Anionic polymerization of optically active propene oxides bearing substituted N,N-diphenylamino moiety

Article abstract of DOI:10.1246/cl.141043

A series of novel chiral propene oxides bearing substituted N,N-diphenylamino moiety were synthesized and anionically polymerized. Most of propene oxides gave polymers possessing high specific rotation and intensive circular dichroism (CD) signals, which

Full text of DOI:10.1246/cl.141043

CL-141043
Received: November 16, 2014 | Accepted: November 28, 2014 | Web Released: December 13, 2014
Anionic Polymerization of Optically Active Propene Oxides Bearing
Substituted N,N-Diphenylamino Moiety
Jin Zhang, Chun-Hui Zhao, Li-Wen Yang, and Nian-Fa Yang*
Key Laboratory of Environmentally Friendly Chemistry and Applications, Ministry of Education,
College of Chemistry, Xiangtan University, Hunan 411105, P. R. China
(E-mail: nfyang@xtu.edu.cn)
A series of novel chiral propene oxides bearing substituted
N,N-diphenylamino moiety were synthesized and anionically
polymerized. Most of propene oxides gave polymers possessing
high specific rotation and intensive circular dichroism (CD)
signals, which were attributed to the helical conformation of
the main chain. Poly(3-(4,4¤-dimethoxy)diphenylaminopropene
20
oxide) exhibited the highest specific rotation (½ꢀꢀ365 = ¹5400).
Synthetic polymers with main-chain helical chiralities have
attracted long-standing interest for their wide applications in
chiral recognition toward racemic compounds, liquid crystal
formation, chiral catalytic activity, and biological mimics.1
So far, a variety of helical polymers have been synthesized.
Examples include poly(triphenylmethyl methacrylate),2 poly-
acrylamides,3 polyaldehydes,4 polyisocyanides,5 polystyrenes,6
polyisocyanates,7 polyacetylenes,8 and so on. But there are still
very few reports available about chiral helical polyethers from
the polymerization of epoxides due to a low barrier to rotation
for carbon-oxygen bonds, especially for helical polyether with
pendant containing nitrogen atoms.9
The helical polyacrylamides including the series of N,N-
diphenylacrylamides have been prepared.10 Although the stereo-
control in the polymerization of acrylamides was difficult, the
bulky N,N-diphenyl side chains of polyacrylamides help the
polymers maintain stable helical conformation in solution,
which encourage us to design, synthesize, and polymerize chiral
epoxides with bulky N,N-diphenyl moiety.
Scheme 1. The polymerization of optically active DPOs.
As Table 1 shows, most of the poly(DPO)s obtained showed
large specific rotation in solution and the optical rotation sign
is opposite to their monomers’. The absolute value of the
20
poly(DPO)s’ specific rotation (½ꢀꢀ365) is 45 times as large as that
of the corresponding monomer (Table 1, Run 11), which implies
that the poly(DPO)s keep a stable helical conformation.
Polymers with single-handed helical conformation often
show Cotton effect in circular dichroism (CD) spectra.11 For
almost all of the obtained poly(DPO)s, obvious Cotton effect
also emerges in the spectrum, which supports that poly(DPO)s
keep prevailing helical conformation in solution. There are also
great differences between poly(DPO)s and the corresponding
monomer in the CD spectrum. The CD signals of most of the
poly(DPO)s are much stronger than the corresponding mono-
mers. Poly((R)-DPO)s and poly((S)-DPO)s exhibited mirror
images in CD spectra (Figure 1).
In this study, chiral epoxides bearing an N,N-diphenyl
moiety (DPOs) were synthesized from the corresponding N,N-
diphenylamine with (R)- or (S)-epichlorohydrin. Anionic poly-
merization of DPOs were performed with potassium hydroxide
as initiator (Scheme 1). Chiroptical properties of the polymers
were investigated by specific optical rotations, circular dichro-
ism (CD) measurements, and g-value.
The anionic polymerization of DPOs bearing methyl groups
at the ortho, meta, and para position of benzene have been inves-
tigated, from 4- to 2-position, the polymerization seems to de-
crease and the poly(4-MeDPO) possessed a high optical rotation
(Table 1, Runs 11 and 12). Interestingly, the phenomenon that the
pendant groups on the polyacrylamides with bulky N,N-diphenyl
side chain had a similar influence as reported in the literature.10
The results of the anionic polymerization of para-disubsti-
tuted DPOs are also summarized in Table 1 (Runs 3-8). The
bulkiness of the substituents is in the order of DPO < 4,4¤-
Me2DPO < 4,4¤-(MeO)2DPO < 4,4¤-(α,α-dimethylbenzyl)2DPO,
and the specific rotation of polymer is increased with that of
the substituent except for poly(4,4¤-(α,α-dimethylbenzyl)2DPO),
since the degree of polymerization of poly(4,4¤-(α,α-dimethyl-
benzyl)2DPO) is too low.
Perhaps the large optical rotation and CD intensity of
poly(DPO)s may arise from the chiral carbons in the polymer
main chain. Based on this hypothesis, all the poly(DPO)s should
have large specific rotation and CD intensity. This seems not to
be favored since the chiroptical properties of poly((S)-2-Me-4-
MeODPO) and poly((S)-3-MeODPO) show no great difference
with their corresponding monomers (Figure 1d; Table 1, Runs
14 and 18), while chiral carbons are also present in these
polyethers.
The epoxy ring of DPOs was opened after polymerization
and the opening of the epoxy ring should have an impact on the
chiroptical properties of poly(DPO)s. However, the experimental
data indicated this impact was not much. When the epoxy ring of
20
¹
(S)-DPO was opened by OH , the ½ꢀꢀ365 of the ring-opening
product was not enough (Scheme 2). Therefore, the change of
250 | Chem. Lett. 2015, 44, 250–252 | doi:10.1246/cl.141043
© 2015 The Chemical Society of Japan

Table 1. Anionic polymerizations of epoxidesa
Monomer
Polymer
PDIc
(Mw/Mn)
Run
20
¹3
20
d
Compounds
½ꢀꢀ365
Yieldb/%
Mn © 10
½ꢀꢀ365
1
2
3
(R)-DPO
(S)-DPO
¹78
+78
¹85
71.9
70.5
53.2
51.6
70.0
71.2
40.2
40.3
42.7
41.8
71.2
70.8
40.8
38.7
46.3
45.2
25.1
24.9
2.1
2.2
2.9
2.7
2.8
2.9
2.7
2.8
2.4
2.3
2.6
2.7
2.3
2.2
2.7
2.7
2.6
2.5
1.28
1.21
1.11
1.12
1.18
1.16
1.15
1.13
1.16
1.15
1.14
1.12
1.20
1.19
1.16
1.17
1.20
1.18
+714
¹702
(R)-4,4¤-Me2DPO
(S)-4,4¤-Me2DPO
(R)-4,4¤-(MeO)2DPO
(S)-4,4¤-(MeO)2DPO
(R)-4,4¤-(α,α-dimethylbenzyl)2DPO
(S)-4,4¤-(α,α-dimethylbenzyl)2DPO
(R)-3-MeDPO
(S)-3-MeDPO
(R)-4-MeDPO
(S)-4-MeDPO
(R)-3-MeODPO
+1196
¹1227
+5344
¹5400
+178
¹188
+880
¹872
+1140
¹1090
+203
¹237
4
+84
5e
6e
7
+973
¹972
¹76
+76
¹80
+80
¹25
+26
¹190
+192
¹44
+44
¹130
+130
8
9
10
11
12
13
14
15
16
17
18
(S)-3-MeODPO
(R)-3,4-Me2DPO
(S)-3,4-Me2DPO
(R)-2-Me-4-MeODPO
(S)-2-Me-4-MeODPO
+1121
¹1092
+95
¹103
b
c
aThe polymer was obtained by bulk polymerization for 24 h at 150 °C using KOH as an initiator. THF-soluble part. Determined by GPC
d
¹1
e
¹1
against polystyrene standards, [M]/[I] = 20/1. C = 4.0 mg mL in THF. C = 0.25 mg mL in THF.
100
100
(R)-DPO
(S)-DPO
(R)-3-MeDPO
(S)-3-MeDPO
(R)-4-MeDPO
(S)-4-MeDPO
poly((R)-3,4-Me DPO)
(R)-4,4′-Me DPO
2
80
60
2
b
a
80
60
poly((S)-3,4-Me DPO)
(S)-4,4′-Me DPO
2
2
poly((R)-4,4′-Me DPO)
(R)-3,4-Me DPO
2
2
poly((S)-4,4′-Me DPO)
2
40
(S)-3,4-Me DPO
40
2
20
20
0
0
-20
-40
-60
-80
-100
-120
-20
-40
-60
-80
-100
-120
(R)-4,4′-(α,α-dimethylbenzyl) DPO
poly((S)-3-MeDPO)
poly((R)-4-MeDPO)
poly((S)-4-MeDPO)
poly((R)-DPO)
poly((S)-DPO)
poly((R)-3-MeDPO)
2
(S)-4,4′-(α,α-dimethylbenzyl) DPO
2
poly[(R)-4,4′-(α,α-dimethylbenzyl) DPO]
2
poly[(S)-4,4′-(α,α-dimethylbenzyl) DPO]
2
260
280
300
320
340
360
380
260
280
300
320
340
360
380
λ/nm
λ/nm
100
80
100
d
80
60
c
(R)-3-MeODPO
(S)-3-MeODPO
poly((R)-3-MeODPO)
poly((S)-3-MeODPO)
60
40
40
20
20
0
0
-20
-40
-60
-80
-100
-120
-20
-40
-60
-80
-100
-120
20
(R)-2-Me-4-MeODPO
(S)-2-Me-4-MeODPO
10
0
(R)-4,4′-(MeO) DPO
2
poly((R)-2-Me-4-MeODPO)
(S)-4,4′-(MeO) DPO
-10
-20
2
poly((S)-2-Me-4-MeODPO)
poly[(R)-4,4′-(MeO) DPO]
2
260 280 300 320 340 360 380
poly[(S)-4,4′-(MeO) DPO]
λ/nm
2
260
280
300
320
λ/nm
340
360
380
260
280
300
320
340
360
380
λ/nm
¹1
Figure 1. The CD spectra of chiral epoxides and their polymers in CHCl3 at 25 °C. Concentration: 3.9 © 10¹5 mol L (referred to
one monomeric unit) for all of CD.
specific rotation after polymerization does not result from the
opening of the epoxy ring.
The conformations of biomolecules and optically active
helical polymers are usually sensitive to temperature variation.12
When the temperature exceeds their “tolerable” limit, their
unique helical conformations can be destroyed and random coils
formed. Thus, the CD spectra of the poly((S)-4-MeDPO) was
measured at different temperature to study their conformational
stability. Figure 2 shows temperature-variable CD spectra of
poly((S)-4-MeDPO) in CHCl3 in the range from 25 to 45 °C.
Chem. Lett. 2015, 44, 250–252 | doi:10.1246/cl.141043
© 2015 The Chemical Society of Japan | 251

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¹1
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The authors thank the National Natural Science Foundation
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252 | Chem. Lett. 2015, 44, 250–252 | doi:10.1246/cl.141043
© 2015 The Chemical Society of Japan