Microporous thermosetting film constructed from hyperbranched polyarylate precursors containing rigid tetrahedral core: Synthesis, characterization, and properties

Article abstract of DOI:10.1021/cm9036617

Porous organic thermosetting film with pore size smaller than 20 A was constructed through the self-cross-linking reaction from hyperbranched polyarylate precursors with rigid tetrahedral skeleton. Using hydroquinone diacetate as the A₂ monomer

Full text of DOI:10.1021/cm9036617

2780 Chem. Mater. 2010, 22, 2780–2789
DOI:10.1021/cm9036617
Microporous Thermosetting Film Constructed from Hyperbranched
Polyarylate Precursors Containing Rigid Tetrahedral Core: Synthesis,
Characterization, and Properties
Bufeng Zhang and Zhonggang Wang*
Department of Polymer Science and Materials, School of Chemical Engineering. Dalian University of
Technology, Dalian 116012, People’s Republic of China
Received December 4, 2009. Revised Manuscript Received March 12, 2010
˚
Porous organic thermosetting film with pore size smaller than 20 A was constructed through the
self-cross-linking reaction from hyperbranched polyarylate precursors with rigid tetrahedral skele-
ton. Using hydroquinone diacetate as the A2 monomer and tetrakis(4-carboxyphenyl)silane as the B4
monomer, the hyperbranched polyarylate precursors were successfully synthesized with high yields,
during which the gelation process was efficiently suppressed by heterogeneous polycondensation
method by means of the elaborate selection of reaction medium. The control of monomer ratio in the
polymerization system led to two types of precursors, i.e., mainly carboxyl-terminated hyper-
branched polyarylate (CTHP) and mainly acetoxy-terminated hyperbranched polyarylate
(ATHP). They had degree of branching values of 0.53-0.69, molecular weights of 10100-34000
g/mol, inherent viscosities of 0.12-0.18 dL/g, and good solubility in the common solvents. The
precursor film, obtained by spin coating from the mixture solution of CTHP and ATHP with
equivalent amount of reactive carboxylic group and acetoxy groups, was readily thermally cured via a
transesterification reaction to form a cross-linked network, which exhibited excellent thermal
stability, good chemical resistance, low birefringence, and low dielectric constant, as well as
2
˚
microporosity, with an average pore size of 11.2 A and surface area of 158 m /g.
Introduction
transformation and chromatographic purification at
every step. In contrast, hyperbranched polymers could
be easily prepared via a one-pot synthesis. The most
common strategy to hyperbranched polymers is the self-
condensation of ABn (n g 2) monomers.8-14 Neverthe-
less, the use of ABn monomers is limited, because they are
usually obtained through tedious synthetic procedures
necessary for asymmetric functionality. To further allow
for convenient and cost-saving scaleup, researchers have
begun to focus toward the polymerization of A2 with
Bn (n>2) monomers, which are all readily available.
The development of polymers with specific topologies
and architectures has continuously been one of the most
interesting subjects in the field of macromolecular engi-
neering, because it provides an original approach to
nanoscience and nanochemistry to achieve a range of
new and special properties, compared to the conventional
polymers.
Dendritic topologies, distinct from the linear type, have
been widely explored in academic and industrial fields
over the past two decades. Dendritic macromolecules
include both dendrimers and hyperbranched polymers,
which are composed of successive branch units.1-7 In
comparison with hyperbranched polymers, dendrimers
have more well-controlled and regularly branched
structures. However, the large-scale production of den-
drimer is difficult, because of the involved multistep
Among them, the notable examples were reported by
15-19
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Long, Voit, Frechet, Jikei, and Fossum.
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ꢀ
*Author to whom all correspondence should be addressed. E-mail:
zgwang@dlut.edu.cn.
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1998, 23, 1.
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r
pubs.acs.org/cm
Published on Web 03/26/2010
2010 American Chemical Society

Article
During the past few years, a new class of macromole-
cular architecture;termed as polymers of intrinsic mi-
Chem. Mater., Vol. 22, No. 9, 2010 2781
PIMs reported in the literature are precipitated powders,
whereas thermosetting PIM films are almost entirely un-
explored with the exception of Economy’s thermosetting
poly(imide-ester) (BET surface area of 83 m2/g).41
In the previous work, we have successfully extended the
concept of microporosity into general thermosetting resin
via polycyanurate chemistry.42 Herein, we make a great
effort to prepare microporous thermosetting films based
on common hyperbranched polymers of commercial
interest, such as polyarylates. For the construction of
polyarylate PIM films, the hyperbranched polyarylates
play three important roles: first, they serve as building
blocks that provide open and accessible cavities; second,
upon heating, they act as thermosetting precursors;
third, the semirigid ester linkers between branchpoints,
rather than full stiff linkers usually used in most micro-
porous networks, could favor the release of internal
stresses within the cross-linked network after curing of
the precursors.
To this end, a rigid tetrahedral monomer tetrakis-
(4-carboxyphenyl)silane, which could efficiently avoid
dense chain packing, was first used as B4 monomer for
preparation of fully aromatic hyperbranched polyester.
Two types of precursors, i.e., mainly carboxyl-terminated
polyarylate (CTHP) and mainly acetoxy-terminated
polyarylate (ATHP), were obtained through the poly-
merization of hydroquinone diacetate (A2) and tetrakis-
(4-carboxyphenyl)silane (B4) monomers. To further form
a cross-linked structure via a transesterification reaction,
a self-curable precursor containing equivalent amount of
carboxylic acid and acetoxy end groups was thus pre-
pared by simply mixing CTHP with ATHP. Therefore,
the chemical component of cured film is only constituted
of polyarylate itself, because no other curing agents are
added in the system.
croporosity (PIMs), which have pore sizes smaller than
˚
20 A;have dramatically emerged as a promising ap-
proach to nanomaterials.20-24 Ascribing to the highly
cross-linked network and fully rigid skeleton within
structure, their applications can be in the field analogous
to zeolites (i.e., catalysis, size-selective absorbents, and
gas storage). In addition, for organic polymers, because
of their outstanding advantage in processability over
inorganic materials, microporous polymer films are sig-
nificant to expend the applications in some advanced
technology fields (e.g., optics, electronics, and membrane
separations).25 Recently, PIMs based on soluble poly-
mers have been explored by Budd et al.26-29
Relative to linear PIMs, the development of thermo-
setting microporous thin films will be more attractive,
because of their good resistance to solvents and moisture,
good dimensional stability, and thermo-oxidative stabi-
lity. Until now, a series of well-established synthesis
strategies;including dioxane-forming polymerization,21
Friedel-Crafts reaction,30 reversible chemistry,20,31-33
transition-metal-catalyzed cross-coupling reaction,24,34-37
Schiff base chemistry,38 and N-alkylation39,40;have been
successfully used to prepare microporous organic networks
with high specific surface areas and well-defined pore
properties. Nevertheless, most of the full rigid cross-linked
^ ꢀ
(20) Cote, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger,
A. J.; Yaghi, O. M. Science 2005, 310, 1166.
(21) McKeown, N. B.; Budd, P. M.; Msayib, K. J.; Ghanem, B. S.;
Kingston, H. J.; Tattershall, C. E.; Makhseed, S.; Reynolds, K. J.;
Fritsch, D. Chem.;Eur. J. 2005, 11, 2610.
(22) Budd, P. M. Science 2007, 316, 210.
(23) Weder, C. Angew. Chem., Int. Ed. 2008, 47, 448.
(24) Jiang, J. X.; Su, F.; Trewin, A.; Wood, C. D.; Campbell, N. L.; Niu,
H.; Dickinson, C.; Ganin, A. Y.; Rosseinsky, M. J.; Khimyak,
Y. Z.; Cooper, A. I. Angew. Chem., Int. Ed. 2007, 46, 8574.
(25) Davis, M. E. Nature 2002, 417, 813.
In this paper, we conduct a detailed study on the
synthesis, characterization of the aforementioned novel
hyperbranched precursors, and the preparation of their
cured film with intrinsic microporosity. Some interesting
effects caused by the unique tetrahedral molecular archi-
tecture on optical and dielectric properties of the film are
also investigated.
(26) Schwab, M. G.; Fassbender, B.; Spiese, H, W.; Thomas, A.; Feng,
€
X.; Mullen, K. J. Am. Chem. Soc. 2009, 131, 7216.
(27) Budd, P. M.; Elabas, E. S.; Ghanem, B. S.; Makhseed, S.;
Mckeown, N. B.; Msayib, K. J.; Tattershall, C. E.; Wang, D.
Adv. Mater. 2004, 16, 456.
(28) Park, H. B.; Jung, C. H.; Lee, Y. M.; Hill, A. J.; Pas, S. J.; Mudies,
S. T.; Wagner, E. V.; Freeman, B. D.; Cookson, D. J. Science 2007,
318, 254.
(29) Yuan, S. G.; Kirklin, S.; Dorney, B.; Liu, D. J.; Yu, L. P.
Macromolecules 2009, 42, 1554.
(30) Wood, C. D.; Tan, B.; Trewin, A.; Su, F.; Rosseinsky, M. J.;
Bradshaw, D.; Sun, Y.; Zhou, L.; Cooper, A. I. Adv. Mater. 2008,
20, 1916.
Experimental Section
Materials. Tetrachlorosilane, n-butyllithium, 1,4-dibromo-
benzene were purchased from J&K Chemical Co., Ltd. Hydro-
quinone diacetate and diphenyl sulfone were purchased from
Shanghai Chemical Reagent Co. Diethyl ether and tetrahydro-
furan (THF) were purified by refluxing over sodium with the
indicator benzophenone complex. Cyclohexanone was purified
by distillation. Diphenyl sulfone was purified by recrystalliza-
tion from ethanol. The other solvents were reagent grade and
used as received.
^ ꢀ
(31) El-Kaderi, H. M.; Hunt, J. R.; Mendoza-Cortes, J. L.; Cote, A. P.;
Taylor, R. E.; O’Keeffe, M.; Yaghi, O. M. Science 2007, 316, 268.
(32) Tilford, R. W.; Mugavero, S. J.; Pellechia, P. J.; Lavigne, J. J. Adv.
Mater. 2008, 20, 2741.
(33) Kuhn, P.; Antonietti, M.; Thomas, A. Angew. Chem., Int. Ed. 2008,
47, 3450.
(34) Weber, J.; Thomas, A. J. Am. Chem. Soc. 2008, 130, 6334.
(35) Jiang, J. X.; Su, F.; Trewin, A.; Wood, C. D.; Niu, H.; James, T. A.;
Khimyak, Y. Z.; Cooper, A. I. J. Am. Chem. Soc. 2008, 130, 7710.
(36) Schmidt, J.; Werner, M.; Thomas, A. Macromolecules 2009, 42,
4426.
€
(37) Stockel, E.; Wu, X.; Trewin, A.; Wood, C. D.; Clowes, R.;
Instrumentation. Fourier transform infrared (FTIR) spectra
were recorded on a Nicolet Model 20DXB IR spectro-
photometer. Sixty four (64) scans were signal-averaged, with
a resolution of 2 cm-1 at room temperature. Samples were
Campbell, N. L.; Jones, J T A.; Khimyak, Y. Z.; Adams, D. J.;
Cooper, A. I. Chem. Commun. 2009, 2, 212.
(38) Schwab, M. G.; Fassbender, B.; Spiese, H. W.; Thomas, A.; Feng,
€
X.; Mullen, K. J. Am. Chem. Soc. 2009, 131, 7216.
ꢀ
(39) Germain, J.; Frechet, J. M. J.; Svec, F. J. Mater. Chem. 2007, 17,
4989.
ꢀ
(40) Germain, J.; Frechet, J. M. J.; Svec, F. Chem. Commun. 2009, 12,
(41) Xu, K.; Economy, J. Macromolecules 2004, 37, 4146.
(42) Zhang, B. F.; Wang, Z. G. Chem. Commun. 2009, 33, 5027.
1526.

2782 Chem. Mater., Vol. 22, No. 9, 2010
Zhang and Wang
prepared by dispersing the complexes in KBr and compressing
the mixtures to form disks.
1H NMR and 29Si NMR were recorded on 400-MHz Varian
Model INOVA NMR spectrometer, with the tetramethylsilane
(TMS) as an internal reference.
(13.04 g, 1.94 mol) and anhydrous diethyl ether (250 mL). 4-
Bromotoluene (159.55 g, 0.933 mol) in anhydrous diethyl ether
(200 mL) was added dropwise over a 8-h period under vigorous
stirring under the protection of nitrogen atmosphere. An im-
mediate exothermic reaction caused the ether to start boiling.
The mixture was stirred for 4 h, and then 17.7 mL of tetrachloro-
silane in 50 mL of anhydrous diethyl ether was added dropwise
over a 4-h period. The mixture was stirred for an additional 2 h
and quenched by the slow addition of 200 mL of 1 N aqueous
HCl at 0 °C. Diethyl ether was removed by rotary evaporation,
the solid was filtered off, and then washed with 400 mL of
methanol and 200 mL of 1 N aqueous HCl. The crude product
was dissolved in chloroform, and the insoluble fraction was
filtered off by filtration. Solvent was evaporated to afford a
white solid. The crude product was recrystallized twice in
cyclohexane and gave as colorless crystals (53.4 g, 87.6%);
mp: 236-236.5 °C. 1H NMR (400 MHz, CDCl3,): d (ppm)
2.45 (s, 12H, Ar-CH3), 7.15 (d, 8H, Ar-H), 7.42 (d, 8H,
Ar-H); IR (KBr, cm-1): 3063 and 3010 (arom. C-H), 2972
and 2863 (Ar-CH3), 1597, 1499 (arom. CdC), 1445, 1107, and
760 (Si-Ph), 803 (arom. p-subst.).
Thermogravimetric analyses (TGA) was performed on a
Netzsch Model TG 209 thermal analyzer both in purified
nitrogen and an air atmosphere, and all samples (with masses
of ∼10 mg) were heated from 25 °C to 700 °C at a rate of
10 °C/min, under the gas flow rates of 60 mL/min.
Differential scanning calorimetry (DSC) measurements were
conducted with a Netzsch Model DSC 204 instrument. The
calorimeter was calibrated with indium metal as a standard.
Samples with masses of ∼10-14 mg were used, at a heating rate
of 10 °C/min under a flow of nitrogen (20 mL/min). The
reported glass-transition (Tg) temperatures corresponded to
the midpoint change in heat capacity in the transition region.
Molecular weights and distributions were determined using
gel permeation chromatography (GPC), with a Water Associ-
ates Model PL-GPC-220 apparatus at room temperature with
tetrahydrofuran (THF) as the eluent, at a flow rate of 1 mL/min,
calibrated with polystyrene standards.
Preparation of Tetrakis(4-carboxyphenyl)silane (TCS).
Tetrakis(4-carboxyphenyl)silane was prepared according to a
modified procedure given in the literature.44 A 2-L three-necked
round-bottom flask in an ice bath was equipped with a thermo-
meter, a mechanical stirrer, and a dropper. To this flask was
charged 960 mL of glacial acetic acid and 400 mL of acetic
anhydride. Maintaining the temperature below 10 °C, sulfuric
acid (60 mL) and TTS (20.8 g) was added successively with
stirring. After that, the temperature was increased to 25 °C, and
Cr2O3 (160 g) was charged in four portions over a 2-h period.
The mixture was stirred for additional hour. The product then
was decomposed with ice-cold water (1.6 L) and filtered. The
green solid was dissolved in dilute aqueous NaOH, and the
solution was acidified with 1 N aqueous HCl until the pH
reached 1. The resultant white solid was washed with dilute
aqueous HCl and distilled water several times. The crude
product was recrystallized from acetic acid three times to give
tetraacid TCS (17.44 g, 67%); 1H NMR (400 MHz, acetone-d6):
δ (ppm) 8.12 (d, 8H, Ar-H), 7.75 (d, 8H, Ar-H); 29Si NMR
(acetone-d6 400 MHz, acetone-d6): δ (ppm) -10.4; IR (KBr,
cm-1): 3025 (arom. C-H), 2300-3400, 2666, 2552 (-COOH),
1694 (-CdO, carboxyl), 1600, 1498 (arom. CdC), 1093, 759
(Si-Ph), 708 (arom. p-subst.).
Preparation of CTHP. Tetrakis(4-carboxyyphenyl)silane
(0.8 g, 1.61 mmol), hydroquinone diacetate (0.435 g, 1.61 mmol),
sodium acetate (0.001 g), and diphenyl sulfone (5 g) were added
into a 50-mL three-neck round-bottom flask that was fitted with a
condenser, nitrogen inlet tube, and mechanic stirrer, to make a
solution of ∼20% solids content. A slow stream of nitrogen was
maintained throughout the entire reaction. The temperature was
increased to 200 °C for 1 h and then was increased to 260 °C, at
which point acetic acid began to evolve as the reaction byproduct.
After 12 h, the mixture became transparent. The polymerization
was continued for an additional 30 min until the mixture became
viscous. After cooling, the product was precipitated in 50 mL of
ethanol, and then it was filtered and washed with hot ethanol for
three times to remove diphenyl sulfone and traces of catalyst. The
crude product was extracted with DMF, using a Soxhlet appara-
tus for 24 h, and the insoluble solid (cross-linked polymers) was
discarded. The clear filtrate was poured into an excess of ethanol.
The inherent viscosities of the samples were measured with an
Ubbelohde viscometer at a concentration of 0.5 g/dL in CHCl3
at 25 ( 0.01 °C.
The atomic force microscopy (AFM) images were recorded
with a PicoPlus II microscope (Agilent, USA) at room tempera-
ture. The images were recorded in the tapping mode to avoid the
possible deformation and indentation of the polymer surface by
the tip. The values of root-mean-square (rms) roughness were
calculated over the entire captured film area using the accessory
software of the instrument.
Nitrogen adsorption-desorption experimentation was con-
ducted using an automated micropore gas analyzer (Autosorb-
1-MP, Quantachrome Instruments). Before sorption measure-
ment, the cured film sample was degassed at 150 °C under high
vacuum overnight. Apparent surface area was calculated from
N2 adsorption data by multipoint BET analysis.
The out-of-plane and in-plane refractive indices of the thin
film were measured with a Sairon Model SPA-400 prism coupler
that was equipped with a He-Ne laser light source (wavelength
= 632.8 nm) and controlled by a computer. The measurements
of the refractive indices were performed in both transverse
electric (TE) and transverse magnetic (TM) modes, with the
appropriate polarization of the incident laser beam. The TE
measurement (in which the electric field was in the film plane)
provided the in-plane refractive index, whereas the TM mea-
surement (in which the electric field was out-of-plane) gave the
out-of-plane refractive index.
The dielectric constants of the film was evaluated with an
optical method based on Maxwell’s equation,
2
ε ¼ 1:1nav
where ε is the dielectric constant and nav is the average refractive
index.
Preparation of Tetra(4-tolyl)silane (TTS). Tetra(4-tolyl)silane
was prepared according to the procedure given in the litera-
ture,43 with some modifications. A 1-L four-necked flask
equipped with a mechanical stirrer, nitrogen inlet and outlet, a
condenser, and a dropper was charged with lithium powder
(43) Michael, C.; Stefan, R.; Martin, D. J. Organomet. Chem. 1992, 427,
(44) Hopff, H.; Deuber, J. M.; Gallegra, P.; Said, A. Helv. Chim. Acta
1971, 54, 117.
23.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2783
The polymer obtained was dried at 100 °C under a vacuum to
constant weight. Yield: 70%. 1H NMR (400 MHz, DMSO-d6): δ
(ppm) 13.2 (s, COOH), 8.40-8.20 (m, Ar-H), 8.13-8.00 (m,
Ar-H), 7.90-8.00 (m, Ar-H), 7.50-7.10 (m, Ar-H), 1.39 (s,
CH3); 29Si NMR (100 MHz, DMSO-d6): δ (ppm) -10.05;10.35;
IR (KBr, cm-1): 3400-2300, 2960, 2880, 1763, 1737, 1700.
Preparation of ATHP. Tetrakis(4-carboxyyphenyl)silane
(0.8 g, 1.61 mmol), hydroquinone diacetate (1.739 g, 6.44 mmol),
sodium acetate (0.001 g), and diphenyl sulfone (10 g) were added
into a 50-mL three-neck round-bottom flask that was fitted with
a condenser, nitrogen inlet tube, and mechanic stirrer, to make a
solution of ∼20% solids content. A slow stream of nitrogen was
maintained throughout the entire reaction. The temperature
was increased to 200 °C for 1 h, and then was increased to
260 °C, at which point acetic acid began to evolve as the reaction
byproduct. After 8 h, the mixture became transparent. The
polymerization was continued for an additional hour until the
mixture became viscous. After cooling, the product was pre-
cipitated in 50 mL of ethanol, and then it was filtered and
washed with hot ethanol for three times to remove diphenyl
sulfone and traces of catalyst. The crude product was extracted
with DMF using a Soxhlet apparatus for 24 h, and the insoluble
solid was discarded. The clear filtrate was poured into an excess
of ethanol. The polymer obtained was dried at 100 °C under a
macromolecular system with unusual properties.45,46 In
this work, tetrakis(4-carboxyphenyl)silane (TCS) was
selected as a B4 monomer to prepare hyperbranched
tetrahedral polyarylates. TCS was prepared according to
the literature with some modifications: p-bromotoluene
was converted to p-tolyllithium and then reacted with
tetrachlorosilane to produce tetrakis(4-tolyl)silane,43 and
the oxidation of methyls using chromium trioxide gave
the corresponding product33 with an overall yield of
58.7%. The chemical structures were confirmed by FTIR,
1H NMR, and 29Si NMR spectra.
The A2 þ Bn type of polymerization usually leads to
gelation. However, if the propagation can be terminated
prior to the gel point, hyperbranched polymers with
satisfactory molecular weights can be obtained. Pre-
viously, several strategies, including dropwise addition
of a diluted monomer solution, partial conversion of
functional groups in dilute solution, and the A2 þ BB02
approach have been reported to delay or even avoid the
occurrence of gelation in the A2 þ B3 polymerization;17,47-49
however, similar results are difficult to achieve via the same
methods in the A2 þ B4 system. According to Flory’s
theory, for equifunctional polycondensations, the critical
conversion (pc) above which gel particles are formed for the
B4 monomer ( pc = 0.58) is much lower than that of the B3
monomer ( pc = 0.71).50 Therefore, it is a great challenge to
determine the suitable reaction conditions that would
enable A2 þ B4 polymerization to yield hyperbranched
polymers with high molecular weight. Most recently, Baek
et al. found that the gelation could be efficiently avoided in
preparation of hyperbranched poly(ether ketone) from
A2 þ B3 approach via the solubility difference of monomer
in the reaction medium.51 They ascribed the result to the
following two factors: (1) self-regulated feeding of the aryl
ether monomers into the system driven by their poor
solubility and phase separation from PPA/P2O5 medium;
(2) reaction-medium-induced isolation of growing macro-
moleculespromotedbythehighbulkviscosity.Thereby,we
speculated that other families of hyperbranched polymers
can also be realized via the elaborate selection of solvents
and control of polymerization condition.
1
vacuum to constant weight. Yield: 75%. H NMR (400 MHz,
DMSO-d6): δ (ppm) 13.2 (s, COOH), 8.41-8.19 (m, Ar-H),
8.12-8.01 (m, Ar-H), 7.89-8.01 (m, Ar-H), 7.48-7.09 (m,
Ar-H), 1.40 (s, CH3); 29Si NMR (100 MHz, DMSO-d6): δ
(ppm) -10.06;10.35; IR (KBr, cm-1): 3400-2298, 2961, 2878,
1765, 1739, 1701.
Preparation of Cured Film. The substrates used in the experi-
ments were highly polished silicon wafers. Organic residue on
the surface was removed by successive ultrasonic cleaning with
acetone, alcohol, and ion-free pure water. CTHP (0.703 g) and
ATHP (1.5 g) were dissolved in DMF (22 mL) to obtain a 10
wt % mixture solution containing equivalent amounts of car-
boxyl and acetoxy groups. The solution then was filtered
through a 0.22-μm Teflon filter and spin-coated on a silicon
wafer. The spin-coating speeds were in the range of 1000-2500
rpm. To guarantee the quality of the spin-coated film, the entire
spin-coating procedure was conducted in a 1000-class ultraclean
room. The resulting film was dried at 60 °C (30 min) and 120 °C
(2 h) to remove the residual solvent under vacuum. Thermal
cross-linking was then performed in a vacuum oven by stepwise
heating at 150 °C for 1 h, 180 °C for 2 h, 220 °C for 5 h, 250 °C for
5 h, and 320 °C for 1 h. Finally, the sample was cooled slowly to
room temperature.
Given the previously mentioned considerations, herein,
we tried to synthesis hyperbranched polymers with a rigid
tetrahedral core by step-growth heterogeneous polycon-
densation, based on kinetic control from A2 þ B4 mono-
mers (see Scheme 1). For this purpose, diphenyl sulfone,
which is a poor solvent for TCS, was selected as the
reaction medium for the esterification reaction between
TCS and hydroquinone diacetate. The polymerization
was conducted at 260 °C in a fixed concentration of
20 wt %. With the reaction proceeding, the TCS was
slowly dissolved into the reaction system and engaged in
the polymerization process. It can be clearly observed
Results and Discussion
Synthesis of Monomer and Hyperbranched Polarylates.
Among numerous structural motifs for the construction
of novel functional nanometer-sized materials, tetrahe-
dral core compounds such as tetraphenylmethane and
tetraphenylsilane have intensively aroused considerable
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bilities for construction of diamond-like and other intri-
guing molecular architectures that might be translated to a
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2784 Chem. Mater., Vol. 22, No. 9, 2010
Zhang and Wang
Scheme 1. Synthesis of Hyperbranched Tetrahedral Polyarylate
via Polymerization of A2 and B4 Monomers
Figure 1. FT-IR spectra of the hyperbranched polyarylates ATHP and
CTHP.
that the heterogeneous mixture turned gradually to
homogeneous and transparent solution within 8-12 h.
The polymer was precipitated in ethanol, and the col-
lected product was washed with hot ethanol for several
times to remove any residual catalyst, unreacted mono-
mers, and low-molecular-weight oligomers. Subse-
quently, the product was extracted with DMF using a
Soxhlet apparatus for 24 h to remove the insoluble
fraction. The extract liquid was precipitated in ethanol
and the white product was collected and dried at 100 °C
under a vacuum to give a yield of 70%-80%. The high-
yield soluble products demonstrated that the gelation
could be suppressed indeed by the slow feeding of the
TCS monomer into the polymerization system, because
of its poor solubility. In addition, by adjusting the
monomer ratio with A2/B4 < 2, mainly carboxyl-termi-
nated hyperbranched polyarylate (CTHP) could be ob-
tained. Similarly, a monomer ratio of A2/B4 > 2 led to the
mainly acetoxy-terminated hyperbranched polyarylate
(ATHP). The polymers showed good solubility in various
solvents such as THF, CH2Cl2, CHCl3, acetone, DMF,
NMP, DMSO, and DMAc.
Figure 2. 1H NMR spectra of the hyperbranched polyarylates ATHP
and CTHP.
the carboxylic acid and that at 1763 cm-1 corresponded to
the carbonyl stretching of acetate group. For ATHP, the
peak at 1763 cm-1 was stronger than that at 1700 cm-1
,
indicating that it was mainly acetoxy-terminated. In con-
trast, in the FTIR spectrum of ATHP, the absorption at
1700 cm-1 was stronger than that at 1763 cm-1. Besides,
therewasan apparent absorption band at 2500-3600 cm-1
assigned to the vibration of OH in the carboxylic acid
group, meaning that the end groups of CTHP were mainly
Characterization of Hyperbranched Polyarylates. The
chemical structures of the hyperbranched polyarylates
were analyzed by FT-IR spectra of Figure 1. The presence
of aromatic ester groups at 1737 cm-1 provided evidence
for the formation of polyarylate. The careful observation of
enlarged peak at 1737 cm-1 showed that it consisted of the
other two absorptions: the peak at 1700 cm-1 attributed to

Article
Chem. Mater., Vol. 22, No. 9, 2010 2785
Table 1. Polymerization Results for the Synthesis of the Hyperbranched Polyarylates via the A2 þ B4 Approach
polyarylate
yield (%)
ηinh (dL/g)
Mw (g/mol)
Mn (g/mol)
PDI
n(-COOH) (mmol/g)
n(-OCOCH ) (mmol/g)
DB
Tg (°C)
3
ATHP
CTHP
75
70
0.18
0.12
33290
10134
8963
5827
3.71
1.71
0.582
0.741
0.769
0.343
0.69
0.53
146
155
carboxyls. This conclusion was further demonstrated by 1H
NMRspectra of Figure 2, whereall the peaks were assigned
to the protons labeled in the structure of hyperbranched
polymer (see Scheme 1). The signals of the aromatic pro-
tons in polymers are divided into four groups, i.e., silarylene
protons adjacent to aromatic ester linker (a and b), silaryl-
ene protons adjacent to carboxyl group (c and d), pheny-
lene protons adjacent to acetate group (g and f), and
phenylene protons adjacent to aromatic ester linker (e).
In the spectrum of CTHP, the obvious carboxylic proton
appearedat 13.2ppm, whereasthissignalwasveryweakfor
ATHP. Moreover, using cyclohexane as an internal stan-
dard (at 1.39 ppm), the content of terminal carboxylic acid
group and acetoxy group could be calculated using the
integral areas of peaks at 13.2 ppm, corresponding to the
carboxylic proton, and at 2.3 ppm, corresponding to the
acetate proton, and the results are listed in Table 1, which
showed that there were more acetoxy groups than carboxyl
groups in ATHP, and the result was opposite for CTHP,
agreeing well with the observation of FTIR spectra.
The degree of branching (DB) was calculated according
to eq 1, as described by Frey:52
Figure 3. Illustrations of the sp3-hybridized carbon and silicon cores in
the hyperbranched polymers: (a) pentaerythritol or trimethylol propane
core, (b) tetraphenylsilane core, and (c) σ-π conjugation of arylsilane.
D þ T - N
2D
DB ¼
¼
ð1Þ
D þ T þ L - N
2D þ L
where N refers to the number of molecules, and D, T, and
L refer to the number of dendritic, terminal, and linear
units in the polymer, respectively. Usually, DB is deter-
mined from 1H NMR or 13C NMR spectra by comparing
the integral area of the peak of the respective unit in the
hyperbranched polymer. However, in our study, because
of the tetrahedral symmetric character of the branched
unit in the hyperbranched polyarylates, no branching
Figure 4. 29Si NMR spectra of the hyperbranched polyarylates ATHP
and CTHP.
1
signal could be observed from the H NMR spectra.
Fortunately, the hyperbranched polymers contain a sp3-
silicon core, while the Si atom can give an σ-π conjuga-
tion and support the transport of electrons,53 as illu-
strated in Figure 3. Thus, in the expanded Si signal (at
approximately -10 ppm) of the 29Si NMR spectra
(Figure 4), the resonance could be resolved into four
resonances with slight overlap. The assignments of D1,
D2, L, and T signals of these resonances were made in
reference to the resolution of quaternary carbons of
hyperbranched polymers with sp3-carbon cores, such as
pentaerythritol and trimethylol propane in the 13C NMR
spectra.54 For an accurate integration of each resonance,
the overlapping peaks were separated by Origin software,
using a Gaussian-Lorentzian cross-fitting function. For
mainly acetoxy-terminated hyperbranched polyarylate
(ATHP), the contents of D1, D2, T, and L were 29.01%,
13.7%, 19.09%, and 38.2%, respectively, and the calcu-
lated DB value, according to eq 1, was 69%. For mainly
carboxyl-terminated hyperbranched polyarylate (CTHP),
the percentages of D1, D2, T, and L were 19.91%, 5.43%,
28.92%, and 45.74%, respectively, resulting in its DB value
of 53%. These DB values indicated that both polymers
exhibited a nearly ideal hyperbranched structure (DB close
to 0.5) rather than a linear (DB close to zero) or dendritic
structure (DB close to unity).
The molecular weights and distributions of the hyper-
branched polyarylates were characterized by GPC, using
THF as an eluent. Figure 5 shows the GPC traces, and the
results are given in Table 1. The weight-average molecular
weights of polymers ATHP and CTHP were 33 290 and
10 134 g/mol, respectively, which are comparable to that
of hyperbranched polyarlyates prepared from ABn or
A2 þ B3 monomers.9-12 Notethat, becausehyperbranched
polymers have a smaller radius of chain gyration than the
€
(52) Holter, D.; Burgath, A.; Frey, H. Acta Polym. 1997, 48, 30.
(53) Uhlig, W. Prog. Polym. Sci. 2002, 27, 255.
(54) Jena, K. K.; Raju, K. V. N.; Prathab, B.; Aminabhavi, T. M.
J. Phys. Chem. B 2007, 111, 8801.

2786 Chem. Mater., Vol. 22, No. 9, 2010
Zhang and Wang
ATHP and CTHP the higher free volume and enhanced
segmental motion ability. In addition, it was observed
that the Tg value of CTHP was higher than that of the
mainly acetoxy-terminated hyperbranched polyarylate
(ATHP) because of the much denser intermolecular
hydrogen-bonding caused by the carboxylic acid groups
in CTHP. However, the Tg increase was not so significant,
probably because the CTHP has a lower molecular weight
than ATHP.
Preparation of Precursor Film and Its Curing Reaction.
Hyperbranched polymers generally have poor film-form-
ing ability, because of the lack of chain entanglement.
However, if the large number of terminal functional
groups can be reacted with appropriate difunctional
compounds and connected via chemical bonds, tough
films can be obtained.56 In this study, because of the
presence of two types of reactive groups, i.e., the car-
boxylic acid and acetoxy terminal groups in the hyper-
branched polyarylate, the self-cross-linking reaction can
occur without needing any additional curing agent, which
means that the chemical composition of the cured film
would be homogeneous and only constituted of pure
polyarylate itself. To this end, a reactive precursor with
an equivalent amount of two reactive terminal groups was
easily prepared by simply mixing polymer ATHP with
CTHP in the following weight ratio:
Figure 5. GPC traces of the hyperbranched polyarylates ATHP and
CTHP.
h
i
n
n
- n
ð- OCOCH3Þ
ð- COOHÞ
mATHP
mCTHP
CTHP
ATHP
h
i
¼
ð2Þ
- n
ð- OCOCH3Þ
ð- COOHÞ
where mATHP and mCTHP are the mass (in grams), whereas
[n n(-COOH) ATHP and [n(-COOH)- n
]
]
(-OCOCH3) CTHP
(-OCOCH3)-
Figure 6. DSC curves of the hyperbranched polyarylates ATHP and
CTHP.
(given in units of mol/g) are the net molar number of
acetoxy and carboxyl groups per mass unit for the
hyperbranched ATHP and CTHP, respectively.
linear polystyrene standards, the actual molecular weights
may be higher than the measured results. Moreover,
the low inherent viscosity of 0.12-0.18 dL/g reflected the
typical characteristic of hyperbranched polymer. Because
of the presence of two types of branching units including
D1 and D2 units in the structure, hyperbranched poly-
arylates will lead to increased diversity of structural topo-
logies. Therefore, ATHP and CTHP exhibited the slightly
rough GPC traces, indicative of multimodal character,
because of the existence of different molecular-weight
products with highly irregular structures, and this pheno-
menon was more obvious for ATHP with relatively high-
er molecular weight.
As shown in the DSC curves in Figure 6, the Tg values
of polymers ATHP and CTHP were 146 and 155 °C,
respectively. Even though a rigid tetraphenylsilane core is
present, ATHP and CTHP exhibited rather low Tg values,
compared to their analogue hyperbranched polyarylate
prepared from terephthaloyl chloride (A2 monomer) and
1,1,1-tris(4-hydroxyphenyl)-ethane (B3 monomer).55 The
reason might be ascribed to that the rigid tetrahedral
unit could open up the polymer segments, leading to
To fabricate a cross-linking film, as illustrated in
Figure 7, the DMF solution of precursor was filtered
through a 0.25-μm Teflon filter, and the spin-coating
operation was performed in a Class 1000 ultraclean room.
The curing process of the precursor was monitored by
DSC. As shown in Figure 8, the endothermic peak of
curing reaction started at ∼200 °C and ended at ∼350 °C.
Therefore, the cure schedule was set as 1 h at 150 °C, 2 h at
180 °C, 5 h at 220 °C, 5 h at 250 °C, and 1 h at 320 °C to
ensure a complete reaction between the carboxylic acid
and acetoxy terminal groups. The FTIR spectrum in
Figure 9 showed that, after curing, the methyl absorption
at 2960 and 2880 cm-1 disappeared. Moreover, there was
only a singlet symmetric peak at ∼1737 cm-1, and the
former carbonyl peaks at 1700 cm-1 (COOH group) and
at 1763 cm-1 (OCOCH3 group) could not be observed,
confirming that the curing reaction was complete.
The AFM height image (Figure 10) exhibited that the
surface of cured film was smooth with a roughness of
∼4 nm. The solvent resistance of the film was evaluated
by immersing it into various solvents, including
(55) Fan, Z.; Lederer, A.; Voit, B. Polymer 2009, 50, 3431.
(56) Kim, Y. H. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 1685.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2787
Figure 9. FT-IR spectra of the cured film.
Figure 10. AFM image of the surface for the cured film.
of the cured film could not be detected during heating from
100 °C to 400 °C in the DSC thermogram, because of the
highly restricted segmental mobility after cross-linking. As
illustrated in Figure 11, the cured film exhibited excellent
thermal stability with a 5% decomposition temperature
(Td5%) of >483 °C. The sharp decomposition curve con-
firmed the homogeneous network structure of the film.
Thermosetting Film of Intrinsic Microporosity. As
shown in the nitrogen adsorption/desorption isotherms
for the cured network in Figure 12, the film sample has a
high uptake at low relative pressure, exhibiting the char-
acteristics of substantially microporous materials.57 The
isotherms shows significant hysteresis with desorption
curve lying above the adsorption curve, which can be
classified as Type 1B. The measured results showed that
the cured film has an apparent surface area of 158 m2/g,
Figure 7. Schematic representation of the preparation of a microporous
thermosetting film.
˚
an average pore size of 11.2 A, and a total pore volume of
0.1 cm3/g, as calculated from the amount of gas adsorbed
at P/P0 = 0.99. The present microporous thermosetting
polyarylate film was constructed with the rigid tetrahe-
dral units held together by aromatic ester linkers
(Figure 13). Compared with precipitated PIM powders
Figure 8. DSC scan of the hyperbranched precursor just (a) prior to
curing and (b) after curing.
chloroform, THF, NMP, DMSO, DMAc, etc., for 24 h,
and no visible change can be observed. The glass transition
(57) Rouqueol, F.; Roquerol, J.; Sing, K. Adsorption by Powders and
Porous Solids; Academic Press: London, 1999.

2788 Chem. Mater., Vol. 22, No. 9, 2010
Zhang and Wang
Figure 13. Rigid tetrahedral core (panel a) in the cured cross-linked
network (panel b).
Figure 11. TGA curve of the cured film.
applications such as wave guides, optical fibers, and
optical storage elements, the optically isotropic charac-
teristic of a material is very important. The three-dimen-
sional cross-linking structure using highly symmetrical
building blocks, i.e., tetraphenylsilane, as branch point
imparts isotropic properties to the cured film well,
exhibiting a low birefringence of only 0.0035.
The porous materials has been an active research area
for development of film with low dielectric constant.58
For the modern microelectronic industry with feature size
of <0.13 μm, porous film materials possessing low di-
electric constant, uniform porosity, and small porous size
(<2 nm), as well as good film formation and excellent
thermal stability, are strongly desired. In the present
work, the dielectric constant of the thermosetting film
can be estimated from the refractive indices, according to
the following equation:
Figure 12. Nitrogen adsorption/desorption isotherms at 77 K for the
cured film.
2
ε ¼ 1:1nav
where ε is the dielectric constant at 1 MHz and nav is the
average refractive index.59 As expected, the determined ε
value was as low as 2.45. Compared with conventional
porous polymeric materials, in the present work, the
microporosity was engineered within the thermosetting
polyarylate film via the direct polymerization reaction of
a precursor containing a specific rigid tetrahedral skele-
ton, without the use of any porogen, cross-linker, and
catalyst, showing promising potential application in
integrated circuit (e.g., as an interlayer insulating layer).
reported in the literature, the amount of microporosity in
the studied film was relatively modest. This might be
ascribed to the ease of rotation about the ester bonds that
enabled the polymer to be densely packed. It could be
proposed that, if the semirigid aromatic ester linker was
substituted by a full rigid rod one, a large surface area
would be obtained. However, such fully rigid networks
were unfavorable for the formation of tough films, be-
cause of the difficulty in releasing internal stresses in-
duced by the curing reaction. In this regard, it still remains
a great challenge to synthesize microporous polymer
networks with the excellent combination of film-forming
properties, huge surface area, and narrow pore size
distribution.
Optical and Dielectric Properties of Cured Film. The
optical properties of the thermosetting film were mea-
sured by a prism-coupling method. The refractive indices
for in-plane and out-plane of the film were 1.4962 and
1.4927, respectively. The microporous structure in the
film resulted in much-lower refractive indices than most
linear and cross-linked polymers. The birefringence is
defined as the difference in refractive indices between
in-plane and out-of-plane of a film. For many optical
Conclusions
In this work, we synthesized a novel class of hyper-
branched macromolecular architectures from hydroquinone
diacetate as the A2 monomer and tetrakis(4-carboxyphe-
nyl)silane as the B4 monomer by step-growth heterogeneous
polycondensation based on kinetic control. Depending on
the monomer feed ratio, two type of polymers;i.e., mainly
carboxyl-terminated hyperbranched polyarylate (CTHP)
and mainly acetoxy-terminated hyperbranched polyarylate
(58) Maier, G. Prog. Polym. Sci. 2001, 26, 3.
(59) Maex, K.; Baklanov, M. R.; Shamiryan, D.; Lacopi, F.; Brongersma,
S. H.; Yanovitskaya, Z. S. J. Appl. Phys. 2003, 11, 8793.

Article
Chem. Mater., Vol. 22, No. 9, 2010 2789
(ATHP);were obtained in good yields. The resulting
polymers had degree of branching (DB) values that
ranged from 0.53 to 0.69, moderate molecular weights,
low inherent viscosity, and good solubility in the common
solvent. The self-curable precursor with an equivalent
amount of carboxylic acid and acetoxy-terminated
groups was prepared by simply mixing the CTHP with
ATHP. Thermal curing of the precursor led to a micro-
porous thermosetting film, which own good film forma-
tion, excellent thermal stability, good solvent resistance,
low birefringence, and low dielectric constant, showing
promising potential in the applications such as separation
membrane, optical and electronic materials, etc.
Acknowledgment. We thank the National Science Foun-
dation of China (through Nos. 50673014 and 20874007) and
the Program for New Century Excellent Talents in Univer-
sity of China (through No. NCET-06-0280) for financial
support of this research. We appreciate help from Jia You
and Hao Yu for the AFM and optical measurements.
Supporting Information Available: This section contains seven
figures (Figures S1-S7), including the FTIR and 1H NMR
spectra of compounds TTS and TCS, the 29Si NMR spectra of
hyperbranched polyarylates ATHP and CTHP, and the pore
size distribution for cured polyarylate film as calculated by the
Horvath-Kawazoe (HK) method. This material is available
free of charge via the Internet at http://pubs.acs.org.