Living-like helix-sense-selective polymerization of an achiral substituted acetylene having bulky substituents

Article abstract of DOI:10.1016/j.polymer.2013.02.009

By using a new achiral phenylacetylene having bulky substituents as a monomer, living-like helix-sense-selective polymerization (HSSP) has been achieved. This is the first example of the HSSP of substituted acetylenes where the degree of polymerization was controlled. The resulting one-handed helical living polymer initiated polymerization of a second monomer to successfully yield a block copolymer.

Full text of DOI:10.1016/j.polymer.2013.02.009

Polymer 54 (2013) 1729e1733
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Polymer
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Polymer communication
Living-like helix-sense-selective polymerization of an achiral substituted
acetylene having bulky substituents
,
b
a
Yu Zang a, Kazuki Nakao a, Hiroki Yotsuyanagi a, Toshiki Aoki a , Takeshi Namikoshi , Toyokazu Tsutsuba ,
*
Masahiro Teraguchi a, Takashi Kaneko a
a Department of Chemistry and Chemical Engineering, Graduate School of Science and Technology, and Center for Transdisciplinary Research, Niigata University, Ikarashi 2-8050,
Nishi-ku, Niigata 950-2181, Japan
b Material Science and Engineering, Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido 090-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
By using a new achiral phenylacetylene having bulky substituents as a monomer, living-like helix-sense-
selective polymerization (HSSP) has been achieved. This is the first example of the HSSP of substituted
acetylenes where the degree of polymerization was controlled. The resulting one-handed helical living
polymer initiated polymerization of a second monomer to successfully yield a block copolymer.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 1 December 2012
Received in revised form
3 February 2013
Accepted 7 February 2013
Available online 13 February 2013
Keywords:
One-handed helical living polymer
Helix-sense-selective polymerization
Bulky substituents
1. Introduction
been realized by using precisely designed initiators [17–23]. How-
ever, living polymers could not be obtained by polymerization of
Conjugated polymers like polyacetylenes have aroused interest
because of their noteworthy physical properties such as conduc-
tivity, organomagnetism, and optical non-linear susceptibility.
Recently chiral polyacetylenes [1e8] have attracted much attention
since the chiral structure can enhance the unique properties and
add new functions. Many kinds of polymers having chiral structures
in their main chains and/or pendant groups have been synthesized.
Among them, soluble chiral conjugated polymers whose chiral
structures arise solely from the one-handed helical conformation of
the conjugated main chains (therefore, they have no asymmetric
carbons) and are stable alone in solution have so far only been
synthesized by the helix-sense-selective polymerization (HSSP) of
achiral phenylacetylene monomers (I in Chart 1) [9] by using a
chiral catalytic system developed in our laboratory [9e14]. After our
discovery, we reported the other two kinds of monomers (II and III
in Chart 1) suitable to the HSSP [15,16].
the monomers suitable to the HSSP when such initiators were used
[24]. It was thought to be caused by very high polymerization rates.
To realize living HSSP of substituted acetylenes, other strategy is
needed.
In this communication, we report our trial to find a new achiral
monomer which produces a one-handed helical living polymer by
using a binary catalytic system, i.e., to cause living polymerization
and HSSP in an identical polymer at the same time. To find such a
monomer, we synthesized four new achiral phenylacetylenes
having bulky substituents such as tert-butyl groups as R groups in
III (Chart 1) and polymerized them by using the catalytic system
because bulky substituents can suppress polymerization rates.
2. Experimental section
2.1. Materials
Living polymerization is a very useful method to obtain well-
organized polymers and to prepare block copolymers. For
substituted terminal acetylenes, some living polymerizations have
All the solvents used for monomer synthesis and polymerization
were distilled as usual. Dry tetrahydrofuran (99.5% purity) and dry
diethyl ether (99.5% purity) purchased from Kanto Chemical was
used as received. The polymerization initiator, [Rh(nbd)Cl]2
(nbd ¼ 2,5-norbornadiene), purchased from Aldrich Chemical was
used as received. N-(2-bromophenyl)-O-benzyl carbamate (S2 in
* Corresponding author. Tel./fax: þ81 25 262 7280.
E-mail address: toshaoki@eng.niigata-u.ac.jp (T. Aoki).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.02.009

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Y. Zang et al. / Polymer 54 (2013) 1729e1733
Fig. 1. (a) Timeeconversion curves, (b) conversioneDPn plots, and (c) first-order plots for polymerization of 4a (-), 4b (C), 4c (,), 4d (B), and SPA (
6).
Scheme S1) was synthesized according to literature [25]. 4-
Dodecyloxy-3,5-bis(hydroxymethyl)phenylacetylene (1) [9] and
4-(trimethylsilyl)phenylacetylene (SPA) [26] were synthesized ac-
cording to our report.
at ꢀ78 ꢁC. After the mixture was stirred for 1 h at the same
temperature, chlorotrimethylsilane was added dropwise to the
reaction mixture. The resulting mixture was stirred at the same
temperature for 6 h and then at room temperature for 12 h. The
reaction mixture was quenched with 2 N HCl aq. and stirred for
1 h. After separation of the two layers, the aqueous layer was
extracted with diethyl ether. The organic layer was dried over
anhydrous MgSO4 and concentrated to give crude product as
yellow liquid. The product was used for synthesis of 3c without
further purification.
2.2. Syntheses of monomers (Scheme 1 and Scheme S1)
2.2.1. Synthesis of 4-dodecyloxy-3,5-diformyphenylacetylene (2)
(Scheme 1) [13]
A solution of 1 (3.26 g, 9.41 mmol) in dichloromethane (200 mL)
was added dropwise to a solution of pyridinium dichromate (17.7 g,
47.0 mmol) in dichloromethane (100 mL) at room temperature. The
solution was stirred for 48 h. The resulting solution was filtered
through a bed of celite, and then the filtrate was concentrated by
evaporation. The brown residue was purified by silica-gel column
chromatography (eluent; hexane: ethyl acetate ¼ 2:1) to give 2 as a
white solid. Yield: 86.6% (2.79 g). Rf: 0.65 (hexane:ethyl
The mixture of the obtained product (1.23 g) and 10% Pd/C
(10 wt%, 123 mg) was stirred in H2 at room temperature for
24 h. The resulting mixture was diluted with diethyl ether
(20.0 mL) and filtrated. After concentration, the crude product
was purified by Al2O3 column chromatography (hexane: ethyl
acetate ¼ 20:1) to give 3c as yellow liquid. Yield: 12.7%
(210 mg). Rf: 0.20 (hexane:ethyl acetate ¼ 20:1). 1H NMR
acetate ¼ 2:1). IR (KBr): 1692 (C]O) cmꢀ1
;
1H NMR (400 MHz,
(400 MHz, CDCl3, ppm) d: 7.30 (dd, 1H, PhH), 7.17 (dt, 1H, PhH),
6.77 (dt, 1H, PhH), 6.63 (d, 1H, PhH), 3.77 (br, 2H, NH2), 0.34 (s,
9H, Si(CH3)3).
CDCl3, ppm) d: 10.4 (s, 2H, CHO), 8.17 (s, 2H, PhH), 4.15 (t, 2H, Oe
CH2CH2), 3.14 (s, 1H, C^CH), 1.81 (tt, 2H, OeCH2eCH2eCH2), 1.27e
1.57 (m, 18H, CH2e(CH2)9eCH3), 0.88 (t, 3H, CH2e(CH2)9eCH3).
2.2.3. Synthesis of 2-(heptyldimethylsilyl)aniline (3d) (Scheme S1)
[27,28]
2.2.2. Synthesis of 2-trimethylsilylaniline (3c) (Scheme S1) [27,28]
A hexane solution of n-BuLi (1.59 M, 13.8 mL, 22.0 mmol) was
added dropwise to S2 (3.06 g, 10.0 mmol) in diethyl ether (45 mL)
A similar procedure to 3c was carried out. See the Supporting
information.
Fig. 2. GPC charts of poly(4b) (A) and poly(4c) (B). (1) Before addition of SPA; (2) 1 h after addition of SPA. (SPA*: Monomer peak appeared just after addition.)

Y. Zang et al. / Polymer 54 (2013) 1729e1733
1731
Fig. 3. (a) Timeeconversion curves, (b) conversioneDPn plots, and (c) first-order plots for polymerization of 4b with (R)-DMPEA (-), TEA (C), (S)-PEA (:), or (S)-PEA/TEA (B).
2.2.4. Synthesis of 4-dodecyloxy-3,5-bis(2-
isopropyphenyliminomethyl)phenylacetylene (4a) (Scheme 1) [13]
A similar procedure to 4b was carried out. See the Supporting
information.
2.3. Polymerization (Scheme 2) [29]
All the procedures were performed under dry nitrogen. A
typical polymerization procedure is as follows: A solution of
[Rh(nbd)Cl]2 (0.38 mg, 0.83
phenylethylamine (DMPEA) (34
(0.33 mL) was added to a solution of 4b (50 mg, 83
m
mol) and (R)-N,N-dimethyl-1-
L, 0.21 mmol) in toluene
mol) in
2.2.5. Synthesis of 4-dodecyloxy-3,5-bis(2-tert-
butylphenyliminomethyl)phenylacetylene (4b) (Scheme 1) [13]
m
m
A
mixture of
2
(1.50 g, 4.39 mmol), 2-tert-butylaniline
toluene (0.50 mL). The reaction solution was stirred at room
temperature for 3 h. The crude polymer was purified by precipi-
tation of the toluene solution into a large amount of methanol and
dried in vacuo to give a red polymer.
Polymerizations of 4a, 4c, and 4d were carried out similarly. The
results are summarized in Table 1. Other amines were also used as a
cocatalyst. The results are summarized in Table 3.
(1.51 mL, 9.66 mmol), and Al2O3 (2.24 g) in toluene (20.0 mL) was
stirred for 18 h at room temperature. After the mixture was
filtered, the solvent was removed. The crude product was purified
by Al2O3 column chromatography (hexane:ethyl acetate ¼ 5:1) to
give 4b as yellow solid. Yield: 88.7% (2.36 g). Rf: 0.47 (hex-
ane:ethyl acetate ¼ 5:1). IR (KBr): 1616 (C]N) cmꢀ1
;
1H NMR
(400 MHz, CDCl3, ppm) d: 8.64 (s, 2H, CH]N), 8.39 (s, 2H, PhH),
7.44 (dd, 2H, NePhH), 7.18e7.25 (m, 4H, NePhH), 6.83 (d, 2H, Ne
PhH), 3.96 (t, 2H, OeCH2CH2), 3.14 (s, 1H, C^CH), 1.84 (tt, 2H, Oe
CH2eCH2eCH2), 1.47 (s, 18H, C(CH3)3), 1.21e1.26 (m, 18H, CH2e
(CH2)9eCH3), 0.88 (t, 3H, Oe(CH2)11eCH3). Anal. Calcd for
C42H56N2O: C, 83.39; H, 9.33; N, 4.63. Found. C, 83.23; H, 9.47; N,
4.47.
2.4. Synthesis of a block copolymer (poly(4b-block-SPA)) by
polymerization of SPA initiated by living poly(4b) (Scheme 2)
SPA was added to the polymerization solution of monomer 4b as
described above. The resulting solution was stirred for 60 min
(polymerization condition: in toluene, room temperature;
[4b]0 ¼ 0.1 M, [SPA] ¼ 0.67 M, [4b]0/[[Rh(nbd)Cl]2] ¼ 100, [(R)-
DMPEA]/[[Rh(nbd)Cl]2] ¼ 250). The polymerization solution was
added to a large amount of acetone to precipitate the block copol-
ymer. In order to remove homopolySPA, the crude block copolymer
was dissolved in ethyl acetate where cannot dissolve homopolySPA.
The precipitate was dried in vacuo to give a red polymer.
2.2.6. Synthesis of 4-dodecyloxy-3,5-bis(2-
trimethylsilylphenyliminomethyl)phenylacetylene (4c) [13]
A similar procedure to 4b was carried out. See the Supporting
information.
2.2.7. Synthesis of 4-dodecyloxy-3,5-bis(2-
heptyldimethylsilylphenyliminomethyl)phenylacetylene (4d)
(Scheme 1) [13]
2.5. Measurements
A similar procedure to 4b was carried out. See the Supporting
information.
Average molecular weight (Mn) was estimated by gel perme-
ation chromatography (tetrahydrofuran as an eluent, polystyrene
Fig. 4. CD and UVevis spectra of the poly(4b) polymerized by using (R)-DMPEA (a) (Table 3, no. 1), (S)-PEA/TEA (b) (Table 3, no. 4), (R)-PEA (c) (Table 3, no. 3), (S)-PEA (d),
respectively as a cocatalyst.

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Y. Zang et al. / Polymer 54 (2013) 1729e1733
Table 1
Polymerization of phenylacetylene having bulky groups (4aed).a
No. Monomer Conversion (%)b Mn (ꢂ105)b DPn
Mw/Mn
g (ꢂ10ꢀ5
)
b
b
c
1
2
3
4a
4b
4c
4d
SPA
91.7
98.7
43.1
23.7
90.0
2.38
1.15
0.118
0.0277
2.56
413
190
18.5 1.60
3.4 1.10
1470 1.96
2.85
1.46
0
1.3
0
0
0
4
Chart 1. Examples of three kinds of achiral monomers suitable to the HSSP we re-
5d
ported before.
a
Polymerized at room temperature in toluene. [monomer]0
¼
0.10 mol/L,
[monomer]0/[[Rh(nbd)Cl]2] ¼ 100, [(R)-DMPEA]/[[Rh(nbd)Cl]2] ¼ 250. Polymeriza-
tion time: 3, 3, 48, 48, and 3 h, for nos. 1e5, respectively.
b
By GPC.
c
g
¼
([q]/3300)/ε, where q ) is molar ellipticity and ε
(deg cm2 dmolꢀ1
(l molꢀ1 cmꢀ1) is molar absorption coefficient.
d
SPA: p-trimethylsilylphenylacetylene. [TEA]/[[Rh(nbd)Cl]2] ¼ 250.
Mn value of poly(4b) was smallest among the three monomers (4ae
c) that produced high-molecular-weight polymers. In addition, the
polymerization of 4b proceeded in first order with respect to
monomer concentration, indicating a bimolecular reaction be-
tween monomer 4b and the propagating end (Fig. 1c) [23]. There-
fore, polymerization of 4b was found to have more number of
characteristics as living polymerization.
Scheme 1. A synthetic route to 4aed. (PDC ¼ pyridinium dichromate).
In order to confirm the living nature of the resulting polymer
prepared by polymerization of 4b by the binary catalyst, (R)-
DMPEA/[Rh(nbd)Cl]2, we tried to synthesize a block copolymer,
poly(4b-block-SPA), prepared by polymerization of SPA by using
the living poly(4b) as a polymer initiator (Scheme 2). As shown in
Fig. 2A and Table 2, 1 h after SPA was added to the living poly(4b),
SPA was completely consumed and the GPC peak of the original
poly(4b) (Fig. 2A(1)) moved to higher MW position (Fig. 2A(2)) with
maintaining the shape. Since the composition was the same as that
in the feed (Table 2) and the product was soluble in ethyl acetate
where polySPA was insoluble, formation of poly (4b-block-SPA) was
confirmed. Therefore, the living nature was confirmed.
In the case of poly (4c) prepared by polymerization for 48 h in
Fig. 2B(1), at that time the conversion did not increase even if re-
sidual 4c was present, i.e., no further homopolymerization was
initiated. Therefore, no active initiation system such as
Rh(nbd)((R)-DMPEA)(4c) remained in the polymerization system.
However, 1 h after SPA monomer having no bulky groups was
added to the polymerization system, the SPA monomer peak dis-
appeared completely and a new peak at a higher MW appeared
(Fig. 2B(2)). Therefore, the new peak must be poly(4c-block-SPA).
However, since original poly(4c) peak still remained, original
poly(4c) was found to be partly active. Since the substituents were
too bulky to yield polymers having high DPn, termination or chain
transfer reaction partly happened more frequently than
propagation.
calibration) using JASCO Liquid Chromatography instruments with
PU 2080, DG 2080 53, CO 2060, UV 2070, CD2095, and two poly-
styrene gel columns (Shodex KF 807L). NMR spectra were recorded
on a JEOL GSX 270 at 400 MHz for 1H NMR. IR spectra were
recorded on a JASCO FTIR 4200 spectrometer. CD spectra were
measured with a JASCO J 720 spectropolarimeter.
3. Results and discussion
In general, polymerization rates of phenylacetylenes which
were suitable to the helix-sense-selective polymerization (HSSP)
we found and developed [9e16] were very high and therefore the
control of their degree of polymerizations (DPn) was impossible
[24]. To suppress the polymerization rates, four new achiral
monomers having bulky substituents (4aed in Scheme 1) were
designed and synthesized according to a synthetic route as shown
in Scheme 1. These monomers (4aed) have i-propyl(iPr) (4a), t-
butyl(tBu) (4b), trimethylsilyl(SiMe3) (4c), and dimethylheptylsilyl
(SiC7H15Me2) (4d) groups, respectively. The increasing order of
bulkiness is iPre < tBue < eSiMe3 < eSiC7H15Me2.
They were polymerized by the chiral catalytic system, a binary
catalyst consisting of (R)-N,N-dimethylphenylethylamine (DMPEA)
and [Rh(nbd)Cl]2 (nbd ¼ 2,5-norbornadiene), for the HSSP we
developed before [9e16]. Table 1 and Fig. 1 list the results together
with those of p-trimethylsilylphenylacetylene (SPA) having no
bulky groups. The conversion of 4a and 4b reached almost 100%,
while the others were low (Table 1 and Fig. 1a). Only the degree of
polymerization (DPn) of 4b could be controlled by changing con-
version and polymerization time (Fig.1b and Figs. S8 & S9). The Mw/
To optimize the living polymerization condition, other amines
such as triethylamine (TEA), phenylethylamine (PEA), and their
mixture (PEA/TEA) were examined for the polymerization of 4b as
cocatalysts in the binary catalytic system. In all the amines used,
Table 2
Preparation of block copolymer, poly(4b-block-SPA) prepared by polymerization of
SPA initiated by living poly(4b).
No. 4b conv.a (%) SPA conv.a (%) SPAfeed (mol%) SPAunitcompositionb (mol%)
1c
89.0
e
0
40
0
40
2d 90.9
99.0
a
By GPC.
b
By 1H NMR.
c
Scheme 2. Polymerization of 4aed and synthesis of block copolymers poly(4aed-
block-SPA). (Chiral amine ¼ (R)-N,N-dimethyl-1-phenylethylamine (DMPEA) or (R)- or
(S)-phenylethylamine (PEA)).
Polymerized for 3 h at room temperature in toluene. [4b]0 ¼ 0.10 mol/L, [4b]0/
[[Rh(nbd)Cl]2] ¼ 100, [(R)-DMPEA]/[[Rh(nbd)Cl]2] ¼ 250.
d
SPA was added to the resulting solution of no. 1, and copolymerized for 1 h.

Y. Zang et al. / Polymer 54 (2013) 1729e1733
1733
Table 3
Acknowledgement
Polymerization of phenylacetylene having t-butyl groups (4b) in the presence of
different amines.a
Partial financial support through a Grant-in-Aid for Scientific
Research (B) (No. 19350054) from the Japan Society for the Pro-
motion of Science is gratefully acknowledged.
Conversion (%)b Mn (ꢂ106)b DPn
Mw/Mn g (ꢂ10ꢀ5
)
b
b
c
No. Amine
1
2
3
(R)-DMPEA 99.4
TEA
0.120
0.197
2.00
198 1.50
326 1.60
3310 2.12
1130 1.84
1.3
0
4.7
ꢀ1.7
94.2
46.6
(R)-PEA
4d (S)-PEA/TEA 73.5
0.686
Appendix A. Supplementary data
a
Polymerized at room temperature in toluene. [4b]0
[[Rh(nbd)Cl]2] ¼ 100, [amine]/[[Rh(nbd)Cl]2] ¼ 250. Polymerization time: 3, 3, 18,
¼
0.10 mol/L, [4b]0/
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.02.009.
and 12 h, for nos. 1e4, respectively.
b
By GPC.
c
g ¼ ([
q]/3300)/ε.
d
[TEA]/[(S)-PEA] ¼ 10.
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In conclusion, by using a new achiral phenylacetylene having
bulky tert-butyl substituents as a monomer, living-like helix-sense-
selective polymerization (HSSP) has been achieved. This is the first
example of HSSP of substituted acetylenes whose degree of poly-
merization was controlled. The one-handed helical living polymer
could initiate polymerization of the second monomer, p-trime-
thylsilylphenylacetylene (SPA) to successfully yield a block copol-
ymer. The polymerization rate of a monomer (4a) having less bulky
substituents, iso-propyl groups was too high to control the mo-
lecular weight, while the polymerization rates of monomers (4c, d)
having more bulky substituents such as trimethylsilyl or dime-
thylheptylsilyl groups were too low to maintain the activity of the
propagation species during polymerization. In addition, polymeri-
zation of the monomer (4b) having tert-butyl substituents gave a
one-handed helical polymer judging from their CD and UV.
Therefore, the bulkiness of tert-butyl group was the best to produce
polymers simultaneously having one-handed helix and controlled
DPn, that is, to realize living HSSP.