A novel type of optically active helical polymers: Synthesis and characterization of poly(α,β-unsaturated ketone)

Article abstract of DOI:10.1002/pola.23915

New αβ-unsaturated ketone monomers, menthyl vinyl ketone (MVK), and 1-menthylbut-3-en-2-one (MBEK) were synthesized. The monomers were polymerized using butyllithium as an initiator. The polymer derived from MVK (poly-MVK) had a tremendous specific optical rotation α] ₃₈₅²⁵, which was as 32 times large as that of its monomer MVK. Poly-MVK was confirmed to keep a prevailing helicity of backbone in solution by means of comparing the specific optical rotation and the CD spectra with that of MVK and the model compound such as ethyl menthyl ketone (EMK) and n-hexyl menthyl ketone (n-HMK). This conclusion was also confirmed by the fact that the specific optical rotation and the CD signal intensity of poly-MBEK were not enough large dueto backbone flexibility caused by the effective isolation of the main chain from the bulky menthyl. The excess value of onehanded helicity of poly-MVK decreased with the increase of polymerization temperature.

Full text of DOI:10.1002/pola.23915

A Novel Type of Optically Active Helical Polymers: Synthesis and
Characterization of Poly(a,b-unsaturated ketone)
SHI WU, NIANFA YANG, LIWEN YANG, JING CAO, JI LIU
Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan, Hunan 411105, People’s Republic of China
Received 13 September 2009; accepted 23 December 2009
DOI: 10.1002/pola.23915
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: New a,b-unsaturated ketone monomers, menthyl
vinyl ketone (MVK), and 1-menthylbut-3-en-2-one (MBEK) were
synthesized. The monomers were polymerized using butyllithium
as an initiator. The polymer derived from MVK (poly-MVK) had a
tremendous specific optical rotation [a]23655, which was as 32 times
large as that of its monomer MVK. Poly-MVK was confirmed to
keep a prevailing helicity of backbone in solution by means of
comparing the specific optical rotation and the CD spectra with
that of MVK and the model compound such as ethyl menthyl ke-
tone (EMK) and n-hexyl menthyl ketone (n-HMK). This conclusion
was also confirmed by the fact that the specific optical rotation
and the CD signal intensity of poly-MBEK were not enough large
due to backbone flexibility caused by the effective isolation of the
main chain from the bulky menthyl. The excess value of one-
handed helicity of poly-MVK decreased with the increase of poly-
C
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merization temperature. 2010 Wiley Periodicals, Inc. J Polym
Sci Part A: Polym Chem 48: 1441–1448, 2010
KEYWORDS: anionic polymerization; a,b-unsaturated ketone;
chiral; helical polymers; synthesis
INTRODUCTION Helical structure is the most fundamental
structures in naturally occurring polymers and plays an im-
portant role in biological activities.1–2 Polymers with single
handed helical sense exhibit potential functions, such as chiral
recognition toward racemic compound, liquid crystal forma-
tion, and chirally catalytic activity,3–4 and have attracted great
attention in the past few years as the single handed helical
polymer poly(triphenylmethyl methacrylate) (TrMA) was used
as a chiral stationary phase for high-performance liquid chro-
matography (HPLC).3–7 At present, a large amount of polymers
which keep a single handed helical conformation in solution
have been prepared from aldehydes,8–9 isocyanates,10–13 iso-
cyanides,14–17 styrenes,18–24 acrylamides,25–29 acetylenes,30–33
N-Propargylphosphonamidates,34 and so on. More recently,
Deng et al.35 reported optically active helical poly(N-propargy-
lureas) and our group have prepared successfully optically
active helical polyether.36–37 However, to our knowledge, there
was little information about the single handed helical polymer
derived from a,b-unsaturated ketone so far. In this work, a,b-
unsaturated ketone menthyl [(1R,2S,5R)-2-isopropyl-5-meth-
ylcyclohexyl] vinyl ketone (MVK) was synthesized and then
polymerized and a novel type of optically active helical poly-
mer poly(a,b-unsaturated ketone) was obtained (Scheme 1).
from Alfar Aesar and n-BuLi, vinylmagnesium bromide and
(ꢀ)-sparteine (Sp) were from Acros. Other chemicals were
commercial analytic reagent. All of chemicals were used as
received without further purification excepting that THF and
toluene were refluxed over benzophenone/Na under argon
atmosphere and were distilled before use. The synthesized
monomers were purified several times until the purity was
not less than 99% (checked by NMR) before use.
Measurements
NMR spectra were recorded on a BRUKER ARX400 MHz
spectrometer using tetramethylsilane (TMS) as internal
standard in CDCl3.
Elemental analyses were carried out with an Elementar Vario
EL instrument.
The number-average molecular weights (Mn) were measured
on a WATERS1515 gel permeation chromatography (GPC)
instrument with a set of HT3, HT4, and HT5. The l-styragel
columns used CHCl3 as the eluent (1.0 mL/min) at 38 ꢁC.
The calibration curve was obtained with linear polystyrene
as standards.
EXPERIMENTAL
Optical rotations (OR) were measured using a PerkinElmer
341 LC polarimeter.
Materials
(ꢀ)-Menthol ([a]2D5 ¼ ꢀ49.7ꢁ in EtOH) was purchased from
Circular dichroism (CD) spectra were measured on Jasco
J-810 CD instrument using CHCl3 as solvent.
Nantong Menthol Factory Company, China, methacrolein was
Additional Supporting Information may be found in the online version of this article. Correspondence to: N. Yang (E-mail: nfyang@xtu.edu.cn)
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Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 48, 1441–1448 (2010) 2010 Wiley Periodicals, Inc.
NOVEL OPTICALLY ACTIVE HELICAL POLYMERS, WU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
solvent was removed in vacuo to afford crude menthyl vinyl
ketone (MVK) (3.37 g, yield 87%). The crude menthyl vinyl
ketone was purified by column chromatography (silica gel,
30% CH2Cl2/hexane). [a]23655 ¼ þ26.9ꢁ (c ¼ 0.010, toluene).
1H NMR (400 MHz, CDCl3, d, ppm): 0.74–1.74 (m, 18 H,),
2.69–2.74 (m, 1 H, ACHCO), 5.76 (d, J ¼ 10.0 1 H, CH2¼¼),
6.27 (d, J ¼ 17.6 1 H, CH2¼¼), 6.40–6.47 (m, 1 H, ACH¼¼). 13C
NMR (100 MHz, CDCl3, d, ppm): 16.2, 21.4, 22.3, 24.0, 28.9,
32.5, 34.6, 38.0, 44.0, 51.4, 127.8 (CH2¼¼), 135.7 (¼¼CHA),
204.4 (C¼¼O).
SCHEME 1 The polymerization of MVK.
The UV-vis spectra were obtained from a PerkinElmer
Lambda 25 spectrometer using CHCl3 as solvent.
Anal. Calcd. for C13H22O: C 80.35, H 11.41. Found: C 80.22, H
11.43.
Synthesis of Menthyl Vinyl Ketone
Menthyl chloride was prepared from (ꢀ)-menthol according
to the method reported by Smith38 (see Supporting informa-
tion S1 for characterization for menthyl chloride) and then
converted to the 0.85 mol/L menthylmagnesium chloride so-
lution in THF by means used by Desmond.39 The concentra-
tion of menthylmagnesium chloride solution was mensurated
by titration.40
Synthesis of 1-Menthylbut-3-en-2-one
To the mixture of CuI (1.9 g, 10 mmol), CH2Cl2(28 mL, 382
mmol), and THF (15 mL), was added the solution of men-
thylmagnesium chloride (0.85 mol/L, 150 mL, 127 mmol) at
0
ꢁC with stirring under N2 atmosphere. Then, the mixture
ꢁ
was stirred at room temperature (about 25 C) for 12 h after
the addition. The reaction mixture was poured into 10% hy-
drochloric acid with stirring and then extracted with diethyl
ether (3 ꢂ 50 mL). The combined ethereal extracts were
washed with 5% Na2S2O3 (3 ꢂ 50 mL), then washed with
water (3 ꢂ 20 mL) and dried over Na2SO4. The solvent was
removed by distillation and the residue was distilled
in vacuo to get menthyl chloromethane (5.4 g, 137 ꢁC/2.9
kPa, yield: 23%). Menthylacetic acid was prepared from
menthyl chloromethane by the method used in preparation
of menthylformic acid. The yield of menthylacetic acid was
60%. To the mixture of AlLiH4 (2.85 g, 75 mmol) and THF
(100 mL), was added the THF solution (50 mL) of menthyl-
acetic acid (6.0 g, 30 mmol) at 0 ꢁC under N2 atmosphere
with stirring. The mixture was refluxed for 10 h with stirring
and then cooled to room temperature. The reaction mixture
was addeꢁd dropwise into 10% hydrochloric acid with stir-
ring at 0 C, then was extracted with ethyl acetate (6 ꢂ 100
mL). The combined ethyl acetate layers were washed with
water (3 ꢂ 50 mL), dried over Na2SO4. The solvent was
removed in vacuo to obtain 2-menthyl ethanol (5.3 g, yield
96%). The solution of 2-menthyl ethanol (3.6 g, 20 mmol) in
CH2Cl2 (40 mL) was added dropwise to the solution of pyri-
dinium chlorochromate (PCC, 8.6 g, 40 mmol) in CH2Cl2
(60 mL) with stirring. The mixture was stirred for 1.5 h at
room temperature after the addition. Then the bulk of sol-
vent was removed. The PCC was precipitated by adding a
large amount of diethyl ether and isolated by filtration. The
filtrate was evaporated in vacuo and the residue was purified
by column chromatography (silica gel, 30% CH2Cl2/hexane)
to afford the menthyl acetaldehyde (2.6 g, yield 61%). The
solution of menthyl acetaldehyde (1.72 g, 9.5 mmol) in THF
(15 mL) was added to the solution of vinylmagnesium bro-
mide (0.7 mol/L in THF, 14 mL) at 0 ꢁC. The mixture was
stirred for 4 h. Then 10% hydrochloric acid was added to
terminate the reaction. The mixture was extracted with ether
(3 ꢂ 50 mL). The combined ethereal extracts were washed
with brine and dried over Na2SO4. The solvent was evapo-
rated in vacuo and the residue was purified by column
Carbon dioxide gas was bubbled slowly through the solution
of menthylmagnesium chloride with stirring at ꢀ80 ꢁC for
15 h. The reaction mixture was poured into 10% hydrochlo-
ric acid with stirring and then was extracted with diethyl
ether for three times. The combined ethereal extracts were
extracted with 10% NaOH solution for three times (3 ꢂ 50
mL). The combined basic extracts were acidified to pH ¼ 2
with 10% hydrochloric acid to result precipitate. The precipi-
ꢁ
tate was filtrated and dried at 40 C in vacuo to obtain men-
thylformic acid. Yield: 40%, based on menthyl chloride.
25
[a]
¼ ꢀ50.9ꢁ (c ¼ 0.010, CHCl3). m.p. ¼ 69.7–70.1 ꢁC.
365
(lit.39 m.p. 60–63 C)
ꢁ
1H NMR (400 MHz, CDCl3, d, ppm): 0.80 (3H, d, J ¼ 6.8),
0.90 (3H, d, J ¼ 6.4), 0.91 (3H, d, J ¼ 6.8), 0.9–1.1 (2H, m),
1.21 (1H, q, J ¼ 12.2), 1.35 (1H, m, ACHMe2), 1.50 (1H, J ¼
3.0, J ¼ 11.6), 1.66–1.77 (3H, overlapping ms), 1.90–1.94(1H,
m), 2.32 (dt, 1H, J ¼ 11.7, J ¼ 3.21, ACHClA). 13C NMR (100
MHz, CDCl3, d, ppm): 16.0 (CH3), 21.3 (CH3), 22.3 (CH3),
23.8 (CH2), 29.3 (CH), 32.0 (CH), 34.5 (CH2), 38.8 (CH2),
44.2 (CH), 47.7 (CH), and 183.0 (CO2H).
Into 10 mL of thionyl chloride, was added menthylformic
acid (10 g, 54 mmol). The mixture was refluxed for 6 h, then
the excess thionyl chloride was removed by distillation and
the residue was ꢁdistilled in vacuo to get menthylformyl chlo-
ride (9.3 g, 163 C/2.9 kPa). Yield: 85%.
Ethylene gas was bubbled through the mixture of menthyl-
formyl chloride (6.1 g, 30 mmol), AlCl3 (4.0 g, 30 mmol) and
CH2Cl2 (100 mL) with stirring at ꢀ25 ꢁC for 6 h and then
the mixture was dealt with 10% hydrochloric acid carefully.
The organic layer was separated, washed with water, and
dried over Na2SO4. The solvent was removed ꢁin vacuo and
the residue (4.6 g, 20 mmol) was stirred at 25 C for 29 h in
CH2Cl2 (48 mL) in the present of triethylamine (6.8 mL, 48
mmol). Then the mixture was poured into 10% hydrochloric
acid with stirring. The organic layer was separated, washed
with 10% hydrochloric acid to remove the triethylamine,
washed with water to pH ¼ 7 and dried over Na2SO4. The
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ARTICLE
chromatography (silica gel, 60% CH2Cl2/hexane) to get 1-
menthylbut-3-en-2-ol (1.68 g, yield 85%).
1H NMR (400 MHz, CDCl3, d, ppm): 0.75–1.71 (m, 29 H,),
2.37–2.48 (m, 3 H, ACH2ACOACHA). 13C NMR (100 MHz,
CDCl3, d, ppm): 14.0, 16.0, 21.4, 22.4, 22.5, 23.4, 23.8, 28.8,
28.9, 31.6, 32.5, 34.6, 38.7, 42.1, 44.0, 54.4, 215.2 (C¼¼O).
To the solution of Dess-Martin periodinane (DMP) (10 g, 24
mmol) in CH2Cl2 (100 mL), was added a solution of the ear-
lier obtained 1-menthylbut-3-en-2-ol (4.2 g, 20 mmol) solu-
tion in CH2Cl2 (20 mL) under N2 atmosphere with stirring.
10% Na2S2O3 was added after the mixture was stirred 6 h at
Anal. Calcd for C17H32O: C 80.95, H 12.69. Found: C 80.70, H
12.58.
ꢁ
Polymerization (Typical Procedure)
25 C. The mixture was extracted with CH2Cl2 (3 ꢂ 50 mL).
The polymerization was performed in a thermostated bath
under N2 atmosphere with stirring. The polymerization was
terminated by the addition of some drops of methanol and
the polymer was precipitated by adding a large amount of
methanol and isolated by filtration. The precipitate was
The combined organic extracts were washed with water and
dried over Na2SO4. The solvent was evaporated in vacuo to
give crude 1-menthylbut-3-en-2-one (MBEK). The crude
product was purified by column chromatography (silica gel,
30% CH2Cl2/hexane) to afford the pure MBEK. (1.34 g, yield
ꢁ
25
365
¼ ꢀ83.8ꢁ (c
washed with methanol and dried in vacuo at 40 C to a con-
24% based on menthyl chloromethane). [a]
¼ 0.010, toluene).
stant weight to obtain the polymer overall yields. The poly-
mer was extracted by refluxing in THF for 15 min. After the
insoluble polymer was removed, the THF in the extraction
solution was evaporated to obtain THF-soluble part of the
polymer.
1H NMR (400 MHz, CDCl3, d, ppm): 0.65–1.86 (m, 19 H,
menthyl), 2.23–2.25 (m, 1 H, ACH2A), 2.74–2.78 (m, 1 H,
CH2A), 5.79 (d, J ¼ 10.4, 1 H, CH2¼¼), 6.20 (d, J ¼ 17.6, 1 H,
CH2¼¼), 6.33–6.40 (m, 1 H, ACH¼¼). 13C NMR (100 MHz,
CDCl3, d, ppm): 15.3, 21.6, 22.5, 24.4, 27.1, 32.6, 35.2, 36.2,
42.3, 44.3, 47.4, 127.6 (CH2¼¼), 137.0 (ACH¼¼), 201.4 (C¼¼O).
RESULTS AND DISCUSSION
About the Configuration and the Purity of MVK, EMK, n-
HMK, and MBEK
Anal. Calcd. for C14H24O: C 80.71, H 11.61. Found: C 80.65, H
11.54.
The structures of MVK, EMK, n-HMK, and MBEK are shown
in Figure 1. The MVK, EMK, and n-HMK were prepared from
menthylformic acid, which was prepared by the carboxyla-
tion of the Grignard reagent derived from menthyl chloride
with CO2. In the carboxylation of the Grignard reagent, 1-
position carbon of menthyl may epimerize to yield menthyl-
formic acid and neomenthylformic acid. By comparing the
physical constants with that of menthylformic acid reported
by D. Cunningham et al.,39 the carboxylation product
obtained in our experiment is menthylformic acid. D. K. Dill-
ner et al.41 reported the NMR datum of menthylformic acid
and neomenthylformic acid. The NMR data of our carboxyla-
tion product are completely consistent with that of menthyl-
formic acid reported by D. K. Dillner et al. (see Supporting
information S2). There is no any signal at 2.90 ppm in the
1H NMR spectrum of our carboxylation product, which is the
characteristic signal of neomenthylformic acid.41 These indi-
cates that the carboxylation product is pure menthylformic
Synthesis of Ethyl Menthyl Ketone
Into a 50 mL round-bottom flask, were added MVK (0.97 g,
5 mmol), ammonium formate (0.75 g, 15 mmol), 10% Pd-C
catalyst (0.1 g) and methanol/THF (2:1, 30 mL). The mixture
was refluxed with stirring for 4 h and then was filtrated af-
ter being cooled to room temperature. The filtrate was
evaporated in vacuo and the residue was purified by column
chromatography (silica gel, 30% CH2Cl2/hexane) to afford
0.75 g of ethyl menthyl ketone (EMK) (0.75 g, yield 76%).
25
365
[a]
¼ þ3.1 (c ¼ 0.010, toluene).
1H NMR (400 MHz, CDCl3, d, ppm): 0.76–1.72 (m, 21H,),
2.39–2.50 (m, 3H, ACH2ACOACHA). 13C NMR (100 MHz,
CDCl3, d, ppm): 7.6, 16.0, 21.4, 22.4, 23.9, 29.0, 32.4, 34.6,
35.1, 38.8, 44.0, 54.0, 215.6 (C¼¼O).
Anal. Calcd. for C13H24O: C 79.59, H 12.24. Found: C 79.67, H
12.18.
Synthesis of n-Hexyl Menthyl Ketone
Into 10 mL of dry THF, was added n-BuLi/n-hexane solution
(2 mL, 2.5 mol/L). A solution of MVK (0.97 g) in dry THF (5
mL) was added to the solution of the n-BuLi solution with
strongly stirring at ꢀ78 ꢁC under N2 atmosphere. The mix-
ture was sequentially stirred for 2 h after the addition.
Water was added to terminate the reaction. The reaction
mixture was extracted with ether (3 ꢂ 20 mL). The com-
bined ethereal extracts were washed with brine and dried
over Na2SO4. The solvent was evaporated in vacuo and the
residue was purified by column chromatography (silica gel,
30% CH2Cl2/hexane) to obtain n-hexyl menthyl ketone
25
365
(n-HMK) (0.52 g, yield 54%). [a]
toluene).
¼ þ15.2ꢁ (c ¼ 0.010,
FIGURE 1 The structures of MVK, EMK, n-HMK, MBEK, poly-
MVK, and Poly-MBEK.
NOVEL OPTICALLY ACTIVE HELICAL POLYMERS, WU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Anionic Polymerizations of MVKa
Polymer
THF-Soluble Part
Yieldb (%)
Yield (%)
Mn ꢂ 10ꢀ3
[a]
365
c
d
25
Run
Initiator
Solvent
Temp. (ꢁC)
1
2
3
4
5
6
7
a
n-BuLi
THF
ꢀ78
ꢀ30
0
63.7
64.5
72.6
78.8
59.0
77.4
85.1
20.6
35.2
72.6
78.8
10.0
64.0
12.5
2.7
3.3
4.2
4.9
3.0
5.5
6.5
828
764
749
662
839
867
863
n-BuLi
THF
n-BuLi
THF
n-BuLi
THF
22
n-BuLi
toluene
toluene
THF
ꢀ78
ꢀ78
ꢀ78
n-BuLi/Spe
n-BuLi/Spe
d
e
[monomer]0 ¼ 0.5 mol/L, [initiator]0 ¼ 0.05 mol/L, reaction time: 24 h.
MeOH-insoluble part.
c ¼ 4.0 mg/mL in toluene.
b
c
Sp ¼ (–)-sparteine, n-BuLi:Sp ¼ 1:1.2.
Determined by GPC.
acid and there is no neomenthylformic acid in it. Therefore,
there is no epimer in MVK, EMK, and n-HMK. The configura-
tion of MBEK was determined by folling experiment: The in-
termediate menthyl chloromethane for synthesis of MBEK
can also be obtained by reducing menthylformic acid and
then converting the reducing product to the corresponding
chloride. NMR data show that the configuration of the chlo-
ride obtained from the reaction of the Grignard reagent with
CH2Cl2 is the same as that derived from menthylformic acid
(see Supporting information S3).
uene, the polymerization goes more slowly but the tacticity
is better than in THF.
Chiroptical Properties and the Secondary Structure of
Poly(unsaturated ketone)s
MVK is optically active due to the chiral menthyl and its spe-
25
cific optical rotation [a]
is þ26.9ꢁ. After polymerization,
365
25
365
the specific optical rotation [a]
25–32 times (Table 1). The minimum [a]
was amplified to about
25
of the polymer
365
was 662ꢁ and the maximum came to 863ꢁ. Based on the
van’t Hoff Optical Rotation Addition Theory,43 the optical ac-
tivity of a chiral compound is the total contribution of every
chiral carbon atom. Thus, the contribution of chiral menthyl
groups in polymers to optical rotation should be similar to
that in the monomer MVK when they are measured under
identical condition. Therefore, it is unlikely that the large
specific optical rotation of the polymer comes from the chiral
pendant group menthyl.
Anionic Polymerization of MVK
The anionic polymerizations of MVK were carried out at dif-
ferent conditions. The results are summarized in Table 1.
The obtained polymers were fractionated to two parts: THF-
insoluble part and THF-soluble part. This study deals mainly
with the THF-soluble part.
To further rule out that the large specific optical rotation of
poly-MVK (see Figure 1) came from the chiral pendant group
menthyl, model compound EMK, and n-HMK were synthe-
sized and then their specific optical rotation were measured.
The specific optical rotations of MVK, EMK, and n-HMK are
shown in Table 2.
As being shown in Table 1, the MVK is highly active for ani-
onic polymerization and the THF-soluble parts of the
obtained polymers have moderately high molecular weights.
The polymerization is significantly affected by the polymer-
ization temperature, solvent and initiator. The overall yield
of anionic polymerization increases as the polymerization
temperature increases (Table 1 run 1-4). The yield and the
number-average molecular weights (Mn) of the THF-soluble
part increase with increase of the polymerization tempera-
ture (Table 1 run 1-4). Maybe, the polymer tacticity becomes
bad as the polymerization temperature increases and the
poor tacticity results in a poor polymer crystal degree and a
better polymer solubility.42
25
365
The specific optical rotations [a]
of MVK, EMK, and
n-HMK, which are similar to each other in structure, are
þ 26.9ꢁ, þ3.0ꢁ, and þ 15.2ꢁ, respectively, and the difference
is not too much! This fact indicates that the contribution of
menthyl to the specific optical rotations of MVK, EMK, and
n-HMK is similar. The repeat unit of poly-MVK has the same
structure as EMK and n-HMK, therefore, the contribution of
menthyl to the specific optical rotations of poly-MVK should
be similar to that of EMK and n-HMK. In other words, the
tremendous specific optical rotation of the poly-MVK is by no
means of resulting from the chiral pendant group menthyl.
The solvent had a great influence on polymer yield and Mn.
When the polymerization was conducted in THF at ꢀ78 ꢁC,
the overall polymer yield and the THF-soluble polymer yield
were 63.7% and 20.6%, respectively. While these yields
were 59.0% and 10.0%, respectively, in the circumstance of
using toluene as a polymerization solvent at the same tem-
perature (run 1 and 5 in Table 1). This indicates that, in tol-
The C¼¼C bond of MVK transformed to CAC bond after poly-
merization and this transformation should have an influence
on the specific optical rotations of poly-MVK. However, the
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ARTICLE
TABLE 2 Optical Rotation of Poly-MVK, Poly-MBEK, and Correlative Compound in Toluenea
[a]25
Run
Compound
365 nm
436 nm
546 nm
578 nm
589 nm
1
2
3
4
5
6
a
MVK
þ26.9
þ3.0
þ15.2
ꢀ83.8
þ828.0
þ93.0
ꢀ50.9
ꢀ38.3
ꢀ27.8
ꢀ54
ꢀ47.8
ꢀ32.4
ꢀ27.4
ꢀ33.6
þ51.0
ꢀ26.3
ꢀ44.1
ꢀ29.6
ꢀ27.0
ꢀ32.8
þ36.5
ꢀ25.9
ꢀ43.4
ꢀ28.9
ꢀ25.2
ꢀ28.8
þ33.0
ꢀ25.6
EMK
n-HMK
MBEK
poly-MVKb
Poly-MBEKb
þ213.5
ꢀ15.2
b
The concentration was 0.0100 mg/mL (run 1–3), 0.0040 mg/mL (run
4–6).
The polymerization was conducted for 24 h at ꢀ78 ꢁC in THF using
n-BuLi as initiator.
difference between the specific optical rotation of MVK and
that of EMK is not much, indicating that this influence is
very small.
bone and, thereout, resulted in the less specific optical rota-
tion. The Mn of poly-MBEK obtained in the presence of BuLi
at 78 ꢁC was 6000 while that of poly-MVK was 2700
obtained under the same condition. The molecular weight
difference of the two polymers indicates that MBEK is more
active for anionic polymerization than MVK, which results in
the poor helical sense selectivity of the polymerization of
MBEK.
Therefore, there must be another factor that is responsible
for the tremendous additional specific optical rotation of the
poly-MVK and this factor should be the one-handed helical
backbone.34,44–45
The vinyl carbon atoms, which connect to the carbonyl
become new chiral atoms after the polymerization of MVK,
which may have a contribution to optical rotation of the
polymer. However, these new-formed chiral atoms become
‘‘pseudo’’ asymmetric after the polymer grows to a long
chain and have rarely contribution to optical rotation of the
polymer. This conclusion has been verified by Okamoto’s
experiment.5,46 Okamoto reported that poly(methyl meth-
acrylate) (poly-MMA) derived from poly(triphenylmethyl
methacrylate) (poly-TrMA), which have large specific rotation
showed almost no optical rotation and suggested that the
large specific rotation of poly-TrMA was attributed to its
one-handed helical conformation.
There were other facts that supported that poly-MVK kept
one-handed helical backbone in solution.
25
365
The specific optical rotations [a]
of poly-MVK decreased
as polymerization temperature increased (runs 1 to 5 in
Table 1). The increase of the polymerization temperature
resulted in a poor selectivity on helix sense and, subse-
quently, the excess of one-handed helical sense of the poly-
mer decreased. Another evidence for the formation of one-
handed helical backbone in poly-MVK was that the specific
25
365
optical rotations [a]
of poly-MVK was promoted by adding
chiral ligand (ꢀ)-sparteine (Sp) (comparing run 1 and run 7,
run 5 and run 6 in Table 1), in other words, the chiral ligand
helped enhance helix sense selectivity.
It has been reported that the specific rotation absolute value
of the polymer of optically active phenyl-2-pyridyl-o-tolyl-
methyl methacrylate (PPyTMA) was four times that of its
monomer PPyTMA and Okamoto confirmed that the specific
rotation increase of the polymer was attributed to the forma-
UV-vis and CD spectra confirm further that poly-MVK forms
a
helical conformation. The UV-vis and CD spectra of
poly-MVK, poly-MBEK, n-HMK, EMK, and MVK are shown in
Figure 2.
tion of the one-handed helicity of the polymer chain but not
The UV-vis spectra of n-HMK and EMK are almost the same
and the kmax is at 288 nm (the n-p* transition of C¼¼O). In
the UV-vis spectra of MVK, the kmax is at 337 nm (the n-p*
transition of C¼¼O). After polymerization, the n-p* transition
of C¼¼O shifts hypsochromically to 293 nm, which is longer
than that of n-HMK and EMK. What is more, the absorption
intensity of the C¼¼O of poly-MVK is much stronger than that
of its monomer and, moreover, stronger than that of n-HMK
and EMK. This strong absorption peak should be attributed
to the helical conformation of the polymer main chains.35
to the chiral lateral chains.47 The specific rotation [a]
of
25
365
poly-MVK is about 25–30 times that of its monomer. Com-
paring specific rotation differences between poly-MVK and
MVK and between poly-PPyTMA and PPyTMA, one could
conclude that poly-MVK should also take one-handed helical
backbone.
25
365
The fact that the specific optical rotation [a]
of the Poly-
MBEK, (see Figure 1) which had a methene CH2 between
the carbonyl and menthyl in pendant group of the polymer,
was much less than that of poly-MVK also supported that
poly-MVK kept an one-handed helical backbone in the solu-
tion. The backbone of poly-MBEK was more flexible than
that of poly-MVK because the bulky menthyl in the poly-
MBEK was isolated from the backbone by CH2, which
resulted in the less excess of the one-handed helical back-
All the CD spectra of poly-MVK, MVK, EMK, and n-HMK
show a positive Cotton effect, which results from the asym-
metric transition of the n-p* of C¼¼O group. The CD spec-
trum of poly-MVK shows a very strong Cotton effect signal
whereas the Cotton effects of MVK, EMK, and n-HMK are
very weak. The maximum molar ellipticity values of poly-
NOVEL OPTICALLY ACTIVE HELICAL POLYMERS, WU ET AL.
1445

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 3 The g-Values of MVK, EMK, n-HMK, and poly-MVK
Substance MVK EMK n-HMK
Poly-MVKa
g-value
2.0 ꢂ 10ꢀ3 2.0 ꢂ 10ꢀ3 1.9 ꢂ 10ꢀ3 6.8 ꢂ 10ꢀ2
a
The polymer was obtained by polymerization for 24 h at ꢀ78 ꢁC in
THF using n-BuLi as initiator.
model compound n-HMK, indicating that the anisotropy of
poly-MVK is much more tremendous than that of its mono-
mer and model compounds.
As the contribution of menthyl to the anisotropy of C¼¼O
group is the same in poly-MVK, MVK, EMK, and n-HMK, the
large anisotropy differences between the polymer and MVK
and between the polymer and model compounds must come
from the asymmetry of polymer backbone. This polymer
backbone asymmetry must be attributed to the excess one-
handed helical sense of the polymer backbone because there
is not other factor that results in so tremendous anisotropy
apart from helical structure.
In the CD spectrum of poly-MBEK, the intensity of the Cotton
effect signal is only one third of that of poly-MVK, indicating
that the excess one-handed helical sense conformation of
poly-MBEK is much less than that of poly-MVK. The less
excess of one-handed helical conformation of poly-MBEK is
due to the flexible backbone, which, in turn, provides
another evidence to support the helical structure of poly-
MVK.
The polymerization temperature has an influence on the Cot-
ton effect of poly-MVK. The dependence of the Cotton effect
on the polymerization temperature is shown in Figure 3. The
Cotton effect signal of poly-MVK decreases as the polymer-
ization temperature rises, indicating that the excess value of
the one-handed helical conformation of the polymer
decreases when the polymerization temperature rises.
FIGURE 2 UV-vis and CD spectra of poly-MVK, poly-MBEK and
their corresponding compounds in CHCl3 at 25 ꢁC. Concentra-
tion: 3.6 ꢂ 10ꢀ4 mol/L (referred to one monomeric unit)for all
of UV-vis spectra measure and for CD spectra of poly-MVK and
poly-MBEK, 2.6 ꢂ 10ꢀ3 mol/L for the UV-vis and CD spectra of
n-HMK, MVK and EMK.
MVK (referred to one monomeric unit), MVK, EMK, and
n-HMK are [h]293 1.47 ꢂ 104, [h]277 1.05 ꢂ 102, [h]290 2.63 ꢂ
102, and [h]290 3.52 ꢂ 102, respectively. The maximum molar
ellipticity value of poly-MVK is 140 times as high as that of its
monomer MVK and 56 times as high as that of the model
compounds EMK. The Cotton effect signal of poly-MVK is so
strong that the Cotton effect signal of MVK, EMK, and n-HMK
can be ignored. These large differences indicate that the ani-
sotropies of C¼¼O bond in the polymer, monomer MVK, model
EMK, and model n-HMK have tremendous difference.
For comparing the anisotropy differentia, the g-values of the
polymer, the monomer, and the model compounds were
measured. The g-values are shown in Table 3.
FIGURE 3 CD of poly-MVK obtained at different temperature in
CH3Cl at 25 ꢁC. Concentration: 3.6 ꢂ 10ꢀ4 mol/L (referred to
one monomeric unit).
The g-value of the polymer is 34 times as high as its mono-
mer and model compound EMK and 36 times as high as
1446
INTERSCIENCE.WILEY.COM/JOURNAL/JPOLA

ARTICLE
11 Yoshiba, K.; Hama, R.; Teramoto, A.; Nakamura, N.; Maeda, K.;
It must be noted that P. Pino reported that a large optically
active polymer fraction was obtained from the anionic poly-
merization of (s)-1-methyl-propyl]vinyl Ketone. The specific
rotation differences between the polymer ([a]2D5 ¼ ꢀ104ꢁ)
and its monomer ([a]2D5 ¼ þ32.4ꢁ) and between the polymer
and the corresponding model compound (þ)(s)-4-methyl-
hexan-3-one ([a]2D5 ¼ þ24.08ꢁ) were also large. Comparing
these specific rotation differences with that founded in this
research work on poly-MVK and with that reported by Oka-
moto on the research of poly-PPyMA,47 we believe that the
polymer fraction deriving from the anionic polymerization of
(s)-1-methyl-propyl]vinyl ketone should also take helical
backbone.
Okamoto, Y.; Sato, T. Macromolecules 2006, 39, 3435–3440.
12 Sakal, R.; Otsuka, I.; Satoh, T.; Kakuchi, R; Kaga, H; Kakuchi,
T. J Polym Sci Part A: Polym Chem 2006, 44, 325–334.
13 Pijper, D.; Feringa, B. L. Angew Chem Int Ed Engl 2007, 46,
3693–3696.
14 Beijnen, A. J. M. V.; Nolte, R. J. M.; Drenth, W.; Hezemans
A. M. F. Tetrahedron 1976, 32, 2017–2019.
15 Kamer, P. C. J.; Nolte, R. J. M.; Drenth, W. J Am Chem Soc
1988, 110, 6818–6825.
16 Chen, J. P.; Gao, J. P.; Wang, Z. Y. Polym Int 1997, 44, 83–87.
17 Kajitani, T.; Okoshi, K.; Sakurai, S.-I.; Kumaki, J.; Yashima,
E. J Am Chem Soc 2006, 128, 708–709.
CONCLUSIONS
18 Habaue S.; Ajiro H.; Okamoto, Y. J Polym Sci Part A: Polym
Chem 2000, 38, 4088–4094.
The anionic polymerization of MVK can take place smoothly
without any problem. The poly-MVK has a large specific opti-
cal rotation and its CD spectrum shows an intense Cotton
effect, indicating that the poly-MVK has a helical conforma-
tion with a prevailing helicity of the backbone in solution.
The lower the polymerization temperature, the larger the
excess value of the prevailing helicity of the obtained poly-
mer. The poorer excess value of the prevailing helicity of the
poly-MVK obtained at higher polymerization temperature is
due to the poorer helical-sense selectivity caused by the rise
of the teperature.
19 Zhi, J. G.; Guan, Y.; Cui, J.; Liu, A.; Zhu, Z. G.; Wan, X.;
Zhou, Q. F.
2408–2421.
J Polym Sci Part A: Polym Chem 2009, 47,
20 Cui, J. X.; Lu, A. H.; Wan, X. H.; Zhou, Q. F. Macromolecules
2009, 42, 7678–7688.
21 Zhi, J. G; Zhu, Z. G; Liu, A. H.; Cui, J. X.; Wan, X. H.; Zhou,
Q. F. Macromolecules, 2008, 41, 1594–1597.
22 Cui, J. X; Liu, A. H.; Zhi, J. G.; Zhu, Z. G.; Guan, Y.; Wan, X. H.;
Zhou, Q. F. Macromolecules, 2008, 41, 5245–5254.
23 Liu, A. H.; Zhi, J. G.; Cui, J. X.; Wan, X. H; Zhou, Q. F. Mac-
We gratefully thank the National Science Foundation of China
(20772102), and the Higher Education Doctoral Science
Foundation of China (No. 20060530002), and Hunan Provin-
cial Innovation Foundation for Postgraduate (S2008yjscx08)
for financial support of this work.
romolecules 2007, 40, 8233–8243.
24 Yu, Z. N.; Wan, X. H.; Zhang, H. L.; Chen, X. F.; Zhou, Q. F.
Chem Commun 2003, 8, 974–975.
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