Depolymerization of poly(2,6-dimethyl-1,4-phenylene oxide) under oxidative conditions

Article abstract of DOI:10.1002/chem.200204669

Depolymerization of an engineering plastic, poly(2,6-dimethyl-1, 4-phenylene oxide) (PPO), was accomplished by using 2,6-dimethylphenol (DMP) under oxidative conditions. The addition of an excess amount of DMP to a solution of PPO in the presence of a CuCl/pyridine catalyst yielded oligomeric products. When PPO (M_n = 1.0 × 10⁴, M _w/M_n = 1.2) was allowed to react with a sufficient amount of DMP, the molecular weight of the product decreased to M_n = 4.9 × 10² (M_w/M_n = 1.5). By a prolonged reaction with the oxidant, the oligomeric product was repolymerized to produce PPO essentially identical to the starting material, making the oligomer useful as a reusable resource. During the depolymerization reaction, an intermediate phenoxyl radical was observed by ESR spectroscopy. Kinetic analysis showed that the rate of the oxidation of PPO was about 10 times higher than that of DMP. These results show that a monomeric phenoxyl radical attacks the polymeric phenoxyl to induce the redistribution via a quinone ketal intermediate, leading to the substantial decrease in the molecular weight of PPO, which is much faster than the chain growth.

Full text of DOI:10.1002/chem.200204669

FULL PAPER
Depolymerization of Poly(2,6-dimethyl-1,4-phenylene oxide)
under Oxidative Conditions
Kei Saito, Toru Masuyama, Kenichi Oyaizu, and Hiroyuki Nishide*[a]
Abstract: Depolymerization of an engi-
neering plastic, poly(2,6-dimethyl-1,4-
phenylene oxide) (PPO), was accom-
plished by using 2,6-dimethylphenol
longed reaction with the oxidant, the
oligomeric product was repolymerized
to produce PPO essentially identical to
the starting material, making the
oligomer useful as a reusable resource.
During the depolymerization reaction,
an intermediate phenoxyl radical was
observed by ESR spectroscopy. Kinetic
analysis showed that the rate of the
oxidation of PPO was about 10 times
higher than that of DMP. These results
show that a monomeric phenoxyl radical
attacks the polymeric phenoxyl to in-
duce the redistribution via a quinone
ketal intermediate, leading to the sub-
stantial decrease in the molecular weight
of PPO, which is much faster than the
chain growth.
(
DMP) under oxidative conditions. The
addition of an excess amount of DMP to
a solution of PPO in the presence of a
CuCl/pyridine catalyst yielded oligomer-
4
ic products. When PPO (M ˆ 1.0  10 ,
n
Keywords: depolymerization
¥
M /M ˆ 1.2) was allowed to react with a
w
n
green chemistry ¥ polymerization ¥
sufficient amount of DMP, the molec-
ular weight of the product decreased to
polyphenyleneoxide
mechanisms
¥
reaction
2
M ˆ 4.9  10 (M /M ˆ 1.5). By a pro-
n
w
n
Introduction
ization of 2,6-unsubstituted phenols was accomplished by
suppressing the coupling reaction at the 2,6-positions by
means of a steric crowding effect by employing tyrosinase
model complexes as catalysts.[ On the other hand, copper
complexes with enhanced oxidizing ability have been found to
catalyze the oxygen-oxidative polymerization of phenols 2,6-
disubstituted with electron-withdrawing groups. This has led
Oxidative polymerization of 2,6-dimethylphenol (DMP) cat-
alyzed by copper± amine complexes, discovered in 1959 by
provides a convenient method to synthesize
poly(2,6-dimethyl-1,4-phenylene oxide) (PPO). The other
93
A. S. Hay,[
1±±3
product of this reaction is formed by CÀC coupling of two
monomeric phenols, 4-(±,5-dimethyl-4-oxo-2,5-cyclohexadie-
nylidene)-2,6-dimethyl-2,5-cyclohexadienone (DPQ) which
degrades the polymer upon further processing at high
temperature. The reaction conditions are mild, and the by-
to the first synthesis and characterization of high molecular
weight poly(2,6-difluoro-1,4-phenylene oxide).[
10, 113
It should
be pointed out that the polymerization mechanism is still not
fully resolved. The reaction is considered to involve the
carbon ± oxygen coupling of aryloxy radicals, but the polymer-
ization does not occur simply by the coupling of polymeric
aryloxy radicals and monomer radicals. Based on several
experimental results, it was concluded that the most likely
product is only H O by the suppression of the formation of
2
DPQ. Oxidative polymerization is an atom economical
reaction that does not require any leaving groups and removal
of the by-product from the resulting polymer. Because of the
increasing importance of low waste and reusable polymers in
green chemistry, oxidative polymerization of DMP is one of
the ideal polymerization processes. However, in spite of this
the synthesis and properties of PPO have focused on its use as
an engineering plastic.[ In particular, the unraveling of the
pathway involves the formation of a quinone ketal inter-
mediate.[
12±1ꢁ3
This mechanism can be divided into rearrange-
ment and redistribution steps. The rearrangement is a
concerted Claisen-type rearrangement, which is a special case
43
[163
of a sigmatropic rearrangement proposed by Ionescu et al.
reaction mechanism for the extension of the monomer has
This rearrangement results in the quinone group being shifted
over the backbone of the oligomeric species, until eventually
the end is reached. According to this mechanism, two dimeric
phenols could give a tetramer as the primary product. The
other mechanism, the redistribution, follows a pathway in
which the newly formed CÀO bond in the quinone ketal
long been a target.[
2, 5±ꢀ3
Recently, the oxidative polymer-
[
a3 Prof. Dr. H. Nishide, K. Saito, T. Masuyama, Dr. K. Oyaizu
Department of Applied Chemistry, Waseda University
Tokyo 169-ꢀ555 (Japan)
Fax : (‡ꢀ1)±-±209-5522
intermediate may dissociate to reform two dimeric species, or
the other ether bond may be cleaved to form a monomeric
E-mail: nishide@waseda.jp
Supporting information for this article is available on the WWW under
http://www.chemeurj.org or from the author.
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DOI: 10.1002/chem.200204669
Chem. Eur. J. 2003, 9, 4240 ± 4246

4
240±4246
and a trimeric species from the two dimeric phenols
depolymerization of PPO. With the addition of DMP to PPO,
the depolymerization of PPO occurs and yields oligomeric
products. The oligomeric products are identical to those
produced during the polymerization of DMP, and thus the
repolymerization can subsequently take place. The reaction
time and the ratio of DMP to PPO were varied to adjust the
molecular weight of the oligomeric products. The molecular
weight decreases were determined by gel permeation chro-
matography (GPC). Throughout this study, CuCl/pyridine
was used as the catalyst. The time course of the decrease in
molecular weight, the effect of the oxidizing agent, the
detection of an intermediate radical, and the depolymeriza-
tion rate constant were determined. The kinetics of the
depolymerization are discussed in the context of determining
the driving force that governs the equilibrated system.
(
Scheme 1).
There are a few studies that have focused on the mechanism
of the redistribution.[
1ꢀ±223
White et al. reported the equilibra-
tion reaction of PPO with phenolic compounds by the
redistribution mechanism, in which DPQ, tert-butyl perben-
zoate, and benzoyl peroxide acted as active initiators. In the
presence of the initiator, PPO reacts with phenolic com-
pounds to form a mixture of low molecular weight hydroxyl
arylene ethers,[ and a bifunctional polymer is formed when
low molecular weight PPO is allowed to react with DPQ.[193
Based on the reversibility of the redistribution mechanism,
one can consider that the reaction of PPO with a large number
of phenolic compounds will induce the depolymerization of
PPO. Recently, the redistribution reactions have been ex-
ploited for the tail end functionalization of PPO. When using a
para-functionalized phenol, the reaction yields a tail-func-
tionalized PPO.[ In this reaction, DPQ, CuCl/4-(dimethyl-
amino)pyridine, and Cu(NO ) /N-methylimidazole were used
1ꢀ3
2±3
Results and Discussion
±
2
as the catalyst, depending on the phenolic compound, the
desired reaction time, and the product purity. The redistrib-
ution reaction using PPO and DMP was attempted, but 1 ± ±
weeks were needed to depolymerize PPO substantially. On
the other hand, the phase-transfer-catalyzed depolymeriza-
tion of PPO in the presence of either 2,4,6-trimethylphenol or
Depolymerization of PPO with DMP using a CuCl/pyridine
4
catalyst: Depolymerization of PPO (M ˆ 1.0  10 , M /M ˆ
n
w
n
1.2) with DMP in toluene was examined in the presence of a
catalyst (CuCl/pyridine ˆ 1:100). The reactions were carried
out under air at room temperature and at several different
PPO and DMP concentrations. The molecular weight of PPO
was significantly decreased to produce an oligomeric product
[243
4-tert-butyl-2,6-dimethylphenol have also been reported,
±
for which a radical-anion mechanism was presented.
(M ˆ 1.6  10 , M /M ˆ 2.±) at a reaction time of 1 min, as a
n
w
n
It should be noted that previous studies on the depolyme-
rization of PPO did not focus on their potential utility from
the viewpoint of green chemistry, and had the drawback that
the products were not reusable resources due to the lack of
reactivity with respect to repolymerization. Depolymerization
of PPO to a repolymerizable oligomer should have great
advantages as a sustainable technology in the development of
green chemistry.
consequence of the depolymerization of PPO by DMP at
À1
À1
initial concentrations of 0.25 unitmolL and 0.025 molL ,
respectively. The molecular weight changes during the course
of the depolymerization are shown in Figure 1. The molecular
weight of PPO readily decreased to a M of 550 (M /M ˆ 1.6)
n
w
n
at a reaction time of 10 min when the initial concentrations of
À1
À1
PPO and DMP were 0.25 unit molL and 0.25 molL ,
respectively.
Herein, we focus on the depolymerization and repolyme-
rization of PPO in the development of oxidative polymer-
ization of DMP as a green chemistry polymerization process.
We chose DMP as the phenolic compound to induce the
The molecular weight remained at this low value for several
minutes, but gradually began to increase as the reaction time
increased. This result indicates that the low molecular weight
state is a transient state during the ongoing redistribution and
Scheme 1. Redistribution mechanism.
Chem. Eur. J. 2003, 9, 4240 ± 4246
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H. Nishide et al.
FULL PAPER
4
Figure 1. Time course of the molecular weight of PPO (M
n
ˆ 1.0  10 , M
w
À1
/
)
À1
M
n
ˆ 1.2, 0.25 unit molL ) during the reaction with DMP (0.025 molL
in the presence of the CuCl/pyridine catalyst (CuCl/pyridine 1:100) in
toluene under air at room temperature.
rearrangement process. The ratio of pyridine to copper was
selected (CuCl/pyridine ˆ 1:100) in this reaction to suppress
formation of DPQ (ꢀ1%; based on UV/Vis spectroscopy
(
l
ˆ 421 nm) of the reaction solution as reported by
[ꢀ3 [±3
max
Driessen and Reedijk et al ). In addition, Reedjik et al.
reported that the formation of DPQ could be reduced in the
oxidative polymerization by starting from mixtures of DMP
and PPO oligomers. This result does not conflict with the
suppression of DPQ formation in our reaction.
[
253
Figure 2. GPC traces obtained from the reaction mixture of PPO and
DMP with the time passage of 0, 1, ±, and 10 min. In the presence of the
CuCl/pyridine catalyst in toluene under air at room temperature. PPO ˆ
GPC traces obtained with solutions during the depolyme-
rization (Figure 2) clearly show that the original two peaks of
PPO and DMP are merged into a one peak, which supports
the overall decrease in the molecular weight of PPO.
À1
À1
0
.25 unit molL , DMP ˆ 0.25 molL
The depolymerization of PPO is based on the rapid
distribution of phenoxyl radicals (vide infra). The subsequent
slow increase in the molecular weight indicates the concom-
itant rearrangement of the resulting oligomers. The transient
state at the lowest molecular weight would correspond to the
transition state at which these two processes are balanced. If
DMP is added when the molecular weight is at the lowest
value, a further depolymerization is expected to occur to give
even lower molecular weight products. Based on this consid-
eration, the stepwise depolymerization of PPO (M ˆ 1.0 Â
n
4
1
0 , M /M ˆ 1.2) with DMP in toluene was examined in the
w
n
presence of the catalyst. Thus, PPO and DMP (0.1 mol per
PPO unit mol) were allowed to react in the first step. Then,
0.1 mol of DMP per PPO unit mol was added to the solution
at the minimum point of the molecular weight of PPO. This
procedure was repeated 10 times such that the same amount
of DMP was finally added to the same mol amount of PPO.
The molecular weight of PPO efficiently decreased to produce
Figure ±. Molecular weight changes of PPO during the stepwise depoly-
merizations with DMP (See text for experimental details). * ˆ determined
value, * ˆ calculated value
2
an oligomeric product (M ˆ 4.9  10 , M /M ˆ 1.5) without
n
w
n
yielding the rearranged polymers (Figure ±).
According to the redistribution reaction, if all the DMP
added to the solution attacked PPO, the degree of polymer-
ization, n', of the resulting oligomer could be calculated by
Equation (1), when x mol of PPO with the degree of polymer-
ization of n is allowed to react with y mol of DMP.
nx ‡ y
n' ˆ
(1)
x ‡ y
Figure ± also shows that the determined molecular weights
are quite close to the calculated values, which means that the
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Chem. Eur. J. 2003, 9, 4240 ± 4246

Depolymerization of under Oxidative Conditions
4240±4246
molecular weight can be modulated due to the quantitative
progress of the reaction. An important aspect with regard to
the nature of the polymerization is derived from the result:
the quantitative redistribution of the phenoxyl radical in-
dicates the lack of concomitant rearrangement during the
reaction which would increase the molecular weight from that
calculated according to Equation (1). It seems that the
rearrangement is initiated only when an apparent steady-
state of the redistribution is accomplished with statistical
distribution of the molecular weights of the oligomer.
Repolymerization of the oligomeric product using the CuCl/
pyridine catalyst: Repolymerization of the oligomeric product
of the depolymerization was attempted. An admixture of the
oligomeric product and a small amount of the catalyst in
toluene afforded PPO after stirring under O . The polymer
2
was obtained as an off-white powder after precipitation from
methanol. While the polymerization of DMP under the same
conditions yields PPO with a molecular weight of M ˆ ±.0 Â
n
2
1
0 (M /M ˆ 1.4), the molecular weight of the product from
w
n
4
the oligomer was significantly higher (M ˆ 1.0  10 , M /
n
w
M ˆ 1.6). Based on this result, oligomeric products are
n
reusable resources and indeed more reactive for the oxidative
polymerization.
ESR spectrum of the depolymerized PPO: Although the
depolymerization of PPO with DMP has been established, it
can be considered that the coupling of the polymeric phenoxyl
radical and the monomeric phenoxyl radical is not kinetically
favored due to the low concentration of the terminal
polymeric phenoxyl radical. Percec et al. reported a radical
anion mechanism for the depolymerization.[ They thought
that the redistribution reaction of the depolymerization
occurred by the coupling of a polymeric phenolate and a
monomeric phenoxyl radical. However, the experimental
evidence for this interpretation seems to be lacking. To
elucidate the depolymerization mechanism, the ESR spec-
trum of the solution was recorded during the depolymeriza-
tion. The solution of PPO and DMP in toluene in the presence
of the catalyst was used as the sample solution. The ESR
spectrum of the phenoxyl radical gave a sharp signal at g ˆ
Figure 4. a) ESR spectrum of the depolymerization toluene solution.
À1
À1
CuCl ˆ 0.01 molL
,
CuCl/pyridine 1:100, PPO ˆ 0.25 unit molL
,
À1
DMP ˆ 0.025 molL . b) ESR spectrum of the toluene solution of PPO in
À1
the presence of the catalyst. CuCl ˆ 0.01 molL , CuCl/pyridine 1:100,
243
À1
PPO ˆ 0.25 unit molL . c) ESR spectrum of the toluene solution of DMP
À1
in the presence of the catalyst. CuCl ˆ 0.01 molL , CuCl/pyridine 1:100,
À1
DMP ˆ 0.025 molL . d) ESR spectrum of the toluene solution of divalent
À1
copper. CuCl ˆ 0.01 molL , CuCl/pyridine 1:100. e) Part of the ESR
spectrum of depolymerization toluene solution near g ˆ 2.0. CuCl ˆ
0.01 molLÀ1
0.025 molL
CuCl/pyridine 1:100, PPO ˆ 0.25 unit molLÀ1
,
,
DMP ˆ
À1
.
Kinetic analysis by UV/Vis spectra: Having established that
the depolymerization mechanism of PPO is a radical ± radical
coupling, one may consider that the polymerization of the
monomer could prevail over the depolymerization due to the
low concentration of the terminal polymeric phenoxy radical.
In the first step of the catalytic cycle of the polymerization, the
substrate (i.e., the phenolate ion) coordinates to the copper
complex and one electron is transferred from the substrate to
the copper(ii) ion. The activated substrate dissociates from the
catalyst and the reduced copper(i) catalyst is oxidized to the
original copper(ii) complex by oxygen. UV/Vis spectroscopy
revealed that a steep rise in the absorbance of the d ± d
transition occurred when DMP was added to the solution of
the CuCl/pyridine catalyst solution.[ This change in the
absorbance was caused by the change in the molar optical
density of the d ± d transition due to the coordination of DMP
to the Cu catalyst and could be measured by UV/Vis
spectroscopy. The absorbance due to the d ± d transition
provides the amount of the copper(ii) ion during the reaction,
2
.0044, which showed a hyperfine structure caused by the
interaction with six protons of the methyl group (Figure 4).
The ESR spectrum of PPO in toluene in the presence of the
catalyst showed a similar signal, but there was no ESR signal
from the solution of DMP under the same conditions. The
ESR absorption ascribed to a divalent copper ion was
recorded in every case. These results suggest that the
phenoxyl radical observed from the solution during the
depolymerization is ascribed to the polymeric phenoxyl
radical. A similar ESR spectrum of the independently
prepared 2,6-dimethylphenoxy radical has been reported.[
These results show that the major mechanism of depolyme-
rization involves the radical ± radical coupling of the poly-
meric phenoxyl radical and the monomeric phenoxyl radical.
The radical concentration of the polymer terminal groups was
determined to be 20% under the depolymerization condi-
tions.
263
2ꢁ3
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4243

H. Nishide et al.
FULL PAPER
that is, the rate of electron transfer from the substrate to the
copper ion, which gives the rate constant k for the formation
of the phenoxy radical (Figure 5).
Figure 6. Cyclic voltammograms of the CH
2
Cl
2
solutions in the presence of
À1
the 1 mmolL of the monomer, the oligomer, and the polymer with
À1
À1
0
2
.1 molL (C
5 mVs
4
H
9
)
4
NBF
4
and 1 mmolL 2,6-diphenylpyridine. Scan rate:
À1
.
Figure 5. Kinetic plots for the decrease of the d ± d absorption. CuCl ˆ
DMP, the polymeric phenoxyl radical and the monomeric
phenoxyl radical are both generated in the solution. The
monomeric phenoxyl radical attacks the para position of the
terminal group of PPO with the n degree of polymerization to
induce the redistribution via the quinone ketal intermediate.
After the formation of the quinone ketal intermediate, the
redistribution could take place to yield the n À 1 polymeric
phenoxyl radical and dimer phenoxyl radical. The dimeric
phenoxyl radical can attack the n À 1 polymeric phenoxyl
radical again. By repeating this redistribution, the depolyme-
rization occurs and the molecular weight reduces. It should be
added that only tace amounts of DPQ were formed in this
depolymerization reaction. This result could be also explained
based the fact that PPO is 10 times more reactive than DMP.
The formation of DPQ is caused by the coupling of the two
monomeric phenoxyl radicals. But in this depolymerization
reaction, the monomeric phenoxyl radical is reacts readily
with the abundant polymeric radical and does not react with
the other monomeric phenoxyl radical. By way of this
mechanism, DPQ formation was efficiently suppressed in
the depolymerization.
Interestingly, the depolymerization can only occur accord-
ing to the redistribution mechanism. When PPO and DMP are
mixed under the oxidization conditions, first the redistribution
occurs exclusively. After the redistribution has occurred and
the molecular weight has reached a minimum, the molecular
weight then abruptly increases according to the rearrange-
ment. If additional DMP is added when the molecular weight
is at a minimum, the subsequent increase in the molecular
weight is effectively suppressed. Therefore, when DMP is
gradually added, the molecular weight is effectively reduced.
À1
À1
0
.01 molL , CuCl/pyridine 1:100, PPO ˆ 0.2 unit molL toluene solution,
under air at room temperature. Inset: Visible spectral changes for the
reaction solution recorded at t ˆ 0, 0.25, 2, ꢀ, 20, 25, ±0, 60, and ꢁ0 min.
The rate constant indicates that the polymeric phenoxyl
À1
radical (k ˆ 0.5± s ) is produced 10 times faster than the
À1
monomeric radical (k ˆ 0.0±4 s ). Therefore, it was clearly
shown that PPO tends to be oxidized about 10 times faster
than DMP. This result indicates that the slowly generated
monomeric phenoxyl radical, once formed, reacts readily with
the abundant polymeric radical in the solution, resulting in the
depolymerization of the polymeric radical rather than the
polymerization of the monomeric radical. Added support for
this interpretation was provided by the consideration of
oxidation potentials (vide infra).
Oxidation potential from electrochemistry: One can expect
that the oxidation potential of the phenoxyl radicals would
become lower as the molecular weight increases. The
potentials for the oxidation of PPO, DMP and 4-(2,6-
dimethylphenoxy)-2,6-dimethylphenol (dimer) were meas-
ured by cyclic voltammetry. Cyclic voltammograms of DMP,
4-(2,6-dimethylphenoxy)-2,6-dimethylphenol and PPO in
CH Cl showed irreversible oxidation peaks at room temper-
2
2
ature (Figure 6).
At a constant sweep rate, the oxidation peak potentials
(
versus Ag/AgCl) were observed at 1.6ꢁ, 1.±4, and 1.20 V for
DMP, 4-(2,6-dimethylphenoxy)-2,6-dimethylphenol (dimer),
and PPO, respectively. The oxidation peak potential became
lower as the molecular weight increased. This result also
clearly shows that PPO is more reactive than DMP and the
oligomer.[
253
Depolymerization mechanism: Based on the above results, we
propose the depolymerization mechanism given in Scheme 2.
The high concentration of the terminal phenoxyl radical
may be balanced by the slowly generated DMP which is
present in excess. Because PPO is 10 times more reactive than
Conclusion
Based on the reversibility of the redistribution reaction, the
depolymerization of PPO with DMP using the CuCl/pyridine
catalyst was accomplished to yield an oligomeric product. The
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Chem. Eur. J. 2003, 9, 4240 ± 4246

Depolymerization of under Oxidative Conditions
4240±4246
Scheme 2. Depolymerization mechanism.
depolymerization proceeds almost quantitatively by the
redistribution of the phenoxyl group, and the final molecular
weight can be modulated. The oligomeric products obtained
from the depolymerization are reusable and are also more
reactive compounds for the synthesis of PPO. The polymeric
phenoxyl radical and the monomeric phenoxyl radical are
both generated under the proposed oxidation conditions. The
monomeric phenoxyl radical attacks the phenoxyl radical of
PPO and thereby induces the redistribution via a quinone
ketal intermediate. Repetition of this redistribution is con-
cluded as the depolymerization mechanism.
chloroform. The toluene/chloroform layer was collected, and the solvent
was evaporated. An oligomeric product was obtained as an off-white
powder. The same reaction are performed again and stopped after 10 min.
An oligomeric product (±.00 g, yield: 100%) was obtained as an off-white
powder after the same treatment. Products were dissolved in chloroform to
1
determine the molecular weight by GPC. H NMR: (500 MHz, CDCl
±
,
TMS): d ˆ 6.44 (s, 2H; aromatic C-H), 6.±6 (m; aromatic C-H head end
group), ꢁ.09 (m; aromatic tail end group), 4.22 (m; -OH), 2.09 ppm (s, 6H; -
2
CH
±
); M
n
ˆ 5.5  10 , M
w
/M
n
ˆ 1.6.
4
Stepwise depolymerization of PPO: PPO (1.5 g, M
n
ˆ 1.0  10 , M
w
/M
n
ˆ
1
.2, 12.5 unit mmol) and DMP (0.15 g, 1.25 mmol) were dissolved in
toluene (40 mL). A mixture of CuCl (0.124 g, 1.25 mmol) and pyridine
(10 mL, 1.25 mmol) was added to the solution which was constantly stirred
(
1
±00 rpm). An oligomeric product was obtained as mentioned above (yield:
00%). The molecular weight of the product was determined by GPC and
the minimum point of the molecular weight of PPO was determined.
Subsequently, DMP (0.15 g, 1.25 mmol) was added to the solution at the
minimum point of the molecular weight. The next minimum point of the
molecular weight and its molecular weight were then established. This
operation was repeated 10 times in total (DMP: 12.5 mmol).
4
Experimental Section
Materials: Methanol, toluene, pyridine, copper(i) chloride, and DMP were
obtained from the Kanto Chemical Co. Chloroform was of GPC quality
from Merck. Solvents were purified by distillation prior to use. DMP was
purified by repeated recrystallization from n-hexane.
UV/Vis spectral detection of DPQ: PPO (1.5 g, M
n
ˆ 1.0  10 , M
w
/M
n
ˆ
1.2, 12.5 unit mmol) and DMP (0.15 g, 1.25 mmol) were dissolved in
Measurements: The 1H and 1±C NMR spectra were recorded on a Jeol
toluene (40 mL). A mixture of CuCl (0.124 g, 1.25 mmol) and pyridine
(10 mL, 1.25 mmol) was added to the solution which was constantly stirred
(±00 rpm). A 1 mL sample was taken at different reaction times (0.5, 5, 10,
20, and ±0 min) and diluted to 10 mL with toluene. This diluted solution
gave a UV/Vis absorption with a maximum at 421 nm, which was ascribed
to DPQ (see Supporting Information). The molar extinction coefficient e
1
1±
JNM-LA500 (500 MHz and 125 MHz for
spectrometer with chemical shifts downfield from tetramethylsilane as
the internal standard. All spectra were obtained in CDCl at room
H and C, respectively)
±
temperature. Molecular weights were determined by gel permeation
chromatography (GPC) using a Tosoh highly sensitive GPC system HLC-
220GPC equipped with a UV-ꢀ220 at a detection absorbance set at
À1
±
À1
ꢀ
2
was determined to be ±ꢀ000 mol dm cm in toluene/pyridine (49:1, v/v)
by using the pure DPQ. The amount of DPQ formed in the depolymeriza-
tion reaction solution was estimated to be 0.9 Æ 0.1% versus the amount of
DMP added, or 0.09 Æ 0.01% versus the obtained oligomeric product. The
54 nm. The measurements were made at 408C using the UV detector.
Tosoh TSK GEL SuperHZ2000 Â 2, SuperHZ±000 Â 1, and SuperHZM-
À1
M Â 1 with chloroform as the solvent (0.±5 mLmin ) were used to analyze
the PPO and depolymerized products, in which the calibration curves were
obtained by using polystyrene standards.
amount of DPQ was almost constant during the whole reaction. The same
À1
depolymerization reaction was performed with PPO (0.25 unit molL
)
À1
and DMP (0.25 molL ): Amount of DPQ formed was 0.9 Æ 0.1% versus
the amount of DMP added, or 0.45 Æ 0.05% vs. the obtained oligomeric
product.
Depolymerization of PPO with DMP using a CuCl/pyridine catalyst:
Depolymerization experiments were performed with different PPO and
DMP concentrations. Typical experimental conditions for the reaction of
À1
À1
PPO (0.25 unit molL ) and DMP (0.25 molL ) are as follows. PPO (1.5 g,
Repolymerization of the oligomeric product using the CuCl/pyridine
catalyst: A mixture of CuCl (0.124 g, 1.25 mmol) and pyridine (10 mL,
4
M
n
ˆ 1.0  10 , M
w
/M
n
ˆ 1.2, 12.5 unit mmol) and DMP (1.5 g, 12.5 mmol)
were dissolved in toluene (40 mL). A mixture of CuCl (0.124 g, 1.25 mmol)
and pyridine (10 mL, 1.25 mmol) was added to the solution which was
constantly stirred (±00 rpm) for 6 h. The reaction was performed under air
at room temperature. A 0.2 mL sample was taken at different reaction
times, which was then neutralized with cold 2.5n HCl and diluted with
1.25 mol) was added to a solution of the oligomeric product (1.5 g, M
n
ˆ
2
4.9 Â 10 , M
w
/M
n
ˆ 1.5, 12.5 unit mmol) in toluene (50 mL). The reaction
was performed under air at room temperature. After 10 min, the reaction
mixture was poured into methanol to precipitate the product. The polymer
1
was obtained as an off-white powder (yield: 100%). H NMR: (500 MHz,
Chem. Eur. J. 2003, 9, 4240 ± 4246
www.chemeurj.org
¹ 200± Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4245

H. Nishide et al.
FULL PAPER
CDCl
±
, TMS): d 6.44 (s, 2H; aromatic C-H), 2.09 ppm (s, 6H; -CH
±
); M
n
ˆ
[53 a) E. Tsuchida, M. Kaneko, H. Nishide, Makromol. Chem. 1972, 151,
221 ± 2±4; b) E. Tsuchida, H. Nishide, Makromol. Chem. 1975, 176,
4
1.0 Â 10 , M
w
/M
n
ˆ 1.6.
1
±49 ± 1±5ꢀ; c) E. Tsuchida, H. Nishide, Adv. Polymer Sci. 1977, 24,1 ±
Detection of radical species by ESR spectra: The ESR spectra were
4
ꢀꢁ.
recorded of the toluene solution of PPO (M
n
ˆ 1.0  10 , M
w
/M
n
ˆ 1.2,
À1
À1
[63 W. Koch, W. Risse, W. Heitz, Makromol. Chem., Suppl. 1985, 12, 105 ±
12±.
[ꢁ3 V. Percec, J. H. Wang, R. S. Clough, Makromol. Chem., Macromol.
Symp. 1992, 54/55, 2ꢁ5 ± ±12.
[ꢀ3 P. J.Baesjou, W. L. Driessen, G. Challa, J. Reedijk, J. Mol. Catal. A:
Chem. 1996, 110, 195 ± 210.
0
.25 unit molL ) and DMP (0.025 molL ) in the presence of the catalyst
À1
(
CuCl/pyridine 1:100), the toluene solution of PPO (0.25 unit molL ) in
À1
the presence of the catalyst, the toluene solution of DMP (0.025 molL ) in
the presence of the catalyst, and the toluene solution of the copper catalyst
À1
(
CuCl ˆ 0.01 molL , CuCl/pyridine 1:100). The ESR spectra were deter-
mined by using a JEOL JES-TE200 ESR spectrometer with 100 kHz field
modulation under air at room temperature. The radical concentration in
the sample solution was determined by a careful integration of the ESR
signal standardized with that of a TEMPO (2,2,6,6-tetramethyl-1-piper-
idinyloxyl) solution.
[
93 H. Higashimura, K. Fujisawa, Y. Moro-oka, M. Kubota, A. Shiga, A.
Terahara, H. Uyama, S. Kobayashi, J. Am. Chem. Soc. 1998, 120,
ꢀ
529 ± ꢀ5±0.
103 K. Oyaizu, Y. Kumaki, K. Saito, E. Tuschida, Macromolecules 2000,
3, 5ꢁ66 ± 5ꢁ69.
113 R. Ikeda, H.Tanaka, H. Uyama, S. Kobayashi, Macromolecules 2000,
3, 664ꢀ ± 6652.
[
[
[
3
Kinetic analysis: The kinetics of the depolymerization was based on the
UV/Vis spectra of the toluene solution of PPO (M
4
n
ˆ 1.0  10 , M
w
/M
n
ˆ
3
À1
1.2, 0.25 unit molL ), the catalyst (CuCl/pyridine 1:100), and DMP
123 G. D. Cooper, H. S. Blanchard, G. F. Endres, H. Finkbeiner, J. Am.
Chem. Soc. 1965, 87, ±996 ± ±99ꢁ.
1±3 D. A. Bolon, J. Org. Chem. 1967, 32, 15ꢀ4 ± 1590.
143 W. J. Mijs, O. E. van Lohuizen, J. Bussink, L. Vollbracht, Tetrahedron
À1
(
2.5 mmolL ) under the same conditions obtained with the Shimadzu
UV-2100 spectrometer under air at room temperature. The depolymeriza-
tion was monitored by the spectra recorded at different reaction times.
[
[
Electrochemical measurements: Cyclic voltammetry was carried out in a
conventional two-compartment cell. A glassy carbon disk-platinum ring
was used as the working electrode and polished before each experiment
with 0.05 mm alumina paste. The auxiliary electrode, a coiled platinum wire,
was separated from the working solution by a fine-porosity frit. The
reference electrode was a commercial Ag/AgCl electrode immersed in a
1
967, 23, 225± ± 2264.
[
[
[
153 G. D. Cooper, J. G. Bennet, J. Org. Chem. 1972, 37, 441 ± 44ꢁ.
163 M. Ionescu, A. B. Mihis, Macromol. Symp. 1997, 122, 249 ± 256.
1ꢁ3 a) P. J. Baesjou, W. L. Driessen, G. Challa, J. Reedijk, J. Am. Chem.
Soc. 1997, 119, 12590 ± 12594; b) P. J. Baesjou, W. L. Driessen, G.
Challa, J. Reedijk, Macromolecules 1999, 32, 2ꢁ0 ± 2ꢁ6.
À1
salt brige consisting of 0.1 molL tetrabutylammonium tetrfluoroborate
[
1ꢀ3 a) D. M. White, J. Org. Chem. 1969, 34, 29ꢁ ± ±0±; b) D. M. White, J.
Polym. Sci., Part A 1971, 9, 66± ± 6ꢁ5.
(
4 9 4 4
(n-C H ) NBF ), which was placed in the main cell compartment. The
formal potential of the ferrocene/ferrocenium couple in dichloromethane
[
[
[
[
[
193 D. M. White, J. Polym. Sci., Polym. Chem. Ed. 1981, 19, 1±6ꢁ ± 1±ꢀ±.
203 W. Risse, W. Heitz, Makromol. Chem. 1985, 186, 1ꢀ±5 ± 1ꢀ5±.
213 W. Chen, G. Challa, J. Reedijk, Polym. Commun. 1991, 32, 51ꢀ ± 52±.
223 H. S.-I. Chao, J. M. Whalen, J. Appl. Polym. Sci. 1993, 49, 15±ꢁ ± 1546.
2±3 a) H. A. M. van Aert, M. E. M. Burkard, J. F. G. A. Jansen, M. H. P.
van Gendersen, E. W. Meijer, H. Oevering, G. H. Werumeus Buning,
Macromolecules 1995, 28, ꢁ96ꢁ ± ꢁ969; b) H. A. M. van Aert, M. H. P.
van Gendersen, G. J. M. L. van Steenpaal, L. Nelissen, E. W. Meijer,
Macromolecules 1997, 30, 6056 ± 6066.
À1
was 0.±4 V s versus this reference electrode. The voltammetric inves-
À1
tigation was carried out in CH
2
Cl
2
in the presence of a sample (1 mmolL ),
À1
À1
(
n-C
4
H
9
)
4
NBF
4
(0.1 molL ) and 2,6-diphenylpyridine (1 mmolL ), and
all potentials were quoted with respect to this Ag/AgCl refernce electrode
at a scan rate 25 mV. A Nikko Keisoku DPGS-1 dual potentiogavanostat
and a Nikko Keisoku NFG-± universal programmer were employed with a
Graphtec WX2400 X-Y recorder to obtain the voltammograms.
[
243 a) V. Percec, J. H. Wang, Polym. Bull. 1990, 24, 6± ± 69; b) V. Percec,
J. H. Wang, Polym. Bull. 1990, 24, ꢁ1 ± ꢁꢀ.
Acknowledgements
[
253 F. J. Viersen, J. Renkema, G. Challa, J. Reedijk, J. Polym. Sci., Part A
1992, 30, 901 ± 911.
This work was partially supported by a Grant-in-Aid for Scientific
Research (No. 1±450±ꢀ4) from MEXT, Japan.
[263 S. Tsuruya, K. Kinumi, K. Hagi, M. Masai, J. Mol. Catal. 1983, 22, 4ꢁ ±
0.
2ꢁ3 E. Tsuchida, H. Nishide, T. Nishiyama. J. Polym. Sci., Symp. 1974, 47,
ꢁ ± 54.
6
[
4
[
13 A. S. Hay, H. S Blanchard, G. F. Endres, J. W. Eustance, J. Am. Chem.
Soc. 1959, 81, 6±±5 ± 6±±6.
23 A. S. Hay, J. Polym. Sci. 1962, 58, 5ꢀ1 ± 591.
±3 A. S. Hay, J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 505 ± 51ꢁ.
43 D. Aycock, V. Abolins, D. M. White, Encyclopedia of Polymer Science
and Engineering, Vol. 13, 2nd ed., Wiley, New York, 1986, pp. 1 ± ±0.
Received: December 16, 2002
Revised: March ±1, 200± [F46693
[
[
[
4246
¹ 200± Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
Chem. Eur. J. 2003, 9, 4240 ± 4246