Synthesis and polymerization of disiloxane Si–O–Si-linked phthalonitrile monomer

Article abstract of DOI:10.1016/j.mencom.2016.11.023

A novel low melting highly hydrolytically stable phthalonitrile monomer containing Ar–Si–O–Si–Ar bonds with T_g = 4 °C exhibits good rheological properties. Its cured resin possesses high heat deflection temperature (T_hd = 471 °C) and

Full text of DOI:10.1016/j.mencom.2016.11.023

Mendeleev
Communications
Mendeleev Commun., 2016, 26, 527–529
Synthesis and polymerization of disiloxane
Si–O–Si-linked phthalonitrile monomer
a
b
a,c
Pavel B. Dzhevakov, Roman F. Korotkov, Boris A. Bulgakov,*
Alexander V. Babkin, Alexey V. Kepman and Viktor V. Avdeevc
a,c
a,c
a
b
c
Institute of New Carbon Materials and Technology, 119991 Moscow, Russian Federation.
E-mail: bbulgakov@gmail.com
Department of Materials Science, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow,
Russian Federation
DOI: 10.1016/j.mencom.2016.11.023
A novel low melting highly hydrolytically stable phthalo­
nitrile monomer containing Ar–Si–O–Si–Ar bonds with
Tg = 4°C exhibits good rheological properties. Its cured resin
possesses high heat deflection temperature (Thd = 471°C) and
demonstrates high thermal and thermooxidative stabilities
NC
NC
O
O
CN
CN
Me2SiCl2
Me Me
O
Thermoset
Si
Si
High heat deflection
temperature
High thermal stability
High thermooxidative
stability
Me Me
Low glass transition temperature
Tg = 4 °C
(
T5% = 503°C, TOS5% = 495°C, Y = 76%) that are typical of
c
phthalonitrile matrices.
Phthalonitrile resins are a unique class of high-temperature (up
to 400°C) materials which find applications in aerospace, marine
and microelectronic industry and can be used as matrices for high-
temperature composite tooling for carbon fiber-reinforced plastics
production. In addition, cure mechanism of formation of these
systems ensures that no volatiles evolve during the polymeriza-
tion resulting in a highly cross-linked, void-free network poly-
mers. Due to this, they should have good mechanical properties,
as high temperature thermosetting matrix for fiber reinforced
plastics.
We attempted to extrapolate the data on the preparation and
curing of the low-melting siloxane-bridged phthalonitriles based
on benzyl alcohol derivatives 1, 2 and synthesized phenol-based
siloxane-bridged phthalonitriles 3, 4 to estimate whether the
1
–7
methylene group or siloxane linker reduced T of the monomers
g
1
7
reported.
outstanding thermo-oxidative stability at elevated temperatures,
high fire resistance, and low water absorption.8
–10
NC
CN
CN
Composite
applications require resin systems which are liquid or have a
rather low melting point, show a large processing window which
is defined as the interval between the melting point and a tem-
perature of polymerization, and possess low viscosity for manu-
facturing composite components by cost effective methods such
as resin transfer molding (RTM) and vacuum infusion molding
O
O
NC
O
Si
O
Ph
R
1 R = Ph
2 R = Me
Tg = 27 °C
Tg = 12 °C
NC
NC
CN
CN
(VIM).
Ph
R
The crucial factor for the development of polymers with high
Si
O
O
O
O
oxidative stability is the incorporation of thermostable structural
units such as siloxane linkage between the terminal phthalonitrile
3
4
R = Ph
R = Me
Tg = 29 °C
Tg = 28 °C
6,11,12
units.
Hydrolytically unstable polysiloxanes derived from the
corresponding diphenol and dichlorosilane containing Si–O–C
bonds have been described.1 Noshay claims that some of the
polymers and block copolymers containing Si–O–C-linkers are
hydrolytically stable and points out that care should be exercised
when extrapolating the characteristic properties of a given chemical
bond from polymers of one architectural type to those of a dif-
3,14
15
It was found that the obtained phenolic monomers containing
disiloxane Si–O–Si linkages indeed possessed T slightly higher
g
than those of benzyl alcohol derivatives, however these monomers
turned out to be hydrolytically unstable in the air and decomposed
into the initial diphenol and a mixture of unidentifiable sub-
stances if kept outside of glovebox for several days in solid state or
15
ferent type. For example, polynorbornenes with pendant Si–O–C
groups are steady, while the monomer is moisture-sensitive.
Low-melting siloxane-based phthalonitrile monomers containing
16
18
for several hours in a solution. By this reason it was impossible
to cure these monomers without foaming indicating decomposi-
tion of the monomers during polymerization.
1
7
O–Si–O-bridge were synthesized by our research group recently.
The phthalonitriles possessed the lowest glass transition tempe-
rature of 12°C. Both the monomers and the polymers synthesized
exhibit high hydrolytic stability.
We assumed that the bond Si–O–Si should be as flexible as
O–Si–O bond, and being used as a linker should provide flexibility
to the monomer structure. At the same time, Si–C and Si–O bonds
are resistant to moisture and other chemical treatments so it was
expected to obtain low-melting and stable phthalonitrile monomer.
Herein, we describe synthesis of low-melting hydrolytically
stable Si-bridged phthalonitrile monomer and estimate its propertie
s
©
2016 Mendeleev Communications. Published by ELSEVIER B.V.
–
527 –
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.

Mendeleev Commun., 2016, 26, 527–529
Table 1 Thermal characteristics of the cured monomers.
Monomer
Structure
Polymer (cured monomer)
Atmosphere Thd/°C T5%/°C T10%/°C Y (%)
Tg/°C
Reference
c
Me
Si
Me
Si
Me
O
Me
NC
NC
CN
CN
Inert
Air
503
495
538
579
76
22
This
article
4
471
441
O
O
NC
O
O
CN
CN
Inert
Air
493
486
–
–
62 (1000°C)
–
4
2
12
NC
4
Ph Me
Si
NC
CN
CN
Inert
Air
537
520
–
–
79 (900°C)
12 (1000°C)
O
O
1
4
2
0
428
428
17
23
NC
NC
O
O
CH2
Si
O
O
CN
Me
Inert
Air
541
543
627
597
82.4 (1000°C)
11.3 (1000°C)
NC
N
N
H
CN
H
NC
NC
Me
Si
Inert
Air
420a
420a
–
–
58
–
O
210a
3
5
24
O
O
Me Me
Me
2
a
Cured at a maximum temperature of 300°C (8 h) with 10% bis[4-(3-aminophenoxy)phenyl]sulfone.
To check the validity of the judgments, we synthesized the desired
monomer 5 and investigated the effect of disiloxane linker on
thermal properties of this monomer and cured polymer matrix.
Several methods of disiloxane group formation are known.
Disiloxanes can be successfully prepared from the corresponding
trisubstituted silane and formamide using carbonyl complexes of
transition metal as a catalyst.19 Alkoxysilanes react with methyl-
allylsilanes in the presence of scandium(iii) trifluoromethane-
sulfonate to yield disiloxanes and isobutene.20 Also synthesis of
Si–O–Si linkage is possible by acidic catalytic dimerization
Grignard reagent which was then coupled with Me SiCl to give
2 2
silanol 7 after aqueous treatment. Self-condensation of silanol 7
occurred with simultaneous deprotection and yielded disiloxane 8
with free phenolic groups. The final step was nucleophilic substi-
tution of nitro group in 4-nitrophthalonitrile with alkali salt of
compound 8 in DMA to yield the desired product.
The glass transition temperature T = 4°C of the monomer
g
prepared, determined by differential scanning calorimetry (DSC)
(Figure 1), is lower than that for any reported phthalonitrile mono-
mers (Table 1). Obviously, the Si–O bond is more mobile with
a less sterically hindering substituent at the Si atom.
2
1
of silanols. The latter method was considered as the most
convenient one due to availability of starting material. It made
possible to synthesize product 5 in four steps without using transi-
The rheometric analysis of the monomer during its poly-
merization was performed in the temperature range of 80–300°C.
The results showed that the viscosity of the monomer gradually
decreased with temperature up until wide plateau from 120°C
to 220°C where the viscosity was not above 100 mPa s. A slow
gradual rising of the complex viscosity begins from 220°C. It is
†
tion metal complexes (Scheme 1). The THP-protected 4-bromo-
6
22
phenol was used as a starting material. It was converted into the
Me
OH
25
Br
Si
known that phthalonitrile resins in the absence of curing agent
polymerize slowly at relatively low temperature.
Me
Br
O
ii
O
Rheometric study of curing process of monomer 5 in the presence
of 4 wt% 1,3-bis(4-aminophenoxy)benzene (APB) was carried out
isothermally at 120°C, which revealed that the pot life of the resin
was up to 48 h and viscosity was in the range of 100–300 mPa s.
i
9
8%
95%
HO
O
O
6
7
exo
Glass transition:
Me
Me
Si
Onset:
Mid:
0.4°C
3.9°C
O
–1.6
Si
iii
iv
Inflection: 3.6°C
–
–
1.8
2.0
End:
∆C
6.7°C
Me Me
2.788 J g K–1
–1
9
3%
50%
p
:
HO
OH
O
8
–2.2
–
–
2.4
2.6
Me
Me
Si
O
NC
NC
Si
CN
CN
–
10
0
10 20 30 40 50 60
Me Me
T/°C
O
5
Figure 1 The DSC thermogram of the monomer.
Scheme 1 Reagents and conditions: i, dihydropyran, HCl; ii, Mg, THF,
†
then Me SiCl –Et N, then H O; iii, 5% HCl, EtOH; iv, 4-nitrophthalonitrile,
For procedures and characteristics of compounds 5–8, see Online
2
2
3
2
K CO , DMA, 70°C.
Supplementary Materials.
2
3
–
528 –

Mendeleev Commun., 2016, 26, 527–529
After two days the reaction mixture was heated up to 200°C and
then isothermal study was held at a given temperature.After 120 min
the viscosity of the reaction mixture started to rise and after 140 min
Si–C and sterically hindered Ph–O–Ph bonds. At the tempera-
tures >640°C, fast oxidation of the polymer occurred. A sharp
weight loss of up to 21.8% at 1000°C is in good agreement with
,
,
we believe, polymer was gelled so that further viscosity measure-
ment was impossible. Thus, there is 2 h period for degassing sample
at reduced pressure and filling tool before the sample starts cure.
The monomer containing Si–O–Si-bridge demonstrated excel-
lent rheological properties, namely, a wide processing window
with low viscosities in the temperature range of 80–220°C and had
an increased lifetime at processing temperatures. This makes it
perfectly suitable for composite manufacturing by the RTM and
VIM methods.
theoretical amount of SiO (21.1%) which should be formed
after complete oxidation of the matrix.
In conclusion, a new low-melting highly hydrolytically stable
silicon-containing phthalonitrile monomer was prepared. The
pure monomer demonstrates excellent rheological properties
2
due to very low for phthalonitriles T = 4°C and could be used
g
for FRP production by RTM or VIM techniques. The study of the
curing process in the presence of 4 wt% of APB by rheometric
analysis showed that the considered thermosetting resin possessed
excessively long pot-life at 120°C and at 200°C gel time was
~2 h. These facts are very promising for high temperature poly-
mer matrix for composite applications. The flexible disiloxane
linkage enhances processability without compromising thermal
and oxidative stability. The cured resin possesses high heat deflec-
tion temperature Thd of 471°C. In argon atmosphere 5% weight
loss of the cured polymer occurs at 503°C, and with 76% residual
mass at 900°C.
Phthalonitrile curing occurs in the presence of amines or
phenols as initiators to yield 3D-network consisting of isoindoline,
triazine and phthalocyanine structures.2 Monomer curing was
performed by heating at 200°C for 24 h followed by slow heating
6,27
–
1
‡
(
2 K h ) up to 375°C with 8 h hold at final temperature. Such
slow program was applied to keep the external temperature lower
than current T of the sample, which is necessary to lower shrinkage
g
and solid samples formation. Due to the presence of highly flexible
Si–O–Si-bridges in the polymer matrix one could expect lowering
of heat deflection temperature of the cured samples. However,
The work was supported by the Russian Federation Ministry
of Education and Science within the contract no. 02.G25.31.0114.
the resulting cured resin demonstrated excellent value of Thd
=
=
471°C featured to phthalonitrile resins, which is higher than
those for reported phthalonitriles (Table 1). This can be explained
by different measurement method, namely TMA in penetration
mode, which usually gives uprated values.
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi:10.1016/j.mencom.2016.11.023.
Thermal stability of the obtained polymer, evaluated by TGA
in air and argon atmosphere, are at the same high level as those of
reported phthalonitriles. According to TGA curves (Figure 2) and
the data in Table 1, in an inert atmosphere the polymer possessed
high thermal stability (curve 1) and did not loose weight up to
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Figure 2 TGA thermogram of the cured resin 5 in Ar and air.
‡
For more detail, see Online Supplementary Materials.
Received: 5th July 2016; Com. 16/4983
529 –
–