M. Hasegawa, et al.
Reactive and Functional Polymers 139 (2019) 181–188
without molecular fluidity. For example, PIs derived from benzophe-
nonetetracarboxylic dianhydride (BTDA) and alkyl-substituted aro-
matic diamines produce crosslinks upon ultraviolet-visible irradiation
via interchain hydrogen abstraction of the excited triplet-state BTDA-
based diimide unit from the adjacent alkyl groups [29]. However, the
hydrogen donors and acceptors must be very close (in other words,
there must be a very high content of these reactive units) for effective
hydrogen abstraction.
the solution was vigorously stirred at room temperature for 1 h. A white
precipitate formed, and this was collected by filtration, repeatedly
washed with water, and dried at 100 °C for 12 h under vacuum (yield:
−1
77%). FT-IR (KBr plate method, cm ): 3350/3168 (amide, N-H
stretching), 1676 (amide-I, C=O stretching), 1631 (NH ,
2
deformation), 1550 (amide-II, C=O + NO
2
asymmetric stretching),
1
1345 (NO
2
symmetric stretching). H NMR [400 MHz, DMSO-d
6
, δ,
ppm]: 9.07 (sd, 2H, J = 2.0 Hz, 2,6-protons of 3,5-dinitorobenzamide
Takeichi et al. [30] synthesized ethynyl (acetylene)-containing
diamines and obtained high-molecular weight crosslinkable PIs using
these diamines, where the crosslinkable ethynyl groups were in-
troduced into the main chains. An ethynyl-containing tetracarboxylic
dianhydride [4,4′-(ethyne-1,2-diyl)diphthalic anhydride], which has
only recently become commercially available, can also be used to ob-
tain analogous crosslinkable PIs [31]. These crosslinkable PIs contain a
rather high content of ethynyl groups in the main chains to ensure ef-
fective intermolecular encountering/collision between the reactive
groups during the curing reactions. Thus, when the content of ethynyl
groups is low, the crosslinking reaction does not occur easily in the non-
molten states. If a new type of monomer that can exert crosslinking
functionality even when present in low concentrations were to become
available, a focused PI system could be modified significantly by co-
polymerization with a minor content of such a functional monomer
while maintaining the chain structures and the inherent properties of
the original PI.
(3,5-DNBA)], 8.96 (st, 1H, J = 2.0 Hz, 4-proton of 3,5-DNBA), 8.67 (s,
1H, CONH
a
H
b
), 8.03 (s, 1H, CONH
a
H ). The melting point was
b
determined from the endothermic peak by differential scanning
calorimetry (DSC) and found to be 183 °C. These data confirm that
the product is the desired dinitro compound (3,5-DNBA).
The nitro groups of 3,5-DNBA were reduced as follows. In a three-
necked flask, 3,5-DNBA (3.05 g, 14.43 mmol) was dissolved in ethanol
(60 mL), and Pd/C (0.33 g) was added as a catalyst. The reaction mix-
ture was refluxed at 80 °C for 8 h in a hydrogen atmosphere, and the
reaction progress was monitored by thin layer chromatography. After
the reaction, the catalyst residue was filtered out, and the filtrate was
concentrated with an evaporator. The precipitate was collected by fil-
tration, and dried at 100 °C for 12 h under vacuum. A reddish-brown
product was obtained with a yield of 60%. FT-IR (KBr plate method,
−
1
1
cm ): 3409/3213 (NH
DMSO‑d
2
), 1648 (amide, C=O). H NMR [400 MHz,
6
, δ, ppm]: 7.44 (s, 1H, CONH
a
H
b
), 6.90 (s, 1H, CONH
a
H ),
b
6.23 (sd, 2H, J = 2.0 Hz, 2,6-protons of 3,5-DABA], 5.93 (st, 1H,
J = 2.0 Hz, 4-proton of 3,5-DABA), 4.81 (s, 4H, 3,5-NH ). Elemental
PI systems with linear/rigid backbone structures have been ac-
cepted to provide PI films with low CTE values in the X–Y direction (in-
plane CTE) [32,33]. This is attributed to the high degree of PI chain
alignment in the X–Y direction (in-plane orientation), which is induced
during the thermal imidization of the PI precursor films formed on
substrates [12,13] or the simple casting process (coating and drying) of
PI solutions [34,35]. If these PIs could be chemically crosslinked while
maintaining a low in-plane CTE property, the thermal expansion be-
havior in the thickness (Z) direction or three-dimensions could also be
controlled. However, crosslinking in the molten states, which is usually
indispensable for ensuring sufficient diffusional motions of the reactive
groups, causes orientational relaxation of the main chains; conse-
quently, the original low in-plane CTE property disappears. Therefore, a
key strategy to solve this issue is to create novel PI systems where
crosslinking reactions can occur effectively even in non-molten states
with low content of crosslinkable groups. Thus, the potential applica-
tions of PIs with three-dimensionally controlled CTEs can be con-
sidered; e.g., their use as interlayer dielectrics in multi-layered circuit
boards having a dramatically increased number of layers.
2
−
1
analysis: Calcd. (%) for C
7
H
9
O
1
N (151.17 g mol ): C, 55.62; H, 6.00;
3
N, 27.80, Found: C, 55.67; H, 5.97; N, 27.73. Melting point (DSC):
151 °C. These data confirm that the product is the desired diamine (3,5-
DABA).
2.1.1.2. Resorcinol bis(trimellitate). An ester-linked tetracarboxylic
dianhydride (TA-RC) was synthesized from trimellitic anhydride
chloride and resorcinol (RC) in anhydrous tetrahydrofuran in the
presence of pyridine in a similar manner to the procedures described
in our previous paper [34]. The product was purified by
recrystallization from a mixed solvent (acetic anhydride/toluene, 11/
−1
50, v/v). FT-IR (KBr plate method, cm ): 3110/3064 (CAr–H), 1855/
1772 (dicarboxylic anhydride, C=O), 1740 (ester, C=O), 1479
1
(phenyl), 1224 (Ar-O).
H
NMR [400 MHz, DMSO‑d , δ, ppm]:
6
8.67–8.63 [m, 4H (3.98H), 3,3′- + 5,5′-protons of the phthalic
anhydride (PAn) unit], 8.29 [d, 2H (2.00H), J = 7.8 Hz, 6,6′-protons
of PAn], 7.65 [t, 1H (1.00H), J = 8.3 Hz, 5-proton of the RC unit], 7.56
(st, 1H (0.97H), J = 2.2 Hz, 2-proton of RC], 7.42 [dd, 2H (2.00H),
J = 8.2, 2.2 Hz, 4,6-protons of RC]. Elemental analysis: Calcd. (%) for
In this study, we present a new diamine and report the effects of the
crosslinking reaction on the thermal properties of the resultant cross-
linkable PIs.
−1
C
24
H O10 (458.34 g mol ): C, 62.89; H, 2.20, Found: C, 62.51; H,
10
2.47. Melting point (DSC): 210 °C. These data confirm that the product
is the desired compound (TA-RC).
2
. Experimental
2
.1.2. Other monomers and raw materials
The abbreviations, commercial sources, pre-treatment conditions,
2.1. Materials
and melting points for the other monomers and raw materials used in
this study are listed in Supporting Data 1. The structures of the
monomers are shown in Fig. 2.
2
.1.1. Monomer synthesis
2
.1.1.1. 3,5-Diaminobenzamide
(3,5-DABA). This
amide-pendant
diamine was synthesized according to the reaction scheme shown in
Fig. 1. In detail, in a 500 mL-flask, 3,5-dinitrobenzoylchloride (3,5-
DNBC, 13.86 g, 60 mmol) was dissolved in toluene (144 mL). To this
solution, a 28% ammonia aqueous solution (100 mL) was added, and
2
.1.3. Polymerization and thermal imidization for PI film preparation
The PI precursors [poly(amic acid)s (PAAs)] were prepared by the
equimolar polyaddition of tetracarboxylic dianhydrides and diamines
according to the scheme shown in Fig. 2. A typical polymerization
procedure was carried out as follows. The diamine (2 mmol) was dis-
solved in anhydrous N-methyl-2-pyrrolidone (NMP). Then, tetra-
carboxylic dianhydride powder (2 mmol) was added to the diamine
solution at room temperature with continuous stirring. The initial total
solid content was 30 wt%. The reaction mixture was stirred at room
temperature in a sealed bottle until it became homogeneous and had
the maximum solution viscosity (typically after 72 h). If necessary, the
Fig. 1. Reaction scheme for the synthesis of 3,5-DABA.
182