Benhalima et al.
881
rephtalate is usually obtained in an amorphous form, which
does not crystallize easily unless submitted to annealing,
stretching, or solvent-induced crystallization. It has been pro-
posed that the key in its amorphous phase stability lies in the
fact that the lowest energy conformation is not the all-trans
conformation found in the crystal form and favoured by pack-
ing interactions, but a gauche conformation.6–8 A similar ori-
gin for the limited crystallization may exist for PES.
With limited data on the PES crystalline form itself, crystal
structures of model compounds become of utmost interest.
Few have been studied in the literature, and the present work
testifies to the difficulty in crystallizing model compounds
composed of a little as two repeat units. A four-ring oligomer
bearing amine terminal groups was studied by Bocelli and
Rizzoli.9 Cyclic ethersulfone oligomer crystal structures were
reported by the group of Colquhoum and Williams,3 as these
yield high-quality single crystals. However, packing of these
cyclic analogues is incompatible with that of polymer chains,
and ring constraints may favour conformations different from
those adopted in long polymeric chains. From the study of
these cyclic oligomers, it was observed that face-to-face
-stacking and C–H··· interactions provided the main stabi-
lization mechanism for crystallization.3 Whether such interac-
tions are at play in PES is unknown.
end groups were deprotected by the addition of BBr3 in
CH2Cl2. Molecular weights of ethersulfones oligomers were
determined by MALDI-TOF mass spectroscopy in positive
linear mode, using a Bruker Autoflex mass spectrometer
equipped with an UV laser ( ϭ 337 nm, 3 ns second pulse).
NaCl was used as a cationizating agent in a dithranol matrix.
Mn was found to be 1600 g mol–1 and the polydispersity index
was 1.15. Oligomer crystallization was performed following
the work of Blackadder et al.1 by first dissolving the PES
copolymer in dichloromethane and then lowering the temper-
ature of the 20 wt % solutions to 5 °C for 24 h. The resulting
powder was dried in air until the powder did not feel sticky
any more, then introduced into a 1 mm diameter glass capillary
(Charles Supper Company), and sealed using a match. X-ray
diffraction diagrams were registered right away on a Bruker
diffractometer equipped with a Kristalloflex 760 generator, a
three-circle goniometer, and a Hi-Star area detector. The gen-
erator produced a graphite-monochromatized copper radiation
(Cu K␣ ϭ 1.5418 Å) at 40 kV and 40 mA. For the MPDS
model compound, the X-ray powder pattern was also calcu-
lated from the x, y, and z coordinates fo the resolved crystal
structure using the Mercury 2.312 program (Cambridge Crys-
tallographic Data Centre) and imposing a constant full-width
at a half-height of 0.4°.
In recent years, our group has focussed on the design of
rigid–flexible copolymers with rigid block lengths close to the
thickness of a single crystalline lamellae.10 These rigid blocks
adopt the same crystal structure as the homopolymers and,
because of their lower molecular weights, crystallize more
readily. Furthermore, during the course of the synthesis, var-
ious low molecular weight model compounds were synthe-
sized and attempts to crystallize them were made. The present
article therefore reports the crystal structure of a new model
compound, X-ray diffraction of an oligomer as well as con-
formational analysis, in the aim of shedding light on the
crystallization of polyethersulfones for future applications.
Single crystal determination of MPDS
A yellow crystal of MDPS having approximate dimensions
of 0.16 mm ϫ 0.15 mm ϫ 0.07 mm was mounted on a glass
fiber using Paratone N hydrocarbon oil. Measurements were
made at 200(2) K on a Bruker APEX II area detector diffrac-
tometer equipped with graphite-monochromated Mo K␣ radi-
ation. Frames corresponding to an arbitrary hemisphere of data
were collected using scans of 0.5° counted for a total of
30 seconds per frame. An orientation matrix corresponding to the
data acquisition, was obtained from a least-squares refinement
using the measured positions of 3181 centered reflections in the
range 2.23° Ͻ Ͻ 18.44°. The APEX 2 program was used for
cell parameter retrieval and data collection.13
Experimental
Data were integrated using the SAINT program.14 The data
were corrected for Lorentz and polarization effects. A multi-
scan absorption correction was performed using the SADABS
program.15 The structure was solved and refined using
SHELXS-97 and SHELXL-97.16 All non-H atoms were re-
fined anisotropically. Hydrogen atoms were placed at ideal-
ized positions. Neutral atom scattering factors were taken from
the International Tables for X-ray Crystallography.17 All cal-
culations and drawings for this crystal structure were per-
formed using the SHELXTL package.18 The final model was
checked either for missed symmetry or voids in the crystal
structure using the PLATON software.19,20 None was found.
The crystal structure gives a satisfactory chekcif report (avail-
able in the Supplementary data).
Synthesis and crystallization of
4,4=-bis(p-methoxyphenoxy)diphenyl sulfone (MPDS)
4,4=-Bis(p-methoxyphenoxy)diphenyl sulfone, abbreviated
MDPS, was synthesized by reacting 5.085 g (20.0 mmol) of
4,4=-difluorodiphenyl sulfone, 4.967 g (40.0 mmol) of
4-methoxyphenol, and 6.08 g (44.0 mmol) of potassium car-
bonate in a round-bottomed flask. Thirty millilitres of dimeth-
ylacetamide (DMAc) was added to this mixture and the
resulting solution was heated to 160 °C overnight under argon
atmosphere. The reaction mixture was diluted with dichlo-
romethane and washed with saturated NaCl water three times
to remove DMAc and potassium carbonate. The organic layer
was washed three times with distilled water and dried over
MgSO4, filtered, evaporated, and dried in vacuo at 60 °C to
give a white solid (MDPS). The MDPS was crystallized by
solvent evaporation of the toluene/methanol solutions (3:1).
Results and discussion
Synthesis and characterization of polyethersulfone
oligomers
PPS oligomers were synthesized by chain-growth polymer-
ization of 4-fluoro-4=-hydroxydiphenylsulfone, following the
work of Yokozawa et al.,11 which was end-capped by the
addition of 4-methoxy phenol in dimethylacetamide. Methoxy
Polyethersulfone crystallinity
Experimental data on PES crystallization present in the
literature is limited, to the best of our knowledge, to mainly
two publications, which report X-ray diffraction of a sample
crystallized at low temperature in the presence of a solvent1
and spherulite-like structures.21 The same crystallization
Published by NRC Research Press
Benhalima, Abdelkader
