Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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POLYSILANES
The invention relates to novel polysilanes.
Polysilanes have been known for a long time and
include different types of materials. Examples of known
polysilanes are linear permethylated polysilanes, cyclic
permethylated polysilanes, branched polysilanes and cage
permethyl polysilanes. Certain polysilanes with substi-
tuents 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 substituents. Also known are polysilanes
where only hydrogen atoms are found on the silicon atoms.
The size of polysilanes can vary widely from the disilane
to polysilanes having a large number of silicon atoms
attached to each other. Linear polysilanes have usually
les~ than 10 silicon atoms in the chain, whilst cyclic and
polycyclic polysilanes often have a large number of silicon
atoms. G.B. specification 2 081 290 describes polysilanes
having the average formula [(CH3)2Si][CH3Si] in which poly-
silane there are from 0 to 60 mole percent (CH3)2Si= units
and 40 to 100 mole percent C~3Si units, wherein there is
also bonded to the silicon atom other silicon atoms and
additional alkyl radicals of 1 to 4 carbon atoms or phenyl.
It is an object of the present invention to provide
novel polysilanes. Accordingly the invention provides a
polysilane of the general formula (RSi)n wherein each R is
independently selected from the group consisting of alkyl,
aryl, alkaryl or aralkyl group having from 4 to 18 carbon
atoms and n has a value of at least 8.
In the general formula of the polysilanes of this
invention R may be for example butyl, pentyl, octyl,
dodecyl, hexadecyl, phenyl, naphthyl, tolyl, phenylethyl,
methylphenyl and propylphenyl.
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Polysilanes generally can be prepared by several
routes. One of the earliest published methods was that
described in UOS. Patent Specification 2 380 995 in the
name of Rochow, in which disilanes were produced by contac-
ting silicon metal with an alkylhalide ~mder specifiedconditions. The most common route for the production of
cyclopolysilanes involves the reductive condensation of a
dialkyldihalosilane with an alkali metal. This has been
described in many articles and patent specifications, for
example U.S. Patent 4 052 430. If an alkyltrihalosilane is
included in the reaction mixture as described above
co-condensation of ~hese silanes can orm cage polysilanes
under certain conditions. Another route for making polysi-
lanes employs low molecular weight polysilanes which are
reacted under anhydrous conditions with a Grignard reagent,
as described for example in G.B. specification 2 081 290.
Such low molecular weight polysilanes are those present in
the direct process residue obtained during the commercial
production of chlorosilanes. However, the direct process
residue is not pure or well defined and the preparation of
polysilanes therefrom involves the extra step of purifi-
cation of the direct process residue.
The polysilanes of the present invention may be
prepared by a process wherein at least one trihalosilane 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 as defined above and
X is a halogen atom, preferably Cl. Such silanes are well
known in the art and they may be prepared e.g~ by 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
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Silicones by W. Noll. Examples of the trihalosilanes
which may be used in the process of the invention include
for example phenyltrichlorosilane, tertiary butyltrichloro-
silane and dodecyltrichlorosilane.
The alkali metal which may be used in such process
can be e.g. Na, K and Li. Li is the preferred metal as it
gives the highest yield of polysilanes. The amount of
alkali metal used in the reaction is about three moles per
mole of the silane utilised. In order to ensure the
completion of the reaction it is preferred to add a slight
excess of the alkali metal.
The organic liquid medium in which the reaction
takes place may be any solvent in which the trihalosilane
reactant is soluble. Preferably the organic liquid medium
is also a solvent for the polysilanes of the invention.
These solvents include hydrocarbon solvents such as toluene
or paraffins, ethers such as tetrahydrofuran and dioxan and
nitrogen containing solvents such as ethylenediamine.
Preferably tetrahydrofuran is used. The by-product alkali
metal halides are normally insoluble and these can be
easily removed by filtration. The amount of solvents used
in the process is not critical although larger amounts o
solvent can result in lower molecular weight polysilanes.
The process may be carried out at any temperature
but preferably the reaction temperature is maintained
below 50C. The reaction which occurs is exothermic and
is preferably initiated at room temperature, no external
heat being supplied, during the reaction. As the tempera-
ture is increased an increase in the molecular weight of
the formed polysilanes is usually observed.
When the reaction has proceeded to the deslred
degree the polysilane may be recovered from the reaction
mixture by any suitable method. If the polysilane is
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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.
The polysilanes of this invention are solid materials
having a three dimensional structure wherein every silicon
atom is linked to at least one other silicon atom and
possibly to an R group. The exact structure of the polysi-
lane has not been defined but is believed to include such
structures as dodecahedron and open cage structures.
One of the more important uses of polysilanes is
their use as precursors for silicon carbide'. It is
desirable to shape these polysilanes prior to their trans-
formation. It is therefore preferred that the polysilanes
should be soluble in a liquid carrier material such as an
organic solvent. However, it has proved difficult to
prepare polysilanes with a silicon to carbon ratio of 1 in
a way that they are soluble in most solvents.
We have now found that those polysilanes of the
invention wherein R is a large or sterically hindered
alkyl, aryl, aralkyl or alkaryl group are soluble at
ambient tem~erature in certain organic liquid media. Such
polysilanes are preferred and accordingly the invention
provides in another of its aspects polysilanes of the
general formula (RSi)n w~erein R is selected from the group
~5 consisting of large alkyl, aryl, alkaryl, aralkyl, steri-
cally hindered alkyl, sterically hindered aryl~ sterically
hindered alkaryl and sterically hindered aralkyl as herein-
above defined and which is soluble in an organic liquid
medium at ambient temperature. The invention also provides
a composition comprising an organic liquid medium and
dissolved therein a polysilane of the general formula
(RSi)n, wherein R and n are as defined for the preferred
polysilanes.
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In these pref~rred polysilanes R may be a large and/
or sterically hindered alkyl, aryl, alkaryl or aralkyl
group as herein defined. By the expression 'sterically
hindered' is meant those groups which are branched or
cyclic for example cycloalkyl groups, isobutyl, isopentyl,
neopentyl, tertiary butyl and 2,4,6 trimethylphenyl and
having at least 4 carbon atoms. By the expression 'large'
group is meant those having more than 5 carbon atoms. The
most preferred polysilanès of the invention are those
wherein R represents tertiary bu~yl or phenyl. The maximum
value of _ to ensure the solubility of the preferred poly-
silanes at ambient temperature is dependent on -the nature
of R. For example phenylpolysilanes of the invention are
soluble in an organic liquid medium when n has a value not
higher than 30, whilst t-butylpolysilanes are still soluble
when n has a value of 68.
Suitable organic liquid media in which such preferred
polysilanes are soluble include hydrocarbon, ether and
nitrogen containing solvents. Examples of such solvents
are toluene, paraf~ins, such as hexane and dodecane, tetra-
hydrofuran, dioxan, ethylenediamine, triethylamine and
N,N,N',N'-tetramethylethylenediamine. Their solubility
enables such polysilanes to be shaped more easily when they
are employed for example as precursors in the formation of
silicon-carbide articles or surface coatings.
The following examples in which parts and percen~-
ages are expressed by weight, 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
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warmed up as the exothermic reaction took place and became
dark brown. When all 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 filtrate was poured into 800ml of
methanol. A precipitate formed and this was filtered off,
washed with water and methanol and dried under vacuum.
The reaction yielded 10.58g of a polysilane solid
material, Analysis of this material showed 67.35% C and
4.71% H. The molecular weight was determined by GPC as
2276. Infrared and NMR analysis showed the presence of Ph
and Si-Ph and Si-Si bonds.
Exame~ 2
To a suspension of Li (2.25g, 0.32 mole) in lOOml of
tetrahydrofuran (Thf) a solution of t-BuSiC13 (18.62g,
0.097 mole) in lOOml of Thf was slowly added. The mixture
warmed up as the exothermic reaction took place and became
dark brown. When all the solution had been added the
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
585~. Infrared and NMR analysis showed the presence of
t-Bu and Si-C bonds.
