B. Díez et al.
Reactive and Functional Polymers 127 (2018) 38–47
o-substituted imide moiety to benzoxazole happens by losing CO
2
or
from toluene. Terephthaloyl chloride (TPC) and isophthaloyl chloride
(IPC) were purchased from Sigma-Aldrich and purified by re-
crystallization from hexane and sublimated at 70 °C and 60 °C, respec-
tively, prior to use. 2,2-Bis[4-carboxyphenyl)hexafluoropropane and
4,4′-sulfonyldibenzoic acid were purchased from TCI and Sigma
Aldrich, respectively. Thionyl chloride was purchased from Scharlau
and distilled under reduced pressure before use. All solvents were ob-
tained from Sigma-Aldrich.
other entities, depending of the employed precursor. This thermal re-
arrangement is additionally complemented by a process of crosslinking
that produces a material with superb resistance to plasticization and
reduced physical aging [23]. Moreover, a huge excess of controlled
fractional free volume (FFV) is attained. Thus, outstanding gas se-
paration properties with permeability values above 1000 barrer are
observed along with a satisfactory permeselectivity [17]. In addition,
the final structure, polybenzoxazole (TR-PBO), is a chemically stable
structure, what permits its use in harsher conditions (high temperature,
presence of water or acidic molecules) than most of classical polymeric
structures.
Aromatic polybenzoxazoles are directly made from aromatic o-di-
hydroxydiamines and aromatic diacids (or their corresponding diacid
chlorides) by reaction in polyphosphoric acid or Eaton's reagent (a
mixture of phosphorous pentoxide and methanesulfonic acid) at tem-
peratures around 200 °C [24]. Another convenient and neat way of
making PBOs consists of the reaction of aromatic o-dihydroxydiamines
and aromatic diacid chlorides in a polar aprotic solvent at room tem-
perature to form poly(o-hydroxyamide)s, HPAs [25]. Afterwards, the
purified HPA is cast as a film and thermally treated at temperatures
around 300–375 °C [26,27]. This process produces materials with PBO
conversion above 95–98%.
2.1.1. Synthesis of acid dichlorides
2 3 3
The two acid dichlorides with SO and C(CF ) as hinge groups were
synthesized by reaction of their corresponding diacids, 4,4′-sulfonyldi-
benzoic acid and 2,2-bis(4-carboxyphenyl)hexafluoropropane with
2
thionyl chloride (3-fold SOCl mol/acid mol) in the presence of some
drops of N,N-dimethylformamide (DMF), which was used as catalyst, at
60 °C for 6 h. The excess of thionyl chloride was distilled off at reduced
pressure and the remaining solid was then purified. Thus, 4,4′-sulfo-
nyldibenzoyl dichloride (DBSC) was recrystallized from toluene and
sublimated at 160–170 °C [32,33], whereas 2,2-bis(4-chlor-
ocarbonylphenyl)hexafluoropropane (6FC) was recrystallized from
pentane and sublimated at 80–90 °C [34,35].
2.2. Synthesis of 2,2-bis(3-amino-4-hydroxyphenyl) propane (APA)
Gas separation properties of membranes formed by casting PBO
solutions are not good, because the formation of charge transfer com-
plexes between the aromatic rings produces materials with high cohe-
sive energy and consequently with low fractional free volume, and in
addition they are not easy to process [28]. However, when the mem-
brane is formed by thermal treatment of as-cast poly(o-hydroxyamide),
better properties are observed whether the thermal procedure is well
optimized [29]. These thermally obtained PBOs from HPAs, are named
as β-TR-PBOs in the field of gas separation. These materials have lower
FFV than common α-TR-PBO materials but they have excellent me-
chanical properties, superlative thermal stability, very high chemical
stability, and they can be processed from soluble HPAs. Several re-
searchers demonstrated that β-TR-PBOs are valid materials for pre-
combustion applications because it is possible to modulate the size
cavity of the FFV units to tailor-made a membrane able to separate
hydrogen from other gases at temperatures around 200 °C without
suffering any degradation of the macromolecular structure at these
operation conditions [29–31].
In this study, two nucleophilic monomers (o-hydroxyamines) were
reacted with four acid chlorides in order to get insight on the TR process
of poly(o-hydroxyamides). Both monomers are quite similar but the
hinge of the two aromatic rings is, for one of them, an electron-with-
drawing hexafluoroisopropylidene group (APAF) whilst for the other
monomer it is an electron-donating isopropylidene moiety (APA). In
order to find new materials with tailor-made properties, some proper-
ties have been related with the electronic properties of monomers.
Throughout the paper, for clarity sake, the acronym TR-PBO will be
used instead of β-TR-PBO.
The monomer was synthesized from 2,2-bis(4-hydroxyphenyl) pro-
pane (1) following the two step methodology depicted in Fig. 1.
Synthesis of 2,2-bis(3-nitro-4-hydroxyphenyl) propane (2):
Concentrated nitric acid (15 mL) was added dropwise over 1 h to a
stirred solution of toluene (75 mL), 2,2-bis(4-hydroxyphenyl) propane
(1) (15.0 g, 0.066 mol) and glacial acetic acid (50 mL) at 0–5 °C. After
stirring for 1 h more at that temperature, the mixture was allowed to
warm up to room temperature and then stirred for 2 h. The solid was
then filtered, washed with cold water and dried at 90 °C for 12 h under
vacuum. The dinitro compound was obtained as a yellow powder: yield
94%; mp. 284 °C (DSC); 1H NMR (400 MHz, DMSO‑d
): δ (ppm) 10.80
6
(s, 1H, OH), 7.72 (d, J = 8.8 Hz, 1H, Har), 7.33 (dd, J = 8.8 Hz, 2.5 Hz,
1H, Har), 7.04 (d, J = 2.5 Hz, 1H, Har), 4.3 (s, 2H, NH), 1.63 (s, 3H,
CH
3
).
Synthesis of 2,2-bis(3-amino-4-hydroxyphenyl) propane (3): The
dinitro compound (2) (10,000 g, 0.0314 mol) was heated to reflux in
absolute ethanol (100 mL) in the presence of wet 10% Pd/C catalyst
(0.650 g). Hydrazine monohydrate (20 mL) was then added dropwise
over a period of 30 min, and the reaction was maintained at reflux for
®
24 h. The hot solution was filtered through Celite to eliminate the
catalyst and partially concentrated under reduced pressure. Afterwards,
the concentrate was poured into cold distilled water. The crude product
was filtered, washed with water, dried under vacuum and recrystallized
twice from methanol to give (3). The diamino compound was obtained
as a white powder: yield: 88%; mp. 263 °C (DSC); Analysis: Calculated
for C15
H, 7.15%; N, 11.02%. H NMR (400 MHz, DMSO‑d
1H, OH), 6.43 (d, J = 8.1 Hz, 1H, Har), 6.34 (d, J = 1.8 Hz, 1H, Har),
18 2 2
H N O : C, 69.74%; H, 7.02%; N, 10.84%. Found: C, 69.35%;
1
6
): δ (ppm); 8.57 (s,
In addition, the combination of techniques, such as Fourier trans-
form infrared, thermogravimetric analysis and differential calorimetry
scan, has allowed establishing the temperature range where the TR-
process occurs. Finally, the performances of these materials as gas se-
6.21 (dd, J = 8.1 Hz, 1.8 Hz, 1H, Har), 4.23 (s, 2H, NH) 1.37 (s, 3H, CH
3
13
f
). C NMR (100 MHz, DMSO‑d
113.5, 113.4, 40.8, 30.9.
6
): δ (ppm) 142.4, 141.4, 135.4, 114.4,
paration membranes for several gas pairs (O
and He/CO ) have been tested.
2 2 2 4 4
/N , CO /CH , He/CH
2
2.3. Synthesis of polymers (HPAs)
2
. Experimental
A general procedure was carried out as follows: 5.0 mmol of dia-
mine were dissolved in 5 mL of 1-methyl-2-pyrrolidinone (NMP) at
room temperature in a 50 mL three-necked flask, equipped with me-
chanical stirrer and nitrogen inlet and outlet. The solution was cooled
down to 0 °C, and then 11 mmol of trimethylsylil chloride and 11 mmol
of pyridine (1.1 mol/mol of amine group) were slowly added.
Afterwards, the reaction temperature was allowed to rise to room
temperature and 5.0 mmol of the acid dichloride and 5 mL of NMP were
2.1. Materials
2
,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) was
purchased from Apollo Scientific and purified by sublimation at
20–225 °C before use. 2,2-Bis-(4-hydroxyphenyl)propane (bisphenol
A) was purchased from Sigma-Aldrich and purified by recrystallization
2
39