Polymeric hydridochlorosilazanes and processes for their preparation

The invention relates to novel polymeric hydridochlorosilazanes and a process for their preparation. The compounds according to the invention can be converted into polyhydridosilaznes by reaction with ammonia, and these can in turn be pyrolyzed to ceramic material containing silicon nitride. To prepare the polymeric hydridochlorosilazanes, oligohydridoalkylsilazanes (R.sup.1 SiHNH).sub.n are reacted with a dichlorohydridoalkylsilane R.sup.2 SiHCl.sub.2.

DESCRIPTION 
The invention relates to novel polymeric hydridochlorosilazanes and a 
process for their preparation. The polymeric hydridochlorosilazanes 
according to the invention can be converted into polymeric 
hydridosilazanes ("polyhydridosilazanes") by reaction with ammonia, and 
these can in turn be pyrolyzed to ceramic material containing silicon 
nitride. 
The pyrolysis of polysilazanes to give ceramic material containing silicon 
nitride has already been described in the literature (R.R. Wills et al., 
Ceramic Bulletin, Volume 62 (1983), 904-915). 
Chlorosilanes are as a rule used as starting materials for the preparation 
of polysilazanes and are reacted with ammonia or primary or secondary 
amines (U.S. Pat. No. 4,540,803, U.S. Pat. No. 4,543,344, U.S. Pat. No. 
4,595,775, U.S. Pat. No. 4,397,828 and U.S. Pat. No. 4,482,669). According 
to U.S. Pat. No. 4,482,669, a dichlorohydridoalkylsilane of the formula 
RSiHCL.sub.2 is reacted with NH.sub.3 to give oligohydridoalkylsilazanes 
(RSiHNH).sub.n, and these are then condensed, for example with the aid of 
KH, to give polysilazanes, hydrogen being eliminated. 
The present invention provides novel starting materials for 
polyhydridosilazanes, that is to say polymeric hydridochlorosilazanes. 
The present invention relates to a process for the preparation of polymeric 
hydridochlorosilazanes, which comprises reacting 
oligohydridoalkylsilazanes of the general formula (R.sup.1 SiHNH).sub.n, 
in which n is about 3 to 12 and R.sup.1 denotes an alkyl group having 1 to 
6 carbon atoms, with a dichlorohydridoalkylsilane of the general formula 
R.sup.2 SiHCL.sub.2, in which R.sup.2 denotes an alkyl group having 1 to 6 
carbon atoms, at 30.degree. to 300.degree. C. Readily volatile by-products 
are formed by this process. These by-products are removed during the 
reaction. 
The oligohydridoalkylsilazanes (R.sup.1 SiHNH).sup.n, in which n is about 3 
to about 12 and which are used as starting substances, can be obtained by 
reacting a dichlorohydridoalkylsilane of the formula R.sup.1 SiHCL.sub.2, 
in which R.sup.1 has the above meaning, with excess NH.sub.3 in a solvent, 
as described in U.S. Pat. No. 4,482,669 (see in particular columns 4, 5, 7 
and 8 therein). A mixture of linear and cyclic oligomers having various 
chain lengths n is thereby in general formed. 
The radicals R.sup.1 and R.sup.2 in the oligohydridoalkylsilazanes (R.sup.1 
SiHNH).sub.n (also called "oligosilazanes" for short below) and in the 
dichlorohydridoalkylsilane R.sup.2 SiHCL.sub.2 (also called 
"dichloroalkylsilane" for short below) can be identical or different and 
they preferably have 1 to 3 carbon atoms. 
Particularly preferably, R.sup.1 =R.sup.2 =CH.sub.3. The molar ratio of the 
reactants in the above reaction of dichloroalkylsilane: R.sup.1 SiHNH unit 
of the oligosilazane is preferably about 0.2 : 1 to 1.5 : 1, in particular 
0.3 : 1 to 1 : 1. 
To react the reactants with one another, the oligosilazanes are preferably 
introduced first and the dichloroalkylsilane is added. Since the reaction 
is exothermic, the temperature is preferably kept initially at 30.degree. 
to 50.degree. C. when the reactants are brought together. The mixture is 
then heated to temperatures of 100.degree. to 300.degree. C., preferably 
to 120.degree. to 250.degree. C. 
The low-boiling components formed as by-products, such as RSiHCL.sub.2, 
RSiClH.sub.2, RSiCL.sub.3, HCL, H.sub.2 and NH.sub.3 (in which R =R1 or 
R.sup.2) partly escape during the reaction. When the reaction has ended, 
the remaining low-boiling constituents are in general removed from the 
reaction vessel by applying a vacuum. 
The majority of the NH.sub.4 CL likewise formed in the reaction sublimes 
out of the reaction mixture in the course of the reaction. Any residue of 
NH.sub.4 CL which remains can be removed from the polymeric 
hydridochlorosilazane prepared according to the invention by extraction 
with an inert organic solvent, such as n-hexane, toluene or ether. 
The duration of the reaction depends on the rate of heating up and on the 
reaction temperature. A reaction time of 5 to 7 hours is in general 
sufficient. 
It is also possible to carry out the reaction in an organic solvent. 
Suitable solvents are those which are inert towards the reactants and have 
a sufficiently high boiling point, that is to say, for example, saturated 
aliphatic or aromatic hydrocarbons, such as n-decane, decalin, xylene or 
toluene, chlorinated hydrocarbons, such as chlorobenzene, or ethers, such 
as dibenzyl ether or diethylene glycol diethyl ether. If a solvent in 
which the NH.sup.4 Cl formed is insoluble is used, the latter can be 
separated off by filtration. The polymeric hydridochlorosilazanes 
according to the invention are then obtained by distilling off the solvent 
under reduced pressure. 
If appropriate, the process can also be carried out under reduced pressure. 
It can also be carried out under pressures in the range from 1 to 10 
atmospheres. 
The process can also be carried out continuously. 
The novel polymeric hydridochlorosilazanes prepared have a molecular 
structure which can be represented by the formula 
##STR1## 
in which the free valencies on the nitrogen atoms are saturated with H 
atoms or silyl radicals R*SiXN&lt;(X =H, CL or N&lt;). R, R', R" and R* denote 
alkyl groups having 1 to 6, preferably 1 to 3, carbon atoms and a, b an c 
denoted the molar fractions of the particular structural units. a +b+c=1. 
Particularly preferably, R =R'=R"=R* =CH.sub.3. The polymeric 
hydridochlorosilazanes have a network structure. 
Accordingly the present invention also relates to polymeric 
hydridochlorosilazanes of the formula 
##STR2## 
in which the free valencies of the nitrogen atoms are saturated with H 
atoms or silyl radicals R*SiXN&lt;(X =H, CL or N&lt;) and in which R, R', R" and 
R* denote alkyl groups having 1 to 6 carbon atoms and a, b and c denote 
the molar fractions of the particular structural units. 
The values of the molar fractions b and c are higher (and correspondingly 
the value of a is lower) the greater the ratio of dichloroalkylsilane: 
R.sup.1 SiHNH unit of the oligosilazane. The particular values of a, b and 
c present can be determined by integration of the .sup.1 H-NMR spectra and 
by elemental analysis. The values a, b and c are in general 0.1 to 0.8, 
and a +b +c =1. Those polymeric hydridochlorosilazanes in which the values 
for a and b are 0.1 to 0.5, in particular 0.2 to 0.4, are preferred. The 
preferred values of c are 0.1 to 0.6, in particular 0.3 to 0.6. As 
mentioned, these values can be adjusted via the relative content of the 
dichloroalkylsilane in the reaction mixture and checked by the analytical 
methods mentioned. The preferred values just mentioned for a, b and c have 
proved particularly appropriate if a fiber is to be produced as the end 
product of the pyrolysis (after conversion of the polymeric 
hydridochlorosilazanes into polyhydridosilazanes). 
The invention furthermore relates to polymeric hydridochlorosilazanes which 
are prepared by reacting oligohydridoalkylsilazanes of the general formula 
(R.sup.1 SiHNH).sub.n, in which n is about 3 to about 12 and R.sup.1 
denotes an alkyl group having 1 to 6 carbon atoms, with a 
dichlorohydridoalkylsilane of the general formula R.sup.2 SiHCL.sub.2, in 
which R.sup.2 denotes an alkyl group having 1 to 6 carbon atoms, at 
30.degree. to 300.degree. C. The readily volatile by-products formed are 
removed during the reaction. 
The novel polymeric hydridochlorosilazanes (also called 
"polyhydridochlorosilazanes") can be converted into polyhydridosilazanes 
by reaction with ammonia (ammonolysis"), and these can in turn be 
converted into ceramic material containing silicon nitride by pyrolysis. 
The ammonolysis can be carried out in liquid NH.sub.3. However, it is 
advantageous to carry it out in an organic solvent. Suitable solvents are 
all those which are inert towards the polyhydridochlorosilazanes. 
Preferred solvents are those in which the ammonium chloride obtained as a 
by-product has a low solubility and can easily be separated off, for 
example ethers, aliphatic and aromatic hydrocarbons and chlorinated 
hydrocarbons. The reactants can be fed into the reaction vessel in any 
desired sequence for the ammonolysis. However, it is usually advantageous 
to initially introduce the polyhydridochlorosilazane initially in solution 
and to introduce gaseous ammonia or to add liquid ammonia. If the 
polyhydridochlorosilazanes according to the invention have been prepared 
in a suitable organic solvent, the ammonolysis can be carried out in this 
solvent without prior removal of the NH.sub.4 CL. The ammonolysis is 
preferably carried out with an excess of NH.sub.3, in order to ensure that 
the reaction is complete and the end products are substantially free from 
chlorine as far as possible. Twice the stoichiometric amount is in general 
sufficient for this purpose. 
The reaction is in general carried out at a temperature of about 
-50.degree. to +100.degree. C., preferably at -20.degree. to +30.degree. 
C. and in particular at room temperature (the mixture being cooled with 
ice). However, it is also possible to carry out the reaction above room 
temperature, for example at the boiling point of the solvent used, or 
below room temperature, for example at -33.degree. C., if liquid NH.sub.3 
is used. 
When the ammonolysis has ended, the excess NH.sub.3 is removed, if 
appropriate, and the ammonium chloride obtained is filtered off. To 
increase the yield, the precipitate can be washed with one of the 
abovementioned organic solvents. After the solvent has been distilled off 
under reduced pressure, the polyhydridosilazanes according to the 
invention are obtained directly as white powders. The polyhydridosilazanes 
are soluble in the above organic solvents, so that these can be used both 
for coating surfaces and for the production of fibers. 
The polyhydridosilazanes can be pyrolyzed by pyrolysis in an inert nitrogen 
or argon atmosphere at temperatures of 800.degree. to 1200.degree. C. to 
give amorphous dense materials which essentially consist of Si, N and C 
and can also contain traces of H and 0. At pyrolysis temperatures above 
1200.degree. C., for example in the range from 1200.degree. C. to 
1400.degree. C., partly amorphous microcrystalline ceramic materials 
containing .alpha.-Si.sub.3 N.sub.4 as the crystalline phase are formed. 
It is a particular advantage that the polyhydridosilazanes can be shaped by 
various processes before the pyrolysis to give three-dimensional shaped 
articles. 
An important method of shaping is drawing of fibers. Specifically fibers 
can be drawn from highly viscous solutions of the polyhydridosilazane in 
solvents such as toluene, tetrahydrofuran or hexane. The fibers are 
advantageously drawn by means of spinnerets 80 to 150 .mu.m in diameter. 
The threads are narrowed by subsequent stretching, so that a very solid 
thread of 2 to 20 .mu.m, in particular 5 to 15 .mu.m, in diameter is 
formed after the pyrolysis. The fibers produced by subsequent pyrolysis 
are used as mechanical reinforcing inclusions in fiber-reinforced 
aluminum, aluminum alloys and ceramic components. 
Another important processing possibility for the polyhydridosilazanes is 
the production of dense, firmly adhering, amorphous or microcrystalline 
ceramic coatings on metals, in particular steels, or on ceramics, such as 
AL.sub.2 0.sub.3, ZrO.sub.2, MgO, SiC or Si.sub.3 N.sub.4. Coating is 
effected with the aid of a solution of the polyhydridosilazane in organic 
solvents, such as toluene, tetrahydrofuran and hexane. The pyrolytic 
conversion into an amorphous or microcrystalline layer is carried out in 
the same temperature range of 800.degree. to 1200.degree. C. or 
1200.degree. to 1400.degree. C. under an inert gas as described above for 
three-dimensional shaped articles. 
Because of their outstanding adhesion, good hardness and surface quality, 
the ceramic coatings are particularly suitable for surface-finishing of 
machine components subjected to mechanical and chemical stresses. 
The polyhydridosilazanes described above can furthermore also be pyrolyzed 
in an NH.sub.3 atmosphere, instead of in an inert gas, with an equivalent 
ceramic yield of 70 to 90%. The result is a practically carbon-free, 
glass-clear colorless material. On pyrolysis in NH.sub.3 at 1000.degree. 
C. or more, the C content is less than 0.5% by weight. Depending on the 
pyrolysis temperature, the pyrolysis product consists of practically pure 
amorphous silicon nitride (pyrolysis below 1200.degree. C.) or crystalline 
Si.sub.3 N.sub.4 (pyrolysis above 1200.degree. C., in particular above 
1300.degree. C.). The pyrolysis in NH.sub.3 can be used on all the shaped 
articles produced by the shaping processes described above, that is to say 
articles, fibers and coatings shaped from powders. 
EXPERIMENTAL REPORT 
Preparation of oligohydridomethylsilazane (CH.sub.3 SiHNH).sub.n 
100 ml (0.97 mol) of methyldichlorosilane were dissolved in 800 ml of 
absolute tetrahydrofuran and ammonia was passed in for 3 hours (rate of 
introduction: 0.5 1/minute). The reaction temperature was kept in the 
range from 20.degree. to 25.degree. C. by cooling with an ice-bath. The 
mixture was stirred at room temperature for 1 hour in order to bring the 
reaction to completion and the ammonium chloride was then removed under 
argon. The precipitate was washed twice with 350 ml of tetrahydrofuran 
each time and the combined tetrahydrofuran solutions were concentrated 
under reduced pressure. A clear, highly mobile oil of (CH.sub.3 
SiHNH).sub.n was obtained in a yield of 44.5 g =78% of theory.

EXAMPLES 
Preparation of polymeric hydridochlorosilazane 
Example 1 
176.1 g (1.53 mol) of methyldichlorosilane were added to 108.8 g (1.84 mol) 
of oligohydridomethylsilazane at 30.degree. to 45.degree. C. and the 
mixture was heated to 200.degree. C. in an oilbath in the course of 7 
hours. During this the internal temperature rose from 46.degree. C. to 
164.degree. C. Above 100.degree. C., vigorous evolution of gas started. 
When the reaction had ended and the mixture had cooled, a brittle resin 
was obtained. The total contents of the reaction flask were 134 g. The 
resin was extracted with 500 ml of tetrahydrofuran, the residue was then 
washed with 50 ml of n-hexane and the organic solvent was stripped off in 
vacuo. 109 g of a white powder having the chemical composition 
.sup.C.sub.1 H.sub.3.62 CL.sub.0.38 N.sub.0.8 Si.sub.1 remained. 
Elemental analysis (% by weight): Found: 19.1% of Cl; 39.8% of Si; 16.2% of 
N; 16.5% of C; 6.8% of H Calculated: 19.7% of Cl; 41.0% of Si; 16.4% of N; 
17.6% of C; 5.3% of H Molar mass: 1,865 g/mol, determined by osmometry in 
benzene 
.sup.1 H--NMR (100 MHz,CDCL.sub.3, ppm): .delta.0.2-0.8 (br,3H,SiCH.sub.3), 
1.5-1.9 (br,0.1H,NH), 4.5 (br), 4.7-5.0 (br) and 5.1 (br,0.4H,SiH). 
IR: (KBr,cm.sup.-1) 3380 (sh), 3150 (br,vs), 3050 (s), 2840 (w), 2160 (s), 
1410 (vs), 1270 (vs), 1200-950 (br), 900 (br,vs), 760 (br,s). 
Example 2 
1.55 g (135 mmol) of methyldichlorosilane were added to 11.3 g (190 mmol) 
of oligohydridomethylsilazane. During this the internal temperature 
thereby rose to 50.degree. C. The reaction mixture was heated to an 
internal temperature of 160.degree. C. in the course of 30 minutes and 
kept at this temperature for 1.5 hours. Heating was then continued at an 
internal temperature of 180.degree. to 190.degree. C. for 4 hours. After 
the low-boiling reaction products had been stripped off, the cooled 
residue was extracted with 150 ml of n-pentane, and, after evaporation 
10.1 g of a white soluble powder consisting of polyhydridochlorosilazane 
were obtained. 
Elemental analysis (% by weight): 
41% of Si 17.6% of N 15.3% of Cl 
Example 3 
10.9 g (184 mmol) of oligohydridomethylsilazane and 17.6 g (153 mmol) of 
methyldichlorosilane were boiled under reflux in an oilbath at a bath 
temperature of 225.degree. to 235.degree. C. for 7 hours. Low-boiling 
constituents were stripped off at an internal temperature of 80.degree. C. 
by applying a vacuum. The residue was dissolved in 50 ml of 
tetrahydrofuran and the solution was filtered. After the solvent had been 
stripped off, 10.0 g of a white soluble powder consisting of 
polyhydridochlorosilazane remained. 
Elemental analysis (% by weight): 
34% of Si 15.9% of N 22% of Cl