Stabilization of higher-order structure of poly(phenyleneethynylene)s by metathesis polymerization at the side chains

Article abstract of DOI:10.1016/j.polymer.2012.04.028

Novel poly(m-phenyleneethynlene-p-phenyleneethynylene)s bearing polymerizable diene or norbornene groups were synthesized by the Sonogashira-Hagihara coupling polymerization of the corresponding d-hydroxyphenylglycine-derived diiodo monomers with p-diethy

Full text of DOI:10.1016/j.polymer.2012.04.028

Polymer 53 (2012) 2559e2566
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Stabilization of higher-order structure of poly(phenyleneethynylene)s
by metathesis polymerization at the side chains
,
Akinobu Hashimoto a, Hiromitsu Sogawa a, Masashi Shiotsuki b, Fumio Sanda a
*
a Department Of Polymer Chemistry, Graduate School Of Engineering, Kyoto University, Katsura Campus, Nishikyo-Ku, Kyoto 615-8510, Japan
b Molecular Engineering Institute, Kinki University, Kayanomori, Iizuka, Fukuoka 820-8555, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel poly(m-phenyleneethynlene-p-phenyleneethynylene)s bearing polymerizable diene or norbor-
nene groups were synthesized by the SonogashiraeHagihara coupling polymerization of the corre-
sponding D-hydroxyphenylglycine-derived diiodo monomers with p-diethynylbenzene. These polymers
exhibited strong Cotton effects derived from a predominantly one-handed helical conformation in CHCl3
and tetrahydrofuran, but exhibited weak or no Cotton effects in N,N-dimethylformamide. The metathesis
polymerization of the diene and norbornene moieties was performed at the side chains of the polymers
under diluted conditions in the presence of a chain-transfer agent, if necessary. The reaction took place
intramolecularly, which was confirmed by size exclusion chromatography (SEC) measurements. The
polymers exhibited stronger Cotton effects even in polar media after the intramolecular crosslinking,
which indicated stabilization of the predominantly one-handed helical structures.
Received 13 February 2012
Received in revised form
13 April 2012
Accepted 16 April 2012
Available online 21 April 2012
Keywords:
Higher-order structure
Poly(phenyleneethynylene)s
Metathesis polymerization
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
This helical structure was more stable to heating than common
poly(phenyleneethynylene)s (i.e., the CD and UVevis spectroscopic
A helix is a typical regulated secondary structure of biopolymers
such as amylose, protein and DNA. Synthetic polymers also form
helical structures. A helical structure in artificial polymers was first
discovered in isotactic polypropylene synthesized using the
ZieglereNatta catalyst, which dates to the 1950s [1]. Since the
discovery of helical poly(triphenylmethyl methacrylate) synthe-
sized by asymmetric anionic polymerization [2], a wide variety of
synthetic helical polymers have been reported, including poly-
acetylenes [3], polyisocyanates [4], polyisocyanides [5], polysilanes
[6] and poly(phenyleneethynylene)s [7]. These polymers adopt
helical conformations due to non-flexible conjugated backbones
and/or steric repulsion between the side chains and garner signif-
icant attention because of their possible applications as chiral
sensors, asymmetric catalysts and optical resolution materials [8].
We recently synthesized novel poly(m-phenyleneethynlene-p-
phenyleneethynylene)s via the SonogashiraeHagihara coupling
signals exhibited almost no changes between 0 and 50 ꢀC in CHCl3)
[10]. The thermal stability was assumed to be brought about by
regulated intramolecular hydrogen bonding between the amide
groups at the ith and (i þ 6)th units on the side chains, as well as by
amphiphilicity and p-stacking between the phenylene main chains.
However, the helical structure was susceptible to polar solvents
such as water and methanol, presumably due to collapse of the
amphiphilic balance and intramolecular hydrogen bonding strands.
Stability in polar solvents is an important property with
respect to the application of D-hydroxyphenylglycine-based helical
poly(phenyleneethynylene)s to functional materials. Clipping or
crosslinking the side chains of the polymer is effective at stabilizing
the conformation.
Hydrocarbon staplingdthe ring-closing metathesis of helical
peptides bearing olefinic side chainsdprovides a useful strategy
for the experimental and therapeutic modulation of proteine
protein interactions in many signaling pathways [11]. Metathesis
reactions are also effective for the fixation of self-assembled
supramolecules [12]. For example, a helical poly(m-phenyleneethy
nylene) bearing cinnamate groups has been covalently stabilized by
[2 þ 2] photodimerization reactions at the side chains [13]. Using
this methodology, we herein report the synthesis of novel helical
poly(phenyleneethynlene)s bearing polymerizable groups at
the side chains and their subsequent polymerization to obtain
polymerization of D-hydroxyphenylglycine-derived diiodo mono-
mers with p-diethynylbenzene [9]. The obtained polymers formed
helices with a hydrophobic exterior (due to alkyl groups) and
a hydrophilic interior (from phenol groups) in nonpolar solvents.
* Corresponding author. Tel.: þ81 75 383 2591; fax: þ81 75 383 2592.
E-mail address: sanda.fumio.5n@kyoto-u.ac.jp (F. Sanda).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2012.04.028

2560
A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
polymers that exhibit high helix stability in both nonpolar and
polar solvents.
temperature. The solution was evaporated after being stirred
overnight. To the residue, CH2Cl2 was added and washed with 0.5 M
HCl, saturated aq. NaHCO3, H2O and brine. The organic layer was
dried over anhydrous MgSO4 and concentrated under reduced
pressure to afford a brown solid mass. The residue was purified by
silica-gel column chromatography eluted with CHCl3/AcOEt ¼ 19/
1 (v/v) and then hexane/AcOEt ¼ 19/1 (v/v), followed by recrys-
2. Experimental procedure
2.1. Measurements
1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on
a JEOL EX-400 or JEOL AL-400 spectrometer. IR spectra were
measured on a JASCO FT/IR-4100 spectrophotometer. Melting
points (mp) were measured on a Yanaco micro melting point
apparatus. Mass spectra were measured on a Thermo Fisher
tallization to obtain 1 as a yellowish solid. Mp 163e164 ꢀC, [
a] e
D
49.4ꢀ (c ¼ 0.1 g/dL, CHCl3). 1H NMR (400 MHz, CDCl3):
d 7.63 [s, 2H,
Ar], 5.82e5.43 [m, 5H, CONH, OCONH, OH, CH2]CH], 5.12e4.82 [m,
5H, C*H, CH2]CH], 4.05 [m, 1H, CH2CH], 2.26e2.07 [m, 4H, CH2],
1.42 [s, 9H, C(CH3)3] ppm. 13C NMR (100 MHz, CDCl3):
d 168.7, 155.0,
Scientific EXACTIVE mass spectrometer. Specific rotations ([
a
]D)
153.6, 138.0, 134.6, 133.8, 133.4, 118.4, 118.3, 82.5, 80.4, 56.7, 48.5,
38.5, 38.2, 28.3 ppm. IR (cmꢂ1, KBr): 3383, 3309, 2977, 2938, 1715,
1644, 1166. HR-ESI-MS (m/z): [M þ H]þ Calcd for C20H27O4N2I2:
613.0060, found 613.0044.
were measured on a JASCO DIP-1000 digital polarimeter. Number-
and weight-average molecular weights (Mn and Mw) of polymers
were determined by SEC (Shodex columns KF805 ꢁ 3) eluted with
THF and calibrated using polystyrene standards at 40 ꢀC. CD and
UVevis absorption spectra were recorded on a JASCO J-820
spectropolarimeter.
2.3.3. Synthesis of monomer 2
Monomer 2 was synthesized in a manner similar to that used
for 1, except 2,6-diallyl-4-methylaniline was used instead of 1,6-
heptadienyl-4-amine. Yield: 40% (yellowish solid). Mp
2.2. Materials
197e199 ꢀC, [
CDCl3): d 7.76 [s, 2H, Ar], 6.90 [s, 2H, Ar], 6.85 [br, 1H, OH],
a
]
D e 58.9ꢀ (c ¼ 0.1 g/dL, CHCl3). 1H NMR (400 MHz,
1,6-Heptadienyl-4-amine [14], 2,6-diallyl-4-methylaniline [15]
and N-(2-aminoethyl)-5-norbornene-endo,endo-2,3-dicarboximide
[16] were synthesized according to methods reported in the litera-
ture. N,N-Dimethylformamide (DMF), Et3N, tetrahydrofuran (THF)
and CHCl3 were distilled prior to use. All other reagents were
commercially obtained and used as received without purification.
5.82e5.73 [m, 4H, CONH, OCONH, CH2]CH], 5.10 [br, 1H, C*H],
5.01 and 4.81 [d, J ¼ 10.2 Hz and d, J ¼ 17.0 Hz, 4H, CH2]CH],
3.16e3.03 [m, 4H, CH2], 2.28 [s, 3H, CH3], 1.43 [s, 9H, C(CH3)3] ppm.
13C NMR (100 MHz, CDCl3):
d 168.2, 155.1, 153.8, 137.9, 137.8, 136.8,
136.4, 134.2, 129.8, 128.9, 115.7, 82.8, 80.6, 56.6, 36.3, 28.4,
21.1 ppm. IR (cmꢂ1, KBr): 3313, 2978, 2926, 1660, 1163. HR-ESI-MS
2.3. Monomer synthesis
(m/z): [M
689.0355.
þ
H]þ Calcd for C26H31O4N2I2: 689.0373, found
2.3.1. Synthesis of N-
diiodo- -phenylglycine
a
-tert-butoxycarbonyl-40-hydroxy-30,50-
D
2.3.4. Synthesis of monomer 3
This compound was synthesized using a modified literature
method [17]. Briefly, ICl (25.0 g, 154 mmol) was added dropwise to
a suspension of D-p-hydroxyphenylglycine (12.0 g, 71.9 mmol) in
Monomer 3 was synthesized in a manner similar to that used for 1
using N-(2-aminoethyl)-5-norbornene-endo,endo-2,3-dicarboximide
instead of 1,6-heptadienyl-4-amine. Yield: 24% (white solid). Mp:
AcOH (95 mL) under nitrogen. After the suspension was stirred at
room temperature for 72 h, the reaction mixture was poured into ice
water (1 L). The precipitated solid was collected by filtration and
washed with ice water (500 mL ꢁ 2). The filtrate was dried under
116e118 ꢀC.[
a
]D e 66ꢀ (c¼ 0.1g/dL, CHCl3).1HNMR(400MHz,CDCl3):
d
7.67 [s, 2H, Ar], 6.36e5.78 [m, 5H, CONH, OCONH, OH, CH]CH], 4.92
[1H, C*H], 3.54e3.20 [m, 8H, CONHCH2CH2N, CHCONCOCH, CH-CH]
CH-CH], 1.73 [d, J ¼ 9.04 Hz, 1H, bridge position], 1.54 [d, J ¼ 8.80 Hz,
1H, bridge position], 1.42 [s, 9H, C(CH3)3] ppm. 13C NMR (100 MHz,
reduced pressure to afford 40-hydroxy-30,50-diiodo-
D-phenylglycine
in 65% yield (19.6 g, 46.8 mmol). 1H NMR (400 MHz, DMSO-d6):
CDCl3): d 178.0, 169.5, 154.9, 153.6, 137.9, 134.4, 134.3, 82.7, 80.4, 56.6,
d
8.45 [br, 4H, COOH, NH2, OH], 7.75 [s, 2H, Ar], 4.24 [s,1H, C*H] ppm.
Asolutionofdi-tert-butyldicarbonate(DIBOC, TOKUYAMA, 6.64g,
52.3, 45.8, 44.9, 39.8, 37.2, 28.3 ppm. IR (cmꢂ1, KBr): 3311, 2978, 2941,
1693,1185,1161. HR-APCI-MS (m/z): [M þ H]þ Calcd for C24H28O6N3I2:
708.0067, found708.0058.
30.4 mmol) in 1,4-dioxane (50 mL) was added to a solution of 40-
hydroxy-30,50-diiodo-
D-phenylglycine (10.5 g, 25.1 mmol) and
Na2CO3 (3.30g, 31.1mmol)inH2O(50mL)atroom temperature. After
theresultingsolutionwasstirred overnight, thereaction mixturewas
concentrated under reduced pressure. AcOEt was added to the
residue, and the resulting mixturewas extracted with H2O. The water
layer was washed with AcOEt, acidified to pH ¼ 3 by the addition of
citric acid and extracted with AcOEt. The organic layer was washed
with brine, dried over anhydrous MgSO4 and concentrated under
2.4. Polymerization
2.4.1. SonogashiraeHagihara coupling polymerization [9]
Typical procedure: a solution of 1 (1.22 g, 1.99 mmol), p-dieth-
ynylbenzene (0.251 g, 1.99 mmol), PdCl2(PPh3)2 (25.7 mg,
0.0366 mmol), CuI (44.7 mg, 0.235 mmol) and PPh3 (44.7 mg,
0.170 mmol) in Et3N (8 mL) and DMF (12 mL) was stirred at 80 ꢀC for
24 h. The reaction mixture was poured into MeOH/acetone [5/1 (v/
v), 600 mL]. The precipitate was separated by filtration using
a membrane filter (ADVANTEC H100A047A) and then dried under
reduced pressure.
reduced pressure to afford N-
30,50-diiodo-
-phenylglycine (11.6 g, 22.3 mmol) in 89% yield. The
product was used in the next step without purification. 1H NMR
(400 MHz, CDCl3): 8.20 [1H, NH], 7.75 [s, 2H, Ar], 5.81 [br, 1H, OH],
a
-tert-butoxycarbonyl-40-hydroxy-
D
d
4.98 [d, J ¼ 4.8 Hz, 1H, C*H], 1.29 [s, 9H, C(CH3)3] ppm.
2.4.2. Spectroscopic data for polymers
2.3.2. Synthesis of monomer 1
1: 1H NMR (400 MHz, CDCl3):
d 7.89e7.33 [br, 2H, Ar], 6.08e5.43
1,6-Heptadienyl-4-amine (2.35 g, 21.1 mmol) and 4-(4,6-
dimethoxy-1,3,5-triazine-2-yl)-4-methylmorpholinium chloride
(TRIAZIMOCH, TOKUYAMA, 4.55 g, 19.4 mmol) were added to
[br, 5H, CONH, OCONH, OH, CH2]CH], 5.31e4.58 [br, 5H, C*H, CH2]
CH], 4.18e3.91 [br, 1H, CH2CH], 2.59e1.98 [br, 4H, CH2], 1.84e1.09
[br, 9H, C(CH3)3] ppm.
a solution of N-
a
-tert-butoxycarbonyl-40-hydroxy-30,50-diiodo-
D
-
2: 1H NMR (400 MHz, CDCl3):
d
7.82e7.40 [br, 2H, Ar], 6.98e6.47
phenylglycine (13.2 g, 25.4 mmol) in THF (50 mL) at room
[br, 3H, Ar, OH], 5.86e5.50 [br, 4H, CONH, OCONH, CH2]CH],

A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
2561
Scheme 1. Synthesis of monomers 1e3.
5.08e4.53 [br, 5H, C*H, CH2]CH], 3.22e2.86 [br, 4H, CH2],
2.37e2.10 [br, 3H, CH3], 1.84e1.11 [br, 9H, C(CH3)3] ppm.
a membrane filter (ADVANTEC H100A047A) and dried under
reduced pressure. The yield was quantitative. 1H NMR (400 MHz,
3: 1H NMR (400 MHz, CDCl3):
d
7.80e7.32 [br, 2H, Ar], 6.23e5.69
CDCl3): d 8.00e6.48 [br, 2H, Ar], 5.92e5.35 [br, 4H, Ar, CONH,
[br, 5H, CONH, OCONH, OH, CH]CH], 5.35e5.10 [br, 1H, C*H],
3.88e2.84 [br, 8H, CONHCH2CH2N, CHCONCOCH, CH-CH]CH-CH],
2.01e1.12 [br, 11H, bridge position, C(CH3)3] ppm.
OCONH, CH]CH], 5.18e4.82 [br, 1H, C*H], 4.09e3.77 [br, 1H,
CH2CH], 2.46e1.85 [br, 4H, CH2], 1.83e0.87 [br, 9H, C(CH3)3] ppm.
The ADMET reaction for the preparation of 2b was performed in
a manner analogous to that used for 1a in CHCl3 at 50 ꢀC for 24 h.
Yield: 80%. 1H NMR (400 MHz, CDCl3):
d7.88e7.39 [br, 2H, Ar],
2.5. Metathesis polymerization at the side chains
6.97e6.61 [br, 3H, Ar, OH], 5.90e5.47 [br, 4H, Ar, CONH, OCONH,
CH]CH], 5.04e4.54 [br, 1H, C*H], 3.24e2.95 [br, 4H, CH2],
2.46e2.05 [br, 3H, CH3], 2.03e0.92 [br, 9H, C(CH3)3] ppm.
2.5.1. Acyclic diene metathesis (ADMET) polymerization at the side
chains of 1a and 2a
A solution of 1a [26.0 mg, 0.0539 mmol (monomer unit)] and
2.5.2. Ring-opening metathesis polymerization (ROMP) at the side
chains of 3a
the second-generation Grubbs’ catalyst (1.3 mg, 1.5 mmol) in THF
(26 mL) was stirred at 45 ꢀC for 18 h. Excess ethyl vinyl ether was
added, and the solution was concentrated to a volume of approxi-
mately 2 mL. The residual solution was poured into MeOH (100 mL)
to precipitate the polymer. The precipitate was filtered using
A solution of 3a [116 mg, 0.200 mmol (monomer unit)], second-
generation Grubbs’ catalyst (11.1 mg, 13.1
mmol) and cis-1,4-
dichloro-2-butene (20 L, 0.19 mmol) as a chain-transfer agent in
m
Scheme 2. SonogashiraeHagihara coupling polymerization of 1e3 with p-diethynylbenzene.

2562
A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
Table 1
The polymers were soluble in CHCl3, THF, DMF and DMSO, but
insoluble in hexane and MeOH.
Polymerization of 1e3 with p-diethynylbenzene.a
Yieldb (%)
Mn
PDIc
c
Monomer
1
2
3
72
64
96
6300
6300
6700
2.4
2.4
2.0
3.3. Chiroptical properties
a
Conditions: [1e3]0 ¼ [p-diethynylbenzene]0 ¼ 0.1 M, [PdCl2(PPh3)2] ¼ 0.002 M,
[CuI] ¼ 0.012 M, [PPh3] ¼ 0.008 M in Et3N/DMF (2/3,v/v), 80 ꢀC, 24 h.
The CD and UVevis spectra of 1ae3a were measured in CHCl3,
THF and DMF at room temperature to obtain information on the
secondary structures. All of the polymers exhibited strong split
Cotton effects in CHCl3 and THF (Fig. 1). The CD and UVevis signals
allowed the assignment of a chirally ordered conjugated polymer
main chain because the lmax values of the polymers appeared at
longer wavelengths than those of the monomers (1e3:
lmax ¼ 224 nm) and p-diethynylbenzene (lmax ¼ 276 nm). The
inflection point from the first positive to second negative CD sign
agreed with the lmax from the UVevis absorption, which indicated
the CD split was caused by exciton coupling of the chromophores
[19]. The polymer solutions exhibited the same CD and UVevis
spectroscopic patterns after they were filtered using a PTFE
b
MeOH/acetone ¼ 5/1 (v/v)dinsoluble fraction.
c
Determined by SEC eluted with THF and calibrated with polystyrene standards.
THF (100 mL) was stirred at 50 ꢀC for 24 h [18]. The solution was
then concentrated to a volume of approximately 2 mL. The residue
was poured into MeOH (100 mL) to precipitate the polymer, which
was collected using a membrane filter (ADVANTEC H100A047A)
and dried under reduced pressure. Yield: 93%. 1H NMR (400 MHz,
CDCl3):
d 8.53e6.87 [br, 2H, Ar], 6.36e5.59 [br, 3H, CONH, OCONH,
OH], 5.51e5.00 [br, 3H, CH]CH, C*H], 4.41e2.90 [br, 8H,
CONHCH2CH2N, CHCONCOCH, CH-CH]CH], 2.58e0.86 [br,11H, CH-
CH2-CH, C(CH3)3] ppm.
membrane with a pore size of 0.45 mm [20], and patterns exhibited
no concentration dependence in the range from 0.02 to 0.05 mM.
Therefore, the CD signals probably originated predominantly from
right-handed helical conformations instead of from chirally
aggregated structures [21].
3. Results and discussion
3.1. Monomer synthesis
In DMF, 1a and 2a exhibited Cotton effects weaker than those in
CHCl3 and THF, and 3a did not exhibit Cotton effects. The lmaxs of
1ae3a red-shifted in DMF compared to those in CHCl3 and THF, but
the degree of red-shift is much smaller than the cases regarding
transformation of helical poly(phenyleneethynylene)s into random
ones [22]. This fact indicates that the small CD intensities of the
present polymers in DMF compared to those in CHCl3 and THF are
mainly caused by the decrease of helix sense predominance, and
partly caused by transformation of helix into random coil. The low
one-handed helicity in DMF is explainable by the higher polarity of
DMF relative to the other two solvents; the higher polarity of DMF
led to a collapse of regulated >NeHꢃꢃꢃO]C< hydrogen bonding
strands between the side chains, which play an important role in
helix formation [9,23]. This hypothesis was also supported by the
CD and UVevis spectroscopic measurements of 3a in a mixed
solvent of THF/H2O (see the Supplementary content, Fig. S1).
Monomers 1e3 were successfully synthesized by the conden-
sation of N-
nylglycine with the corresponding amines bearing diene and
norbornene moieties (Scheme 1). N-
-tert-bButoxycarbonyl-40-
hydroxy-30,50-diiodo-
-phenylglycine was synthesized by diioda-
tion of hydroxy- -phenylglycine using ICl followed by N-tert-
a D-phe-
-tert-butoxycarbonyl-40-hydroxy-30,50-diiodo-
a
D
D
butoxycarbonylation using DIBOC. All the monomers were
obtained as yellowish-white powders and were characterized by
1H, 13C NMR, IR and high-resolution mass spectrometry.
3.2. Polymerization
The SonogashiraeHagihara coupling polymerization of 1e3
with p-diethynylbenzene afforded the corresponding poly(m-phe-
nyleneethynlene-p-phenyleneethynlene)s 1ae3a that bear poly-
merizable groups at the side chains with Mn values that range from
6300 to 6700 (PDI ¼ 2.0e2.4) in 64e96% yields (Scheme 2, Table 1).
Specifically, the Kuhn’s dissymmetry factor (g value ¼
which ]/3298) of 3a decreased with increasing H2O content
ε ¼ [
and approached 0 when the H2O content reached 10% as shown in
Dε/ε, in
D
q
Fig. 1. CD and UVevis spectra of 1ae3a measured in CHCl3, THF and DMF at room temperature (c ¼ 0.03 mM).

A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
2563
spectroscopic measurements (see the Supplementary content,
Fig. S2). The ADMET conversion of 2a was determined to be 10%
(see the Supplementary content, Note S1). No significant changes in
Mn or PDI values (Table 2) were observed before and after ADMET,
which indicates the absence of intermolecular crosslinking and side
reactions, such as metathesis reactions, that involve the triple
bonds of the main chain. The conjugation through the triple bonds
and steric hindrance appear to effectively suppress such side
reactions.
Polymer 3a was subjected to ROMP using the second-generation
Grubbs’ catalyst in THF (see the Supplementary content, Note S2).
In this case, cis-1,4-dichloro-2-butene was added as a chain-
transfer agent because the degree of polymerization at the side
chains was expected to be no more than 2 based on the Mn of 3a
(see the Supplementary content, Note S3). The conversion of the
norbornene moieties was 60%, which was determined by 1H NMR
spectroscopy (see the Supplementary content, Fig. S3). Based on the
SEC data, the reaction did not take place intermolecularly, but
intramolecularly in a fashion similar to the previously discussed
ADMET of 1a and 2a.
3.5. Chiroptical properties of the polymers after metathesis
Fig. 3 depicts the CD and UVevis spectra of polymers 1be3b
obtained from metathesis at the side chains of 1ae3a. No
Fig. 2. Relationship between g values at 381 nm and H2O content [H2O/(H2O þ THF),
volume ratio] of
temperature.
a
solution of 3a and 3b in THF/H2O (c
¼
0.03 mM) at room
Table 2
Metathesis reactions of 1ae3a.a
Polymer Before metathesis [Ru]b Solvent Temp Time After metathesis
Fig. 2. Therefore, the helicity of 3a was highly susceptible to the
polarity of solvents.
(mM)
(ꢀC)
(h)
d
Mn
PDI
Yieldc Mn
PDId
(%)
1a
6300
6300
6700
2.4
2.4
2.0
0.06 THF
0.08 CHCl3 50
0.13 THF 50
45
18
24
24
Quant 5900 2.2
2a
80
93
6900 2.0
6600 1.8
3.4. Polymerization at the side chains
3ae
a
[M]0 ¼ 2 mM (monomer unit).
Grubbs’ second-generation catalyst.
MeOH-insoluble fraction.
Determined by SEC eluted with THF and calibrated with polystyrene standards.
cis-1,4-Dichloro-2-butene (2 mM) was added.
Polymers 1a and 2a were subjected to ADMET polymerization
using the second-generation Grubbs’ catalyst in THF and CHCl3,
respectively. The polymerizations were performed at high dilution
[2 mM (monomer unit)] to suppress intermolecular reactions. The
progress of ADMET (Scheme 3, top) was followed by 1H NMR
b
c
d
e
Scheme 3. ADMET and ROMP at the side chains of 1ae3a.

2564
A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
Fig. 3. CD and UVevis spectra of 1be3b measured in CHCl3, THF and DMF at room temperature (c ¼ 0.03 mM).
Fig. 4. Top and side views of possible conformers 1a and 1b (12-mers). Geometries were fully optimized using the MMFF94 method.

A. Hashimoto et al. / Polymer 53 (2012) 2559e2566
2565
significant difference was observed between the intensities of the
Cotton effects of 1a and 1b measured in THF and those of 2a and 2b
measured in CHCl3, whereas the intensities of 1b and 2b became
larger than those of 1a and 2a in DMF. These results suggested that
the helical structures were partly fixed by ADMET polymerization
and that the stability toward DMF was partly enhanced. These
results were also supported by the comparison of CD intensities of
1a and 1b in DMF/THF (19% and 27%) and those of 2a and 2b in
DMF/CHCl3 (21% and 44%).
The difference between 1a / 1b (þ8%) and 2a / 2b (þ23%)
was presumably caused by the difference in degrees of ring-closing
metathesis (RCM). As noted (see the Supplementary content, Note
S1), 1a was more likely to induce RCM than was 2a, which resul-
ted in better helix stability for 1b than for 2b.
ROMP at the side chains, the Mn values of the polymers were
unchanged, which indicated that the metathesis reactions occurred
intramolecularly, not intermolecularly. The synthesized polymers
exhibited more intense Cotton effects in DMF than did the poly-
mers prior to metathesis. Interestingly, 3b exhibited CD signals in
DMF comparable in intensity to those taken in THF. Moreover, the g
value of 3b was almost constant in a THF/H2O mixed solvent at any
composition. These results indicated that the predominantly one-
handed helical conformation of 3b was successfully fixed by
intramolecular ROMP. We are currently investigating the chiral
recognition properties of polymers with fixed conformations.
Acknowledgments
The trend of helix stabilization was apparent in the conversion
of 3a to 3b. In DMF, 3a exhibited no CD signal, whereas 3b exhibited
signals comparable in intensity to that in THF. Thus, we concluded
that the helix stability of 3b was greatly enhanced due to intra-
molecular crosslinking by ROMP. This conclusion was also
confirmed by the CD and UVevis spectra measured in mixed THF/
H2O solvent. The Kuhn’s dissymmetry factor (g value) of 3a
remarkably decreased when the H2O content was increased from
0 to 10% (Fig. 2). In contrast, the g value of 3b only slightly decreased
when the H2O content was increased. These results clearly indi-
cated that helix stability was enhanced by ROMP at the side chains.
This research was supported by a Grant-in-Aid for Science
Research (B) (22350049), Kyoto University Global COE Program
“International Center for Integrated Research and Advanced
Education in Materials Science” from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and the Kurata
Memorial Hitachi Science and Technology Foundation. We are
grateful to Prof. Kenneth B. Wagener at University of Florida for
helpful discussions.
Appendix A. Supplementary material
3.6. Conformation analysis
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.polymer.2012.04.028.
The conformations of 1ae3b were analyzed using the MMFF94
method [24]. Based on the first positive and second negative
exciton-coupled CD signals (Figs. 1 and 3), the polymers adopted
a predominantly right-handed helical conformation. Fig. 4 depicts
the top and side views of 12-mers of 1a and 1b (see the
Supplementary content, Figs S4 and S5 regarding 2ae3b). In each
case, the conformer formed a helically folded structure, and each
turn consisted of six monomer units. The interatomic distance
between the ith amide carbonyl oxygen and the (i þ 6)th amide
hydrogen (>C]OꢃꢃꢃHeN<) was approximately 2 Å (i.e., the amide
groups of the polymers formed i þ 6 / i intramolecular hydrogen
bonds). The carbamate groups also formed hydrogen bonds in
a similar fashion. The interatomic distances between the diagonal
oxygen atoms and the pitch of the helix were 20e22 Å and 4 Å,
respectively. The helical structures appeared to be stabilized by
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