Synthesis of diethoxy(phenyl)silane and its polycondensation in acetic acid

Article abstract of DOI:10.1007/s11172-013-0096-z

Low-temperature reaction between phenylmagnesium chloride and triethoxysilane at lowered affords diethoxy(phenyl)silane, whose polycondensation in acetic acid gives oligomeric (phenyl)hydrosiloxanes.

Full text of DOI:10.1007/s11172-013-0096-z

Russian Chemical Bulletin, International Edition, Vol. 62, No. 3, pp. 705—709, March, 2013
705
Synthesis of diethoxy(phenyl)silane and its polycondensation in acetic acid*
S. A. Milenin, A. A. Kalinina, N. V. Demchenko, N. G. Vasilenko, and A. M. Muzafarov
N. S. Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences,
70 ul. Profsoyuznaya, 117393 Moscow, Russian Federation.
Fax: +7 (495) 335 9000. Eꢀmail: aziz@ispm.ru
Lowꢀtemperature reaction between phenylmagnesium chloride and triethoxysilane at
lowered affords diethoxy(phenyl)silane, whose polycondensation in acetic acid gives oligoꢀ
meric (phenyl)hydrosiloxanes.
Key words: silanes, hydrosilanes, hydrosiloxane oligomers, polycondensation.
One of the urgent modern tendencies in the synthesis
of organosilicon polymers is the use of organoalkoxysilanes
as the starting compounds instead of their chlorosilane
analogs. Accessibility of new types of organoalkoxysilanes
plays the determining role in the further development of
structural diversity of polymers based on these organoꢀ
alkoxysilanes. The latter are usually obtained by the alkꢀ
oxylation of the corresponding chlorosilanes, such as tetraꢀ
chloroꢀ and trichloromethylꢀ or trichlorophenylsilanes.1
Hydrideꢀcontaining alkoxysilanes form a special group,
and the methods for their synthesis, on the whole, are
insufficiently developed. (Alkoxy)(alkyl)hydrosilanes are
fairly accessible, since they can be synthesized by the esꢀ
terification of the corresponding commercial chlorosilanes.
Similar prospects for the production of phenylꢀcontaining
alkoxyhydrosilanes are less reliable, because the respective
chlorosilanes are formed as minor admixtures in the synꢀ
thesis of trichlorophenylsilane.2 Another way seems more
efficient: the synthesis of these compounds using the Grigꢀ
nard reaction from largeꢀtonnage triethoxysilane. At the
same time, its application in the Grignard reactions is
impeded by disproportionation processes to form silane,
which makes these reactions dangerously explosive.3 Unꢀ
fortunately, similar reactions are insufficiently studied;
in addition, literature data on the use of (alkoxy)(phenyl)ꢀ
hydrosilanes for synthesis of oligomers and polymers
are lacking.
position of siloxane hydrideꢀcontaining composites is
promising from the viewpoint of controlling the reactivity
of hydrosilyl fragments. These circumstances are espeꢀ
cially important for the synthesis of siloxane composites
with a high refractive index.4
Several methods for synthesis of (dialkoxy)(organyl)ꢀ
hydrosilanes are known to date5,6; however, none of them
has preparative significance. The purpose of this work is
the synthesis of hydrideꢀcontaining alkoxyphenylsilanes
and functional oligomers based on them. It seemed urgent
to develop a convenient method for the synthesis of
hydrideꢀfunctional alkoxyphenylsilanes by the Grignard
reaction from industrial triethoxysilane (Scheme 1).
Scheme 1
The study showed that the determining parameter, in
this case, is the reaction temperature, which depends, in
turn, on the rate of addition of the Grignard reagent to the
reaction mixture: a solution of phenylmagnesium chloride
in THF should be added to pure triethoxysilane in such
a way that the temperature should not exceed –30 C.
When the temperature is decreased to –60 C, the target
reaction proceeds smoothly and is not accompanied by
side rearrangement that afford dangerous silane. As the
temperature increases to ambient, tetraethoxysilane, which
could be formed by triethoxysilane disproportionation, was
observed in the reaction mixture in the presence of ethoxyꢀ
magnesium chloride (Scheme 2).
Meanwhile, oligomers and polymers with (phenyl)ꢀ
hydrosilane units seem to be rather promising for a whole
series of reasons. Among the most substantial ones are the
problem of compatibility of organosilicon polymers with
organic polymers in various composites and compositions.
In this case, the use of phenylꢀcontaining siloxanes has
explicitly more advantages over methylsiloxane analogs.
The introduction of (phenyl)hydrosilyl units into the comꢀ
Scheme 2
* Dedicated to the Academician of the Russian Academy of
Sciences I. P. Beletskaya on the occasion of her anniversary.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 0705—0709, March, 2013.
1066ꢀ5285/13/6203ꢀ705 © 2013 Springer Science+Business Media, Inc.

706
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Milenin et al.
Undesirable reactions were inhibited after the process
ceased by immediate dilution with hexane and filtration
off of the solid residue. The filtrate containing the target
product was stable, and no evolution of gaseous products
from side reactions was observed. Thus, a mixture containꢀ
ing 35—40% PhSiH(OEt)2 and 45—50% Ph2SiH(OEt)
was obtained. Individual compounds were isolated by recꢀ
tification in vacuo.
2
1
The synthesized phenylꢀsubstituted hydrosilanes are
promising for the synthesis of earlier inaccessible polyꢀ
mers. First, a molecule of PhSiH(OEt)2 can be considered
as bifunctional monomer in the condensation of alkoxy
groups. Based on this monomer, one can obtain oligomers
with the latent functional Si—H groups of both linear and
cyclic structure and branched oligomer in a mixture with
other alkoxysilanes. Second, a Ph2SiH(OEt) molecule is
a compound monofunctional with respect to the alkoxy
group and can be used as blocking agent containing the
screened hydrosilyl group.
We have previously shown7,8 that the condensation of
bifunctional alkoxysilanes in anhydrous acetic acid, which
acts as both solvent and reagent, is a promising method for
the selective synthesis of linear or cyclic siloxane oligoꢀ
mers and polymers. Therefore, it is promising to study the
condensation of the obtained compounds by the indicated
method. In this case, the main problem is a fairly high
lability of the hydrosilyl group, which can result, under
the proposed conditions, in undesirable secondary reacꢀ
tions of this bond cleavage and, hence, in the appearance
of defect branching units.
5
10 /min
Fig. 1. GPC curves for oligo(phenyl)hydrosiloxane obtained in
entry 1 (see Table 1) before (1) and after (2) blocking the termiꢀ
nal groups by trimethylchlorosilane.
characteristic of OH groups, the GPC curves of the polyꢀ
condensation product before and after blocking almost
coincided (Fig. 1). The quantitative determination of hydrꢀ
oxysilyl groups in the synthesized compounds is possible
from the ratio of intensities of signals from protons of the
1
methyl and phenyl groups in the H NMR spectra of the
blocked samples.
An analysis of the product of condensation of diethoxyꢀ
(phenyl)silane in acetic acid with reflux (see Table 1, enꢀ
try 1) by the procedures described above showed that the
synthesized oligosiloxane is a mixture of oligomers with
the monomodal distribution (from 40 000 to 500 amu)
(see Fig. 1) with the maximum intensity at 1000 amu. The
content of hydroxy groups in the synthesized product was
~0.5 wt.%.
A significant decrease in the content of hydrosilyl
groups was observed upon condensation. According to the
1Н NMR spectroscopic data, the conversion of hydrosilyl
groups due to side reactions was ~30%. An additional inꢀ
troduction of anhydrous ethanol into the reaction mixture
on reflux of diethoxy(phenyl)silane in acetic acid (see
Table 1, entry 2) resulted in an increase in the conversion
of hydrosilyl groups to 50%. The obtained oligo(phenyl)ꢀ
hydrosiloxane was characterized by a broader molecular
weight distribution (from 75 000 to 500 amu) (Fig. 2) and
contained the same 0.5 wt.% of OH groups. It was shown
previously7 that the introduction of alcohol into the reaction
mixture on reflux is an efficient method for enhancing
The course of condensation was monitored by a change
in the intensity of signals from protons of the ethoxysilyl
1
groups in the Н NMR spectrum of samples of the reacꢀ
tion mixture. After cessation of the process, i.e., after the
complete conversion of the Si—OEt groups, the reaction
1
product was analyzed by GPC, Н NMR spectroscopy,
and IR spectroscopy.
According to the IR spectroscopic data, the oligomerꢀ
ic products of alkoxysilane condensation in an active meꢀ
dium always contains residual hydroxysilyl groups. To
evaluate their amount and to obtain the stable product,
the synthesized oligomers were blocked with trimethylꢀ
chlorosilane (Scheme 3) under the conditions that, as
shown previously,8 do not distort the structure of the reacꢀ
tion products.
Scheme 3
5
10
/min
In fact, after the complete conversion of hydroxysilyl
groups, which was confirmed using IR spectroscopy by
the absence of absorption in the region of 3100—3600 cm–1
Fig. 2. GPC curves for oligo(phenyl)hydrosiloxane obtained in
entry 2 (see Table 1).

Diethoxy(phenyl)silane oligomers
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 3, March, 2013
707
Table 1. Characterization of the products of diethoxy(phenyl)silane condensation in an active medium
Entry
Conditions of condensation
Characteristics of product
Ta/C
Cb (%)
Nc (%)
nD
20d
Composition of reaction mixture
(molar ratio)
1
2
3
PhSiH(OEt)2—AcOH
(1 : 10)
PhSiH(OEt)2—AcOH—EtOH
(1 : 3 : 0.5)
PhSiH(OEt)2—AcOH—H2O
(1 : 10 : 5)
B.p.
B.p.
~20
30
50
10
0.50
0.51
0.14
1.5732
1.5670
1.5682
a Condensation temperature.
b Percentage of losses of silyl hydride moieties calculated from the 1Н NMR spectrum of the product.
с Number of terminal OH groups calculated from the 1Н NMR spectrum of the condensation product blocked
by trimethylchlorosilane.
d Refractive index of the obtained condensation product.
selectivity of condensation towards cyclic product formaꢀ
tion. Unfortunately, in our case, the introduction of alcohol
only increased the undesirable loss of silyl hydride moieties.
Since hydrosilyl groups of PhSiH(OEt)2 are unstable
on reflux in acetic acid, they cannot be considered as
a latent functional group. At the same time, the controlled
character of their conversion can be used for the synthesis
of branched polycyclic structures. A simple comparison of
the molecular weight of the obtained products, conversion
of hydrosilyl groups, and residual hydroxy groups suggests
that a branched polycyclic structure of the obtained polyꢀ
mer was formed (Scheme 4).
If we assume that this is the esterification stage which
requires to carry condensation at elevated temperatures,
the addition of water should allow the process to be carried
out in acetic acid at ambient temperature. An analysis by
1H NMR spectroscopy of the condensation product with
the ratio of the initial reactants PhSiH(OEt)2—AcOH—H2O
1 : 10 : 5 showed that the ratio of intensities of signals from
protons of the hydrosilyl and phenyl groups of the product
corresponded to the initial one, which indicates the stabilꢀ
ity of the hydride group at the silicon atom under these
conditions. The products obtained under these conditions
possessed, according to the GPC data, the bimodal
molecular weight distribution (Fig. 3). The lowꢀmolecuꢀ
larꢀweight fraction was ~60% of the weight of the whole
product and was characterized by a narrow dispersion
distribution with Мpeak = 600 amu and a minimum
(0.14%) amount of hydroxy groups. The GLC method
established that this fraction was a mixture of cisꢀ and
transꢀisomers of hydrideꢀcontaining phenylcyclotriꢀ and
tetrasiloxanes (see Table 1, entry 3). Summating the
obtained results, one can conclude the predominantꢀ
ly cyclic structure of the synthesized product and the
Scheme 4
It has earlier been shown8 that for the processes carried
out under the conditions of active medium, i.e., in an
excess of anhydrous acetic acid, the reaction follows the
mechanism of hydrolysis condensation. Its main distincꢀ
tive feature is that the all water necessary for the reaction
is formed in the course of esterification of acetic acid with
alcohol that is evolved by the acetoxylation of alkoxyꢀ
silane (Scheme 5).
Scheme 5
5
10
/min
Fig. 3. GPC curves for oligo(phenyl)hydrosiloxane obtained at
ambient temperature (see Table 1, entry 3).

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Milenin et al.
tained from 0.5 mole of (56.5 g) of chlorobenzene and 0.55 mole
(13.5 g) of magnesium in THF (400 mL) and then transferred
with a needle to a dropping funnel under argon, was added with
permanent stirring in an continuous argon flow for 4 h to 164 g
(1 mol) of triethoxysilane at –60 C. A white precipitate was formed
during the addition. Then the temperature was brought to amꢀ
bient, after which anhydrous hexane (300 mL) was immediateꢀ
ly added to the obtained reaction mixture. The precipitate
was filtered off and washed with hexane (3Ѕ100 mL). Hexꢀ
ane and a triethoxysilane excess were distilled off in vacuo
(247 Torr). Target PhSiH(OEt)2 was isolated by fractionation
in vacuo, the yield was 38 g (40%), colorless liquid, b.p.
97—98 С (12 Torr). 1H NMR (CDCl3), : 7.35—7.67 (m, 5 Н,
(C6H5)(H)Si); 4.93 (s, 1 H (H)Si); 3.84—3.93 (m, 4 Н,
(CH3CH2O)Si); 1.27 (t, 6 H, (CH3CH2O)Si)). The second prodꢀ
uct Ph2SiH(OEt) was isolated continuing distillation at 0.75 Torr,
the yield was 25 g (45%), colorless liquid, b.p. 67 С (0.75 Torr).
1H NMR (CDCl3), : 7.37—7.68 (m, 10 Н, (C6H5)(H)Si); 5.45
(s, 1 H (H)Si); 3.84—3.92 (m, 2 Н, (CH3CH2O)Si); 1.28 (t, 3 H,
(CH3CH2O)Si)).
absence of side reactions involving hydrosilyl groups
(Scheme 6).
Scheme 6
Thus, in the course of the performed study, we proꢀ
posed an efficient method for the synthesis of hydrideꢀ
containing ethoxyphenylsilanes and found the conditions
for the preparation from them branched polycyclic polyꢀ
mers formed due to the complete conversion of ethoxy
groups and partial conversion of hydrosilyl groups during
condensation in an active medium. It was found that the
hydrolytic condensation in acetic acid at ambient temperꢀ
ature can produce products of predominantly cyclic strucꢀ
ture. In this case, the complete conversion of ethoxysilyl
groups is achieved but, unlike the previous case, the hydroꢀ
silyl groups are not involved in the reaction.
Condensation of PhSiH(OEt)2 in acetic acid in a molar ratio
of 1 : 10. A mixture of PhSiH(OEt)2 (2.0 g, 0.01 mol) and anꢀ
hydrous AcOH (6.1 g, 0.10 mol) was refluxed with vigorous stirꢀ
ring until the signals from protons of the ethoxy groups comꢀ
pletely disappeared from the 1H NMR spectra of samples of the
reaction mixture. Then volatiles were removed at 20—25 C
(0.75 Torr). The yield of the product was 1.26 g (99%). 1H NMR
(CDCl3), : 4.75—5.25 (m, 1 H, H(C6H5)Si); 7.10—7.80 (m, 5 H,
H(C6H5)Si). IR (CCl4), /cm–1: 3300 (ОН) w. The amount of
terminal hydroxy groups in the obtained poly(phenyl)hydrosilane
calculated by the 1H NMR spectroscopic data of the samples
Experimental
20
blocked with trimethylchlorosilane was 0.50%. nD = 1.5732.
GPC: 40000—500 amu, Мpeak = 1000 amu. Found (%): Si, 22.90;
С, 57.06; H, 4.50. C6H5.67O1.17Si. Calculated (%): С, 57.88;
H, 4.56; Si, 22.50.
Tetrahydrofuran, hexane, acetic acid, and ethanol were puꢀ
rified according to earlier described procedures.9 Triethoxysiꢀ
lane with the content of the major component 98.5% was used
without additional purification.
1Н NMR spectra were recorded on a Bruker WPꢀ250 SY
spectrometer (250.13 MHz) using Me4Si as an internal standard
and CDCl3 as a solvent. A ChromatecꢀCrystal 5000 chromatoꢀ
graph (Russia) was used for GLC (catharometer served as a deꢀ
tector, helium as a carrier gas, columns 2 m Ѕ 3 mm, stationary
phase SEꢀ30 (5%) on ChromatonꢀHꢀAW).
A GPC analysis was carried on a chromatographic system
consisting of a Staier seriya 2 highꢀpressure pump (Akvilon, Rusꢀ
sia), a RIDK 102 refractometric detector (Czechia), and a Jetꢀ
stream 2 Plus thermostat of columns (Knauer, Germany). The
thermostatting temperature was 40 С ( 0.1 С); THF as an
eluent, flow rate 1.0 mL min–1; column 300Ѕ7.8 mm packed
with the sorbent Phenogel (Phenomenex, USA), particle size
5 m, pore size 103 Å (passport rage of separation was up to
75 000 D). The data were recorded and calculated using the
UniChrom 4.7 program (Belarus). The molecular weight was
estimated from the ratio to linear polystyrene standards.
IR spectra were recorded on an Equinox 55/S IR spectroꢀ
meter (Bruker). Liquid cells with KBr glasses were used for
measurements (solvent CCl4).
Condensation of PhSiH(OEt)2 in acetic acid with an addition
of ethanol in a molar ratio of 1 : 3 : 0.5. A mixture of PhSiH(OEt)2
(9.1 g, 0.047 mol), AcOH (8.5 g, 0.141 mol), and anhydrous
EtOH (1.1 g, 0.024 mol) was refluxed with vigorous stirring until
the signals from protons of the ethoxy groups completely disapꢀ
peared from the 1H NMR spectra of samples of the reaction
mixture. All volatiles were distilled off. The yield of the product
was 5.9 g (99%). 1H NMR (CDCl3), : 4.75—5.25 (m, 1 H,
H(C6H5)Si); 7.10—7.80 (m, 5 H, H(C6H5)Si). IR (CCl4),
/cm–1: 3300 (ОН) w. The amount of terminal hydroxy groups
of obtained poly(phenyl)hydrosiloxane calculated from the data
of 1H NMR spectroscopy of the samples blocked with trimethylꢀ
chlorosilane was 0.51%. nD20 = 1.5670. GPC: 75000—500 amu,
Мpeak = 1800 amu. Found (%): Si, 22.70; С, 56.8; H, 4.40.
C6H5.55O1.26Si. Calculated (%): Si, 22.27; С, 57.3; H, 4.41.
Condensation of PhSiH(OEt)2 in acetic acid with an addition
of water in a molar ratio of 1 : 10 : 5. A mixture of PhSiH(OEt)2
(1.36 g, 0.007 mol), AcOH (4.16 g, 0.069 mol), and water (0.62 g,
0.035 mol) was stirred at ~20 С to the complete disappearance
of signals from protons of the ethoxy groups in the 1H NMR
spectra of samples of the reaction mixture. Then the reaction
mixture was dissolved in toluene, washed off with distilled water
to neutral pH, dried over sodium sulfate, and evacuated on heatꢀ
ing to 50 С. The yield of the product was 0.86 g (97%). 1H NMR
(CDCl3), : 4.75—5.25 (m, 1 H, H(C6H5)Si); 7.10—7.80 (m, 5 H,
H(C6H5)Si). IR (CCl4), /cm–1: 3300 (ОН) w. The amount of
An IRFꢀ454B2M refractometer was used to measure the reꢀ
fractive index.
Diethoxy(phenyl)silane and ethoxy(diphenyl)silane. A soluꢀ
tion of phenylmagnesium chloride (0.5 mol), which was obꢀ

Diethoxy(phenyl)silane oligomers
Russ.Chem.Bull., Int.Ed., Vol. 62, No. 3, March, 2013
709
terminal hydroxy groups of obtained poly(phenyl)hydrosiloxane
calculated by the 1H NMR spectroscopic data for samples
blocked with trimethylchlorosilane was 0.14%. nD = 1.5682.
GPC: 40%: 75000—2000 amu, Мpeak = 18000 amu; 60%:
2000—400 amu, Мpeak = 600 amu. GLC of the lowꢀmolecularꢀ
weight fraction: 20% [HPhSiO]3, 80% [HPhSiO]4. Found (%):
Si, 22.80; С, 58.88; H, 4.92. C6H6.01O1.01Si. Calculated (%):
Si, 22.92; С, 58.93; H, 4.92.
Blocking of terminal hydroxy groups of polyphenylsiloxane
(general procedure). A 20% solution of poly(phenyl)hydroꢀ
siloxane (0.65 g, 0.005 mol) in anhydrous toluene was added
dropwise to a solution of trimethylchlorosilane (1.33 g, 0.012 mol)
and pyridine (1.97 g, 0.012 mol) in anhydrous toluene (10 mL),
and the mixture was refluxed with stirring for 4 h. The obtained
product was washed off with distilled water to neutral pH, dried
over sodium sulfate, and evacuated with heating to 50 С
(0.75 Torr). The yield of the products was 1.2—1.3 g (90—98%).
1H NMR (CDCl3), : 4.75—5.25 (m, 1 H, H(C6H5)Si);
7.10—7.80 (m, 5 H, H(C6H5)Si); 0.10—0.25 (m, 9 Н, (CH3)3Si).
IR (CCl4), /cm–1: signals from OH groups in a region of
3300 cm–1 are absent.
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This work was financially supported by the Council on
Grants at the President of the Russian Federation (Proꢀ
gram for State Support of Leading Scientific Schools of
the Russian Federation, Grant NSh 116.2012.3) and the
Russian Foundation for Basic Research (Project No. 11ꢀ03ꢀ
12095_ofi_m).
Received October 4, 2012;
in revised form February 26, 2013