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INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 71
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
CARBORANE–SILOXANES: SYNTHESIS AND PROPERTIES. NEW POSSIBILITIES FOR STRUCTURE CONTROL
Cite this: INEOS OPEN,
2018, 1 (2), 71–84
DOI: 10.32931/io1806r
Received 10 April 2018, Accepted 5 July 2018
http://ineosopen.org
A. A. Anisimov,a A. V. Zaitsev,
a V. A. Ol'shevskaya,
a M. I. Buzin,
a
V. G. Vasil'ev,a O. I. Shchegolikhina,
a and A. M. Muzafarov*
a,b
a Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, 119991 Russia b Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences,
ul. Profsoyuznaya 70, Moscow, 117393 Russia
Abstract This review considers recent trends in the development of
carborane–siloxane structures. Both individual and polymer
carborane-containing siloxane derivatives are presented. The
main synthetic approaches to these compounds are described. The
potential of their use in different fields of science and engineering
is demonstrated.
Key words: polyhedral carboranes, silsesquioxanes, oligo- and poly(carborane–siloxanes), new generation materials.
1. Introduction
Synthesis of new polymeric materials—films, coatings,
elastomers, fibers, rigid plastics and oils is the most important
challenge of modern materials science. The use of polyhedral
carboranes for creation of these products is a discernable trend.
This is due to a unique combination of electronic and steric
properties of carboranes, which can impart radiation, thermal
and thermo-oxidative stability to the resulting materials [1].
Carborane-containing polymers can be divided into two
large groups:
(1) polymers bearing carborane polyhedra in main chains
(A) (the most studied group);
(2) polymers bearing carborane polyhedra in side
substituents (B) (Fig. 1).
Figure 1. Carborane-containing polymers (C – carborane polyhedron).
The syntheses of different classes of macromolecular
compounds bearing carborane polyhedra in their structures have
been reported to date. They include the following types:
– carborane-containing polyarylenes [2–7];
– carborane-containing polyesters [8, 9];
– carborane-containing polyamides and polyimides [10, 11];
– heteroelement-substituted carborane polymers [12, 13];
– carborane-containing polyphosphazenes [14–17];
– branched macromolecular systems [18–21].
Among the mentioned classes of macromolecules, siloxane
polymers with organoelement structural units amount to one of
the most extensively studied compounds. The diversity of their
structures allows one to greatly extend a range of useful
properties of new polymers and, as a consequence, to increase
the potential of their application in different fields of science
and engineering. At the same time, it is important to produce
new derivatives with improved characteristics (thermal, thermo-
oxidative, radiation and mechanical stabilities, etc.) according to
the modern principles, such as atom economy and green
chemistry. The realization of these goals can be achieved by
modifying polymer chains of siloxanes, for example, with
organoelement structural units. Nowadays, this is one of the
most efficient approaches to produce new polysiloxanes with the
desired properties. A literature survey has revealed a large
number of publications devoted to the introduction of various
modifiers into polysiloxane structures in order to improve their
properties [22–29]. For example, different metals, such as
aluminum, titanium, and iron were included in the main chains
of polysiloxanes, and the resulting polymers exhibited ultrahigh
melting and glass-transition points [30]. Furthermore, the
polysiloxanes with arylene units in the main chains were shown
to possess high decomposition points [31]. An important class of
organoelement polymers consists of carborane-containing
polysiloxanes. They found wide application in modern fields of
industry, engineering and medicine owing to their unique
properties, such as thermal stability, radiation resistance,
nontoxicity, and biological inertness [1]. The improved thermal
stability of these structures is caused by the ability of a bulky
carborane polyhedron to inhibit the cyclization of a linear
polysiloxane at elevated temperatures, whereas no
depolymerization takes place. At the same time, the high
thermal stability of a siloxane bond is far from being fully
realized in carborane siloxanes; therefore, further investigations
are required to develop preparative methods for the synthesis of
carborane–siloxanes to address this issue.
2. Polydimethylsiloxanes with carboranyl
substituents
The works on the synthesis of carborane–siloxanes
developed in the 1960s [32, 33] led to the creation of
commercially available products. These polymers were obtained
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 72
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
using meta-carboranes. The high-temperature stationary phases
for gas chromatography under the trademarks of DEXSIL and
UCARSIL were elaborated. The beginning of this century was
marked by the reports on synthesis and investigation of new
carborane–siloxane polymers for gas chromatography (Stx-500,
HT-8) [34–37]. They afforded the improvement of separation
characteristics of columns and service lives. In turn, modern
analytical equipment provided better understanding of the
structures and properties of the known polymers [38–40].
One of the methods for synthesis of carborane–siloxanes is
FeCl3-catalyzed polycondensation of 1,7-
[(MeO)Me2Si]2C2B10H10 with bis-(chlorosilyl)-m-carboranes,
organochlorosilanes, or organochlorosiloxanes. These
conditions provide copolymers with molecular masses of about
10000. A drawback of the method is side gelation that hampers
the formation of high-molecular linear polymers [41, 42]. In the
case of polycondensation, this problem was solved by the use of
phenyl-substituted dimethoxy-m-carboranes, which gave rise to
elastomers featuring molecular masses of about 150 kDa [43].
The thermal properties of the polymers with arylene carborane
moieties were reviewed by Vinogradova et al. [44].
There are also some other methods for the synthesis of
carborane–siloxanes that do not require the use of metal-
containing catalysts.
One of the methods is based on the synthesis of silanol
derivatives of carboranes 1,7-[(HO)Me2Si]2C2B10H10 which
react further with compounds of a general formula SiR2L2,
where R = amino, carbamate or ureido groups and L = NMe2 or
an amide group (Scheme 1) [45–48]. Thus, the reaction of
bis(ureido)silanes results in linear polymers with the molecular
masses over 250000.
Yet another method is based on the reaction of dilithium
derivatives of carboranes with diorganosiloxanes bearing
terminal ≡Si–Cl groups [47]. A limitation of this method is side
reactions that are accompanied by the cleavage of siloxane
bonds.
Zhang et al. [49] suggested a new method for production of
poly(carborane–siloxanes) which is based on the reaction of a
disilanol derivative of m-carborane with
hexaorganocyclotrisilazane in the presence of (NH4)2SO4
(Scheme 2). The authors stated that this method is more
convenient for the synthesis of poly(carborane–siloxanes) than
the previously reported approaches, since in this case the
process is not accompanied by side reactions.
An essential contribution to the production of monomeric
[50–52] and polymeric [53–55] organosilicon derivatives of
carboranes was made by B. A. Izmaylov and his colleagues.
Besides carborane–siloxane liquids, carborane–siloxanes
show great promise for the production of polymer networks,
which, in turn, can be used to create materials with unique
properties. The most popular industrial method for obtaining
silicon resins is hydrosilation. This approach was used also for
the production of carborane–siloxane network polymers. Houser
et al. [56] obtained in 1998 the first materials based on 1,7-
bis(vinyltetramethyldisiloxy)-m-carborane and linear
polydimethylsiloxane, which contаin the hydride functional
groups (Scheme 3). This is the most striking example of the use
of hydrosilation for the synthesis of carborane–siloxanes. The
N
O
N
Ph
Si(Me)R
2
HOSi
CB10H10C
Me Me
SiOH
Me Me
+
HOSi
CB10H10C
Me Me
SiO
Me Me
N
O
NHPh
2
+
SiO
SiCB10H10C
Me Me
SiOH
Me MeMe R
n
Scheme 1. Synthesis of carborane–siloxanes by heterofunctional
condensation of silanol and ureido groups.
Scheme 2. Synthesis of carborane–siloxanes by heterofunctional
condensation of silanol groups with hexaorganocyclotrisilazane.
CCSi SiSiSi
OO
Me
Me
Me
Me
Me
Me
Me
Me
Si Si SiOO
Me
Me
Me
H
Me
Men
+
SiSiSiO O
Me
Me
Me Me
Me
Si Si SiOO
Me
Me
MeMe
Men
n
C
C
Si
Si
Si
Si
O
O
MeMe
MeMe
MeMe
MeMe
H2PtCl6
Scheme 3. Synthesis of a carborane-containing siloxane resin.
Figure 2. Hydride-containing siloxane cross-linking agents.
reaction was carried out at different ratios of the reagents.
Chloroplatinic acid was used as a catalyst. The formation of a
spatial network proceeded very slowly and lasted several days.
The reaction product was a transparent solid material.
Kolel-Veetil et al. suggested another approach [57] that
consists in the use of branched siloxanes depicted in Fig. 2 as
hydride components and the more active Karstedt catalyst. The
reaction was carried out in hexane for several hours, which
resulted in a flexible transparent material. The decomposition
points of these films ranged from 500 to 550 oC. There is no
explanation for the flexibility of this material in the presence of
a fine network, which should not facilitate it. The authors only
compare the results obtained with those reported by
Houser et al.
3. Poly(carborane–siloxane–acetylene)
copolymers
The first example of successful introduction of a diacetylene
moiety into a siloxane chain was described in 1994 by
Henderson and Keller [58, 59]. Thus, a linear polymer was
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 73
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
obtained in a high yield upon interaction of dilithium derivative
of diacetylene with 1,7-bis(chlorotetramethyldisiloxy-m-
carborane (Scheme 4).
At the temperatures above 250 oC, the condensation by the
acetylene units takes place that results in a plastic insoluble
material.
In 1998 Houser et al. synthesized a linear ferrocene-
functionalized carborane–siloxane–diacetylene polymer in
which a part of the carborane moieties was replaced for the
ferrocene ones (Fig. 3) [60].
The resulting polymer has the molecular mass of 10000. At
350 oC in an inert atmosphere it converts to a black flexible
material and loses 2 wt %. Further heating to 1000 oC affords a
black ceramic material in 75% yield.
To improve the physicochemical properties of
poly(carborane–siloxane–acetylene) copolymers, it was
suggested to introduce a phenyldiacetylene moiety into the
structure of a macromolecule main chain [61, 62]. The syntheses
of these polymers were carried through the production of an
organometallic derivative of phenyldiacetylene as an
intermediate product according to Scheme 5.
CCSiSi
OO
Me
Me
Cl
Me
Me
Cl
n-BuLi
Cl Cl
Cl
Cl
ClCl
Li Li
CCSi SiSi
OO
Me
Me
Me
Me
Me
Me
n
Si
Me
Me
Si Si
Me
MeMe
Me
Scheme 4. Synthesis of poly(carborane–siloxane–acetylene)
copolymers.
CCSi SiSi
OOMe
Me
MeMe
Fe
CCSi Si
Si
OO
Me
Me
Me
Me
MeMe
Me
Me
n
Si
Me
Me
Si
Me
Me
Figure 3. Unit of ferrocene-containing poly(carborane–siloxane–acetylene) copolymers.
An alternative method for the synthesis of poly(carborane–siloxane–phenylacetylene) copolymers was proposed by
Homrighausen. This method is based on the polycondensation of
bis(dimethylaminodimethylsilyl)butadiene with different
prepolymers bearing silanol groups [63, 64]. The formation of a
linear polymer was accompanied by the release of
dimethylamine.
Kolel-Veetil et al. showed [65, 66] that variation of the
length of a prepolymer chain bearing silanol groups can be used
to control the cross-linking density of a curing product obtained
from these copolymers. According to the authors, the resulting
phenyl-containing copolymers exceed their alkyl analogs in the
operational characteristics.
4. Organosilsesquioxanes with carboranyl
substituents
Organosilsesquioxanes are organosilicon compounds with
an empirical formula of (RSiO3/2)n, where the substituent at the
silicon atom is hydrogen or an organic group, namely, alkyl,
alkenyl, aryl, arylene, and so on. The structures of
organosilsesquioxanes can be disordered branched, ladder,
partially condensed, or polyhedral (Т8, Т10, Т12).
Ladder polyorganosilsesquioxanes possess a complex of
valuable properties, such as improved thermal stability, film-
forming ability, and good mechanical characteristics [67–70].
Owing to their compositions, branched
oligoorganosilsesquioxanes exhibit more loose structures and do
not crystallize at the specified content of branching points and
more readily form coils upon cooling. Their rheological
properties depend on the molecular mass and temperature to a
lesser extent. Depending on the molar fraction of branches in the
molecule chains, there is observed a minimum of the flowing
point, being close to the glass transition point (at the branching
content of about 15–20 mol %). Therefore, works on the
synthesis of hyperbranched siloxane structures are actively
developed nowadays [71–73].
Of particular interest are polyhedral organosilsesquioxanes
(POSS). These structures represent unique organic-inorganic
matrices and are considered as molecular models of SiO2. They
are also used as nanosized blocks in production of polymeric
materials. Special attention to POSS is stipulated by their
potentially broad application scope [74].
The introduction of a carborane moiety into the structure of
silsesquioxanes has a stabilizing effect owing to the thermo-
oxidative stability of a carborane polyhedron. In turn,
silsesquioxanes serve as a convenient matrix for the introduction
of the required amount of carborane units upon creation of new
materials.
CC
Si SiSiOO
Me
Me
Me
MeCl
Me
Me
Cl
CCSi SiSi
OO
Me
Me
Me
Me
Me
Me
m-C6H4 MgBr2
o-C6H4 MgBr2
n
CCSi SiSi
OO
Me
Me
Me
Me
Me
Me
nC6H4 MgBrC6H4 2 2
EtMgBro,m- o,m-
Si
Me
Me
Si
Si
Me
Me
Me
Me
Scheme 5. Synthesis of phenyl-containing poly(carborane–siloxane–acetylene) copolymers.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 74
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
C
C
R
C
C
R
SiCl3
C
C
R
Si(OEt)3
2 HSiCl3Cat
2 HSi(OEt)3Cat H2O, Cat
DMSO, CHCl3Si
Si Si
HSi
O
O
OO
Si
Si Si
SiO
O
OO
O O
O O
C
CC
C
CC
C C
CC
C
C
C
C
C C
R
R
R
R
R
R
RR
Scheme 6. Two methods for synthesis of an octacarboranyl silsesquioxane polyhedron.
CCSi Si
OMe
Me
Me
Me
Si SiO
Me
Me
Me
Me
Si
Si Si
Si
Si
Si
Si
Si
O
O O
O
O
O
O
O
O
OO
O
OO
O
O
OO
O
SiHHSi
SiH
SiH
HSi
HSi HSi
O
Me
Me
Me
Me
Me
Me
Me MeMe
Me
Me
Me
Me
Me+
[Pt]
HSi
Me
Me
Scheme 7. Synthesis of polymers with network structures based on carboranes and polyhedral silsesquioxanes.
4 HSi(OEt)3
[Pt] CC
SiCl3
SiCl3
n
CC
CC SiSi
O
O
O
Scheme 8. Synthesis of carborane–silsesquioxane network three-dimensional structures by the sol-gel technique.
González-Campo et al. [75] suggested the method for
synthesis of different siloxane and silsesquioxane derivatives of
carboranes. The products obtained were used to prepare
polyanionic compounds. Of particular interest is carborane-
containing POSS (Scheme 6).
The synthesis was carried out using chloro- or alkoxy-
substituted silyl derivatives of carborane, which were obtained
by hydrosilation of allyl carborane under action of the
corresponding silane. Subsequently, these derivatives were
subjected to hydrolysis and condensation. This resulted in the
corresponding polyhedral organosilsesquioxanes in low to
moderate yields (20–50%).
Astruc et al. considered the creation of new organic-
inorganic materials based on three-dimensional network
structures obtained from multifunctional blocks [76].
Compounds with these structures were synthesized by Kolel-
Veetil et al. [77], who used bis(vinyltetramethyldisiloxy)-m-
carborane and POSS with eight dimethylsiloxy groups as
building blocks (Scheme 7). A network structure of the polymer
was formed as a result of hydrosilation in the presence of the
Karstedt catalyst. This afforded the material that can be used as
a coating in electronic and optical devices and gas-separation
membranes.
One more method for production of carborane-containing
silsesquioxanes is the sol-gel technique. González-Campo et al. suggested [78] an approach based on the production of bis-
trichlorosilyl derivative of o-carborane, which undergoes
hydrolysis followed by condensation according to Scheme 8.
According to the authors, these xerogels are of particular
practical importance, and the suggested approach seems to be a
promising route to creation of new generation materials.
Continuing investigations in this field, Spanish and French
researchers developed another method for the synthesis of
xerogels based on carboranes [79]. They synthesized alkoxysilyl
derivatives of carboranes instead of the chlorosilyl one. The
former were also obtained by hydrosilation (Scheme 9). Then
the products obtained were subjected to hydrolytic condensation
according to Scheme 10.
CC
CC
Si(OEt)3
Si(OEt)3
CC
R
Si CC
R
Si
Si(OEt)3
Si(OEt)3
Si(OEt)3
4 HSi(OEt)3[Pt]
6 HSi(OEt)3
[Pt]
Scheme 9. Synthesis of polyalkoxysilyl carborane derivatives.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 75
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
An advantage of this method relative to the chlorine one
consists in the fact that the reaction results in the release of an
alcohol instead of HCl. It appeared to be a step towards
ecologically friendly method for the synthesis of xerogels.
The same authors suggested also the method for preparation
of nanoporous xerogels. Its basic idea is that the carborane is
extracted from the polymer structure during the sol-gel process
under action of a catalyst (NaOH, TBAF), providing nanopores
(Scheme 11).
5. New boron-substituted carborane–siloxanes of various architectures
The appearance of new boron-substituted carborane–siloxane monomers offers great opportunities for the design of
carborane-containing systems. Earlier, carboranes were used as
parts of chemically bound molecular systems. The development
of a library of carborane-containing polymers and molecular
fillers of analogous chemical nature allows one to employ the
processes of self-assembling of different carborane-containing
systems for construction of new materials in order to obtain and
control the properties of composite materials on their base.
The data on boron-substituted carborane–siloxanes have not
been presented in the literature until recently. Boron-substituted
polyhedral carboranes are of particular interest owing to the
possibility of further modification of a carborane core by two
C–H groups which offers new synthetic routes for the use of
already known structures.
For this purpose, we firstly synthesized carborane–siloxanes
based on boron-substituted allyl carboranes according to the
published procedure (Fig. 4) [80]. The presence of an allyl
functional group allowed us to exploit one of the most explored
CC
Si(OEt)3
Si(OEt)3n
1. 3n H2O
2. Cat
THF CC
SiO1.5
SiO1.5
n
Scheme 10. Synthesis of carborane-containing xerogels.
reaction in organosilicon chemistry, namely, hydrosilation and
provided great opportunities for the high-yielding syntheses of
new carborane–siloxanes with predetermined structures, which
can be used as precursors for the synthesis of new thermally
stable rubbers and coatings.
Two monomers depicted in Fig. 4 were used for the
synthesis of a series of carborane-containing
polydimethylsiloxanes differing in the positions of carborane
polyhedra in the macromolecule structures. The effect of this
modification of PDMS on the properties of the resulting
products was estimated.
The synthesis of polydimethylsiloxanes bearing terminal
carboranyl groups was also described [81]. Their properties
were studied, and the effect of bulky substituents on the
physicochemical properties of PDMS was evaluated.
At the first step, carboranyl derivatives 1 and 2 were
prepared (Scheme 12). 9-γ-Chlorodimethylsilylpropyl-m-
carborane 1 was obtained by hydrosilation of 9-allyl-m-
carborane with dimethylchlorosilane in the presence of the
Karstedt catalyst. The hydrolytic condensation of 1 resulted in
1,3-bis(9-propyl-m-carboranyl)tetramethyldisiloxane 2 in 71%
yield.
Disiloxane 2 was involved in cationic polymerization of
octamethylcyclotetrasiloxane (D4) catalyzed by
trifluoromethanesulfonic acid (Scheme 13). This afforded
polymer 3 bearing terminal carboranyl substituents in 75% yield
(MM = 7000).
In order to compare the effect of different bulky terminal
groups on the properties of PDMS, one more type of modifying
agents was synthesized. Thus, α-tris(4-trimethylsilylphenyl)-β-
(dimethylchloro)-disilylethane 6 was prepared according to
Scheme 14 [82].
CH
HC
CH
HC
9-allyl-m-carborane 9,12-diallyl-o-carborane
Figure 4. Structures of allyl-functionalized carboranes.
CC
R
Si
Si(OEt)3
Si(OEt)3
Si(OEt)3
1. 4.5n H2O2. Cat
THFSi
OSi
SiO1.5
SiO1.5O1.5Si
O1.5Si
O1.5Si SiO1.5
n/2
CC
R
H
n+
Scheme 11. Production of nanoporous xerogels.
HC
HC
Si
MeCl
HC
HC
HC
HC
H2OMe2SiClH
~100% 2 71%1
Me Si
O
2Me
Me
Scheme 12. Synthesis of chlorosilyl derivative 1 and disiloxane 2.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 76
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
HC
HC
Si
Me
Me
OSi
Me
Me
CH
HC
CF3SO3H
D4
3 75%
HC
HC
Si
Me
Me
OSi
Me
Me
CH
HC
n
Scheme 13. Synthesis of polymer 3.
Br
Br
1. n-BuLi
2. Me3SiCl
SiMe3
Br
Me3Si Si
1. n-BuLi
2. VinSiCl3
3
Me2SiHCl
[Pt]Me3Si Si
3
SiMe2Cl
6 96%5 75%
4 97%
Scheme 14. Synthesis of phenylene modifiers: tris(4-trimethylsilylphenyl)vinylsilane 5 and α-tris(4-trimethylsilylphenyl)-β-(dimethylchloro)-
disilylethane 6.
SiO
SiOLi
MeMe
LiO
MeMe
D3 SiO
SiOLi
Me
MeLiO
Me
Me
n
SiO
Si
Me
Me
Me
MeSi Si
n
SiMe3Me3Si
3 3
7 85%
SiMe3Si
3
SiMe2Cl
- LiCl
Scheme 15. Synthesis of polymer 7 bearing tris(4-trimethylsilylphenyl)silyl terminal groups.
The molecular structures of compounds 5 and 6 were
elucidated by single-crystal XRD (Fig. 5).
Compounds 5 and 6 were used to synthesize PDMS bearing
tris(4-trimethylsilylphenyl)silyl terminal groups.
Anionic polymerization of hexamethylcyclotrisiloxane (D3)
in the presence of compound 6 afforded PDMS with terminal
tris(4-trimethylsilylphenyl)silyl groups in 85% yield (compound
7 (ММ = 6900), Scheme 15). An initiator (lithium
tetramethyldisiloxanolate) was obtained by the reaction of
tetramethyldisiloxane diol with n-BuLi. It should be noted that
this synthetic approach affords polymers with narrow MMD.
To assess the impact of the length of a siloxane chain on the
properties of PDMS having bulky terminal groups, the polymers
with high molecular masses were obtained by heterofunctional
condensation of SKTN-A with compounds 1 and 6 according to
Scheme 16.
Figure 5. Molecular structures of compounds 5 (on the left) and 6 (on
the right).
OHO
TMS
TMS
TMS SiSi
OStSi
O
n
Hn - Py . HCl
St-Cl
St =HC
HC
8 992% 86%
Si
Me
Me
Me
Me
StSi
Me
Me
Me
Me
Scheme 16. Synthesis of PDMS with tris(4-trimethylsilylphenyl)silyl
(8) and carboranyl (9) terminal groups.
Polymers 8 (ММ = 34000) and 9 (MM = 35000) were
obtained in 92% and 86% yields, respectively. The molecular-
mass distribution curves for all the polymers derived are
presented in Fig. 6.
The thermal and rheological behaviors of polymers 3, 7, 8,
and 9 were studied by DSC and rheometry.
DSC studies showed that carboranyl and tris(4-
trimethylsilylphenyl)silyl terminal groups affect the
thermophysical properties of PDMS in different ways (Fig. 7).
Figure 6. Gel-permeation chromatograms of polymers 3, 7, 8, and 9.
-100 -50 0
en
do
He
at
Flo
w0
,4 W
/g
T,oC
exo
1
2
3
Figure 7. DSC curves for PDMS (1) and polymers 3 (2) and 7 (3);
heating rate 10 °С/min.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 77
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
Curve 1 obtained for an unmodified sample of low-
molecular PDMS revealed all the thermal transitions
characteristic of this type of polymers: glass-transition of PDMS
at 23 °C, cold crystallization at –82 °C, and melting at –45 °C.
According to the DSC data (curves 2 and 3), the bulky terminal
substituents in polymers 3 and 7 suppress crystallization of
modified PDMS. The glass-transition point of polymer 3 was
higher than that of PDMS and composed –117 °C (curve 2),
whereas for polymer 7 it remained almost unchanged. In the
case of polymer 3, it can be concluded that the siloxane chain
and the carboranyl terminal groups facilitate the formation of a
single amorphous phase, which does not crystallize; compared
to PDMS, a glass-transition point increases insignificantly. In
the case of polymer 7, tris(4-trimethylsilylphenyl)silyl terminal
groups form a separate crystalline phase. But, compared to the
melting point of neat α-tris(4-trimethylsilylphenyl)-β-
(dimethylchloro)-disilylethane (158 °C), this crystalline phase
melts at a much lower temperature (–10 °С), since the steric
factor hampers ordering of siloxane chains. These data show
that carborane units have higher affinity to a siloxane bond than
tris(4-trimethylsilylphenyl)silyl ones.
The terminal groups in polymers 8 and 9 do not affect
significantly the thermophysical properties of PDMS with
increasing molecular mass of a siloxane fragment (Fig. 8).
In order to estimate the intermolecular interactions in the
resulting polymers (3, 7, 8, and 9), the rheological investigations
were carried out. The corresponding flow curves are depicted in
Fig. 9.
The flow curves obtained indicate that the viscosities of the
new polymers do not depend on the shear rate (Ý); they behave
as Newtonian fluids. Polymer 7 bearing tris(4-
trimethylsilylphenyl)silyl terminal groups demonstrated higher
viscosity values than polymer 3 having carboranyl terminal
groups (curves а and b, respectively). Polymers 8 and 9 with the
higher molecular masses exhibited higher viscosity values
(curves c and d, respectively; Table 1) than their counterparts
with the lower molecular masses.
In general, the introduction of bulky terminal groups into a
polydimethylsiloxane chain increases the chain segment and the
activation energy of viscous flow. Polydimethylsiloxanes 3 and
9 having carboranyl terminal groups feature lower viscosity
values than polymers 7 and 8 bearing tris(4-
trimethylsilylphenyl)silyl terminal groups.
Another structural modification of carborane–siloxanes
obtained in our group based on the new monomers (Fig. 4)
appeared to be poly(carborane–siloxanes) with different
positions and content of carborane polyhedra in the
macromolecule structures.
Hydrosilation of carborane-containing telechelics with 9,12-
diaallyl-o-carborane according to Scheme 17 resulted in
carborane–siloxanes bearing polyhedra in the main chains.
Table 1. Activation energies of viscous flow for polydimethylsiloxanes
3, 7, 8, and 9
Polymer 3 7 8 9
Ea, kJ/mol 15 27 18 18
-100 -50 0
en
do
He
at
Flo
w0
,4 W
/g
T,oC
exo
1
2
3
Figure 8. DSC curves for starting PDMS (MМ = 33000) (1) and
polymers 8 (2) and 9 (3); heating rate 10 °С/min.
Figure 9. Flow curves of polymers 3 (a), 7 (b), 8 (c), and 9 (d) at room
temperature.
Si SiO
Me
Me
Me
Me
HH
m
CHHC
+[Pt]
CHHC
Si SiO
Me
Me
Me
Mem
CHHC
n
Scheme 17. Synthesis of poly(carborane–siloxanes) 10 (m = 15, n = 10),
11 (m = 35, n = 11), 12 (m = 55, n = 11) and 13 (m = 126, n = 5) bearing
carborane moieties in the main chains.
Figure 10. Gel-permeation chromatograms of polymers 10, 11, 12, and
13.
A series of polymers with different lengths of the siloxane
blocks were obtained in 70–90% yields. Gel-permeation
chromatograms of the resulting polymers 10–13 are
demonstrated in Fig. 10.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 78
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
Hydrosilation allows one to control the microstructure of the
resulting polymer, which was confirmed by the NMR
spectroscopic data. Thus the effect of a modifying unit on the
properties of PDMS can be estimated.
DSC studies established that the introduction of a carborane
polyhedron into a siloxane chain to a certain degree of
polymerization suppresses crystallization of the resulting
polymers and increases the glass-transition point compared to
PDMS. The data obtained indicate that the introduction of bulky
carborane moieties into a siloxane chain affects the mobility of
the main chain and hampers macromolecule packing. For
polymer 13 bearing 126 Si–O units in the siloxanes block, the
crystallization was detected (Table 2, Fig. 11).
Investigation of polymers 10–13 by TGA showed that an
increase of the content of carborane units in a chain facilitates an
increase of the residue content and a decrease in the polymer
decomposition onset temperature (Fig. 12).
The activation energies of viscous flow for these polymers
were determined (Table 3). The data obtained do not differ
significantly from those for neat PDMS.
In order to establish whether the physical characteristics of
modified PDMS depend on the positions of polyhedra, we
obtained carborane–siloxanes bearing carborane units as side
substituents at the silicon atoms. The synthesis was carried out
in two steps. At the first step, cationic polymerization was used
to obtain polymers with hydride functions in the chain (Scheme
18). At the second step, hydrosilation of the resulting hydride-
containing polymers with 9-allyl-m-carborane afforded a series
of polymers with variable length of the siloxane chains, which
differed in the content of carborane moieties (Scheme 18).
The molecular-mass characteristics of polymers 14 and 15
are presented in Table 4. Their thermal behaviors were studied
by means of DSC, TGA, and DTA.
The DSC curves (Fig. 13) demonstrated that the introduction
of carboranyl moieties into the structure of PDMS as side
substituents in the polydimethylsiloxane chain suppresses the
crystallization of the resulting products. An increase in the
content of carborane fragments in the structure raises the glass-
transition point. In polymers 14, 15, side carborane substituents
hamper packing of the siloxane chain and, as well as in the case
of polymers 10–13 bearing carborane polyhedra in the main
Table 2. Characteristics and thermal data for compounds 10–13 and
SKTN-A
Polymer m m Tg Tcr
10 15 10 –109
11 35 11 –115
12 55 11 –119
13 126 5 –122 –34
SKTN-A 480 - –125 –44
Table 3. Activation energies of viscous flow for polymers 10–13
Polymer 10 11 12 13 PDMS
Ea, kJ/mol 17.8 19 14.8 14.5 15
Table 4. Molecular-mass characteristics of polymers 14 and 15
Polymer m n Mn Mw/Mn
14 12 86 4396 3.34
15 10 120 12797 2.47
chain, reduce the chain mobility. According to the TGA data,
the thermal destruction of polymer 14 starts earlier than that of
polymer 15 (see Fig. 14).
-150 -100 -50
4
3
exoH
ea
t F
low
T,oC
0,2
W/g
en
do
1
2
Figure 11. DSC curves of polymers 10 (3), 11 (1), 12 (2) and 13 (4);
heating rate 10 оС/min.
200 400 600
20
40
60
80
100
10
11
12
13
Re
sid
ue
co
nte
nt, w
t %
T,oC
Figure 12. TGA curves of polymers 10, 11, 12 and 13; heating rate
5 °С/min.
O OSi
Me
Me
OSi
Me
Hn m C
HC
+[Pt]
Me3Si SiMe3
O OSi
Me
Me
OSi
Me
n mMe3Si SiMe3
CH
CH
Scheme 18. Synthesis of poly(carborane–siloxanes) 14 (m = 12, n = 96)
and 15 (m = 10, n = 240) bearing carborane moieties in the side chains.
-150 -125 -100 -75 -50
exoHe
at
Flo
w
T,oC
0,1
W/g
en
do
1
2
Figure 13. DSC curves of polymers 14 (2) and 15 (1); heating rate
10 °С/min.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 79
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
200 400 6000
20
40
60
80
100
Re
sid
ue
co
nte
nt,
wt
%
T,oC
exo
1
1'
2
2'
Figure 14. TGA and DTA curves for compounds 14 (1 and 1') and 15 (2
and 2'); heating rate 5 °С/min.
HC
HC
HC
HC
HSiCl3
100 %
[Pt]
16
SiCl3
Scheme 19. Synthesis of trichlorosilyl carborane derivative 16.
Si
Si
Si
Si
Si
Si
Si
Si
O
OO
OO
O
O
O
O
O
O
O
HC
CH
CH
CH
HC
CH
HC
HC
CH
HC
CH
CH
CH
CH
HC
CH
HC
HC
SiCl3
EtOH, H2O
38 %17
Scheme 20. Synthesis of octasilsesquioxane cubane 17 bearing closo-
carboranyl substituents.
Figure 15. Molecular structure (on the left) and crystal packing (on the
right) of compound 17.
Figure 16. 2D NMR spectra of compound 17.
Table 5. Activation energies of viscous flow for polymers 14 and 15
Polymer 14 15
Ea, kJ/mol 20 17.3
As can be seen from the data presented, the introduction of
carborane polyhedra into the structure of PDMS affects the
thermophysical properties of the resulting polymers. An increase
in the content of the carborane units leads to a growth of the
glass-transition point.
The activation energies of viscous flow for the resulting
polymers were determined (Table 5).
In the case of the shorter siloxane chain and the higher
content of carborane units (polymer 14), there is observed the
stronger intermolecular interaction than for the polymer with the
longer siloxane chain and the lower carborane content
(compound 15), which is reflected in the value of activation
energy of viscous flow. Although, in general, these values differ
insignificantly from the corresponding parameters of PDMS
with the same molecular mass.
As it was already discussed, the number of publications
devoted to the synthesis of carborane-containing silsesquioxanes
is rather limited, and there are almost no reports on boron-
substituted carborane–silsesquioxanes.
Recently, the synthesis of a boron-substituted polyhedral
carborane–silsesquioxane was accomplished [83]. At the first
step, the hydrosilation of 9-allyl-m-carborane with
trichlorosilane in the presence of the Karstedt catalyst yielded
quantitatively 9-γ-trichlorosilylpropyl-m-carborane 16 (Scheme
19). Hydrolytic condensation of the latter afforded compound 17
in 38% yield (Scheme 20). A cubic structure of this compound
was confirmed using different physicochemical methods. Thus,
XRD analysis of a single crystal afforded the molecular and
crystalline structures of the new compound (Fig. 15).
The structural features of compound 17 were also studied by
NMR spectroscopy using a full set of modern 2D-experimental
techniques. Some of the spectra and correlations are depicted in
Fig. 16.
DSC studies revealed that compound 17 crystallizes at
266 °С. This is evidenced by the presence of a reversible
endothermic peak in the DSC curves (ΔН = 40 J/g), which
corresponds to the melting of a crystalline phase (see Fig. 17,
curve 1).
Figure 17. DSC curves (1 – first heating, 2 – cooling, 3 – second
heating) for compound 17; heating rate 10 °С/min.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 80
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
According to the TGA data, the decomposition of compound
17 in an argon atmosphere occurs in a single step with the
decomposition onset temperature at around 400 °С. The mass of
a solid residue becomes constant at the temperatures above
600 °С (52% from the mass of the initial sample). In air,
compound 17 is less stable. In the temperature range of 250–480
°С there is observed 5% mass loss. Further temperature rise
leads to an essential reduction in the sample mass. The most
intensive process is observed in the temperature range of 480–530 °С; subsequently, it significantly decelerates, but continues
up to 950 °С. At this point, the sample mass exceeds that of the
initial one by 29% (Fig. 18).
In continuation of these studies, we synthesized a carborane
cubane featuring siloxane branching points. Its synthesis was
carried out according to Scheme 21.
Hydrolysis of tetraethoxysilane resulted in octaammonium
derivative which was reacted in situ with dimethylchlorosilane
(Scheme 21) [84]. This resulted in white powder product 18 in
60% yield. Then, the resulting cubane 18 was introduced into
hydrosilation under action of 9-allyl-m-carborane (Scheme 22).
DSC studies showed that compound 19 is crystalline (Fig.
19). Its melting point determined by DSC composes 190 °C (ΔН
= 32 J/g).
The resulting carborane-containing POSS 18 and 19 can be
used as nanofillers for production of thermally stable composite
materials and as multifunctional cores for creation of new three-
dimensional structures with predetermined architectures.
The synthesis of unique stereoregular functional
cyclosilsesquioxanes was described [85]. These compounds can
find application as matrices for production of organic-inorganic
supramolecular systems and materials. In particular, along with
the cubic carborane-containing structures, cyclosiloxanes can be
considered as promising molecular fillers. The synthesis of these
compounds was carried out using individual
organometallosiloxanes [86–99]; investigations on their
synthesis and properties are actively developed at the
Laboratory of Organosilicon Compounds, INEOS RAS.
These compounds are of both theoretical and practical
importance. Owing to the presence of metal atoms, they can be
used as catalysts, molecular magnets, ceramic precursors, and
macrocyclic compounds. The molecules of these compounds
contain one or two stereoregular organosiloxanolate cyclic
ligands bound to the matrix with three to ten metal ions (Fig.
20).
Figure 18. TGA curves for compound 17 (1 – in argon, 2 – in air);
heating rate 10 °С/min.
Si
Si
Si
Si
Si
Si
Si
Si
O
OO
OO
O
OO
O
O
O
O
(Me)2HSi OSiH(Me)2
O
O
O
(Me)2HSi
O
SiH(Me)2
O
SiH(Me)2O
(Me)2HSi
O(Me)2HSi
SiH(Me)2
1. Me4NOH, H2O2. Me2SiClH
60 %18
MeOHSi(OEt)4
Scheme 21. Synthesis of octakis(dimethylsiloxy)octasilsesquioxane
cubane 18.
Si
Si
Si
Si
SiSi
Si
Si
O
OO
OO
O
OO
O
O
O
O
HSi OHSi
O
O
O
SiH
O
SiH
O
HSiO
HSiOSiH
SiH
Si
Si
Si
Si
SiSi
Si
Si
O
OO
OO
O
OO
O
O
O
O
Si OSi
O
O
O
Si
O
Si
O
SiO
SiO
Si
Si
CH
CH
CH
CH
CH
CH
HC CH
HC
HC
HC
HC
CH
HC
CH
HC
[Pt]
90 %19
Si(OEt)4
1. Me4NOH, H2O, MeOH
2. Me2SiClH
60 %18
HC
HC
Scheme 22. Synthesis of octasilsesquioxane cubane 19 bearing closo-
carboranyl substituents with siloxane branching points (at the silicon
atom).
125 150 175 200
3
endo
0,5
W/g
T,oC
Hea
t F
low
exo
1
2
Figure 19. DSC curves for compound 19 (1 – first heating, 2 – cooling,
and 3 – second heating); heating rate 10 °С/min.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 81
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
Figure 20. Structures of cyclic siloxanolate ligands and spatial
arrangement of the matrix from metal ions in metal–siloxane
frameworks.
Figure 21. General scheme for synthesis of functional stereoregular
organosilsesquioxane macrocycles.
Scheme 23. Synthesis of carborane-containing stereoregular
organosilsesquioxane macrocycles 25–29.
Treatment of these organometallosiloxanes with
trimethylchlorosilane resulted in nonfunctional stereoregular
organocyclosilsesquioxanes with different sizes of the siloxane
rings bearing trimethylsiloxy substituents at the silicon atoms
[100–106]. However, the most promising direction seems to be
the production of silsesquioxane macrocylces of various
structures with functional groups at the silicon atom (Fig. 21)
[85].
The yields of these multifunctional macrocyclic
silsesquioxanes reach 85%, which cannot be achieved by the
conventional methods of siloxane chemistry. Table 6 lists the
Table 6. Yields and thermal characteristics of compounds 20–24
Compound Formula MM Yield Tg
20 [PhSi(O)OSiHMe2]4 785 76 -
21 [MeSi(O)OSiHMe2]4 537 87 –139
22 [PhSi(O)OSiHMe2]6 1178 76 –80
23 [PhSi(O)OSiHMe2]12 2356 71 –73
24 [MeSi(O)OSiHMe2]12 1611 65 –139
characteristics of hydride-containing organosilsesquioxanes 20–24, which structures were further functionalized with carborane
substituents. Their thermal properties were studied by means of
DSC and TGA [85].
Hydrosilation of compounds 20–24 with 9-allyl-m-
carborane was carried out according to Scheme 23. The reaction
depicted in this Scheme allows one to selectively introduce
carborane units into the macrocycle structures.
The reaction products were purified by preparative
chromatography which afforded new carborane-containing
silsesquioxanes 25–29 in good yields (60–70%).
Carborane-containing rings were fully characterized by
different physicochemical methods. They represent viscous
transparent liquids (in the case of compounds 25–27 and 29) or a
white powder (in the case of compounds 28).
6. Conclusions
This review highlighted the main trends in the development
of various carborane–siloxane structures. Both individual and
polymer carborane-containing siloxane derivatives were
presented. The main synthetic approaches to the construction of
these compounds were demonstrated. The potential of these
derivatives for application in different fields of science and
engineering was shown. Despite successful commercial use of
carborane–siloxanes, there are some challenges that require
more detailed investigation and development of new synthetic
routes.
The use of boron-substituted carboranes offers ample
opportunities for further functionalization of carborane–siloxanes by the C–H bond, which will afford new
supramolecular systems and new generation materials. The
presence of allyl groups at the terminal carborane units provides
a possibility to use the resulting polymers as precursors in the
synthesis of new thermally stable rubbers and coatings.
There were presented the examples of syntheses of
carborane-containing compounds of various architectures, some
of which are of particular interest as potential polymer matrices
or binders, the others—as nanosized fillers. In this respect, the
problems of compatibility and phase segregation in
compositions involving these carborane units, embedded in
different structural forms, gain growing importance and require
further studies.
Of particular attention are the investigations dealing with
chemical transformations of a carborane core both from the
viewpoint of utilization of the active C–H centers and from the
viewpoint of controlled oxidation of the boron atoms. We
suppose that the developed library of carborane–siloxane
products will extend with the creation of composite materials on
their base and with the development of concepts on assembling
of carborane units in these compositions.
INEOS OPEN – Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences 82
A. A. Anisimov et. al., INEOS OPEN, 2018, 1 (2), 71–84
Acknowledgements
This work was supported by the Russian Foundation for
Basic Research, project no. 16-03-00984.
Corresponding author
* E-mail: aziz@ineos.ac.ru. Tel: +7(499)135-9349 (A. M.
Muzafarov)
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