Macromolecules
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30 °C was 0.46 dL g−1. 1H NMR (300 MHz, DMSO-d6, δ ppm): 9.69
(s, 2H, NH−CO), 8.07 (s, 4H,Ar−H), 7.81 (s, 1H, Ar−H), 6.87
(s, 1H, Ar−H), 3.89 (s, 6H, CH3). IR (ATR, ν, cm−1): 3422 (ν N−H),
2939 (ν CH3), 1651 (amide I type), 1524 (amide II type). Anal. Calcd
for C16H14N2O4: C, 64.41; H, 4.73; N, 9.39. Found: C, 66.74; H, 6.13;
N, 7.74. EA analysis on Me-prePPBO do not agree with the calculated
ones, which suggests that Me-prePPBO contains many end groups,
such as amine and carboxylic acid moieties.
2.3. Synthesis of PPBO Precursor (iPr-prePPBO). Under a
nitrogen atmosphere, 2 (0.224 g, 1.0 mmol) was dissolved in NMP
(4.0 mL) and then cooled to 0 °C, to which terephthaloyl chloride
(0.203 g, 1.0 mmol) and triethylamine (0.3 mL, 2.0 mmol) were
added, and then the mixture was stirred at 0 °C for 24 h. The solution
was poured into 80 mL of methanol to produce a precipitate, which
was filtered and rinsed several times with methanol. The obtained
solid was rinsed with methanol at 90 °C using a Soxhlet extractor and
then dried at 80 °C for 24 h in a vacuum to produce a green powder
(0.336 g, yield 95%). The inherent viscosity of the obtained polymer
measured at a concentration of 0.5 dL g−1 in H2SO4 at 30 °C was
Figure 1. Chemical structures of prePPBO.
polymer materials.11 However, especially in the field of
microelectronics, generation of an organosilicon moiety during
the fabrication is avoided in order to prevent the contamination
of the devices, and thus a PPBO precursor free from the silyl
moiety is strongly requested.
In this paper, we describe the development of a novel soluble
PPBO precursor functionalized with an alkyl group instead of
TBS as a substitution group of poly(o-hydroxyphenyl amide)
(R = alkyl in Figure 1). Especially, the formation mechanism of
the thermal conversion from the o-alkoxyphenylamide12,13 to
the benzoxazole structure has not yet been studied, and the
structural dependency of the alkyl group (methyl vs isopropyl)
was investigated based on the TGA, IR, pyrolysis gas chro-
matography mass spectrometry (Py-GC/MS), and evolved gas
analysis GC/MS (EGA-GC/MS) techniques. Finally, we
evaluated the mechanical and thermal properties of the fabri-
cated freestanding PPBO films from the novel PPBO precursor
films prepared in this study.
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1.21 dL g−1. H NMR (300 MHz, DMSO-d6, δ ppm): 9.60 (s, 2H,
NH−CO), 8.14 (s, 4H, Ar−H), 8.08 (s, 1H, Ar−H), 6.94 (s, 1H,
Ar−H), 4.73 (m, 2H, C−H), 1.37 (d, 13H, CH3). IR (ATR, ν, cm−1):
3419 (ν N−H), 2976 (ν CH−CH3), 1667 (amide I type), 1520
(amide II type). Anal. Calcd for C20H22N2O4 + 1.33 H2O: C, 63.48; H,
5.81; N, 7.40. Found: C, 63.48; H, 4.72; N, 9.19.
2.4. Fabrication of Free-Standing PPBO Films. The iPr-
prePPBO (100 mg) was dissolved in NMP (2 mL) at 140 °C for 2 h.
The solution was then filtered through cotton followed by solution
casting on a glass substrate to obtain a film, which was dried at 60 °C
for 1 h, 70 °C for 1 h, 80 °C for 1 h, 100 °C for 1 h, and finally at
120 °C for 1 h. The film was peeled off the glass substrate by
immersing the substrate in water. The obtained film was dried at 80 °C
for 24 h under vacuum and subsequently heated in a vacuum at 250 °C
for 6 h and then at 300 °C for 6 h. The intrinsic viscosity of the
obtained polymer film measured in methanesulfonic acid at 30 °C was
1.68 dL g−1. IR (ATR, ν, cm−1): 1617 (ν CN), 1361 (ν C−N),
1054 (ν C−O). Anal. Calcd for C14H6N2O2 + 0.084 H2O: C, 71.79; H,
3.15; N, 10.94. Found: C, 71.80; H, 2.56; N, 11.96.
2. EXPERIMENTAL SECTION
2.1. Materials. 1,5-Difluoro-2,4-dinitrobenzene and terephthaloyl
chloride were purchased from TCI. Terephthaloyl chloride was
purified by recrystallization from n-hexane, and the other reagents
were used as received. Palladium on carbon (Pd/C, 10%), potassium
hydroxide (KOH), and triethylamine were purchased from Aldrich
and used as received. Potassium carbonate (K2CO3), ethyl acetate
(EtOAc), methanol, 2-propanol, sulfuric acid (H2SO4), methanesul-
fonic acid, NMP, N,N-dimethylformamide (DMF), N,N-dimethylacet-
amide (DMAc), and dimethyl sulfoxide (DMSO) were purchased
from Wako Pure Chemical Co. and used as received. 1,5-Dimethoxy-
2,4-dinitrobenzene, 4,6-dimethoxy-1,3-diaminobenzene (1), and
4,6-diisopropoxy-1,3-diaminobenzene (2) were synthesized according
to the literature.14 1,5-Diisopropoxy-2,4-dinitrobenzene was synthe-
sized according to the literature15 (for details, see the Experimental
Section in the Supporting Information).
2.2. Synthesis of PPBO Precursor (Me-prePPBO). Under a
nitrogen atmosphere, 1 (0.168 g, 1.0 mmol) was dissolved in NMP
(4.0 mL) and then cooled to 0 °C, to which terephthaloyl chloride
(0.203 g, 1.0 mmol) and triethylamine (0.3 mL, 2.0 mmol) were
added. After a few minutes, the solution changed to a gel, to which
NMP (46 mL) was added followed by stirring at 100 °C for 24 h. The
obtained solution was poured into 800 mL of methanol to produce a
precipitate, which was filtered and rinsed several times with methanol.
The obtained solid was rinsed with methanol at 90 °C using a Soxhlet
extractor and then dried at 80 °C for 24 h in a vacuum to give a brown
powder (0.254 g, yield 85%). The inherent viscosity of the obtained
polymer measured at the concentration of 0.5 dL g−1 in H2SO4 at
2.5. Measurements. The inherent viscosity measurements were
1
carried out using an Ostwald viscometer. The H NMR spectra and
FT-IR spectra were measured using an AV 300 M spectrometer
(Bruker Biospin) and a Spectrum 65 FT-IR (PerkinElmer) spec-
trometer equipped with an ATR apparatus at a resolution of 4 cm−1,
respectively. The mechanical properties of the films were measured
at 25 °C using an EZ-S (Shimadzu) at the displacement rate of
1.0 mm min−1.
2.6. TG Analysis. A thermogravimetric analysis (TGA) was car-
ried out using an EXSTAR TG/DTA 6300 (SII Nanotechnology).
The temperature program was 30−900 °C at the heating rate of
10 °C min−1 under flowing N2 (200 mL min−1), and for the isothermal
TGA, the temperature program was 30−250, 30−300, 30−350, 30−
400, and 30−450 °C at the heating rate of 10 °C min−1 under flowing
N2 (200 mL min−1) and holding the temperature (250, 300, 350, 400,
and 450 °C) for 6 h under flowing N2 (200 mL min−1).
2.7. Py-GC/MS Analysis. A multifunctional double-shot pyro-
lyzer PY-2020iD (Frontier Lab.) mounted on a GC-MS-QP-2010Plus
(Shimadzu) was used under flowing He as the carrier gas. A coated
stainless steel tube (HP-5MS, length = 30 m, inner diameter =
0.25 mm, film thickness = 0.25 μm) (Agilent Tech.) was used as the
GC/MS column. The GC oven was programmed at 40 °C at the
heating rate of 10 °C min−1 to 300 °C. The injection port was in the
constant linear velocity mode at 80 kPa with the split ratio = 1/30.
Scheme 1. Synthesis of Dialkoxydiaminobenzenes
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dx.doi.org/10.1021/ma4024526 | Macromolecules 2014, 47, 2088−2095