Synthesis and properties of an oligomeric divinyl-terminated aromatic ether containing resin

Article abstract of DOI:10.1039/b902581b

The synthesis of a novel oligomeric divinyl-terminated aromatic ether containing resin and polymerization with silane containing compounds utilizing a hydrosilylation reaction is demonstrated. The transparent, clear polymers exhibit high thermal and oxida

Full text of DOI:10.1039/b902581b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and properties of an oligomeric divinyl-terminated aromatic
ether containing resin
*
Matthew Laskoski and Teddy M. Keller
Received 6th February 2009, Accepted 19th March 2009
First published as an Advance Article on the web 31st March 2009
DOI: 10.1039/b902581b
The synthesis of a novel oligomeric divinyl-terminated aromatic ether containing resin and
polymerization with silane containing compounds utilizing a hydrosilylation reaction is demonstrated.
The transparent, clear polymers exhibit high thermal and oxidative stability while exhibiting
elastomeric and plastic properties depending on the curing temperature and curing agent.
In this paper, we will discuss the synthesis, polymerization,
Introduction
and thermal stability of an oligomeric divinyl-terminated
aromatic ether containing resin system and show the conversion
of this resin to a transparent polymer via a room temperature
hydrosilylation reaction.
High-temperature polymers are an important class of materials
that offers a variety of opportunities for new technological
applications especially in the modern aerospace, marine, energy,
and electrical industries.1 Two important requirements of elas-
tomeric materials for these applications in extreme environments
are good thermal stability up to several hundred degrees and the
materials ability to maintain good structural integrity at elevated
temperatures. Polysiloxanes have been shown to be good
candidates as high-temperature elastomers because of their
flexible and heat-resistant backbone.1 However, it is sometimes
difficult to obtain tough materials based solely on the siloxane
backbone.
Experimental
General information
All starting materials were of reagent grade and used without
further purification. Differential scanning calorimetric (DSC)
analysis was performed on a TA Instruments DSC 2920 modu-
lated thermal analyzer at a heating rate of 10 ꢀC minꢁ1 and
a nitrogen purge of 50 cm3 minꢁ1. The glass transition tempera-
ture (Tg) was reported as the temperature centered at the
midpoint between the linear baselines of a DSC scan. Ther-
mogravimetric analysis (TGA) was performed on a TA Instru-
ments TGA Q50 at a heating rate of 10 ꢀC minꢁ1 under a nitrogen
or air purge of 50 cm3 minꢁ1. Infrared (IR) spectra were recorded
The development of durable high-temperature materials
requires the incorporation of thermally stable aromatic or het-
eroaromatic structural units within the backbone of a polymeric
system.2–6 By designing a polymeric system with these highly
stable structural units and with flexible linkages, the desired
thermo-oxidative stability and structural properties can be ach-
ieved. A simple way to form such systems is by utilizing
a modified Ullmann synthesis for the preparation of aryl con-
taining ethers, developed in our laboratory and extensively
studied for thermosetting systems.7–10 This method allows for the
formation of difunctional hydroxyl-terminated aryl ether oligo-
mers of variable length and composition. We have endcapped the
hydroxyl-terminated oligomers with a vinyl silane group to
obtain liquid monomers. The divinyl-terminated monomers are
then converted to a linear or crosslinked polymer using hydro-
silylation chemistry in the presence of a transition metal catalyst
and a multiple Si–H containing compound.
as films on NaCl disks using a Nicolet Magna FTIR 750 spec-
€
1
trometer. H-NMR was performed on a Bruker ADVANCE 300
spectrometer.
Synthesis of the hydroxyl-terminated oligomeric aromatic ether 4
To a 250 mL three-necked flask fitted with a thermometer,
a Dean–Stark trap with condenser, and an argon inlet were
added bisphenol
A 2
(30.2 g, 132 mmol), 4,40-difluor-
obenzophenone 3 (14.4 g, 66.0 mmol), toluene (20 mL) and N,N-
dimethylformamide (200 mL). The resulting mixture was
degassed thoroughly with argon for 10 min and K2CO3 (27.3 g,
198 mmol) was added in one portion. The reaction mixture was
By exploiting this methodology, the liquid divinyl-terminated
resin systems can be easily processed into shaped composite
components or thin elastomeric films utilizing a cost effective
technique such as resin transfer molding or by spray coating at
ambient temperatures. In essence, by employing hydrosilylation
chemistry, the clear resins are converted to a linear or network
system under ambient conditions.
ꢀ
then refluxed at 135–145 C for 12 h. The water formed in the
reaction was removed by azeotropic distillation. The toluene was
then removed by distillation and the mixture was cooled to
ambient temperature. Water (300 mL) was then added to the
reaction mixture. At this point, the product mixture was slightly
basic and 2 M HCl (300 mL) was added. The mixture was then
extracted twice with ether (100 mL) and the combined ether
layers were washed with 2 M HCl (100 mL) and water (100 mL)
until neutral. Carbon black (2 g) was added and the ether extract
was filtered through a short plug of silica gel to remove the
insoluble components. The solvent was removed at reduced
Chemistry Division, Advanced Materials Chemistry Branch, Code 6127,
Naval, Research Laboratory, Washington DC, 20375-5320, USA.
E-mail: matthew.laskoski@.nrl.navy.mil
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pressure yielding a yellow glassy solid, which was vacuum dried
to afford the hydroxyl-terminated oligomer 4 (n | 1, 16.1 g,
97%). 1H-NMR (300 MHz, CDCl3): d 7.77 (d, aromatic-H), 7.71
(dd, aromatic-H), 7.32 (dd, aromatic-H), 7.28–7.21 (m, aromatic-
H), 7.05–6.94 (m, aromatic-H), 1.75–1.68 (m, CH3). IR [cmꢁ1]: l
3040 (C]CH), 2970 (CH3), 2232 (CN), 1650 (C]O), 1592
(C]C), 1498 (aromatic), 1308 (aromatic), 1279 (C–O), 1246
(CH3), 1160 (C–O), 928 (C–O), 837 (aromatic).
conditions within 60 s and the transparent polymer 6a was
subsequently post-cured at 100 ꢀC for 4 h and at 300 ꢀC for 1 h.
Results and discussion
The hydroxyl-terminated oligomer 4 was prepared by reacting
bisphenol A 2, 4,40-difluorobenzophenone 3, and K2CO3 in the
presence of N,N-dimethylformamide (DMF) and toluene
(Scheme 1). Oligomer 4 was isolated as a yellow glassy solid
in 97% yield. During the reaction, water that formed as a
by-product was removed by azeotropic distillation at 135–145ꢀC.
Oligomer 4 was further reacted with chlorodimethylvinylsilane 5
and triethylamine in dry tetrahydrofuran (THF) to afford 1 as
a clear oil in 95% yield. The length of the aromatic spacer
between the terminal vinyl silane groups can be varied by
changing the ratio between 2 (excess) and 3. The reactions
producing 4 and 1 were monitored by infrared spectroscopy and
1H-NMR.
Synthesis of the divinyl-terminated oligomeric aromatic ether
resin 1
To a 100 mL three-necked flask fitted with a thermometer,
a Dean–Stark trap with condenser, and a nitrogen inlet were
added the hydroxyl-terminated oligomer 4 (10.0 g, 15.7 mmol),
triethylamine (4.72 ml, 33.9 mmol) and anhydrous tetrahydro-
furan (100 mL). The reaction mixture was cooled by means of an
ice bath and vinyl(chlorodimethyl)silane 5 (4.68 ml, 33.1 mmol)
was added dropwise. The resulting mixture was stirred under
ambient conditions for 1 h. The product mixture was then
poured into water and extracted with diethyl ether. After
removing the solvent at reduced pressure, the resulting oil was
dissolved in 1 : 1 CH2Cl2–hexane, dried over anhydrous
Na2SO4, and filtered through a silica plug. The solvent was
removed in vacuo and the clear oil was subjected to a full vacuum
to yield 1 (12.0 g, 95%). 1H-NMR (300 MHz, CDCl3): d 7.78 (d,
aromatic-H), 7.73 (dd, aromatic-H), 7.32 (dd, aromatic-H), 7.27–
7.23 (m, aromatic-H), 7.06–6.95 (m, aromatic-H), 6.27 (dd,
alkene-H), 6.05 (dd, alkene-H), 5.75 (dd, alkene-H), 1.77–1.68
(m, CH3). IR [cmꢁ1]: l 3052 (C]CH), 2967 (CH3), 1654 (C]O),
1593 (C]C), 1500 (aromatic), 1242 (C–O), 1171 (C–O), 834
(aromatic).
Oligomer 1 is readily soluble in most organic solvents due to
the existence of aliphatic groups in the system. Conversion of 1 to
a linear or networked polymer occurs at ambient temperature in
the presence of any compound containing at least two Si–H
moieties and a Pt catalyst using typical hydrosilylation chem-
istry.11 In this case, either 1,1,3,3-tetramethyldisiloxane or tet-
rakis(dimethylsiloxy)silane was reacted with 1 in combination
with a platinum cyclovinylmethylsiloxane complex in xylene to
facilitate the formation of transparent polymeric material 6
(Fig. 1).
In a typical polymerization reaction, a selected ratio of the
divinyl-terminated resin 1 to the silane (Si–H) containing
compound was dissolved in a minimal amount of toluene. The
mixture was heated to dissolve the monomer 1 and the Si–H
containing compound and then cooled to about 0 ꢀC before the
addition of the catalyst. Consolidation to an elastomer or plastic
was achieved by the addition of 0.1 ml of the Pt catalyst with
Polymerization of divinyl-terminated oligomeric aromatic ether
resin 1 with tetrakis(dimethylsiloxy)silane (2 : 1 ratio) with a Pt
catalyst
Divinyl-terminated resin 1 (0.29 g, 0.361 mmol) and tetrakis-
(dimethylsiloxy)silane (0.07 mL, 0.059 g, 0.180 mmol) were
mixed in toluene (1 mL). The reaction mixture was heated to
70 ꢀC to ensure dissolution of the monomer and crosslinker silane
compound. The resulting solution was cooled to about 0 ꢀC, and
0.1 ml of a platinum cyclovinylmethylsiloxane complex in xylene
was added with adequate stirring followed by immediate transfer
to a silicone mold. Gelation occurred under ambient temperature
conditions within 60 s and the transparent polymer 6b was
subsequently post-cured at 100 ꢀC for 4 h and at 300 ꢀC for 1 h.
Polymerization of divinyl-terminated oligomeric aromatic ether
resin 1 with 1,1,3,3-tetramethyldisiloxane (1 : 1 ratio) with a Pt
catalyst
Divinyl-terminated resin 1 (0.54 g, 0.672 mmol) and 1,1,3,3-tet-
ramethyldisiloxane (0.12 mL, 0.09 g, 0.672 mmol) were mixed in
toluene (1 mL). The mixture was heated to 70 ꢀC to ensure
dissolution of the monomer and 1,1,3,3-tetramethyldisiloxane.
The resulting solution was cooled to about 0 ꢀC and 0.1 ml of
a platinum cyclovinylmethylsiloxane complex in xylene was
added with adequate stirring followed by immediate transfer to
a silicone mold. Gelation occurred under ambient temperature
Scheme 1 Synthesis of divinyl silane resin 1 and polymer 6.
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Fig. 1 Photographs of the cured transparent polymer 6b.
adequate stirring followed by pouring into a mold and allowing
the homogenous mixture to react under ambient conditions. The
dissolution of 1 and the silane containing compound in toluene
proved necessary due to the chemical incompatibility of the
monomer with the silane containing compound. After gelation at
ambient temperature had occurred, the resulting polymer 6 was
Fig. 3 FTIR spectra of monomer 1 and 1,1,3,3-tetramethyldisiloxane in
a 1 : 1 ratio cured in the presence of a Pt catalyst. Initial mixing (A);
1,1,3,3-tetramethyldisiloxane (B); reaction at room temperature for 14 h
(C); and reaction at 100 ꢀC for 1 h (D).
ꢀ
ꢀ
post-cured at 100 C for 4 h and at 300 C for 1 h. During the
post-cure heat treatment, volatilization of the toluene occurred
affording a void-free polymer. The transparent elastomeric
polymer 6a prepared from 1 and 1,1,3,3-tetramethyldisiloxane
displayed a glass transition temperature (Tg) of approximately
ꢁ17 ꢀC (Fig. 2, top). A polymeric plastic 6b (Fig. 2, bottom)
prepared from 1 and tetrakis(dimethylsiloxy)silane showed a Tg
of around 60 ꢀC. Additionally, DSC analysis of the polymers did
not show any visible exothermic transitions and it was assumed
that the polymerization reaction was complete.
of thin films of 6a formed from reaction of monomer 1 and
1,1,3,3-tetramethyldisiloxane in a 1 : 1 molar ratio in the pres-
ence of a Pt catalyst. Plot A is a spectrum taken immediately after
mixing 1, 1,1,3,3-tetramethyldisiloxane and Pt catalyst. As
a comparison, plot B is the spectrum of 1,1,3,3-tetramethyldisi-
loxane showing the Si–H peak at approximately 2100 cmꢁ1
.
When the polymerization mixture (plot C) was allowed to sit at
room temperature overnight (14 h), the peak attributed to Si–H
was considerably diminished. Following heating to 100 ꢀC for 1
h, the Si–H peak was absent (plot D) and the polymerization
reaction was assumed to be complete.
IR analysis was used to monitor and to determine the extent of
the polymerization reactions forming 6. Fig. 3 shows the spectra
The thermal and oxidative stability of 6 was assessed by TGA
analyses (Fig. 4). When 1 and tetrakis(dimethylsiloxy)silane were
cured in the presence of 0.1 ml of a Pt catalyst at up to a maximum
ꢀ
temperature of 300 C under inert conditions, the resulting nꢀet-
worked polymer 6b showed weight retention of 95% at 430 C
and an overall char yield of 71% upon heating to 1000 ꢀC under
a nitrogen atmosphere. A sample of 6b exhibited a weight
retention of 95% at 470 ꢀC aꢀnd an overall residue of 40%
remained upon heating to 1000 C under a flow of air. Another
sample of 6b that was post-cured to 400 ꢀC for 1 h displayed
a weight retention 95% at 495 and 490 ꢀC when heated under
nitrogen and air atmospheres, respectively. The post-cured 6b
Fig. 2 DSC curves showing the glass transition temperatures of poly-
mers (top plot) 6a and (bottom plot) 6b.
Fig. 4 TGA thermograms of thermoset 6b post-cured to 300 ꢀC for 1 h:
under N2 (A) and air (B).
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exhibited a similar char yield as the 300 ꢀC cured samples, when
Acknowledgements
ꢀ
heated to 1000 C.
The authors would like to thank the Office of Naval Research for
financial support of this project.
Conclusion
In summary, the liquid properties of monomer 1 along with the
prospect of a room temperature polymeric conversion are
important features of this new resin. These properties may allow
the use of the clear divinyl-terminated aromatic ether containing
resin 1 in a wide variety of applications including coatings,
adhesives, optics, and potentially as a matrix resin in structural
components. Based on the quantity of the reactants 2 and 3 used
in the synthesis of 1, it may be possible to control the flexibility,
rigidity, Tg, and other physical properties of the resulting
hydrosilylated polymer 6 without severely compromising the
thermal and oxidative stability. Additionally, depending on the
reactive Si–H sites in the silane containing compound used in
the hydrosilylation reaction, either plastic or elastomeric mate-
rials can be readily obtained. The procedure to produce oligomer
4 has the potential to be applicable to a variety of bisphenols and
dihaloaromatic compounds. In subsequent papers, we will
explore the physical and mechanical properties of 6 and other
resins produced from different bisphenols.
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