Highly efficient helix-sense-selective polymerization of an achiral phenylacetylene having a bulky group

Article abstract of DOI:10.1246/cl.150863

Novel achiral phenylacetylene having a bulky t-butyl group was synthesized and polymerized by a chiral catalytic system containing [Rh(nbd)Cl]2 (nbd: norbornadiene) and chiral amines to yield a one-handed helical polymer having a much higher g value than polymers with no bulky groups. Highly efficient helix-sense-selective polymerization has been achieved using a bulky monomer and a less bulky chiral cocatalyst.

Full text of DOI:10.1246/cl.150863

Received: September 13, 2015 | Accepted: September 28, 2015 | Web Released: October 3, 2015
CL-150863
Highly Efficient Helix-sense-selective Polymerization of an Achiral Phenylacetylene
Having a Bulky Group
Yanqing Qu,1 Yu Zang,*1 Mingyu Zhang,1 Toshiki Aoki,*1,2 Masahiro Teraguchi,2 Takashi Kaneko,2 Liqun Ma,1 and Hongge Jia1
1College of Materials Science and Engineering, Qiqihar University, Wenhua Street 42, Qiqihar, Heilongjiang 161006, P. R. China
2Faculty of Engineering, Niigata University, 2-8050 Ikarashi, Nishi-ku, Niigata 950-2181
(E-mail: zangyu@qqhru.edu.cn)
A novel achiral phenylacetylene having a bulky t-butyl
group was synthesized and polymerized by a chiral catalytic
system containing [Rh(nbd)Cl]2 (nbd: norbornadiene) and chiral
amines to yield a one-handed helical polymer having a much
higher g value than polymers with no bulky groups. Highly
efficient helix-sense-selective polymerization has been achieved
using a bulky monomer and a less bulky chiral cocatalyst.
Only a few kinds of asymmetric polymerizations of achiral
monomers to give chiral polymers having chiral structures only
in their main chains have been reported.1-4 π-Conjugated
polymers like polyacetylenes5,6 have attracted increasing atten-
tion because of their noteworthy physical properties such as
electric conductivity, nonlinear susceptibility, high gas perme-
ability,7-9 and so on. Recently, chiral polyacetylenes have
received much attention because the chiral structure can enhance
the unique properties and add new functions.10-24 Among them,
soluble chiral π-conjugated polymers whose chiral structures
arise solely from the one-handed helical conformation of the
conjugated main chains (that is, they have no asymmetric
carbons) and are stable in solution have so far only been
synthesized by the helix-sense-selective polymerization (HSSP)
of achiral phenylacetylene monomers using a chiral catalytic
system developed by us.25-30 We reported the first successful
HSSP of a phenylacetylene using a chiral catalytic system.25 The
polymer formed had a one-handed helical conformation fixed by
intramolecular hydrogen bonds, and had no asymmetric carbons
derived from the monomer, initiator, or cocatalyst. In addition,
we reported that a chiral rhodium complex having two chiral
amines may be the true active chiral species when a catalytic
system consisting of [Rh(nbd)Cl]2 (nbd: norbornadiene) and a
chiral amine was used for HSSP.26
The efficiency in the chiral induction during the HSSP of
phenylacetylenes having no bulky group (Chart 1; C8, C12,
C14, and C12B), that is, the intensities of the circular dichroism
(CD) signals (g value) of the resulting polymers we reported
before were small.27,28 Although the role of chiral amine
cocatalysts in the HSSP of a phenylacetylene using a catalytic
system was reported by our group,26 the best selection of chiral
amines has not been considered.
Chart 1. Monomer structures of phenylacetylenes.
Chart 2. Chemical structures of chiral amines.
the HSSP of those monomers was low, that is, the intensities of
the CD signals (g values) of the resulting polymers were small.
Therefore, in order to synthesize polymers having higher CD
intensity (g value), we successfully synthesized an achiral
phenylacetylene monomer having a bulky group (TB) as a white
solid in 84% yield (See Supporting Information, SI-3). The
HSSP of monomer TB was carried out by a chiral catalytic
system consisting of [Rh(nbd)Cl]2 as the initiator and chiral
phenylethylamine (Ia) as the cocatalyst (Scheme 1). A typical
polymerization procedure was as follows: A solution of
[Rh(nbd)Cl]2 (4.00 mg, 8.80 mmol) and Ia (28.2 mL, 220 mmol)
in dry THF (0.440 mL) was added to a dry THF (0.44 mL)
solution of TB (37.0 mg, 88.0 mmol). The reaction solution was
stirred at room temperature for 4 h. The crude polymer was
purified by precipitation of the THF solution into a large amount
of methanol, and the formed solid was dried in vacuo to give
1
poly(TB) as a red solid in 10% yield. H NMR (CCl4/DMSO-
d6 = 5/1 (v/v), TMS): ¤ 7.56-6.81 (br, 10H, PhH), 5.93 (br, cis
proton in the main chain), 4.79 (br, 2H, PhOCH2Ph), 4.37 (br,
4H, Ph (CH2OH)2), 1.51 (br, 9H, C(CH3)3). IR (KBr): 3600-
3100 (OH), 3000-2800 (CH), 1645 (C=N) cm¹1. The other
results are shown in Table 1 (No. 5). The CD and UV spectra of
poly(TB) in THF are shown in Figure 1 (No. 5). The polymer
showed Cotton effects in THF at room temperature. The CD
signal observed around 430 nm is assigned to the one-handed
helical main chain and the peaks around 310 nm may arise from
chiral positions between adjacent pendant phenyl groups. This
In this communication, in order to enhance the efficiency
in the chiral induction during HSSP, that is, to obtain a
poly(phenylacetylene) having a higher g value, a novel achiral
phenylacetylene monomer having a bulky group (Chart 1, TB)
was synthesized and polymerized using six chiral amines with
different basicity and bulkiness (Chart 2) as cocatalysts.
We reported the synthesis of poly(phenylacetylene)s having
no bulky groups by HSSP in our previous study (Table 1, Nos.
1-4).25-27 However, the efficiency in the chiral induction during
Chem. Lett. 2015, 44, 1777–1779 | doi:10.1246/cl.150863
© 2015 The Chemical Society of Japan | 1777

Table 1. HSSP of monomers by Ia at room temperaturea
b
c
[Monomer]/ Yield Mw
g310
b
No. Monomer
Mw/Mn
Solubilityd
[[Rh(nbd)Cl]2] /% (©107)
(©105)
1e C8
50
50
50
50
50
25
10
25
92
28
93
10
57
84
0.10
0.25
0.67
0.79
7.9
5.0
3.8
4.7
4.0
0.15
0.22
0.056 +++
0.019 +++
++
+++
2e C12
3e C14
4e C12B
5
6
7
TB
TB
TB
8.0 ¹0.61
9.3 ¹1.6
13 ¹2.7
++
+
12
2.4
+++
a[Rh(nbd)Cl]2 was dissolved in THF, [M] = 0.08 mol L¹1, [Ia]/
[[Rh(nbd)Cl]2] = 200, for 4 h. bBy GPC correlating polystyrene
c
d
standard (eluent: THF). g = ([ª]/3300/¾) © 0.001. Solubility in
e
THF: +: poor; ++: good, +++: best. refs 27 and 28.
Figure 2. CD and UV spectra of poly(TB) in THF at different
temperatures.
Scheme 1. HSSP of monomer TB.
Figure 3. CD and UV spectra of poly(TB) by using different
chiral amines as cocatalysts (Table 2, Nos. 1-3 and 6) in THF.
almost unchanged on heating the polymer solution, indicating
either hydrodynamically or thermodynamically stable helical
conformation in THF at a very high temperature.
In order to obtain polymers having higher g values, we
examined six chiral amines as chiral cocatalysts, i.e., three
commercially available Ia-Ic and three synthesized alkoxy-
amines IIa-IIc (Chart 1) (for the synthetic routes, see SI-4) with
different bulkiness and basicity. The HSSPs were carried out
using the same conditions as above (Table 1, No. 7), except for
the use of different chiral amines. The polymerization results
are listed in Table 2. All the polymers prepared by the different
amines showed Cotton effects (Figure 3). We found that the
primary amines Ia and IIa showed better performance during
HSSP than the corresponding tertiary N,N-dimethyl-substituted
amines Ib and IIb, respectively (Table 2, for Nos. 1 and 2, Ia:
g = ¹2.7 and Ib: g = ¹0.14; for Nos. 4 and 5, IIa: g = ¹0.067
and IIb: g = ¹0.037). Since the primary amines have lower
bulkiness and higher basicity (higher pKa) than the tertiary
amines, the former ones can coordinate to the rhodium more
effectively. Therefore, the g values for the polymers obtained by
Ia and IIa were higher than those by Ib and IIb, respectively.
Among the four primary amines, Ia, Ic, IIa, and IIc, the g value
of poly(TB) synthesized using IIa as cocatalyst was the lowest
(¹0.067, Table 2, No. 4), while the chiral amines Ia, Ic, and
IIc gave polymers with higher g values (¹2.7, ¹3.7, and 3.3;
Table 2, Nos. 1, 3, and 6). Among the four primary amines,
bulkiness around the amino groups that coordinate to the
rhodium is higher for IIa than for Ia, Ic, and IIc. Therefore, it
Figure 1. CD and UV spectra of poly(TB) in THF (The
numbers in the figure correspond to those in Table 1).
indicates the presence of an excess of the one-handed helical
polyacetylene backbone in the polymer. The g value of
poly(TB) (Table 1, No. 5, g = ¹0.61 © 105) is 3 times higher
than those of other polymers (Table 1, Nos. 1-4, g ¯ 0.22 ©
105). This suggests that the bulkier monomer has a higher
efficiency in the chiral induction during polymerization. That is,
the bulkiness of the monomer should enhance chiral selectivity
during the propagation of HSSP. Consequently, the monomer
having a bulkier group is more suitable for synthesizing
polymers with a higher g value.
When we carried out the HSSP in our previous HSSP
condition (Table 1, No. 5), the yield in the polymerization of
TB was very low. In order to enhance the yield, we decreased
the molar ratio of the monomer to [Rh(nbd)Cl]2. As a result, the
yield of poly(TB) increased from 10% to 84% (Table 1, Nos. 5
and 7). The condition of No. 7 was the best for a higher yield,
larger g value, and better solubility of the polymer.
The stability of the helical structure of poly(TB) was also
investigated in this study. We measured the CD and UV spectra
of poly(TB) in THF from ¹10 to 60 °C. The results are shown
in Figure 2. The CD signal due to the main-chain chirality was
1778 | Chem. Lett. 2015, 44, 1777–1779 | doi:10.1246/cl.150863
© 2015 The Chemical Society of Japan

Table 2. HSSP of monomer TB by using different chiral amines as cocatalysts in THF at room temperaturea
d
e
pKa
(calculated)b
Yield
/%
Mw
g310
d
No.
Chiral amine
Bulkinessc
Mw/Mn
(©107)
(©10 )
¹5
1
2
3
4
5
6
Ia
Ib
Ic
IIa
IIb
IIc
9.04
8.88
++
++
++
+++
+++
+
84
95
51
8.0
83
59
2.4
12
2.6
20
2.9
2.9
13
4.8
6.9
10
32
8.0
¹2.7
¹0.14
¹3.7
¹0.067
¹0.037
3.3
10.89
8.94
f
®
8.93
b
a[M] = 0.08 mol L¹1; [chiral amine]/[[Rh(nbd)Cl]2] = 200; [M]/[[Rh(nbd)Cl]2] = 10 for 4 h. Calculated values using Advanced Chemistry
Development Software V11.02 found in Scifinder. cThe chiral center carbon has a ring and a long linear substituent (+++), a ring and a short
linear substituent (++), and a short linear and a long linear substituent (+). dBy GPC correlating polystyrene standard (eluent: THF). eg = ([ª]/
f
3300/¾) © 0.001. No data.
1977, 39, 1098.
was thought that IIa could not make an active chiral initiator
effectively. This indicates the negative effect of the bulky
substituent in the chiral amine on the g value. The highest g
value (¹3.7) was attained using Ic, which had the highest pKa,
i.e., the strongest basicity, among them (Table 2, No. 3). We
conclude that chiral amines with higher basicity and less bulky
substituents are effective for HSSP. Presumably, the coordina-
tion ability of the chiral amines to rhodium was enhanced by
increasing the basicity and decreasing the bulkiness. Therefore,
Ic was the best chiral cocatalyst to synthesize polymers with the
highest g value.
In conclusion, a novel one-handed helical polymer poly(TB)
having bulky t-butyl groups was successfully synthesized by
HSSP. The monomer TB having a bulky t-butyl group was the
most suitable for HSSP, judging from the g value of the resulting
polymer. Six chiral amines with different basicity and bulkiness
were examined as cocatalysts to obtain polymers having higher
g value. The resulting polymer synthesized using the chiral
amine with the highest basicity and less bulkiness as a
cocatalyst, i.e., Ic, showed the highest g value. Highly efficient
helix-sense-selective polymerization has been achieved using a
bulky monomer and a less bulky cocatalyst with higher basicity.
The HSSP of TB having a bulky group using chiral amine Ic
having the best coordination ability gave polymers with the
highest g value.
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Partial financial support through the National Natural
Science Foundation of China (Nos. 21404064, U1162123,
51103076, and 21376127), through the Overseas Scholars
Foundation of the Education Department of Heilongjiang
province of China (No. 1254HQ008), through the program of
Young Teachers Scientific Research in Qiqihar University
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Chem. Lett. 2015, 44, 1777–1779 | doi:10.1246/cl.150863
© 2015 The Chemical Society of Japan | 1779