Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR THE PREPARATION OF POLYSILANES
The invention relates to a process for the prepara-
tion of polysilanes.
Polysilanes hàve been known for a long tlme and
include different types of materials. Examples of known
polysilanes are linear perme~hylated polysilanes, cyclic
permethylated polysilanes, branched polysilanes and cage
permethyl polysilanes. Polysilanes with substituents
other than methyl, for example phenyl and isobutyl groups,
are also known, as are polysilanes having a mixture of
methyl and other substituents, for example hydrogen,
halogen or phenyl substituen~s. Also known are polysi-
lanes where only hydrogen atoms are found on the silicon
atoms. The size o the polysilane molecules can vary
widely from the disilane to polysilanes having a large
number of silicon atoms attached to each other. Linear
polysilanes have usually less than 10 silicon atoms in the
chain, whilst cyclic and polycyclic polysilanes often have
a larger number of silicon atoms.
Polysilanes can be prepared by several routes. One
of the earliest published methods was that described in
U.S. Patent Specification 2 380 995 in ~he name of Rochow,
in which disilanes were produced by contacting silicon
metal with an alkylhalide under specified conditions. The
most common route for the production o~ cyclopolysilanes
involves the reductive condensation of a dialkyldihalo-
silane with an alkali metal. This route has been
described in, for example, U.S. Patent 4,052,43p. If an alkyl-
trihalosilane is included in the reaction mixture as
described above co-condensation of these silanes can form
cage polysilanes under certain conditions. Another route
for making polysilanes starts from low molecular weight
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polysilane molecules which are reacted under anhydrous
conditions with a Grignard reagent, as described for
example in G.B. specification 2 081 290.
One of the more imp~rtant uses of polysilanes ls as
precursors for siiicon carbide. The polysilanes which
are most preferred in ~his application are those which
have a ratio of carbon atoms to silicon atoms which is as
close as possible to 1. Hence the preparation of cage
polysilanes and branched polysilanes for this purpose.
G.B. specification 2 081 290 describes polysilanes having
the average iormula [(CH3)2Si][CH3Si] in which polysilane
there are irom O to 60 mole percent (CH3)2Si= units and 40
to lOO mole percent CH3Si- units, wherein there is also
bonded to the silicon atom other silicon atoms and addi-
tional alkyl radicals of 1 to 4 carbon atoms or phenyl
radicals. These are prepared by reacting polysilanes
present in the direct process residue obtained during the
production of chlorosilanes, with an alkyl or aryl
Grignard reagent. However, the direct process residue is
not pure or well defined. Making polysilanes according to
the me~hod described in G.s. Specification 2 081 290 involves an extra
step of purifying the direct process re~idue.
Patent Speciiication G.B. 2 077 710 discloses and
claims a method for synthesising an unsubstituted poly-
silane having an approximate composition of -(SiHn)-x
where x is large and n is from 1 to 2 comprising reacting
SiHmX4_m, where X is fluorine, chlorine, bromine or iodine
and m ~ 1, 2 or 3, with lithium in a suspension of liquid
in~rt to the reagents and the product and in which the
polysllane is insoluble. When thi~ reaction was repeated
according to the example given ln Britlsh Specification No. 2 077 710, an
oxygenated silicone product was ob~ained rather than a
polysilane of the general formula -(SiH)X-.
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According to the present invention there is provided
a process for the preparation of polysilanes of the
general formula (RSi)n wherein each R is independently
selected from the group consisting of alkyl, aryl~ alkaryl
and aralkyl groups having ~rom 1 to 18 carbon atoms and _
is at least 8, which comprises the step of reacting at
least one silane o~ the general formula RSiX3 wherein R is
as defined above and X denotes a halogen atom with an
alkali metal in an organic liquid medium in which the
silane is soluble.
The invention provides in another aspect a polysi-
lane of the general formula (RSi)n wherein R and _ are as
defined above when prepared by the process described
above.
In the process of the invention at least one tri-
halosilane is reacted with an alkali metal in an organic
liquid medium. The trihalosilanes which can be used in
the process have the general formula RSiX3 wherein R is an
alkyl, aryl, alkaryl or aralkyl group having from 1 to 18
carbon atoms and X is a halogen atom, preferably Cl.
Examples of the group R are methyl, ethyl, isobutyl,
phenyl, tolyl and phenylethyl. These silanes are well
known in the art and a number of them are commercially
available. They may be made e.g. by direct synthesis
using silicon metal and methylchloride, by the Grignard
synthesis or by the addition of unsaturated alkenes or
aromatic compounds to silanes having a silicon-bonded
hydrogen atom. Such processes are well known and have
been described in e.g. Chemistry and Technology of
Silicones by W. Noll.
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Examples of the trihalosilanes which may be used in
the process of the invention are methyltrichlorosilane,
phenyl~richlorosilane, butyltrichlorosilane and dodecyl-
trichlorosilane. Preferably R is the same for each silane
used in the process.
The alkali metal which may be used in the process of
the invention can be e.g. Na, K and Li. Li is the pref-
erred metal as it gives the highest yield of polysilanes.
The amount of alkali metal used in the reaction is at
least three mole per moles of the silane utilised. In
order to ensur~ the completion of the reaction it is
preferred ~o add an amount slightly in excess of 3 moles of
the alkali metal per mole of the silane.
The organic liquid medium in which the reaction
takes place may be any solvent in w~ich the trihalosilane
reactant is soluble. Preferably the solvent used is one
in which the polysilane which is produced in the process
is also soluble. These solvents include hydrocarbon
801vent8 such ~8 ~oluelle or para~fins, e~her~ and nitrogen
containing solvents for example ethylenediamine, triethyl-
amine and N,N,N',N'-tetramethylethylenediamine. Prefer-
ably tetrahydrofuran is used as the organic liquid medium.
The organic liquid medium is not generally a solvent for
the alkali metal halides which are formed and these can be
easily removed by filtration. The amount of organic
liquid medium used in the process of the invention is not
critical, although the use of progressively larger amounts
can result in polysilanes of progressively lower molecular
weight.
The process may be carried out at any ~e~perature
but preferably the reaction temperature is maintained
below 50C. The reaction which occurs is exotherMic and
is preferably initiated at room temperature, no external
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heat being supplied during the reaction. If the tempera-
ture is increased an increase in ~he molecular weigh~ of
the formed polysilanes is usually observed. This may lead
to the production of polysilanes which are insolu~le in
S the organic liquid medium.
When the reaction has proceeded to ~he desired
degree the polysilane may be recovered from the reaction
mixture by any suitable method. If the polysilane is
insoluble in the liquid organic material in which the
reaction took place it can be filtered out from the
mixture. This is preferably done when other insolubles
such as the alkali metal halides which are formed as a
side product have been removed, for example by scooping or
decanting. Depending on the components of the reaction
lS the solid byproduct may float towards the surface o~ the
mixture whilst the polysilane tends to precipitate. If
the polysilane is soluble in the solvent other insolubles
can be removed by filtration and the polysilane can be
retained in the solvent, purified by washing or dried to a
powder.
Polysilanes produced by the process of the invention
are solid materials having a three dimensional structure
wherein each silicon atom is linked to at least one other
silicon atom and possibly to an R group. The exact struc-
ture of the polysilane has not been defined but isbelieved to include such structures as dodecahedron and
open cage structures. In these polysilanes (RSi)n R may
be an alkyl, aryl, alkaryl or aralkyl group having from 1
to 18 carbon atoms. Preferably R is Cl_6 alkyl or a
phenyl group. The value of _ in the general formula
(RSi)n is at least 8. There is, strictly speaking, no
maximum value for _ bu~ if the value is very high the
polysilanes tend to become insoluble in the organic liquid
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medium. Preferably n has a value of from 8 to about 100
depending on the nature of R. When R denotes for example
a phenyl group, the value of _ is preferably from 8 to
about 30 as these phenylpolysilanes are soluble in the
organic liquid medium. Polysllanes obtained by the
process of the invention which are soluble in hydrocarbon,
ether or nitrogen containing solvents can be shaped more
easily before they are formed into silicon-carbide
materials and are, therefore, the most preferred.
The following examples in which parts and percent-
ages are expressed by weight, Me denotes a methyl group,
t-Bu denotes a tertiary butyl group and Ph denotes a
phenyl group, illustrate the invention.
Example 1
To a suspension of Li (2.8g, 0.4 mole) in lOOml of
tetrahydrofuran (Thf) a solution of PhSiC13 (27.6g, 0.13
mole) in lOOml of Thf was slowly added. The mixture
warmed up as the exothermic reaction took place and became
dark brown. When all of the solution had been added the
mixture was stirred for a further 3 hours at ambient temp-
erature. The excess Li and LiCl which was formed were
filtered off and the iltrate was poured into ~OOml of
methanol. A precipitate formed and was filtered off,
washed with water and methanol and dried under vacuum.
The reaction yielded 10.58g of a solid polysilane
material. Analysis of this material showed 67.35% C and
4.71% H. The molecular weight was determined by ~PC as
2276. Infrared and NMR analysis showed the presence of Ph
and Si-Ph and Si-Si bonds.
Example 2
To a suspension of Li (5.11g, 0.73 mole) in lOOml of
tetrahydrofuran (Thf) a solution of MeSiC13 (30g, 0.20
mole) in 100 ml of Thf was slowly added. The mixture
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warmed up as the e~othermic reaction took place bringing
the Thf to boil. The rest of the solution was added at
a rate sufficient to maintain the reaction mixture at
reflux. Then the mixture was stirred for a further 2
hours at ambient temperature. One litre of methanol was
added to destroy the excess of Li. The solids were
filtered off, washed with water and methanol and dried
under vacuum. The reaction yielded 8.6g of a solid poly-
silane material which was insoluble in Thf. Analysis of
this material showed 26.51% C and 6.12% H.
Example 3
To a suspension of Li (2.13g9 0.3 mole) in 100ml of
N,N,N',N'-tetramethylenediamine (TMEDA) and cooled to
-10C, a solution of MeSiC13 (14.2g, 0.0~5 mole) in 50ml
of TMEDA was slowly added. The mixture was kept at 10C
by external cooling during the addition. When all of the
solution had been added the mixture was stirred for a
further 5 hours at -10C followed by 3 hours at ambient
temperature. The excess Li and the LiCl which was formed,
were filtered off and the filtrate was poured in 1000ml
of methanol. A precipitate formed and this was filtered
off, washed with water and methanol and dried under
vacuum. The reaction yielded 3.21g of a solid polysilane
material. Analysis of this material showed 27.02% C and
6~25% H. The molecular weight was determined by GPC as
1548 Infrared and NMR analysis showed the presence o~ Me
and Si-Me and Si-Si bonds.
Example 4
To a suspension of Li (2.25g, 0.32 mole) in 100ml of
tetrahydrofuran (Thf) a solution of t-BuSiCl3 (18.62g,
0.097 mole) in 100ml of Thf was slowly added. The mixture
warmed up as the exothermic reaction took place and became
dark brown. When all of the solution had been added the
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mixture was stirred for a further 6 hours at ambient
temperature. The excess Li and the LiCl which was formed
were filtered off and the filtrate was poured into lOOOml
of methanol. A precipitate formed and this was filtered
off, washed with water and methanol and dried under
vacuum. The reaction yield 6.86g of a solid polysilane
material. Analysis of this material showed 54.95% C and
9.83~ H. The molecular weight was determined by GPC as
5854. Infrared and NMR analysis showed the presence o~
t-Bu and Si-C bonds.
