature. This
may be because the persistence length of the helical
structure of the polymer chain increases as the tempera-
ture decreases, as already reported for poly(alkyl
isocyanate)s and other poly(phenyl isocyanate)s.5,7ab
On the other hand, the specific rotations of poly-2 and
poly-5 exhibited an unusual temperature dependence.
The specific rotation of poly-2 changed from a positive to
a large negative value on decreasing the temperature, and
As reported in previous papers,9 the temperature-
induced helix–helix transition of poly(phenyl isocya-
nate)s bearing an optically active ester group occurred
only in poly[(S)-3secBuOCPI], which has an asymmetric
center at the g-position from the phenyl group. For the
poly(phenyl isocyanate)s bearing an optically active
alkoxyl group, only poly-2 and poly-5, whose structure
and position of the asymmetric center are equivalent to
those of poly[(S)-3secBuOCPI], exhibited a reversible
helix–helix transition. These results indicate that the
important factor for the helix–helix transition is the
existence of the chiral sec-butyl group at the g-position
from the phenyl group.
In order to investigate the effect of molecular weight
on the main-chain conformation of poly-2, a series of
poly-2 with various molecular weights were prepared. In
Fig. 4, the specific rotations of the polymers at 25°C in
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 361–367

366
K. HINO, K. MAEDA AND Y. OKAMOTO
The specific rotations of poly-2 of DP = 160 and 2200
were measured at various temperatures and the results are
shown in Fig. 5. The inversion of optical activity took
place at almost the same temperature regardless of the
DP, indicating that the helix–helix transition of poly-2
proceeds independently of the molecular weight. Specific
rotation and CD measurements were also performed at a
lower concentration (specific rotation, 0.2 g dl 1; CD,
0.03 mg ml 1) in THF at various temperatures. The
changes in the specific rotation and CD spectra of poly-2
(DP = 160) at a low concentration were nearly the same
as those in Figs. 2 and 3, respectively. These results
exclude the possibility of the aggregation of the polymer
chains for the reason of the above inversion of optical
activity, although it has been reported that an analogous
change in the optical activity of a polypeptide can be
ascribed to an aggregation of polymer chains.12
To examine the solvent effect on the helix–helix
transition of poly-2, the temperature dependence of the
specific rotation was investigated in various solvents,
such as toluene, dichloromethane and diethyl ether (Fig.
6). The specific rotation in toluene changed slightly
without crossing zero at low temperature. In dichlor-
omethane, the specific rotation increased monotonically
as the temperature decreased from 35 to 45°C. On the
other hand, the change in the specific rotation in diethyl
ether was similar in pattern to that in THF, and the
specific rotation crossed zero at a lower temperature than
in THF. In chloroform, an inversion of the sign for the
optical activity was also observed, although a small
positive optical activity was observed only above 40°C.
These results indicate that the conformational change of
poly-2 with change in temperature depends significantly
on the solvents and clear inversion of the helices occurs
in some solvents such as THF and diethyl ether.
Figure 4. Plots of speci®c rotation versus degree of
polymerization of poly-2 in THF
THF are plotted versus the degree of polymerization
(DP). The specific rotation of the polymers increased
with increase in DP, up to DP = 700, and reached a
25
plateau value ([a]365 ꢀ ‡2100°). Green et al. reported
an analogous relationship between the molecular weights
and [a]300 for the optically active poly(alkyl isocyanate)
poly[(R)-1-deutero-1-hexyl isocyanate], with chirality
arising from the difference in deuterium and hydrogen,
in which [a]300 increased up to DP ꢀ 500 and then
maintained a constant value.5b These results suggest that
poly-2 may have a long persistence length of the helical
structure, which is comparable to that of poly[(R)-1-
deutero-1-hexyl isocyanate].
Figure 6. Temperature dependence of speci®c rotation of
poly-2 in toluene (~), diethyl ether (&), THF (*), CHCl3 (~)
and CH2Cl2 (*)
Figure 5. Temperature dependence of speci®c rotation of
poly-2 in THF [(*) DP = 160 and (&) DP = 2200] (c = 1.0 g
1
)
dl
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 361–367

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367
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Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 361–367