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Can. J. Chem. Vol. 86, 2008
cipitation in methanol (2–4), and form II, obtained by
precipitation in water (5–7). The crystal structure of both
forms is the object of considerable debate (8–10). Various
studies indicate that additional PPTA crystalline forms may
exist, although experimental data are even further limited
(11–12). New data is clearly needed to gain a better insight
on the polymorphism of this polymer.
Inclusion of spacers in regular polymers has previously
been shown to allow retention of crystallinity in polyethyl-
ene-based copolymers, although in the case of flexible–flexi-
ble copolymers, length and regularity in the size of the
crystallizable block is critical (13). For PPTA-based hinged
copolymers, length of the crystallizable block seems less of
an issue, i.e., a block as small as one and a half repeat units
(therefore containing three aromatic groups) crystallized in
the same crystal structure as the parent PPTA polymer (1),
whereas for polyethylene, 22 repeat unit blocks were neces-
sary for crystallization to occur (13).
from dry hexane. Tetrahydrofuran was distilled over sodium
metal before use. para-Phenylenediamine was sublimed
under vacuum to remove degradation products. Poly(metha-
phenylene isophtalamide) (mpI) was synthesized as described
by Morgan (18).
Synthesis of para-diphenylene terephthalamide model
compounds
A series of model compounds in which the central aro-
matic ring is C=O substituted have been synthesized accord-
ing to the reaction scheme in Fig. 1. These are abbreviated
3ARA-X, where 3 stands for the number of aromatic rings,
A corresponds to the CO substitution of the central aromatic
ring, and X is the terminal substitution (or salient group
thereof). In all cases, abbreviations are given in the figure.
Synthesis of undecanoyl chloride
Undecanoic acid (1 g) was inserted in a flask containing
10 mL oxalyl chloride under nitrogen atmosphere. The reac-
tive medium was brought to reflux under nitrogen, and the
reaction proceeded for 45 min. Residual oxalyl chloride was
removed by evaporation under vacuum. Final product was
liquid at room temperature. Yield: 100%. FTIR (cm–1): 2922
(νa aliphatic (C–H)), 2851 (νs aliphatic (C–H)), 1775 (acid
These observations prompted many questions. In particu-
lar, would such three aromatic-ring molecules crystallize alone
in the same crystal structure when no flexible spacer was
placed between them, or could flexible end-groups favor a
different crystal phase? What is the influence of the substitu-
tion of the middle para-aromatic ring (NH-linked vs. C=O
linked)?
1
chloride ν(C=O)). H NMR (δ, from TMS in DMSO-d6):
Therefore, a series of aramids, comprising three aromatic
rings, were synthesized in the aim of providing model com-
pounds for PPTA polymporphism. These model compounds
are also possible candidates for organic gelators (14), as
their aspect ratio should favor the existence of fibrils, lead-
ing to sol–gel formation. Only one model structure of this
type has been reported (15–16), the N,N′-p-phenylene
dibenzamide, in which no substitution is present on the ter-
minal aromatic rings. This compound has been shown to ex-
ist in two main forms, one crystal structure exhibiting
hydrogen-bonded sheets of molecules placed in a parallel
fashion, as in PPTA forms I and II, whereas the second ex-
hibited a packing in which molecules overlap in a staggered
arrangement. An infrared and Raman spectroscopy study has
also been reported for this model compound, but in this
study, the focus was put on normal vibrational analysis and
estimation of tensile modulus using diagonal force constants
derived from the spectroscopic data (17). No study has been
reported on three-ring PPTA model compounds bearing para
end-groups at each molecule extremity, and therefore, the ef-
fect of substituents of the crystal structure and chain packing
of such rigid molecules remains to be investigated.
2.23 (t, 2He), 1.22 (q, 2Hf), 1.15–1.25 (m, 18Hg), 0.80 (t, 3Hh).
Synthesis of 3ARA-NO2 and 3ARA-NH2
Synthesis of 3ARA-NO2 was performed as described by
Nakata and Brisson (19).
Synthesis of 3ARA-C10
To a solution of 0.42 (1.22 mmoles) 3ARA-NH2 in 70 mL
NMP were added 500 mg (2.44 mmol) undecanoyl chloride
under nitrogen atmosphere. Reaction medium was mechani-
cally stirred 3 h, during which time the desired product
reprecipitated out. The resulting suspension was poured in
water, the yellow precipitate filtered, washed with water, and
dried under vacuum at 120 °C. Yield: 96%; decomposes
above 350 °C before melting. FTIR (cm–1): 2922 (δa
aliphatic (C–H)), 2851 (δs aliphatic (C–H)), 1656 + 1640
(amide I), 1553 (amide II). 1H NMR (δ, from TMS in
DMSO-d6): 10.10 (s, 2Hb), 9.6 (s, 4He), 8.03 (t, 4Ha), 7.65
(d, 2Hc), 7.51 (d, 2Hd), 2.52 (t, 4Hf), 1.57 (t, 4Hg), 1.15–
1.25 (m, 24Hh), 0.80 (t, 6Hi)
Synthesis of 3ARA-C12
The present article will therefore report the synthesis of
various three-ring aramid PPTA model compounds bearing
para-substituents, and a preliminary investigation to deter-
mine to what extent these substituents and the internal ar-
rangement of the amide–aromatic ring linkages affect crystal
structures.
Synthesis was performed as described for 3ARA-C10, us-
ing 1.8 mg (2.44 mmol) dodecanoyl chloride. Reaction me-
dium was diluted by a factor of two (140 mL NMP instead
of 70). Yield: 84%; decomposes above 350 °C before melt-
ing. FTIR (cm–1): 2922 (νa aliphatic (C–H)), 2849 (νs
1
aliphatic (C–H)), 1657 + 1641 (amide I), 1553 (amide I). H
NMR (δ, from TMS in DMSO-d6): 10.30 (s, 2Hb), 9.80 (s,
4He), 8.05 (t, 4Ha), 7.65 (d, 2Hc), 7.52 (d, 2Hd), 2.25 (t,
4Hf), 1.50 (t, 4Hg), 1.20–1.25 (m, 32Hh), 0.81 (t, 6Hi).
Experimental
All chemicals used were ACS reaction-grade and were
purchased from Aldrich, unless otherwise specified. 1-
Methyl-2-pyrrolidinone was purified by fractional distilla-
tion at 10 Torr (1 Torr = 101 325/760 Pa) to remove water as
benzene azeotrope. Terephthaloyl chloride was crystallized
Synthesis of the N,N′-para-phenylene dibenzamide
model compounds
A second series of model compounds has been synthe-
sized, as depicted in Fig. 2. In this case, the central aromatic
© 2007 NRC Canada