Oxidative coupling polymerization of bishydrazide for the synthesis of poly(diacylhydrazine): Oxidative preparation of oxidatively degradable polymer

Article abstract of DOI:10.1002/pola.26225

The oxidative coupling polymerization of bishydrazide is successfully performed to form poly(diacylhydrazine) (PDAH), which is an oxidatively degradable polymer. Oxone is an effective oxidant, and a mixture of an aprotic polar solvent, water, and acetonitrile or N,N-dimethylacetamide is necessary as the solvent. On treatment of PDAH with sodium hypochlorite solution or hydrogen peroxide, the PDAH is rapidly oxidized and degraded to the corresponding dicarboxylic acid. When hydrogen peroxide is used as the oxidant, the addition of acetonitrile and potassium carbonate is necessary for effective degradation. PDAH exhibits high thermal stability in air and a high T_g value. No oxidation is observed in air. Thus, PDAH is an oxidatively degradable high-performance polymer that is stable toward oxygen.

Full text of DOI:10.1002/pola.26225

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Oxidative Coupling Polymerization of Bishydrazide for the Synthesis
of Poly(diacylhydrazine): Oxidative Preparation of Oxidatively
Degradable Polymer
Kentaro Nagashima, Nobuhiro Kihara, Yusuke Iino
Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka 259-1293, Japan
Correspondence to: N. Kihara (E-mail: kihara@kanagawa-u.ac.jp)
Received 15 April 2012; accepted 18 June 2012; published online
DOI: 10.1002/pola.26225
ABSTRACT: The oxidative coupling polymerization of bishydra-
zide is successfully performed to form poly(diacylhydrazine)
(PDAH), which is an oxidatively degradable polymer. Oxone
is an effective oxidant, and a mixture of an aprotic polar sol-
vent, water, and acetonitrile or N,N-dimethylacetamide is nec-
essary as the solvent. On treatment of PDAH with sodium
hypochlorite solution or hydrogen peroxide, the PDAH is rap-
idly oxidized and degraded to the corresponding dicarboxylic
acid. When hydrogen peroxide is used as the oxidant, the
addition of acetonitrile and potassium carbonate is necessary
for effective degradation. PDAH exhibits high thermal stability
in air and a high Tg value. No oxidation is observed in air.
Thus, PDAH is an oxidatively degradable high-performance
C
V
polymer that is stable toward oxygen.
2012 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 000: 000–000,
2012
KEYWORDS: degradation; high perfomance polymers; hydra-
zide; oxidative coupling polymerization; oxidative degradation;
oxone; poly(diacylhydrazine); polycondensation
INTRODUCTION Recently, we reported that poly(diacylhydra-
zine) (PDAH) 1a, a polyamide for which hydrazine is the dia-
mine component, is a novel degradable polymer that decom-
poses through the action of an oxidant such as sodium
hypochlorite solution (Scheme 1).1
reports.1 Therefore, it is worthwhile to investigate the prop-
erties of PDAH, including their oxidative degradability.
PDAH is synthesized by the method used to synthesize poly-
amides: however, instead of a diamine, hydrazine is used in
the condensation reaction with carboxylic acid deriva-
tives.1(a) Because hydrazine is a volatile, hygroscopic, corro-
sive, and toxic compound, it is convenient to use bishydra-
zide as the hydrazine equivalent.2,3 In both conventional
methods, the nylon-salt technique is not available for the
preparation of PDAH because of the low nucleophilicity of
the terminal amino group in the hydrazide. Therefore, poly-
condensation of bishydrazide with a bis(acid chloride) or
bis(active ester) has been used, although the use of bis(acid
chloride) sometimes causes staining of the product.1 There-
fore, the accessibility of PDAH has been lower than that of
other polyamides.
1a exhibits some unique features as a degradable polymer:
(i) because molecular oxygen does not oxidize the diacylhy-
drazine moiety, PDAH 1a is stable in air even at high tem-
peratures (Td ¼ 320 ꢀC in air). (ii) Because sodium hypo-
chlorite is readily available but is not a naturally occurring
chemical, decomposition hardly takes place under ambient
conditions. However, once 1a is exposed to sodium hypo-
chlorite, oxidative degradation starts promptly. Therefore,
degradation can be initiated at the desired moment. (iii)
PDAH 1a is an ordinary polyamide. Because of the strong
hydrogen-bonding interaction between the polymer chains,
PDAH 1a exhibits good thermal and mechanical properties.
A special method to access diacylhydrazine, that is, the par-
tial oxidation of hydrazine, has been reported.4 Thus, after
hydrazide is oxidized to acyldiimide, the facile nucleophilic
attack of a second hydrazide molecule on the highly reactive
acyldiimide affords diacylhydrazine (Scheme 2). The elimi-
nated diimide is further oxidized to nitrogen and water.
Therefore, theoretically, two moles of two-electron oxidant
are necessary for the formation of one mole of diacylhydra-
zine. Because the oxidation of diacylhydrazine is more
Because of unique features of 1a as a novel degradable poly-
mer, we were interested in the properties of other PDAHs.
PDAHs have been prepared as the precursors for whole aro-
matic poly(1,3,4-oxadiazole)s.2 Although thermal dehydrative
conversion of PDAH to poly(1,3,4-oxadiazole) has been
extensively studied, properties of PDAH itself have not been
paid much attention. Especially, oxidative degradability of
PDAH has not been reported except for our previous
C
V
2012 Wiley Periodicals, Inc.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
1

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
Rigaku Thermo plus TG-8120 instrument with a temperature
gradient of 10 K/min; samples were ramped from room tem-
perature to 500 ꢀC in an aluminum pan under air. Differen-
tial scanning calorimetry (DSC) was performed on a Rigaku
Thermo plus DSC-8230 with a temperature gradient of 10
K/min; samples were placed in a crimped aluminum pan
under air. HPLC analysis was performed using a Shimadzu
LC-20AT system with a TOSOH ODS 100V column placed in a
CTO-20A oven equipped with RID-10A and SPD-20A detec-
tors. The solvent was degassed using a Shimadzu DGU-20A
on-line degasser. Gel permeation chromatography (GPC) was
performed using a Shimadzu LC-10AT system with a Polymer
Laboratories PolyPore column placed in a CTO-10A oven
equipped with RID-10A and SPD-10A detectors. The solvent
was degassed using a DGU-12A on-line degasser.
SCHEME 1 Oxidative degradation of 1a.
difficult than that of hydrazide because of the electron-with-
drawing nature of the acyl group, selective oxidation of hy-
drazide to produce diacylhydrazine is possible.
If this oxidative coupling reaction can be used in polycon-
densation, we can access PDAH directly from bishydrazide.
To produce high-molecular weight PDAH, we have to control
the oxidation very precisely: complete oxidation of hydrazide
is required, whereas the oxidation of diacylhydrazine has to
be thoroughly prevented.
Oxidative Coupling Polymerization: Typical Procedure
Oxone (1.2347 g, 2.01 mmol) was added to a solution of
bishydrazide 3a (246.6 mg, 0.985 mmol) in N-methyl-2-pyr-
rolidone (NMP), water, and acetonitrile (1 mL each). After
being stirred for 24 h, the reaction mixture was poured into
methanol. The precipitate was washed thoroughly with
methanol followed by water and was dried in vacuo to
obtain polymer 1a (156.2 mg, 73% yield) as a white powder.
In this article, we report a novel effective method to prepare
PDAH: the oxidative coupling polycondensation of bishydra-
zide. Because various PDAHs were prepared easily with the
oxidative coupling poycondensation, we also investigate their
oxidative degradability and thermal properties.
Bishydrazide 3g
EXPERIMENTAL
A mixture of ester 4g (2.8233 g, 9.72 mmol) and hydrazine
hydrate (4.5360 g, 90.61 mmol) in ethanol (15 mL) was
Materials
5-tert-Butylisophthalodihydrazide 3a was prepared according
to the method previously reported1(b) and was used after
recrystallization from ethanol/water. Reagent-grade aliphatic
bishydrazides 3b–f are commercially available and were
used without further purification. Dimethyl 5-(octyloxy)i-
sophthalate (4g),5 dimethyl 5-(decyloxy)isophthalate (4h),6
dimethyl 5-phenylisophthalate (4i),7 dimethyl 4,6-bis(octy-
loxy)isophthalate (4j),8 4,40-oxybis(ethyl benzoate) (4k),9
3,30-(1,4-phenylene)bis(ethyl propionate) (4l),10 and 3,30-
(1,3-phenylene)bis(ethyl propionate) 4m10,11 were prepared
according to the literature procedures with some modifica-
tion. All other chemicals and solvents were reagent-grade,
and were used without further purification.
ꢀ
stirred for 18 h. The reaction mixture was cooled at ꢁ30 C.
The precipitate was separated by filtration, washed with a
small amount of cold ethanol, and dried in vacuo to afford
bishydrazide 3g (2.4380 g, 7.56 mmol, 78% yield).
mp 143.8–145.4 ꢀC; 1H NMR (400 MHz, DMSO-d6, d): 9.77
(s, 2H), 7.84 (s, 1H), 7.45 (s, 2H), 4.51 (s, 2H), 4.03 (t, J ¼
6.6 Hz, 2H), 1.69–1.76 (m, 2H), 1.38–1.45 (m, 2H), 1.23–1.36
(m, 8H), 0.83–0.90 (m, 3H); 13C NMR (100 MHz, DMSO-d6,
d): 165.6, 158.3, 135.3, 118.8, 115.7, 68.4, 31.7, 29.2, 29.0,
26.0, 22.6, 14.4.
Bishydrazide 3h
A mixture of ester 4h (5.6470 g, 16.12 mmol) and hydrazine
hydrate (8.0080 g, 160.00 mmol) in ethanol (30 mL) was
Instruments
ꢀ
stirred for 16 h. The reaction mixture was cooled at ꢁ30 C.
NMR spectra were recorded on JEOL JNM-ECP300, JNM-
EX400, and JNM-ECP500 spectrometers using tetramethylsi-
lane as an internal standard. IR spectra were recorded on a
JASCO FT/IR 4100 spectrometer. Mass spectra were recorded
on a JEOL JMS-T-100LC instrument equipped with a time-
of-flight detector and on JMS-AX-505H mass spectrometers.
Melting points were observed on a Yanaco MP-500D instru-
ment. Thermal gravimetric analysis was performed on a
The precipitate was separated by filtration, washed with a
small amount of cold ethanol, and dried in vacuo to afford
bishydrazide 3h (4.7726 g, 13.62 mmol, 85% yield).
mp 108.0–111.5 ꢀC; 1H NMR (300 MHz, DMSO-d6, d): 9.78
(s, 2H), 7.84 (s, 1H), 7.45 (s, 2H), 4.50 (s, 2H), 4.03 (t, J ¼
6.4 Hz, 2H), 1.69–1.76 (m, 2H), 1.38–1.46 (m, 2H), 1.20–1.36
(m, 14H), 0.82–0.88 (m, 3H); 13C NMR (100 MHz, DMSO-d6,
SCHEME 2 Oxidative coupling reaction of hydrazide.
2
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
d): 165.6, 158.9, 135.6, 118.9, 115.7, 68.4, 31.8, 29.4, 29.0,
25.9, 22.6, 14.4.
Bishydrazide 3i
A mixture of ester 4i (1.2177 g, 4.51 mmol) and hydrazine
hydrate (2.0951 g, 41.85 mmol) in ethanol (15 mL) was
stirred for 42 h, and heated at reflux for 4 h. The reaction
SCHEME 3 Oxidative coupling polymerization of 1a.
ꢀ
mixture was cooled at ꢁ30 C. The precipitate was separated
by filtration, washed with a small amount of cold ethanol,
and dried in vacuo to afford bishydrazide 3i (0.6845 g, 2.53
mmol, 56% yield).
Bishydrazide 3m
A mixture of ester 4m (4.1327 g, 14.85 mmol) and hydra-
zine hydrate (7.0561 g, 140.95 mmol) in ethanol (30 mL)
was stirred for 24 h. The reaction mixture was cooled at
ꢁ30 ꢀC. The precipitate was separated by filtration, washed
with a small amount of methanol, and dried in vacuo to
afford bishydrazide 3m (3.2476 g, 12.98 mmol, 87% yield).
mp 199.5–201.0 ꢀC; 1H NMR (300 MHz, DMSO-d6, d): 8.94
(s, 2H), 7.19–7.14 (m, 1H), 7.05–6.99 (m, 3H), 4.16 (s, 4H),
2.77 (t, J ¼ 7.8 Hz, 4H), 2.29 (t, J ¼ 7.8 Hz, 4H) ppm; 13C
NMR (125 MHz, DMSO-d6, d): 170.7, 141.0, 128.1, 125.7,
35.0, 30.9.
mp 189.3–192.1 ꢀC; 1H NMR (500 MHz, DMSO-d6, d): 9.96
(s, 2H), 8.26 (s, 1H), 8.22 (s, 2H), 7.81 (d, J ¼ 7.3 Hz, 2H),
7.52 (t, J ¼ 7.6 Hz, 2H), 7.43 (t, J ¼ 7.3 Hz, 1H), 4.57 (s,
4H); 13C NMR (100 MHz, DMSO-d6, d): 165.7, 140.7, 139.4.6,
134.7, 129.5, 128.6, 127.7, 127.5, 125.8; EIMS (m/z): 270,
255, 239, 224, 209, 181, 152.
Bishydrazide 3j
A mixture of ester 4j (3.1087 g, 6.90 mmol) and hydrazine
hydrate (3.5042 g, 70.00 mmol) in methanol (15 mL) was
ꢀ
stirred for 24 h. The reaction mixture was cooled at ꢁ30 C.
Oxidative Degradation by Sodium Hypochlorite:
Typical Procedure
The precipitate was separated by filtration and washed with
a small amount of cold methanol. The crude product was
recrystallized from methanol to afford bishydrazide 3j
(1.8322 g, 4.07 mmol, 59% yield).
A 5% solution of sodium hypochlorite (30 mL) was added to
a powder of PDAH 1a (86.5 mg, 0.40 mmol-unit). Gas evolu-
tion ceased within 10 min, and the reaction mixture was
stirred for additional 24 h. Concentrated hydrochloric acid
was added to give a pH value of < 1. The precipitate was
separated by filtration, washed with water, and dried in
vacuo to afford carboxylic acid 2a (49.0 mg, 0.22 mmol, 55%
yield).
mp 105.4–108.1 ꢀC (ref. 12, 113 ꢀC); 1H NMR (500 MHz,
CDCl3, d): 9.01 (s, 1H), 8.66 (s, 2H), 6.45 (s, 1H), 4.16–4.13
(m, 8H), 1.95–1.89 (m, 4H), 1.52–1.46 (m, 4H), 1.42–1.29
(m, 16H), 0.89 (t, J ¼ 6.8 Hz, 6H); 13C NMR (125 MHz,
CDCl3, d): 165.7, 160.3, 136.9, 113.7, 96.3, 69.7, 31.7, 29.2,
29.1, 29.0, 26.0, 22.6, 14.1.
Oxidative Degradation by Hydrogen Peroxide:
Typical Procedure
Bishydrazide 3k
Potassium carbonate (138.5 mg, 1.00 mmol) and 30% hydro-
gen peroxide solution (1 mL) were added to a solution of
PDAH 1a (105.4 mg, 0.48 mmol-unit) in NMP (1 mL) and
acetonitrile (0.5 mL). The reaction mixture was stirred for
24 h. Concentrated hydrochloric acid was added to give a pH
value of < 1. The precipitate was separated by filtration,
washed with water, and dried in vacuo to afford carboxylic
acid 2a (75.0 mg, 0.34 mmol, 70% yield).
A mixture of ester 4k (13.8223 g, 43.97 mmol) and hydra-
zine hydrate (22.2724 g, 444.9 mmol) in ethanol (100 mL)
was stirred for 40 h, and heated at reflux for 19 h. The reac-
tion mixture was cooled at ꢁ30 ꢀC. The precipitate was
separated by filtration, washed with a small amount of cold
ethanol, and dried in vacuo to afford bishydrazide 3k
(7.5629 g, 26.42 mmol, 60% yield).
mp 224.7–226.9 ꢀC; 1H NMR (500 MHz, DMSO-d6, d): 9.74
(s, 2H), 7.87 (d, J ¼ 8.8 Hz, 4H), 7.09 (d, J ¼ 8.8 Hz, 4H),
4.47 (s, 4H).
1
IR (KBr): m ¼ 3423, 2967, 1690, 1453, 1281 cmꢁ1; H NMR
(DMSO-d6, 500 MHz, d): 13.24 (s, 2H), 8.31 (s, 1H), 8.18 (s,
2H), 1.41 (s, 9H).
Bishydrazide 3l
A mixture of ester 4l (3.1030 g, 11.15 mmol) and hydrazine
hydrate (5.5336 g, 110.54 mmol) in ethanol (30 mL) was
stirred for 2 days. The reaction mixture was cooled at ꢁ30
ꢀC. The precipitate was separated by filtration, washed with
a small amount of cold methanol, and dried in vacuo to
afford bishydrazide 3l (2.4710 g, 9.88 mmol, 89% yield).
mp 232.7–234.1 ꢀC; 1H NMR (400 MHz, DMSO-d6, d): 8.94
(s, 2H), 7.08 (s, 4H), 4.14 (s, 4H), 2.76 (t, J ¼ 7.8 Hz, 4H),
2.29 (t, J ¼ 7.8 Hz, 4H); 13C NMR (100 MHz, DMSO-d6, d):
171.2, 139.2, 128.6, 35.6, 31.1.
RESULTS AND DISCUSSION
Polymerization Condition
For the optimization of the polymerization condition, the
bishydrazide 3a was used as the monomer (Scheme 3).
Because we have already reported PDAH 1a,1(b) we can eas-
ily identify the product polymer to evaluate the effectiveness
of the oxidation conditions. Polymer 1a is soluble in aprotic
polar solvents. Therefore, the reaction was carried out in
NMP as a polar solvent because of its inertness to the oxida-
tion conditions.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
3

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 1 Oxidative Coupling Polymerization of 3a with Various
faster than that of diacylhydrazine. A small amount of 1a
was obtained when [bis(trifluoroacetoxy)iodo]benzene was
used at room temperature (run 7). Other I(III) oxidants were
ineffective for the polymerization (runs 8 and 9). The reac-
tivity of iodine itself was so high that only over-oxidation
occurred (run 10). We found that Cu(II) acetate4(c) was an
ineffective oxidant for the oxidative coupling polymerization
(run 11). Therefore, we tried other one-electron oxidants
(runs 12–14).4(e) A polymeric product was obtained when
Oxidantsa
Oxidantb
Time
(h)
Yieldd
(%)
Run
(Equivalent)
Additivec
1
Oxone (1)
Oxone (2)
TBHP (1)
Water (1 mL)
Water (1 mL)
None
24
9
2
24
44
0
3
24
4
mCPBA (1)
K2S2O8 (1)
PhI(OAc)2 (1)
PhI(OCOCF3)2 (1)
PhIO (1)
None
24
0
1
Ce(IV) (CAN) was used as the oxidant, although the H NMR
spectrum of the product indicated that the desired PDAH 1a
was not produced (run 12).
5
None
24
0
0e
6
None
48
7
None
48
3
Because oxone was the only selective oxidant among those
we examined, the polymerization conditions were investi-
gated further using oxone as the oxidant to afford the high-
molecular weight PDAH 1a. We found that the solvent signif-
icantly affected the result of the polymerization. The results
are summarized in Table 2.
8
None
48
0f
0g
0g
0
9
Ph(IO)CO2Hf (1)
None
48
10
11
12
13
14
a
I2 (1)
Et3N (2 equiv.)
Water (1 mL)
Et3N (3 equiv.)
None
24
5 ming
Cu(OAc)2 (1)
CAN (1)
24
27h
Ag2O (1)
24
0
0h
Both the water content in the solvent and the concentrations
of substrates greatly affected the yield of the polymer. When
the amount of water was fixed to 1 mL for 1 mmol of 3a,
the best yield of polymer was obtained when 2 mL of NMP
was used (run 2), whereas no polymer was obtained when 3
mL of NMP was used (run 3). No polymer was obtained at
higher temperatures because of over-oxidation (run 4). The
effect of the organic solvent was also examined. CH3CN was
found to be a good solvent as well as NMP (run 6). Surpris-
ingly, the best result was obtained when NMP was used with
CH3CN (run 7). The yield of polymer simply decreased at
lower temperatures (runs 8 and 9); the oxidation was sensi-
tive to the polymerization temperature.
K3[Fe(CN)6] (1)
None
48
Polymerization of 3a (1 mmol) was carried out at room temperature in
NMP (1 mL).
b
One equivalent of oxidant consumes four electrons to produce one
diacylhydrazine moiety.
c
Equivalent to hydrazine moiety.
d
Methanol- and water-insoluble part.
e
5-tert-Butylisophthalic acid was formed.
2-Iodosylbenzoic acid.
f
g
Microwave irradiation (500 W).
h
Complex mixture.
For the oxidative coupling polymerization, we had to deter-
mine the appropriate oxidant. Oxone (2KHSO5ꢂKHSO4ꢂK2-
SO4)4(a) and (diacetoxyiodo)benzene (DAIB)4(b) have been
reported to be an effective oxidants for the coupling of hy-
drazide to obtain diacylhydrazine. Oxidation by copper(II)
acetate under microwave irradiation has also been repor-
ted.4(c) Thus, 3a was treated with these oxidants and related
compounds in NMP. When the oxidant was insoluble in NMP,
water was added as a co-solvent. Polymer 1a was isolated as
the methanol- and water-insoluble part, and the product was
The role of each organic solvent was examined in the NMP/
CH3CN/water tersolvent system. First, CH3CN was replaced
with other organic solvents (runs 10–12). We found that
N,N-dimethylacetamide (DMAc) is as effective as CH3CN (run
11). It should be noted that the NMP/DMAc/water (1/1/1)
system gave better results than the NMP/water (2/1) system
(run 2), in spite of the fact that NMP and DMAc are very
similar aprotic polar solvents. Next, NMP was replaced by
other organic solvents (runs 13–15). Aprotic polar solvents
of the amide type, such as DMF and DMAc, also gave good
results.
1
identified by H NMR spectroscopy. The results are summar-
ized in Table 1.
No polymerization occurred without water. It seems that
water is necessary only to dissolve the oxone. Because water
can be a competitive nucleophile toward the acyldiimide in-
termediate, we expected that the yield and molecular weight
of the polymer would increase in the absence of water. Thus,
the oxidative coupling polymerization was carried out with-
out water, but in the presence of phase-transfer catalyst to
dissolve oxone in nonaqueous media. However, no polymer
was obtained in the presence of 18-crown-6 ether or tetra-
butylammonium hydrogen sulfate, even though oxone par-
tially dissolved in the presence of these phase-transfer cata-
lysts. Although the role of water in the oxidation reaction is
not clear, it was found that water is essential in the oxidative
coupling polymerization of bishydrazide by oxone.
When 1 equivalent of oxone was used as the oxidant, poly-
mer 1a was obtained, albeit in low yield (run 1). The yield
increased as the amount of oxone was increased (run 2).
Other peroxides gave no polymeric product (runs 3–5).4(d)
Although DAIB has been reported to be an effective oxidant
for hydrazide for the formation of diacylhydrazine,4(b) the
oxidative coupling polymerization of 3a with DAIB was
unsuccessful (run 6). The product was an oligomer that was
soluble in methanol and/or 5-tert-butylisophthalic acid 2a,
the over-oxidation product. To examine the reactivity of
DAIB, we treated 1a with DAIB in NMP, and found that 1a
was slowly oxidized by DAIB. Therefore, DAIB also oxidizes
diacylhydrazine, even though the oxidation of hydrazide is
4
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 2 Oxidative Coupling Polymerization of 3a Using Oxone as the Oxidanta
Yieldb (%)
Mn
Mw/Mn
c
c
Run
Solvent (mL)
Temperature
Time (h)
1
NMP/water (1/1)
rt
48
48
48
48
24
24
48
72
48
48
48
48
48
48
48
25
34
0
14,000
21,000
–
6.6
6.9
–
2
NMP/water (2/1)
rt
3
NMP/water (3/1)
rt
60 ꢀC
4
NMP/water (2/1)
0
–
–
5
DMF/water (1/1)
rt
15
27
64
0
15,000
18,000
20,000
–
6.7
8.0
29
–
6
CH3CN/water (1/1)
rt
7
NMP/CH3CN/water (1/1/1)
NMP/CH3CN/water (1/1/1)
NMP/CH3CN/water (1/1/1)
NMP/THF/water (1/1/1)
NMP/DMAc/water (1/1/1)
NMP/MeOH/water (1/1/1)
DMF/CH3CN/water (1/1/1)
DMAc/CH3CN/water (1/1/1)
THF/CH3CN/water (1/1/1)
rt
8
ꢁ40 ꢀC
0
9
ꢀC
41
39
69
24
64
65
48
14,000
23,000
20,000
15,000
25,000
21,000
9,500
18
48
9.6
42
76
59
150
10
11
12
13
14
15
a
rt
rt
rt
rt
rt
rt
Polymerization was carried out using 3a (1 mmol) with oxone (2 mmol).
Methanol- and water-insoluble part.
b
c
Estimated by GPC (DMF, PSt standards).
The effect of polymerization time was then examined. The
results are shown in Figure 1 with GPC profile. As the poly-
merization time was increased, both the yield and molecular
weight of 1a increased, and they were saturated around for
24 h. This behavior is unlikely in step polymerization, but
likely to occur in chain polymerization. At present, the rea-
son for this unusual behavior is not clear.
A very large molecular weight distribution (MWD) was
observed, although no gel fraction was observed even in the
case of long polymerization time. In the GPC chart, the con-
siderable amount of the fraction in the very large molecular
weight region was observed from the early stage of the poly-
merization. It can be deduced that partial crosslinking
occurred as a side reaction. Because an irregular structure
1
was not detected in the H NMR spectra, there is no spectro-
scopic information on the branching structure. On the other
hand, PDAH with a large MWD was also obtained in the pol-
ycondensation system.1(b) Therefore, it can be estimated that
the main chain of PDAH 1a reacts with the active polymer
terminal. We deduce that the crosslinking reaction occurred
via the nucleophilic attack of the diacylhydrazine group
on the highly reactive acyldiimide terminal, as shown in
Scheme 4, to afford partially branched polymer.13
FIGURE 1 The change of the yield and the GPC profile during
the polymerization.
SCHEME 4 Possible crosslinking reaction.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
5

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
SCHEME 5 Oxidative coupling polymerization of aliphatic dihydrazide.
Because the NMP/CH3CN system was superior to other sol-
vents for the oxidative coupling polymerization, 3a was
treated with iodine(III) oxidants in NMP/CH3CN. When [bis
(trifluoroacetoxy)iodo]benzene was used, the desired poly-
mer 1a was obtained in 6% yield (Mn 26,000, Mw/Mn 57).
Although the yield was still low, it was better than that
obtained in NMP. On the other hand, 1a was not obtained
even in NMP/CH3CN when DAIB was used as the oxidant.
showed rather high solubilities in organic solvents, high-mo-
lecular weight PDAH was obtained, whereas the less soluble
3j–m gave low-molecular weight PDAH. Although the effect
of the solvent system was small, the molecular weight of
PDAH tended to be high in NMP/DMAc/water for the less
soluble bishydrazide, because DMAc is a better solvent for
PDAH than acetonitrile. Although the hydrophobic 3j is solu-
ble in chloroform, PDAH 1j was insoluble in chloroform and
DMSO, and only partially soluble in DMF.
Oxidative Coupling Polymerization of Bishydrazides
The oxidative coupling polymerization of the commercially
available alkanodihydrazides 3b–f was examined in NMP/
CH3CN/water using oxone as the oxidant (Scheme 5).
Because of the low solubility of these bishydrazides, how-
ever, every polymerization system was heterogeneous.
Because hydrazides 3c and 3d are insoluble in the solvent
system, no polymerization occurred. When 3b, 3e, and 3f,
which are slightly soluble in organic solvents, were used as
monomers, water- and methanol-insoluble products were
obtained in 58–78% yield. However, the products were insol-
uble in any organic or inorganic solvents, including sulfuric
acid and aqueous ammonia solution.1(a) Therefore, further
characterization of the polymer was not carried out.14
Oxidative Degradation of PDAH
Because various structures of PDAH were obtained, their oxi-
dative degradabilities were examined. First, PDAHs 1a and
1g–m were treated with sodium hypochlorite solution (5%)
at room temperature. Except for 1j, the PDAHs decomposed
immediately with the evolution of nitrogen gas. The decom-
position finished within a few tens of minutes to give a clear
solution. The decomposition of the aliphatic PDAHs 1l and
It was supposed that strong intermolecular hydrogen bond-
ing caused the low solubility of hydrazides and PDAH. There-
fore, some bishydrazide monomers 3g–m, with a bending
structure and hydrophobic side chain to prevent intermolec-
ular hydrogen bonding and the stacking of methylene chains,
were prepared. According to the general synthetic strategy,15
the corresponding diester 4 was treated with excess hydra-
zine hydrate in alcohol to afford the desired bishydrazide 3
in good yield (Scheme 6). Although these bishydrazides are
soluble in aprotic polar solvents, their solubilities were
lower than that of 3a, with the exception of 3i, which has as
good solubility in aprotic polar solvents as 3a. The rather
hydrophobic 3i and 3j are also soluble in chloroform, in
which other bishydrazides are insoluble.
The oxidative coupling polymerization of 3g–m was carried
out in both the NMP/CH3CN/water and NMP/DMAc/water
solvent systems. In the case where the monomer had low
solubility in the solvent system, the solution was heated to
dissolve the monomer as much as possible, and oxone was
added at room temperature before the monomer was pre-
cipitated out. The results of the polymerization are summar-
ized in Table 3.
Using oxidative coupling polymerization, we could access
various PDAHs very easily. Even though MWDs were so
large, no gel fraction was observed. Because 3a and 3g–i
SCHEME 6 Synthesis of bishydrazides.
6
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 3 Oxidative Coupling Polymerization of Bishydrazide 3a
TABLE 4 Oxidative Degradation of PDAH by Sodium
Hypochlorite
Solventb
Yieldc (%)
Mn
Mw/Mn
d
d
Monomer
PDAH
Mn
Yielda (%)
3a
3a
3g
3g
3h
3h
3i
A
B
A
B
A
B
A
B
A
B
A
B
A
B
A
B
73
72
85
69
90
92
69
65
76
80
76
80
56
63
78
68
23,000
20,000
20,000
16,000
10,000
13,000
16,000
20,000
4,700e
4,800e
5,000
58
1a
1g
1h
1i
23,000
20,000
13,000
20,000
4,800
55
9.6
33
81
65
70
35b
0 (18)
38b
36b
56b
61
1j
160
79
1k
1l
11,000
10,000
9,000
3i
34
3j
140e
340e
490
75
1m
a
3j
As dicarboxylic acid 2. Recovery of the starting polymer in
parentheses.
3k
3k
3l
b
Partially chlorinated dicarboxylic acid.
11,000
5,700
180
1,200
140
850
3l
10,000
9,000
neously with the oxidative degradation of PDAH by the
action of hypochlorite. It should be noted that the aromatic
rings in 2i–m have the positions to be attacked by the elec-
trophile, whereas those in 2a, 2g, and 2h are strongly deacti-
vated by the two carbonyl groups. Because the nonregiose-
lective polychlorination was deduced from the 1H NMR
spectra of the chlorinated carboxylic acid, further characteri-
zation was not carried out.
3m
3m
a
6,000
Polymerization was carried out using 3 (1 mmol) with oxone (2 mmol)
at rt for 24 h.
b
A: NMP/CH3CN/water (1 mL each). B: NMP/DMAc/water (1 mL/1 mL/
0.8 mL).
c
Methanol- and water-insoluble part.
d
Estimated by GPC (DMF, PSt standards).
e
DMF-soluble part.
The reaction mechanism for the oxidative degradation of
PDAH is shown in Scheme 7. After the oxidation of the diac-
ylhydrazine moiety, the highly reactive azodicarbonyl moiety
is rapidly hydrolyzed to afford the carboxylic acid and dii-
mide,16 which is easily oxidized to nitrogen gas under the
reaction conditions used. Because of the high hydrophobicity
of 1j, the approach of the hypochlorite ion to 1j was so slow
that 1j was recovered without change.
1m proceeded more rapidly than that of the aromatic
PDAHs. When the reaction mixture was acidified with hydro-
chloric acid, the decomposition product was precipitated out.
The precipitates were collected by filtration, and the yields
are summarized in Table 4.
In the decomposition of 1a, 1g, and 1h, the product was the
corresponding carboxylic acid 2. The decomposition of 1i,
1k, 1l, and 1m also afforded carboxylic acid, and the prod-
ucts were spectroscopically very similar to the expected car-
Next, the oxidative degradation with hydrogen peroxide was
examined. We have reported that the oxidative degradation of
1a with hydrogen peroxide was unsuccessful even in the pres-
ence of transition metal salts.1(b) However, the special solvent
effect of acetonitrile on the oxidative coupling polymerization
prompted us to re-examine the oxidative degradation of PDAH
with hydrogen peroxide. Thus, hydrogen peroxide (30%) was
added to a solution or suspension of 1a (Mn 21,000) at room
temperature. The mixture was stirred for 24 h, and the
1
boxylic acids. However, in the H NMR spectra of the decom-
position products, the integral of the aromatic region was
smaller than the expected value for the carboxylic acid 2.
Further, the Beilstein test was positive for these products.
These observations clearly indicated that the chlorination of
the aromatic ring in the carboxylic acid 2 occurred simulta-
SCHEME 7 Oxidative degradation of PDAH.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
7

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
TABLE 5 Oxidative Degradation of 1a by Hydrogen Peroxide
TABLE 7 Thermal Properties of PDAHs
a
b
PDAH
Mn
Tg
Td5
1a
1g
1h
1i
23,000
20,000
13,000
20,000
4,800
N
N
N
320 ꢀC
306 ꢀC
300 ꢀC
320 ꢀC
298 ꢀC
251 ꢀC
270 ꢀC
257 ꢀC
215 ꢀC
1j
N
N
N
N
Solvent
Additive
Yield (%)
Recovery (%)
Mn
1k
1l
11,000
10,000
9,000
NMP
None
None
K2CO3
K2CO3
K2CO3
0
97
96
90
0
8,000
3,500
10,000
–
NMP/CH3CN
NMP
0
1m
a
0
Measured by DSC (10 K/min, in air). N: not observed under Td.
NMP/CH3CN
CH3CNa
70
86
b
5% weight-loss temperature measured by thermal gravimetric analysis
0
–
(10 K/min, in air).
a
Heterogeneous.
scarcely observed in the oxidation of 1j; the low degradabil-
ity of 1j may be attributed to the low solubility and the high
hydrophobicity of 1j, whereas the strong electron-donating
nature of two alkoxy groups that deactivate the electrophilic
carbonyl groups to reduce the rate of hydrolysis can also be
the reason. Although the decomposition was slower than
that in sodium hypochlorite solution, the decomposition fin-
ished within a few hours to give a clear solution, and the
decomposition product was collected as the precipitate after
acidification of the reaction mixture. The product yields are
summarized in Table 6.
precipitate was collected after acidification of the reaction
mixture. The results are summarized in Table 5.
Although oxidative degradation was very slow in NMP, it was
accelerated by acetonitrile, and the expected dicarboxylic
acid 2a was obtained in the presence of a base such as po-
tassium carbonate. Oxidative degradation also proceeded
without NMP, even though 1a is insoluble in acetonitrile. At
present, the reason for this special effect of acetonitrile is
unclear. We deduce that the formation of peroxyimidic acid
is responsible for the solvent effect of acetonitrile.17
Being different from the oxidative degradation with sodium
hypochlorite, the decomposition of the aliphatic PDAHs 1l
and 1m was slower than that of the aromatic PDAHs, and
the oligomeric PDAH was recovered instead of the carboxylic
acid. In every case, no oxidation of the aromatic ring of the
carboxylic acid was observed. Hydrogen peroxide selectively
oxidized the diacylhydrazine moiety because hydrogen per-
oxide is a mild oxidant.
The oxidative degradation of other PDAHs with hydrogen
peroxide was examined in NMP/acetonitrile in the presence
of potassium carbonate. Except for 1j, the PDAHs decom-
posed with the evolution of nitrogen gas. Degradation was
TABLE 6 Oxidative Degradation of PDAH by Hydrogen
Thermal Properties of PDAH
Peroxide
Stability in air is one of the most important properties of
PDAH as an oxidatively degradable polymer. Thus, thermal
gravimetric and DSC analyses were performed to measure the
thermal properties of the PDAHs. All the measurements were
carried out in air to examine the stability of the newly
obtained PDAHs in air. The results are summarized in Table 7.
PDAH
Mn
Yielda (%)
The high-molecular weight PDAHs showed Td5 values above
300 ꢀC, whereas Td5 values of around 250–270 ꢀC were
observed for the rather low-molecular weight PDAHs. The
decomposition products were colorless. Thus, it was con-
firmed that the PDAHs are thermally stable and are hardly
oxidized in air. For most PDAHs, no Tg and Tm were
observed. Tg was observed only for 1i at 215 ꢀC. It can be
deduced that the othꢀer PDAHs have Tg values that are
greater than Td (>300 C).
1a
1g
1h
1i
23,000
20,000
13,000
20,000
4,800
70
46
31
44
(65)b
1j
1k
1l
11,000
10,000
9,000
63
(56)c
(78)d
1m
a
CONCLUSIONS
Yield of dicarboxylic acid 2. Recovery of polymer in parentheses.
b
c
Mn ¼ 4,800.
Mn ¼ 8,000.
Mn ¼ 2,000.
In this article, we have disclosed the novel oxidative coupling
polymerization of bishydrazide to form PDAH, an oxidatively
d
8
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000

JOURNAL OF
POLYMER SCIENCE
ARTICLE
WWW.POLYMERCHEMISTRY.ORG
4 (a) Kulkarni, P. P.; Kadam, A. J.; Desai, U. V.; Mane, R. B.;
Wadgaonkar, P. P. J. Chem. Res. (S) 2000, 184–185; (b) Prakash,
O.; Sharma, V.; Sadana, A. Synth. Commun. 1997, 27,
3371–3377; (c) Mogilaiah, K.; Prashanthi, M.; Reddy, G. R.
Synth. Commun. 2003, 33, 3741–3745; (d) Hoffman, R. V.;
Kumar, A. J. Org. Chem. 1984, 49, 4014–4017; (e) Mackay, D.;
Marx, U. F.; Waters, W. A. J. Chem. Soc. 1964, 4793–4800.
degradable polymer. The over-oxidation of PDAH can be pre-
vented by using oxone as the oxidant. For effective polymer-
ization, a tersolvent system (aprotic polar solvent/water/ace-
tonitrile or DMAc) should be used. Various PDAHs of high
molecular weight were prepared in good yield when both
monomer and polymer exhibited good solubility in the sol-
vent system.
5 Yang, Y.; Xue, M.; Xiang, J.-F.; Chen, C.-F. J. Am. Chem. Soc.
2009, 131, 12657–12663.
The PDAHs generally showed good oxidative degradability.
When PDAH was treated with sodium hypochlorite solution,
oxidative degradation was rapid, although chlorination of the
electron-rich aromatic ring occurred simultaneously. When
hydrogen peroxide was used as the oxidant in an acetoni-
trile-containing solvent in the presence of potassium carbon-
ate, oxidative degradation proceeded gently, and no side
reaction was observed.
6 Berl, V.; Huc, I.; Lehn, J.-M.; DeCian, A.; Fischer, J. Eur. J.
Org. Chem. 1999, 3089–3094.
7 Muller, W.; Lowe, D. A.; Neijt, H.; Urwyler, S.; Herrling, Paul,
¨
L.; Blaser, D.; Seebach, D. Helv. Chim. Acta 1992, 75, 855–864.
8 Zeng, H.; Miller, R. S.; Flowers, R. A.; Gong, B. J. Am. Chem.
Soc. 2000, 122, 2635–2644.
9 Tomita, M.; Higuchi, H. Yakugaku Zasshi 1954, 74, 1278–1279.
10 (a) Koizumi, T.; Tsutsui, K.; Tanaka, K. Eur. J. Org. Chem.
2003, 4528–4532; (b) Pohl, H. J. Prakt. Chem. 1934, 141, 44–60.
We confirmed that PDAH exhibits high thermal stability in
air. Even though PDAH is an oxidatively degradable polymer,
oxygen is a poor oxidant for the diacylhydrazine moiety. Fur-
ther, the thermal stability of the NAN bond is high enough,
and the PDAHs are as thermally stable as usual polyamides.
Because of the high Tg values, these PDAHs can provide a
novel type of high-performance polymer with high
degradability.
11 Kipping, F. S. J. Chem. Soc. 1888, 53, 21–47.
12 Yang, Y.; Yang, Z.-Y.; Yi, Y.-P.; Xiang, J.-F.; Chen, C.-F.;
Wan, L.-J.; Shuai, Z.-G. J. Org. Chem. 2007, 72, 4936–4946.
13 (a) Cheung, H. T.; Blout, E. R. J. Org. Chem. 1965, 30,
315–316; (b) Imamoto, T. Bull. Chem. Soc. Jpn. 1972, 45,
2216–2217; (c) Bumgardner, C. L.; Purrington, S. T.; Huang,
P.-T. J. Org. Chem. 1983, 48, 2287–2289.
14 (a) Hasegawa, M.; Unishi, T. J. Polym. Sci. Part B: Polym.
Lett. 1964, 2, 237–239; (b) Unishi, T.; Hasegawa, M. J. Polym.
Sci. Part A: Gen. Pap. 1965, 3, 3191–3198.
ACKNOWLEDGMENTS
15 (a) Wagner, R. B.; Zook, H. D. In Synthetic Organic Chemis-
try; Wiley: New York, 1953; pp 569; (b) Shin Jikken Kagaku
Koza, No. 2; Chemical Society of Japan, Ed.; Maruzen: Tokyo,
1977; Vol. 14, pp 1220.
The authors thank the financial support of Grant-in-Aid for
Scientific Research from Japan Society for the Promotion of
Science (JSPS).
16 (a) Bumgardner, C. L.; Purrington, S. T.; Huang, P.-T. J. Org.
Chem. 1983, 48, 2287–2289; (b) Millington, C. R.; Quarrell, R.;
Lowe, G. Tetrahedron Lett. 1998, 39, 7201–7204; (c) Stefane, B.;
Kocevar, M.; Polanc, S. Tetrahedron Lett. 1999, 40, 4429–4432;
(d) Srinivas, R.; Reddy, B. V. S.; Yadav, J. S.; Ramalingam, T.
J. Chem. Res. (S) 2000, 376–377; (e) Peters, C.; Waldmann, H.
J. Org. Chem. 2003, 68, 6053–6055; (f) Takale, B. S.; Telvekar,
V. N. Chem. Lett. 2010, 39, 546–547.
REFERENCES AND NOTES
1 (a) Kihara, N.; Ii, R.; Ogawa, A. J. Polym. Sci. Part A: Polym.
Chem. 2007, 45, 963–967; (b) Kihara, N.; Iino, Y.; Misawa, T. J.
Polym. Sci. Part A: Polym. Chem. 2008, 46, 6255–6262.
2 (a) Hasegawa, M. Encycl. Polym. Sci. Technol. 1969, 11,
169–187; (b) Unishi, T. Shinku Kagaku 1969, 16, 73–77.
3 (a) Frazer, A. H.; Reed, T. A. In Macromolecular Syntheses;
Moore,J. A., Ed.; Wiley: New York, 1977; Coll. Vol. 1, pp 299;
(b) Cambell, T. W. In Preparative Method of Polymer Chemis-
try; Interscience: New York, 1961; pp 103.
17 (a) Payne, G. B.; Williams, P. H. J. Org. Chem. 1961, 26,
651–659; (b) Payne, G. B.; Deming, P. H.; Williams, P. H. J. Org.
Chem. 1961, 26, 659–663; (c) Ogata, Y.; Sawaki, Y. Tetrahedron
1964, 20, 2065–2068.
WWW.MATERIALSVIEWS.COM
JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY 2012, 000, 000–000
9