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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



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



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.



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.



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.



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.



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.



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.



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.



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.



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.



Acknowledgements

This work was supported by the Russian Foundation for

Basic Research, project no. 16-03-00984.

Corresponding author

* E-mail: [email protected]. Tel: +7(499)135-9349 (A. M.

Muzafarov)

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