Cis and trans radicals generated in helical poly(propargyl acetate)s prepared using a [Rh(norbornadiene)Cl]2 catalyst

Article abstract of DOI:10.1016/j.polymer.2010.12.023

The poly(propargyl acetate) (A) having a helical cis-transoid structure was stereospecifically prepared using the Rh complex catalyst, [Rh(norbornadiene) Cl]₂, in MeOH or NEt₃ solvent at 0 and 40 °C in moderate yield. Electron spin resonance (ESR) analysis of the polymer revealed the formation of the cis (B) and trans (C) radicals which were produced through the thermal rotational scission of the helical cis CC bonds in the main-chain during the polymerization. The spatial and geometrical structure was successfully deduced using the two analogues' polymers in which either methyl or methylene group is deuterated, by the aide of computer simulation of the observed ESR spectra together with the calculation of spin density of the two radicals.

Full text of DOI:10.1016/j.polymer.2010.12.023

Polymer 52 (2011) 646e651
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Cis and trans radicals generated in helical poly(propargyl acetate)s prepared
using a [Rh(norbornadiene)Cl]2 catalyst
,
Yoshiaki Yoshida a, Yasuteru Mawatari a, Chigusa Seki a, Toshifumi Hiraoki b, Haruo Matsuyama a
**
,
,
Masayoshi Tabata a
*
a Department of Applied Chemistry, Graduate School of Engineering, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan
b Department of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The poly(propargyl acetate) (A) having a helical cisetransoid structure was stereospecifically prepared
using the Rh complex catalyst, [Rh(norbornadiene)Cl]2, in MeOH or NEt3 solvent at 0 and 40 ꢀC in
moderate yield. Electron spin resonance (ESR) analysis of the polymer revealed the formation of the cis
(B) and trans (C) radicals which were produced through the thermal rotational scission of the helical cis
C]C bonds in the main-chain during the polymerization. The spatial and geometrical structure was
successfully deduced using the two analogues’ polymers in which either methyl or methylene group is
deuterated, by the aide of computer simulation of the observed ESR spectra together with the calculation
of spin density of the two radicals.
Received 12 October 2010
Received in revised form
8 December 2010
Accepted 10 December 2010
Available online 21 December 2010
Keywords:
Poly(propargyl acetate)
Rh catalyst
Ó 2011 Elsevier Ltd. All rights reserved.
Electron spin resonance
1. Introduction
(nbd)Cl]2/NEt3, (nbd ¼ norbornadiene), we describe the polymeri-
zation of PE in MeOH. Radical species involved in the resultant
Substituted polyacetylenes (SPs) have been considered as the
typical conjugated polymers, which have interesting properties, e.g.,
pressure induced cis-to-trans isomerization [1], helix and columnar
formations [2], molecular recognition ability [3e7], and non-linear
optical (NLO) [8,9] and anti-ferromagnetic (AFM) properties due to
the cis and/or trans radical spins generated in the polymers [10,11].
The AFM properties are potentially important for a new type of time
memory materials similar to human’s neuron system [12]. The AFM
property of the polymers was increased at the lower temperature,
e.g., 58e114 K and in the prolonged time when the polymer was
kept without magnetic field at a low temperature and finally the
magnetization was saturated [13]. It was reported that poly(prop-
argyl ester)s (PPE)s has humidity sensitive properties among the
conjugated polymers which can be prepared by using Mo, W, Pd
transition metal catalysts [14]. Recently, the polymerization of
propargyl ester (PE) by using the so-called Zwitter ionic Rh catalyst in
tetrahydrofuran, THF, was reported by Masuda and Noyori [15]. This
catalyst is inactive in MeOH because of its deactivation in alcoholic
solvent. In the present report using a monodentate Rh catalyst: [Rh
cisetransoid poly(propargyl acetate)s (PPA)s were analyzed by
Electron spin resonance (ESR) method and a spatial geometrical
structure of the cis and trans radicals could be successfully deter-
mined. Both radicals (B and C) were produced through rotational
scission of the cis C]C bonds of the cisetransoid polymer (A) by
thermally induced cis-to-trans isomerization during the polymeri-
zation and/or on standing the polymer solution at room temperature
[1d] by the so-called pressure induced cis-to-trans isomerization
[1aec] (see Scheme 1).
It is noteworthy that PPE was prepared as another promising
candidate of the anti-ferromagnetic polymer, i.e., “Polymer Timer”
which is expected to show a large increment of magnetization even
when the PPE was cooled down to more than 114 K without
magnetic field as mentioned above.
2. Experimental section
2.1. Measurements
Number and weight average molecular weights (Mn and Mw) of
polymers were measured using JASCO GPC 900-1 equipped with two
Shodex K-806L columns and RI detector. Chloroform was used as an
eluent at 40 ꢀC and poly(styrene) standards (Mn ¼ 800e1,090,000)
were employed for calibration. 1H NMR and 13C NMR spectra were
* Corresponding author. Tel./fax: þ81 143 46 5963.
** Corresponding author. Tel.: þ81 143 46 5752.
E-mail addresses: matsuyama@mmm.muroran-it.ac.jp (H. Matsuyama), tabata@
mmm.muroran-it.ac.jp (M. Tabata).
0032-3861/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.12.023

Y. Yoshida et al. / Polymer 52 (2011) 646e651
647
4.68 (s, 2H, HC^CeCH2e). 13C NMR (CDCl3, TMS):
74.71, 77.48, 169.78.
d
¼ 20.21, 51.76,
2.2.3. Synthesis of a,a
-dideuterio-propargyl alcohol (30)
To a stirred suspension of 5 g (0.12 mol) of lithium aluminum
deuteride (LAD) (Taiyo Nippon Sanso) in 540 mL of dry ether,
a mixture of 20.6 g (0.21 mol) of ethyl propiolate (TCI) in 160 mL of
dry ether was added slowly dropwise. The reaction mixture was
stirred for 1.5 h at ꢁ78 ꢀC and then quenched carefully with 10 mL of
distilled water, 5 mL of a 15% NaOH solution and 10 mL of distilled
water. After stirring for 12 h, the mixture was passed through a short
alumina column, and the solvent was distilled off. The crude product
was purified by distillation under nitrogen, and the fraction between
60 and 108 ꢀC was collected to give the
a,a-deuterio-propargyl
alcohol (30) as a colorless liquid (4.5 g, 38%), which was used without
further purification in the next step.
2.2.4. Synthesis of
To a stirred mixture of 68 mL of dry ether, 4.5 g (0.08 mol) of the
crude
-dideuterio-propargyl alcohol (30), 8.8 g (0.09 mol) of
a,a-dideuterio-propargyl acetate (3)
a,a
pyridine, and 0.95 g (8 mmol) of 4-(N,N-dimethylamino)pyridine,
a mixture of 7.3 g (0.09 mol) of acetyl chloride in 9.3 mL of ether
was added dropwise at 0e5 ꢀC. After stirring for 12 h at room
temperature, the resulting mixture was washed with 5% HCl solu-
tion and distilled water. The organic layer was dried over anhydrous
sodium sulfate, and the solvent was distillated off under nitrogen.
The crude product was purified by distillation (58 ꢀC/96 mmHg) to
Scheme 1. Polymerization of monomers 1e3 and thermal cis-to-trans isomerization.
measured on a JEOL JNM-EX270 and Bruker DSX-300 using chloro-
form-d as a solvent at room temperature. ESR spectra were recorded
on a JEOL FE3XG (9.44 GHz) with 100 kHz field modulation at room
temperature. A signal due to Mn2þ bedded in MgO was used for
calibration of the magnetic field. In order to observe detailed infor-
mation for the hyperfine coupling constant of the ESR spectra the
second derivative spectra were measured at room temperature using
lower microwave power. The energetically optimal conformation of
PPA together with its spin density in order to explain the ESR spectral
data was deduced by using semi-empirical quantum chemical
calculation AM1 method [16]. Computer simulation of the observed
ESR spectra was performed using our own program [17].
give a colorless liquid (2.3 g, 28%). 1H NMR (CDCl3, TMS):
d
¼ 2.11 (s,
3H, eCH3), 2.47 (s, 3H, HeC^C). 13C NMR (CDCl3, TMS): 20.49,
51.28, 74.66, 77.47, 169.75 (Scheme 2).
2.3. Polymerization
PPAs were obtained by the polymerization of propargyl acetate
(PA) initiated with a monodentate Rh catalyst, [Rh(nbd)Cl]2/NEt3, as
shown in Scheme 1. In a typical procedure, 0.1 ꢂ10ꢁ3 mol of PA and
the calculated amount of the catalyst (0.1 ꢂ10ꢁ3 mol) were dis-
solved in MeOH or NEt3 solvent using a specially designed U-shaped
ampoule [1], and the polymerization was carried out for 4 h at given
temperatures in the solvent with constant shaking. The resulting
solution was poured into excess amount of MeOH to precipitate
a polymer. The resulting polymer was washed with MeOH and dried
under dynamic vacuum, ca. 10ꢁ2 torr for 12 h at RT. The yield of the
polymer was determined by gravimetry (Table 1).
2.2. Materials
2.2.1. Synthesis of propargyl acetate (1)
To a stirring mixture of 15.1 g (0.27 mol) of propargyl alcohol
(TCI), 30.4 g (0.38 mol) of pyridine (KANTO) and 3.3 g (0.03 mol) of
4-(N,N-dimethylamino)pyridine (Aldrich) in 288 mL of dichloro-
methane (DCM), a solution of 25 g (0.32 mol) of acetyl chloride
(KANTO) in 32 mL of DCM was added dropwise at 0e5 ꢀC. After
stirring for 24 h at room temperature, the resulting mixture was
washed with a 5% HCl solution and distilled water. The organic layer
was dried over anhydrous sodium sulfate, and the solvent was
removed by distillation under nitrogen. The crude product was
purified by distillation (44 ꢀC/42 mmHg) to give a colorless liquid
Poly(propargyl acetate): poly(1). 1H NMR (CDCl3, TMS):
d
¼ 2.05
(s, 3H, eCH3), 4.61 (s, 1H, HC]CeCH2e,), 6.28 (s, 1H, HC]Ce). 13
C
(20.7 g, 66%). 1H NMR (CDCl3, TMS):
1H, HeC^C), 4.68 (s, 2H, HC^CeCH2e). 13C NMR (CDCl3, TMS):
¼ 20.52, 51.74, 74.70, 77.48, 169.70.
d
¼ 2.11 (s, 3H, eCH3,), 2.49 (s,
d
2.2.2. Synthesis of propargyl trideuterio acetate (2)
A mixture of 100 mL of benzene, 6.0 g (0.09 mol) of tetradeu-
teroacetic acid (TCI), 8.4 g (0.15 mol) of propargyl alcohol, and 1.7 g
(0.01 mol) of p-toluenesulfonic acid (Wako) was refluxed for 6 h with
DeaneStark apparatus. The resulting mixture was washed with a 5%
sodium hydrogen carbonate solution and distilled water. After the
organic layer was dried over anhydrous sodium sulfate, the solvent
was removed by distillation under nitrogen. The crude product
was purified by distillation (59 ꢀC/102 mmHg) to give a colorless
liquid (3.1 g, 34%). 1H NMR (CDCl3, TMS):
d
¼ 2.49 (s, 1H, HeC^C),
Scheme 2. Synthesis of propargyl acetates (1), (2), and (3).

648
Y. Yoshida et al. / Polymer 52 (2011) 646e651
Table 1
Polymerization of PAs using the [Rh(nbd)Cl]2 catalyst.a
Run Monomer Solvent Cocat.b
([cocat.]/[cat.]) (ꢀC)
Temp. Yieldc Mn
Mw/Mn cise
d
d
(%)
(/104)
(%)
1
2
3
4
5
6
7
1
1
1
1
1
2
3
MeOH
NEt3
NEt3
MeOH 100
MeOH 100
MeOH 100
MeOH 100
e
e
e
40
0
40
0
40
40
40
Trace
43
43
79
57
e
e
e
0.7
0.6
0.5
0.5
0.7
0.8
1.9
1.9
1.8
2.0
1.8
1.9
73
70
70
77
85
82
44
37
a
b
c
[M]0 ¼ 0.1 mol/L, [M]0/[Cat.] ¼ 100.
NEt3 was used as cocatalyst.
Insoluble fraction in methanol.
d
e
CHCl3 soluble part and estimated by GPC(PSt, CHCl3).
CHCl3 soluble part and determined by 1H NMR(CDCl3).
NMR (CDCl3, TMS):
d
¼ 20.6, 66.6, 128.3, 134.4, 170.5. Poly(propargyl
¼ 4.61 (s, 1H,
HC]CeCH2e), 6.28 (s, 1H, HC]Ce). 13C NMR (CDCl3, TMS):
¼ 20.6, 66.6, 128.3, 134.4, 170.5. Anal. Calcd for C5H3D3O2: C,
deuterio-acetate)s: poly(2). 1H NMR (CDCl3, TMS):
d
d
59.39; H(D), 8.97. Found: C, 57.79; H(D), 8.69. Poly(
a
,
a
-dideuterio-
¼ 2.05 (s, 3H,
¼ 20.6, 66.6,
propargyl acetate): poly(3). 1H NMR (CDCl3, TMS):
d
eCH3), 6.28 (s, 1H, HC]Ce). 13C NMR (CDCl3, TMS):
d
128.3, 134.4, 170.5. Anal. Calcd for C5H4D2O2: C, 59.99; H(D), 8.05.
Found: C, 58.72; H(D), 7.80. The elemental analysis for the deuteron
was calculated by using the 1H NMR spectral data of the two
deuterated polymers.
Fig. 1. 1H NMR spectra of poly(1), poly(2), and poly(3) (300 MHz, CDCl3) prepared with
runs 5, 6, and 7 in Table 1.
3. Results and discussion
observed at 6.28 ppm. The spectra of the deuterated polymers, poly
(2) and poly(3), also supported their structures as shown in Fig. 1.
The cis % of the polymers in Table 1 were determined according to
the method of the literature [1].
3.1. Polymerization
Poly(propargyl acetate)s (PPA)s were obtained by polymeriza-
tion of monomers 1, 2, and 3 using the monodentate Rh catalyst: [Rh
(nbd)Cl]2/NEt3 in MeOH or NEt3 as the polymerization solvent. The
yield with number and weight average molecular weights (Mn and
Mw) and its distributions (Mw/Mn), and cis % are listed in Table 1. In
the case of poly(1), Poly(2), and poly(3), cis content was carefully
estimated by comparison of integrated ratio of vinyl proton and
methyl or methylene protons observed in the 1H NMR spectra.
Previously, we have reported that an organic solvent such as
MeOH, THF, and NEt3 with lone pair electrons acts as a good poly-
merization cocatalyst because the solvent stabilizes the Rh catalyst
as the propagation species which was dissociated to a monodentate
Rh catalyst from the bidentate Rh complex catalyst [1,18,19].
However, we found that the highest yield was observed when the
polymerization of monomer 1 was performed under the conditions
of NEt3 as the cocatalyst in MeOH at 0 ꢀC (Table 1, run 4). It is
noteworthy that aliphatic acetylene ester monomers, e.g., alkyl-
propiorates unlike this PA monomer did not polymerize in NEt3 [20].
Poly(1) (Table 1, run 5) prepared in MeOH at 40 ꢀC, poly(2) and poly
(3) (Table 1, runs 6 and 7) deuterated at either methyl or methylene
moieties, respectively, were used in order to unequivocally deter-
mine the spatial geometrical structure of the cis and trans radicals
stabilized in the resulting polymer using the ESR method at room
temperature as mentioned below.
3.3. ESR spectra
Previously, we reported that the ESR spectra of the radicals of
aromatic polyacetylenes were observed before and after compres-
sion of the polymers, and the cis and trans radicals are produced
through the cis-to-trans isomerization via the rotational scission of
the helical cis C]C bond [1a,b,c,d], as shown in Scheme 1. ESR
parameters of poly(1), poly(2), and poly(3) are summarized in Table
2. In addition, the g value of the observed radicals was used to
deduce the geometrical structures, i.e., cis form or trans form, when
one heteroatom such as N, S, and I is involved within the side-chain
of the polyacetylene molecules [1e,21e23]. Such a heteroatom has
a relatively large spin orbit coupling constant, z compared with that
of the hydrocarbon atoms [22e24]. We have also revealed that
these heteroatom can shift the g value to a lower magnetic field,
especially in the cis form, although such a shift is extremely small
for trans form, because of spatial decoupling of the magnetic
interaction due to the unpaired electron between the side-chain
and the planar trans main-chain. This indicates that the side-chain
and the planar trans p-conjugation plane are nearly perpendicular
to each other where such strong magnetic interaction is decoupled
between the heteroatom in the side-chain and the unpaired elec-
trons in the main-chain.
3.2. Polymer structure
The structure of the polymers was elucidated by 1H NMR and 13
C
3.4. Spectral simulation of poly(1)
NMR spectroscopies. The 1H NMR spectra of poly(1), poly(2), and
poly(3) are shown in Fig. 1. The ethynyl proton signal, HeC^,
observed at 2.53 ppm in their monomers (1, 2, and 3) disappeared
completely and the new olefinic protons for polyene main-chain,
which suggest a cisetransoid structure of the main-chain, were
We examined whether two ESR signals corresponding to the cis
and trans radicals were also observable in case of the freshly
prepared poly(1) at room temperature. Consequently, we observed
a fairly strong ESR spectrum as shown in Fig. 2a, although the

Y. Yoshida et al. / Polymer 52 (2011) 646e651
649
Table 2
ESR parameters of PPAs.a
a
b
aH
1,2
b3,4
b
Polymer
Radical
g value
aH (G)
(G)
aH
(G)
aHg (G)
D
Hmsl (G)
Spins/g (/1016
)
Unit/spin (/105)
poly(1)c
cis
trans
cis
trans
cis
trans
2.0044
2.0027
2.0044
2.0027
2.0044
2.0027
e
6.0
e
19.0
e
e
4.5
6.0
4.5
6.0
4.5
6.0
1.91
3.21
2.16
2.45
16.0
e
4.0
e
poly(2)d
poly(3)e
6.0
e
19.0
e
2.75
2.45
16.0
e
4.0
e
6.0
e
2.9
e
16.0
4.0
a
Observed at room temperature.
b
c
DHmsl ¼ line width.
Prepared by using the polymerization conditions of run 5 in Table 1.
Prepared by using the polymerization conditions of run 6 in Table 1.
Prepared by using the polymerization conditions of run 7 in Table 1.
d
e
pressure was not imposed to the polymer. To the best of our
knowledge such ESR spectrum has never been reported up to date.
Therefore, in order to spectroscopically determine whether the two
paramagnetic chemical species, i.e., cis and trans radicals, are
generated in poly(1), the so-called microwave power saturation
effect was also examined at 1 mW and 0.001 mW, respectively
[1d,21]. It was clear that the spectral line shape and its intensity was
changed with decreasing microwave power (see Fig. 2b), by which
fairly small hyperfine coupling (hfc) pattern compared with that of
Fig. 2a were detected. The newly observed coupling pattern may
reflect the existence of the two types of radicals involved in the
polymer, poly(1). Therefore, in order to obtain more detailed hfc data
for each radical the second derivative ESR spectrum was also
measured at a fairly lower microwave power, i.e., 0.001 mW, as
shown in Fig. 2c.
D
Hmsl ¼ 6.0 G and g ¼ 2.0027 ꢃ 0.0005 as shown in Fig. 2e. Thus, the
simulated spectrum, Fig. 2d well agreed with that of the observed
one, Fig. 2c each other.
The ESR spectrum of poly(3) was also simulated by considering
the so-called gyromagnetic ratio of the proton and deuteron, gH
/
gD ¼ 6.5 [22], as shown in Fig. 3b and c. Thus, this experiment
strongly proves that the observed spectra are undoubtedly super-
imposed with those of the cis and trans radicals (B and C). This also
suggests that the highest spin density of the resulting cis radical (B)
localizes at not only main-chain protons, Hb1 and Hb2, but also the
two methylene protons; Hb3 and Hb4, in the side-chain of poly(1) in
Fig. 4a, and this result is similar to poly(p-halogenated phenyl-
acetylene)s [24] and poly(p-sulfoxide phenylacetylene) [21].
As a working hypothesis the ESR spectrum of Fig. 2c is consid-
ered to be superimposed with a tripleetriplet and a doublet line
spectra due to the cis and trans radicals, respectively, as shown in
Fig. 2e with the stick diagram. Such a preliminary assignment may
be supported by our previous experimental facts, i.e., the cis and
trans radicals generated in the substituted polyacetylene polymers
are observed as the sum of the so-called odd and even line ESR
spectra, respectively [1e,5,21].
3.5. Spectral simulation of poly(2)
The poly(2) deuterated at the methyl moiety, eCD3, in the side-
chain gave the completely same ESR spectra as shown in the
Fig. 2aec. This result strongly indicates that the three protons of the
methyl group of the side-chain are never magnetically interacted
with the two chemical species, i.e., the cis and/or trans radicals
which are detectable by the ESR technique.
3.6. Spectral simulation of poly(3)
Another deuterated polymer, poly(3), was also examined whether
the spectral change is induced at room temperature. The second
derivative ESR spectra of poly(3) were also measured at different
microwave power of 1 mW and 0.001 mW, respectively, at room
temperature. It is clear that notably large spectral changes took place
as shown in Figs. 2c and 3b. These spectral change undoubtedly
proves thatthe large tripletline of poly(1) (see Fig. 2e) comes from not
the two vinyl protons of the main-chain but the methylene protons in
the side-chain of poly(3) (see Schemes 1 and 3). Therefore, according
to the stick diagram (see Figs. 2e and 3d) the spectra were successfully
simulated assuming that the cis radical (B) has such the tripleetriplet
b
3
b4
spectrumwith the hfc: aH ¼ aH ¼19.0 G due to the two methylene
protons, and aH ¼ aH ¼ 6.0 G due to two main-chain vinyl protons,
b1
b2
line width,
D
Hmsl ¼ 4.5 G and g ¼ 2.0044 ꢃ 0.0005, and the trans
Fig. 2. ESR spectra of poly(1) (a) observed at 1 mW, (b) observed at 0.001 mW, (c) second
derivative ESR spectrum observed at 0.001 mW, (d) simulated spectrum, and (e) stick
diagram.
radical (C) shows a doublet spectrum with a aH ¼ 16.0 G due to the
a
one main-chain Ha proton, and aH ¼ aH ¼ 4.0 G and line width,
g1
g2

650
Y. Yoshida et al. / Polymer 52 (2011) 646e651
Scheme 3. Possible thermal-induced cis-to-trans isomerization through rotation
hemolysis of the cis double bond.
Fig. 3. Second derivative ESR spectra of poly(3) (a) observed at 1 mW, (b) observed at
0.001 mW, (c) simulated spectrum, and (d) stick diagram.
was adopted. After that the further simplified cis radical model as
depicted in Fig. 4a was used to calculate the spin density of the cis
On the other hand, the doublet line spectrum as shown in Fig. 2e
was reasonably calculated by assuming that only one Ha proton can
magnetically couple with the unpaired electron as the trans radical as
radical. The AM1 calculation showed that in the cis radical the Ca, Cb
,
1
Cb2, and Cb takes a planar plane with a sp2 configuration and an
3
angle, q1 and q2 ¼ 75ꢀ, q3 and q4 ¼ 30ꢀ, respectively. The calculation
revealed that the spin density appeared at not only ether oxygen,
eOe, but also until the carbonyl oxygen, C]O to some extent. This
means that the spin density due to unpaired electron generated
through the rotational scission of the helical cis C]C double bonds in
the main-chain can migrate to the methylene moiety and the OeC]
O moiety in the side-chain (see Scheme 3). Therefore, according to
the AM1 calculation the observed large g value, i.e., 2.0044 ꢃ 0.0005,
due to the cis radical is reasonably explained by the migration of the
spin density to the OeC]O moiety because of its fairly large spin
shown in Fig. 4b. In other words, neither the main-chain Hg1 and Hg
2
protons nor the side-chain CH2 protons magneticallycouples with an
unpaired electron of the trans radical because the 2p orbital of
unpaired electron and the side-chain plane are spatially perpendic-
ular each other, as shown in Fig. 4b. Thus the trans radical was also
proven to show ESR spectrum with the simple double triplet line
unlike the previous case of the trans radicals which were generated
by the so-called pressure induced cis-to-trans isomerization where
the trans radical hold a fairy planar conjugation structure, i.e., a fairly
narrow Lorentzian single line spectrum was observed and suggested
orbit coupling constant, i.e.,
z value as mentioned above and the large
p-conjugation such as the so-called soliton reflecting fast motional
z
value make the g value shift to a lower magnetic field as observed in
narrowing of the unpaired electron [2b,19,26].
Figs. 2 and 3. Thus we can successfully explain the reason why the g
value of the cis radical is fairly large, 2.0044 ꢃ 0.0005 compared to
2.0027 ꢃ 0.0005 of the trans radical. The very large spin density can
be delocalized even on the ether oxygen and carbonyl oxygen atoms
3.7. Semi-empirical quantum chemical calculation
Conformational analysis using the AM1 program [19] was per-
formed assuming a model compound of poly(1) to determine the
most energetically stable conformation of their isomers and distri-
bution of the unpaired electrons produced by the rotational scission
of the pristine cis C]C bond during not only the polymerization but
also on standing the polymer solution at room temperature (see
Scheme 3). In order to calculate the most energetically stable struc-
ture by usingtheAM1calculationa modelstructurewhich comprises
20 monomer sequences with an unpaired electrons for the cis radical
which have also a large orbit couplingconstant
the spin orbit coupling constant responsible for the eOeC]O group
has been unknown up to date.
The doublet line of ESR as shown in Figs. 2e and 3d may also be
reasonably attributed to that of the trans radical stabilized in the
conjugated main-chain, because the resulting trans sequences have
a fairly twisted configuration which can localize the spin density as
0
the p-radical, i.e., a planer plane made by the Ca, Cb , and Cb allows
z
¼ 151 [22], although
the magnetic coupling between the unpaired electron and Ha to

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Thus, the AM1 calculation consistently explains our experimental
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The aromatic polyacetylenes having a small amount of the spin
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4. Conclusion
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The stereospecific polymerization of propargyl acetate and two
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methylene moiety was successfully performed using a Rh catalyst,
[Rh(nbd)Cl]2, MeOH and NEt3 solvents at 0 and 40 ꢀC and the cor-
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moderate yields. The polymerization even at the temperature
induced the cis-to-trans isomerization, which was observed by the
measurement of the ESR spectra. The ESR spectrum is considered to
be composed of the cis radical whose spin density migrate to not
only the main-chain but also the side-chain, i.e., eCH2e and
eOeC]O moiety giving the large g value, g ¼ 2.0044 ꢃ 0.0005. On
the other hand, the spin density of the trans radical is localized in
the only main-chain which affords a smaller g value compared to
that of the cis radical, which is a typical hydrocarbon radical.
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