Synthesis of asymmetrically substituted head-to-head polyacetylenes from 2,3-disubstituted-1,3-butadienes

Article abstract of DOI:10.1002/pola.29215

Asymmetrically substituted head-to-head polyacetylenes with phenyl and triphenylamine, thienyl or pyrenyl side groups were synthesized through anionic or controlled radical polymerization of 2,3-disubstituted-1,3-butadienes and subsequent dehydrogenation process. Anionic polymerizations of the designed monomers bearing pendent triphenylamine and thienyl group gave narrow disperse disubstituted precursor polybutadienes with exclusive 1,4- or 4,1-structure, which were confirmed by GPC and NMR measurements. In addition, the monomers possessing pyrenyl group were polymerized via nitroxide mediated radical polymerization and the resulting polymers were obtained with controlled molecular weight and low polydispersities. These polybutadiene precursors were then dehydrogenated in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone. Thus asymmetrically substituted head-to-head polyacetylenes were obtained as indicated by ¹H NMR. The properties of polybutadiene precursors and the corresponding polyacetylenes were analyzed by UV–vis, DSC, and TGA.

Full text of DOI:10.1002/pola.29215

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Synthesis of Asymmetrically Substituted Head-to-Head Polyacetylenes
from 2,3-Disubstituted-1,3-butadienes
Yunhai Yu, Chengke Qu, Junpo He
The State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University,
Shanghai 200433, People’s Republic of China
Correspondence to: J. He (E-mail: jphe@fudan.edu.cn)
Received 31 May 2018; Accepted 7 August 2018
DOI: 10.1002/pola.29215
ABSTRACT: Asymmetrically substituted head-to-head polyacety-
lenes with phenyl and triphenylamine, thienyl or pyrenyl side
groups were synthesized through anionic or controlled radical
polymerization of 2,3-disubstituted-1,3-butadienes and subse-
quent dehydrogenation process. Anionic polymerizations of the
designed monomers bearing pendent triphenylamine and thie-
nyl group gave narrow disperse disubstituted precursor polybu-
tadienes with exclusive 1,4- or 4,1-structure, which were
confirmed by GPC and NMR measurements. In addition, the
monomers possessing pyrenyl group were polymerized via nitr-
oxide mediated radical polymerization and the resulting poly-
mers were obtained with controlled molecular weight and low
polydispersities. These polybutadiene precursors were then
dehydrogenated in the presence of 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone. Thus asymmetrically substituted head-to-
head polyacetylenes were obtained as indicated by 1H NMR.
The properties of polybutadiene precursors and the corres-
ponding polyacetylenes were analyzed by UV–vis, DSC,
and TGA. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A:
Polym. Chem. 2018
KEYWORDS: anionic polymerization; dehydrogenation; disubsti-
tuted butadiene; nitroxide mediated radical polymerization;
substituted polyacetylenes
only for the preparation of functional elastomers, but also for
the preparation of regiospecific and sequence specific poly-
INTRODUCTION Nowadays, precision polymers are attracting
much attention in polymer science thanks to the advent of vari-
ous controlled or living polymerization techniques.1 These
polymerization systems, in combination with post-modification
and monomer design, have been used to synthesize polymers
with tailored molecular parameters such as molecular weight,
polydispersity, topological architecture, end group function-
ality, and chain microstructures including regiospecificity,
cis–trans configuration and sequence, and so forth.2,3 Among
these approaches, polymer molecular engineering based on
substituted 1,3-butadiene derivatives affords a unique way to
prepare functionalized polybutadienes,4 head-to-head vinyl
polymers,5 substituted polyacetylenes6,7 and sequence con-
trolled polymers.8 These butadiene based monomers can be
polymerized through radical or ionic mechanism depending
on the substitution position and structure.
mers.
A feature of the polymerization of disubstituted
1,3-butadienes was the high regioselectivity exerted by the
steric and electronic effect of the side groups. For monomers
with high steric hindrance, products with linear chain of
exclusive 1,4-addition were obtained. Monomers with identi-
cal side groups, such as methyl, phenyl, N,N-dialkylamino-
methyl, 4-ethoxy-4-oxo-butyl, and branched terphenyl group
at 2,3-position were polymerized through radical or anionic
mechanism, resulting in exclusive 1,4-addition.9–14 Some of
these products were converted by hydrogenation into head-
to-head vinyl polymers,10,15 or by dehydrogenation into
substituted polyacetylenes,6,7,14,16,17 respectively. Sequential
anionic polymerization of 2,3-bisphenyl-1,3-butadiene and sty-
rene followed by hydrogenation afforded regioblock polymer
of styrene containing head-to-head and head-to-tail seg-
ments.18 Cyclic and bicyclic conjugated diene monomers, such
as cyclopentadiene,19 cyclohexadiene,20,21 and tetrahydroin-
dene (THI),22 were polymerized through cationic or anionic
mechanism, respectively. The monocyclic dienes gave prod-
ucts with mixed 1,2-, and 1,4-addition, whereas the bicyclic
THI gives high content of 1,4-addition through cationic
polymerization.22
A number of monosubstituted 1,3-butadienes were synthe-
sized and anionically polymerized to prepare functionalized
polybutadiene elastomers.4 The stereochemistry was of spe-
cial importance in these studies to obtain products with high
cis-1,4-enchainment, and thus better elasticity of the resulting
elastomer. Conversely, disubstituted monomers were used not
Additional supporting information may be found in the online version of this article.
© 2018 Wiley Periodicals, Inc.
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Synthesis and polymerization of asymmetrically substituted
1,3-butadienes with different side groups at 1- and 3-, or 2-
and 3-positions are more challenging. Kamachi et al. reported
the radical polymerizations of ethyl trans-4-ethoxy-2,4-
pentadienoate and trans-4-ethoxyl-2,4-pentadienonitrile. The
resulting polymers were hydrogenated to give alternating
copolymers of ethyl vinyl ether/ethyl acrylate and of ketene/
acrylonitrile, respectively.23 Li et al. reported anionic polymer-
ization of 1-phenyl-3-isopropyl-1,3-butadiene (PhiPrBu). The
polymerization was highly regioselective and the resulting
product was dehydrogenated into substituted polyacetylene
with alternating sequence of the substituents.8 Qu et al. per-
formed cationic polymerization of 1-alkoxy-3-phenyl-1,-
3-butadiene. Exclusive 1,4-addition was observed and the
products were hydrogenated into alternating copolymers of
styrene/vinyl ether or styrene/vinyl alcohol.24 To the best of
our knowledge, there is no report on the synthesis of poly-
merization of 2,3-asymmetrically disubstituted 1,3-butadienes.
In this study, we synthesized three butadiene monomers with
different substituents at 2- and 3-position with the purpose of
preparation of substituted polyacetylene possessing head-to-
head connectivity with two different side groups.
polystyrene standard with molecular weight ranging from
2.2 × 103 to 5.2 × 105 g mol−1. Differential scanning calorime-
ter (DSC, TA Q2000) was carried out unꢀder nitrogen atmo-
sphere from ambient temperature to 250 C at a heating rate
of 10 C min−1. Thermogravimetric analysis (TGA, TA Q5000)
ꢀ
was conducted under nitrogen atmosphere from ambient tem-
perature to 500 ꢀC at a heating rate of 10 ꢀC min−1
.
Synthesis of (4-Acetylphenyl)diphenylamine
Triphenylamine (8.84 g, 0.036 mol), anhydrous AlCl3 (2.45 g,
0.018 mol), and CH2Cl2 (36 mL) were added to a solution of
acetyl chloride (2.82 g, 2.6 mL, 0.036 mol) in CH2Cl2 (8 mL)
with vigorous stirring at ambient temperature over 10 min.
The reaction mixture was refluxed for 12 h. After cooling to
room temperature, 200 mL aqueous HCl (2 mol L−1) was
added. The organic layer was separated, washed with water,
and dried with Na2SO4. The CH2Cl2 was removed using a
rotary evaporator and the residual oil was purified with col-
umn chromatography on silica gel using petroleum ether/
chloroform (v/v, 3:1) as eluent to give a yellow solid
(8.27 g, 80%).
Synthesis of 2-(4-(Diphenylamino)phenyl)-3-phenylbut-
3-en-2-ol
EXPERIMENTAL
A solution of 25 mL α-bromostyrene in THF (1 M, 25 mmol)
was added dropwise to magnesium powder (0.73 g, 30 mmol)
activated with I2 in THF (5 mL) at room temperature under
nitrogen. The reaction mixture was stirred at room tempera-
ture for 1 h. A THF solution (50 mL) of (4-acetylphenyl)diphe-
Materials
Styrene (National Pharmaceutical, 99%), magnesium powder
(J&K, 99%), bromine (Shanghai Ling-feng, 99%), triphenylamine
(J&K, 99%), thiophene (J&K, 99%), pyrene (J&K, 99%),
acetyl chloride (J&K, 99%), anhydrous AlCl3 (J&K, 99%),
tetrakis(triphenylphosphine) palladium (Pd(PPh3)4, J&K, >99%),
p-toluenesulfonic acid monohydrate (National Pharmaceutical,
99%) and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ,
98%) and 2-methyl-2-[N-tert-butyl-N-(diethoxyphosphoryl-2,-
2-dimethylpropyl) aminooxy]propionic acid (BlocBuilder MA,
Arkema, 97%) were used as received. n-Butyllithium (n-BuLi)
(Acros, 2.4 M solution in cyclohexane) and sec-butyllithium (s-
BuLi) (Acros, 1 M solution in cyclohexane) were titrated before
use. Tetrahydrofuran (THF), cyclohexane (CHX) and toluene
ꢀ
nylamine (5.17 g,18 mmol) was then added dropwise at 0 C
to the reaction system. The resulting mixture was stirred at
room temperature for 5 h and the reaction was quenched
with water. The resulting organic layer was extracted with
CH2Cl2 and dried over anhydrous MgSO4. After distillation of
the solvent, a light brown liquid (5.99 g, 85%) was obtained.
The crude product was used for the next reaction step with-
out isolation.
Synthesis of 2-(Triphenylamine)-3-phenyl-1,3-butadiene
To a solution of 2-(4-(diphenylamino)phenyl)-3-phenylbut-
3-en-2-ol (3.91 g, 10 mmol) in dioxane (100 mL) was added
p-toluenesulfonic acid monohydrateꢀ (0.19 g, 1 mmol). The
resulting mixture was heated to 110 C for 3 h. After the reac-
tion mixture was cooled to room temperature, 50 mL water
was added. The resulting organic layer was extracted with
CH2Cl2 and dried over anhydrous MgSO4. The CH2Cl2 was
removed using a rotary evaporator and the crude product
was purified with column chromatography on silica gel using
petroleum ether/ethyl acetate (v/v, 10:1) as eluent to afford
as a light yellow liquid (3.36 g, 90%). 1H NMR (400 MHz,
CDCl3), δ (TMS, ppm): 5.22 (d, 1H), 5.32 (d, 1H), 5.52 (d, 1H),
5.54 (d, 1H), 6.89–7.12 (m, 8H), 7.14–7.31 (m, 9H), 7.36–7.45
(m, 2H). MS: m/z calcd for C28H23N (M + H)+, 374; found, 374.
were passed through
a column of Al2O3 before use.
α-Bromostyrene was synthesized according to a reported
procedure.25
Measurements
High-pressure liquid chromatography was performed on an
instrument composed of a Waters 515 pump, a C-18 column,
and a UV detector with the wavelength set at 254 and
300 nm. Acetonitrile/water (v/v = 83/17) mixture was used
as eluent (1.0 mL min−1) at 40 C. H and 13C NMR measure-
1
ꢀ
ments were carried out on a Bruker (400 MHz) NMR instru-
ment, using CDCl3 as the solvent and tetramethylsilane (TMS)
as the interior reference. Spectrophotometer Lambda 750 (Per-
kinElmer) was used for UV–vis measurements. Size exclusion
chromatography (SEC) analysis was performed on a Waters
system equipped with a guard column and three TSK columns
(Gel H-type, pore size 15, 30, and 200 Å) in series, a Waters
410 RI detector, using THF as the eluent at a flow rate of
The synthetic procedures of 2-thienyl-3-phenyl-1,3-butadiene
(ThPPB) and 2-pyrenyl-3-phenyl-1,3-butadiene (PyPB) were
similar to that of 2-(triphenylamine)-3-phenyl-1,3-butadiene
1
1 mL min−1 at 35 C. The columns were calibrated by narrow
(TPAPB). H NMR result for ThPPB: 5.18 (d, 1H), 5.39 (d, 1H),
ꢀ
2
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5.55 (d, 1H), 5.60 (d, 1H), 6.95–7.18 (m, 8H), 6.73–6.87 (m,
2H), 6.99–7.09 (m, 1H), 7.12–7.27 (m, 3H), 7.36–7.46 (m, 2H).
and stopped by slowly immersing the flask into liquid nitrogen.
The resulting polymer was obtained as a yellow powder after
three times precipitation in methanol and dried under vacuum
1
MS: m/z calcd for C14H12S (M + H)+, 213; found, 213. H NMR
spectral data for PyPB: 4.86 (d, 1H), 5.29 (d, 1H), 5.50 (d, 1H),
5.65 (d, 1H), 7.17–7.65 (m, 5H), 7.93–8.41 (m, 9H). MS: m/z
calcd for C26H18 (M + H)+, 331; found, 331.
at 50 ꢀC. Yield: 0.80 g (35%), Mn,GPC = 2700 g mol−1
.
Dehydrogenation of Poly(TPAPB)
A Schlenk flask was charged with poly(TPAPB) (0.2 g), DDQ
(0.49 g, 2.14 mmol), and 5 mL anhydrous dichlorobenzene
under a nitrogen atmosphere. The flask was degassed by
freeze-pump-thaw cycles, charged with nitrogen, and placed
in a thermostated oil bath at 110 ꢀC for 24 h. The resulting
product was precipitated by pouring the reaction mixture into
500 ml methanol. After dried under vacuum for 24 h, the
dehydrogenated product was afforded as a black powder.
Yield: 0.16 g (80%). The dehydrogenation of poly(ThPPB) and
poly(PyPB) was similar to that of poly(TPAPB).
Anionic Polymerization of TPAPB
A typical polymerization is given in the following example.
Freshly distilled CHX (40 mL) was mixed with a small amount
of THF and a CHX solution of TPAPB (2.00 g, 5.00 mmol) in the
reaction flask. The reaction flask was then heated to 40 ꢀC and
s-BuLi in hexane (0.37 mL, 0.37 mmol) was added. After the
ꢀ
reaction solution was stirred at 40 C for 1 h, the polymeriza-
tion was terminated with degassed methanol and the resulting
polymer was isolated by precipitation into methanol (20-fold
volume excess to reaction mixture). The final product was sep-
arated by filtration and dried under vacuum at 50 ꢀC to a con-
RESULTS AND DISCUSSION
stant weight. Yield: 1.98 g (99%), Mn,GPC = 4200 g mol−1
.
Synthesis of 2,3-Disubstituted-1,3-butadiene Monomers
Three monomers, TPAPB, ThPPB, and PyPB, with electron
donating or conjugating groups were synthesized through the
procedure illustrated in Scheme 1. The process involved
Friedel–Crafts acylation reaction on corresponding aromatic
compounds 1 to form ketone 2, Grignard reaction to obtain
allylic alcohol 3, which was subsequently dehydrated to
obtain monomer 4. The overall yields were ranging from
45–73%. The chemical structures of designed monomers
were confirmed by 1H NMR and GC–MS as shown in the
Experimental section.
The procedure of anionic polymerization of ThPPB was simi-
lar to that of TPAPB.
Nitroxide Mediated Controlled/“Living” Radical
Polymerization of PyPB
BlocBuilder MA (76.2 mg, 0.2 mmol) and PyPB (2.31 g, 7 mmol)
were dissolved in 4 mL toluene. The reaction mixture was
thoroughly deoxygenated by three freeze-pump-thaw cycles.
The polymerization reaction was performed at 110 ꢀC for 16 h
SCHEME 1 Synthesis of asymmetrically substituted head-to-head polyacetylenes from 2,3-disubstituted butadiene monomers.
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(a)
(b)
(c)
Mn = 5600 g/mol, Mw/Mn = 1.09
Mn = 10400 g/mol, Mw/Mn = 1.09
Mn= 4200 g/mol, Mw/Mn = 1.09
Mn= 8500 g/mol, Mw/Mn = 1.19
Mn = 2700 g/mol, Mw/Mn = 1.15
10
12
14
16
18
20
22
24
26
10 12 14 16 18 20 22 24 26 28
10
12
14
16
18
20
22
24
26
Elution time (min)
Elution time (min)
Elution time (min)
FIGURE 1 GPC traces of (a) poly(TPAPB)s prepared by anionic polymerization in CHX/THF, (b) poly(ThPPB)s prepared by anionic
polymerization in THF, and (c) poly(PyPB) prepared by nitroxide mediated radical polymerization in toluene.
Anionic Polymerization of Asymmetrically
2,3-Disubstituted-1,3-butadienes
Anionic polymerization of PyPB gave no polymer in CHX in
the presence of trace amount of THF, and produced only olig-
omers in THF at −78 C (Table 1 and Supporting Information
Fig. S4). Such effects may be attributed to the electron-
transfer and addition reactions between anion and pyrenyl
group.26 In addition, the stabilizing effect of the large conju-
gated system on the anionic species may also reduce the reac-
tivity for further monomer addition. Therefore, PyPB was
polymerized through radical mechanism (vide infra).
ꢀ
The three monomers exhibited very different anionic polymeri-
zation behaviors depending on solvent and temperature. TPAꢀPB
did not polymerize in the presence of s-BuLi in THF at −78 C,
whereas it polymerized smoothly in CHX in the presence of
trace amount of THF (THF/Li ≈ 10/1) to complete conversion
(~99%). As shown in Figure 1(a), the obtained poly(TPAPB)s
exhibited unimodal GPC curves with Mn = 4200 g mol−1 and
8500 g mol−1 with low polydispersity indices of 1.09 and 1.19,
respectively.
Two of the anionic polymerization products, poly(TPAPB) and
poly(ThPPB), were characterized by MALDI-TOF MS. As can
be seen in Supporting Information Figures S2 and S3, there
was only one series of chain species in each of the spectrum,
indicating that side reaction was negligible. The intervals
between two successive peaks were 373.5 and 212.7 Da,
respectively, consistent with molar mass of the repeating unit
from the corresponding monomers. The molar mass of each
individual signal corresponded well with the theoretical value
calculated from one butyl end group derived from sec-BuLi,
n repeating units, and Ag+ from the ionization salt.
The anionic polymerization of ThPPB reached merely 10%
conꢀversion in CHX in the presence of trace amount of THF at
40 C for 4 h. The obtained poly(ThPPB) showed broad distri-
bution (Supporting Information Fig. S1). Nevertheless, ThPPB
ꢀ
polymerized to high conversion in THF at −78 C with differ-
ent (ThPPB)/(s-BuLi) ratio. As shown in Figure 1(b), the GPC
traces of the resulting polymers were unimodal and the poly-
dispersity indices were low.
TABLE 1 Polymerization of Asymmetrically 2,3-Disubstituted-1,3-butadienes.
Time
(h)
Temperature
(ꢀC)
Yield
(%)
Mn,theo.
Mn,GPC
Mn,MALDI
Monomera
TPAPB
Solventb
Initiator/Mediator
(g mol−1
)
(g mol−1
)
(g mol−1
)
PDI
CHX
CHX
THF
CHX
THF
THF
CHX
THF
THF
TOLe
s-BuLid
s-BuLid
s-BuLi
1
1
40
40
99
99
–
4950
9900
–
4200
8500
–
N.D.c
N.D.c
–
1.09
1.19
–
2
−78
40
ThPPB
PyPB
s-BuLid
4
10
93
90
–
1000
4650
9000
–
2000
5600
10300
–
1980
5680
11000
–
1.20
1.09
1.09
–
s-BuLi
4
−78
−78
40
s-BuLi
s-BuLid
8
1
s-BuLi
2
−78
−78
110
d
–
–
–
–
–
s-BuLi
10
16
–
–
–
–
–
BlocBuilder MA
35
4430
2700
3800
1.15
a
Anionic polymerization of 2,3-disubstituted butadienes were carried out
using s-BuLi as the initiator.
[THF]/[s-BuLi] = 10/1 (mol %).
Nitroxide mediated radical polymerization was used instead of anionic
polymerization.
e
b
c
CHX, cyclohexane.
N.D. = not detected.
4
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(a)
b+c+d+e+f+g+h+i+
b'+c'+d'+e'+f'+g'+h'+i'
(b)
d+e+f+i+j+m+n+o+
d'+e'+f'+i'+j'+m'+n'+o'
l+l'
c+h+c'+h'
k'
b+g+b'+g'
k
150 148 146 144 142 140 138 136
a+a'+j+j'
no pendent vinyl proton
a+a'+p+p'
8
7
6
5
4
3
2
1
0
160 140 120 100 80
60
40
20
0
Chemical shift (ppm)
Chemical shift (ppm)
FIGURE 2 (a) 1H NMR and (b) 13C NMR spectra of poly(TPAPB). [Color figure can be viewed at wileyonlinelibrary.com]
The polymerization products of the three monomers were sol-
uble in cyclohexane, THF, CHCl3, and toluene.
occurred during the ionization process, which was often
observed in MALDI-TOF MS analysis of NMRP products.28
Nitroxide Mediated Radical Polymerization of PyPB
Microstructure Analysis of Asymmetrically Disubstituted
Polybutadienes
Although it did not polymerize under aforementioned
anionic polymerization conditions, PyPB can be polymerized
with the radical initiator. To control the molecular weight of
the resulting polymer, nitroxide mediated radical polymeri-
zation (NMRP) of PyPB was performed at 110 ꢀC with a
commercially available alkoxyamine, BlocBuilder MA as the
initiator and the mediating agent. The polymerization was
slow, a feature of NMRP due to the persistent radical
effect.27 The resultant poly(PyPB) exhibited unimodal GPC
trace (Mn = 2700 g mol−1, PDI = 1.15), as shown in Figure 1
(c). MALDI-TOF MS spectrum in Supporting Information
Figure S5 shows a symmetrical series of peaks, the interval
between two successive peaks corresponding to the molar
mass of the repeat unit, 330.4 Da. The average molecular
The microstructures of the polymerization products were ana-
lyzed by 1H and 13C NMR. As shown in Figure 2, no signals
are observed in the region δ = 4−6 ppm in 1H NMR and
δ = 110−120 ppm in 13C NMR of poly(TPAPB), indicating the
absence of pendent vinylic group. Similar result was observed
for poly(ThPPB) in Figure 3. This demonstrated that the
anionic polymerization of the two monomers proceeded with
exclusive 1,4- or 4,1-enchainment. Calculations of trans/cis
ratios for the polybutadiene derivatives were performed on
the basis of the assignments reported previously.7,8 The car-
bons k + k0 [Fig. 2(b)] were chosen for the calculation of cis/
trans ratio because of well-separated doublet peaks. The
chemical shift located at peak k is for trans and peak k0 for cis
carbon. It was found that the poly(TPAPB) consisted of
approximately 48% trans and 52% of the cis isomeric struc-
ture. Similarly, it was estimated from peak g and g0 that the
poly(ThPPB) consisted of about 91% cis and 9% trans struc-
tures [Fig. 3(b)]. The different trans/cis ratios of the two
weight measured by MALDI-TOF MS (Mn,MS = 3800 g mol−1
)
was higher than that measured by SEC in which polystyrene
was used as the calibration standard. In addition, a minor
series of peaks was observed in the MS spectrum, indicating
that homolytic cleavage of C ON bond in the end group
(b)
(a)
b+c+d+e+f+g+
b'+c'+d'+e'+f'+g'
d+e+f+i+j+k+
d'+e'+f'+i'+j'+k'
b+c+h+b'+c'+h'
g'
g
146
144
142
140
138
a+a'+h+h'
a+a'+l+l'
no pendent vinyl proton
8
7
6
5
4
3
2
1
0
160 140 120 100
80
60
40
20
0
Chemical shift (ppm)
Chemical shift (ppm)
FIGURE 3 (a) 1H NMR and (b) 13C NMR spectra of poly(ThPPB). [Color figure can be viewed at wileyonlinelibrary.com]
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(a)
(b)
b+c+d+e+f+g+h+i+j+
d+e+f+i+j+k+l+m+n+o+p+q+
k+l+m+b'+c'+d'+e'+f'+
g'+h'+i'+j'+k'+l'+m'
r+s+t+u+d'+e'+f'+i'+j'+k'+l'+
m'+n'+o'+p'+q'+r'+s'+t'+u'
b+c+h+g+
b'+c'+h'+g'
a+a'+n+n'
a+a'+v+v'
no pendent
vinyl proton
9
8
7
6
5
4
3
2
1
0
160
140
120
100
80
60
40
20
0
Chemical shift (ppm)
Chemical shift (ppm)
FIGURE 4 (a) 1H NMR and (b) 13C NMR spectra of poly(PyPB). [Color figure can be viewed at wileyonlinelibrary.com]
polymers were caused by the different reaction conditions.
Poly(TPAPB) was obtained in the cyclohexane/THF (small
amount of THF served as additive) at 40 ꢀC, whereas the
poly(ThPPB) was synthesized in THF (polar solvent) at low
chlorobenzene, and CHCl3, whereas insoluble in cyclohexane
and toluene. 1H NMR in Figure 5(a) shows the signals of methy-
lene protons of poly(TPAPB) (δ = 1.5–2.5 ppm) disappear and
the signals of the resulting olefinic and aromatic protons shift
to the range of δ = 5.5–7.5 ppm to form a broadband, indicating
that poly(TPAPB) was dehydrogenated completely. In addition,
the dehydrogenation of poly(ThPPB) and poly(PyPB) also pro-
ceeded successfully as illustrated by the disappearance of meth-
ylene signal, as shown in Figure 5(b,c), respectively.
ꢀ
temperature (−78 C). As can be seen in Figure 4, the signals
at the chemical shift δ = 5.5–8.5 ppm are assigned to the aro-
matic protons of poly(PyPB), whereas the peaks in the high
field region are from main chain methylene, as well as tert-
butyl groups in BlocBuilder MA fragments. Furthermore, NMR
results also showed that the enchainment of PyPB polymeriza-
tion is nearly 100% 1,4- or 4,1-style. Unfortunately, further
information on configuration was not available, as the corre-
sponding carbons were not split into two peaks.
The GPC results of the products before and after dehydrogena-
tion are shown in Figure 6. It is noted that the dehydrogenated
products, poly(TPAPA), poly(ThPPA), and poly(PyPA) exhibit
very broad GPC profiles, with a shoulder of higher molecular
weight and prolonged peak elution time in comparison with pre-
cursor polybutadienes, poly(TPAPB), poly(ThPB), and poly
(PyPB), respectively. Similar phenomenon was observed previ-
ously for symmetrically 1,3- and 2,3-disubstituted-1,3-butadiene
systems and the asymmetrically 1,3-disubstituted-1,3-butadi-
ene.7,8,29 The reason for the high molecular weight shoulder may
be ascribed to the aggregation of the conjugated chain. A hint for
this is the higher shoulder on the GPC curve of poly(PyPA) [Fig. 6
(c)] in which higher degree of aggregation may be caused by
Transformation of the Products into Asymmetrical
Substituted Head-to-Head Polyacetylenes by
Dehydrogenation
The above polymerization products, poly(TPAPB), poly(ThPPB),
and poly(PyPB) were white to light yellow powders. The dehy-
drogenation reactions were performed in dichlorobenzene at
ꢀ
110 C in the presence of DDQ. The dehydrogenated products
were obtained as black powders after precipitation in methanol.
These black substances were partly soluble in THF,
(a)
(b)
(c)
THF-d8
CDCl3
a' + b'
THF-d8
a' + b'
a' + b'
Poly(PyPA)
DDQ
Poly(TPAPA)
DDQ
Poly(ThPPA)
DDQ
a + b
a + b
a + b
Poly(TPAPB)
Poly(PyPB)
Poly(ThPPB)
9
8
7
6
5
4
3
2
1
9
8
7
6
5
4
3
2
1
0
9
8
7
6
5
4
3
2
1
0
Chemical shift (ppm)
Chemical shift (ppm)
Chemical shift (ppm)
FIGURE 5 1H NMR spectra of (a) poly(TPAPB) and poly(TPAPA), (b) poly(ThPPB) and poly(ThPPA), and (c) poly(PyPB) and poly(PyPA).
[Color figure can be viewed at wileyonlinelibrary.com]
6
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(c)
(b)
(a)
Poly(ThPPB)
Poly(ThPPA)
Poly(PyPB)
Poly(PyPA)
Poly(TPAPB)
Poly(TPAPA)
10 12 14 16 18 20 22 24 26
12 14 16 18 20 22 24 26 28
10 12 14 16 18 20 22 24 26
Elution time (min)
Elution time (min)
Elution time (min)
FIGURE 6 GPC traces of (a) poly(TPAPB) and poly(TPAPA), (b) poly(ThPPB) and poly(ThPPA), and (c) poly(PyPB) and poly(PyPA).
additional π–π stacking among pyrene moieties. Conversely, the
reason for the peak shift to lower molecular weight region may
be due to chain degradation during the dehydrogenation reac-
tion, or adsorption of the substituted polyacetylenes by the GPC
column beads.
Results of TGA are presented in Figure 8. It is obvious that
the stabilities of polymers before and after dehydrogenation
are significantly different, the former degraded to more than
90% while the latter gave only about 20% weight loss at
500 ꢀC. Among the precursor polymers, the temperatures of
the main weight losses and the residual percentages depended
on the structure of the side groups. Pꢀoly(PyPB) exhibited
highest decomposition temperature (385 C) and highest con-
tent of the residue (ca. 10%), indicative of highest stability
exerted by the pyrene group. It is also noted that poly(PyPB)
A deterministic method to analyze the presence of aggregates
is light scattering. However, the measurement of light scatter-
ing did not give reasonable result due to light absorption by
the sample solution.
ꢀ
showed a minor weight loss at 196 C because of the cleavage
The UV–vis spectra of the precursor polybutadienes and the
corresponding substituted polyacetylenes are given in Figure 7.
It is obvious that all of the substituted polyacetylenes exhibit
very broad absorption bands extending to wavelengths longer
than 600 nm, which are attributable to π–π* electronic transi-
tions of the larger π-conjugated system. The characteristic
peaks of the chromophores are still visible in the spectra of the
dehydrogenated products.
of the nitroxyl end group. For the substituted polyacetylene
samples, poly(ThPPA) seems to be the lowest stability as its
weight loss was most obvious before 200 ꢀC.
CONCLUSIONS
Three kinds of asymmetrically substituted head-to-head poly-
acetylenes were successfully synthesized through the follow-
ing two steps, living anionic polymerization or controlled
radical polymerization of 2,3-disubstituted-1,3-butadienes and
the subsequent DDQ dehydrogenation process. It was found
that the butadienes bearing phenyl/triphenylamine and phe-
nyl/thienyl side groups were polymerizable with the initiation
of s-BuLi, whereas monomer with phenyl/pyrenyl side groups
was suitable for nitroxide mediated radical polymerization. It
is necessary to choose appropriate conditions such as solvent
and temperatures in anionic polymerizations of TPAPB and
ThPPB. Interestingly, both anionic polymerization and radical
DSC measurements were conducted for both the precursors
and the dehydrogenated products. As shown in Supporting
Information Figure S7, the glass transition temperatures (Tg)
of poly(TPAPB) (8500 g mol−1), poly(ThPPB) (10300 g mol−1),
and poly(PyPB) (2700 g mol−1) were about 132 ꢀC, 107 ꢀc,
and 169 ꢀC, respectively. Obviously, larger side group led to
higher Tg. The dehydrogenated products showed no Tg within
the measured temperature range, indicative of rigid backbone
of the resulting polyacetylenes.
(a)
(b)
(c)
Poly(ThPPB)
Poly(ThPPA)
Poly(PyPB)
Poly(PyPA)
Poly(TPAPB)
Poly(TPAPA)
300
400
500
600
700
800
300
400
500
600
700
800
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
FIGURE 7 UV–vis spectra of (a) poly(TPAPB) and poly(TPAPA), (b) poly(ThPPB) and poly(ThPPA), and (c) poly(PyPB) and poly(PyPA).
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(a)
(b)
(c)
100
Td = 369 oC
Td = 349 o
C
Td,1 = 196 o
C
Td,2 = 385 o
C
100
100
80
60
40
20
0
80
60
40
80
60
40
20
0
Poly(ThPPB)
Poly(ThPPA)
Poly(TPAPB)
Poly(TPAPA)
Poly(PyPB)
Poly(PyPA)
20
0
100
200
300
400
500
100
200
300
400
500
100
200
300
400
500
Temperature (oC)
Temperature (oC)
Temperature (oC)
FIGURE 8 TGA results for (a) poly(TPAPB) and poly(TPAPA), (b) poly(ThPPB) and poly(ThPPA), and (c) poly(PyPB) and poly(PyPA).
11 A. Hirao, Y. Sakano, K. Takenaka, S. Nakahama, Macromole-
cules 1998, 31, 9141.
polymerization yielded products with exclusive 1,4-addition,
thus facilitating the transformation of these precursor poly-
mers into corresponding substituted polyacetylenes through
dehydrogenation processes using DDQ as the oxidation agent.
The UV–vis results demonstrated that the head-to-head type
substituted polyacetylenes absorb at longer wavelength,
which is similar to that reported previously.30
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ACKNOWLEDGMENT
This work is subsidized by the National Natural Science Foun-
dation of China (No. 21474016).
19 M. Ouchi, M. Kamigaito, M. Sawamoto, Macromolecules
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20 J. He, K. Knoll, G. McKee, Macromolecules 2004, 37, 4399.
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