Process for producing tris (N-methylamino) methylsilane

A method of producing tris (N-methylamine) methylsilane, including the steps of forming and cooling a liquid solution of methylamine in an inert solvent and under an inert atmosphere at a temperature of about -30.degree. C. and slowly adding a quantity of methyltrichlorosilane while maintaining said temperature. The reaction mixture is then heated for about 60 minutes at a temperature of about 40.degree. C., followed by filtering the solid portion from the liquid portion. The liquid is distilled to remove the solvent, resulting in a high yield of tris (N-methylamine) methylsilane.

FIELD OF THE INVENTION 
This invention relates to an improved process for producing tris 
(N-methylamino) methylsilane, which in turn may be used to produce silicon 
carbide-silicon nitride fibers having desirable properties. 
BACKGROUND OF THE INVENTION 
Recent concern about the safety of carbon fibers has led to a program to 
develop silicon carbide-silicon nitride fibers, generally of the formula 
Si.sub.x N.sub.y C.sub.z. Current use of carbon fibers as reinforcement in 
composite materials has been suggested to be potentially unsafe due to the 
high electrical conductivity of carbon. Si.sub.x N.sub.y C.sub.z fibers 
are 10.sup.6 times more electrically resistive than carbon fibers, and at 
the same time have similar mechanical properties. Si.sub.x N.sub.y C.sub.z 
fibers are produced from the monomer tris (N-methylamino) methylsilane, 
which for convenience is referred to as TNMAMS. 
The preparation of TNMAMS is disclosed in German Patent Disclosure No. 
2,218,960, dated Nov. 8, 1973, in which Dr. Wolfgang Verbeek is listed as 
the inventor. In Verbeek, a process for producing materials from 
homogeneous mixtures of silicon carbide and silicon nitride is disclosed. 
The reaction between methyltrichlorosilane and methylamine is disclosed as 
an example of the general reaction of converting halogen substituted 
silanes with one or more compounds having NH.sub.2 or NH groups. Example 1 
of Verbeek teaches the addition of methylamine to a solution of 
methyltrichlorosilane dissolved in petroleum ether for reaction at 
40.degree. C. After purification by distillation, the product is used to 
manufacture a resin. Example 2 teaches the use of another silane to which 
is added methylamine for reaction at 40.degree. C., with purification in 
the same manner. Verbeek is silent about the yield of monomer. 
Aliphatic chlorosilanes are also reacted with large excesses of gaseous 
ammonia in U.S. Pat. No. 2,579,416, to N. D. Cheronis, under anhydrous 
conditions. Additionally, U.S. Pat. No. 2,579,417, also to Cheronis 
discloses the use of a gaseous primary amine, in excess, in a similar 
reaction. Reaction temperatures for both Cheronis references are between 
-10.degree. C. and -15.degree. C. In U.S. Pat. No. 4,255,549 to 
Christophliemke et al, a reaction of methyltrichlorosilane with a great 
excess of ammonia is taught, where the reaction is accomplished at above 
0.degree. C. under pressure. Christophliemke et al is stated to be an 
improvement on the Cheronis work. Other patents teach the reaction of 
organohalosilane at temperatures above 0.degree. C., such as Johannson, 
U.S. Pat. No. 2,429,883; Breedervald et al, U.S. Pat. No. 2,807,635; 
Takamizawa et al, U.S. Pat. No. 3,927,057; and Berger et al, U.S. Pat. No. 
3,700,716. 
Efforts to produce a significant quantity of TNMAMS which is suitable for 
use in the production of useful resins has not been possible using the 
methods described in the prior art. Several of these references teach the 
production of laboratory quantities of polymerizable monomers, but none 
provide a process which can produce large quantities of monomer of high 
yield and suitable purity. Scale-up of these prior art methods leads to 
substantial difficulties such as poor yields, difficult purification, loss 
of reactants and wasted by-products. 
DISCLOSURE OF THE INVENTION 
It is an object of this invention to provide a method of producing tris 
(N-methylamino) methylsilane in sufficiently high yield to justify and 
permit large scale production. 
Another object is to provide a process for producing TNMAMS without loss of 
starting materials and by-products, to thereby have an economically 
acceptable process. 
These and other objects of this invention will become apparent from a 
reading of the detailed description contained hereinafter. 
DETAILED DESCRIPTION OF THE INVENTION 
It has now been discovered that the above and other objects of this 
invention may be accomplished in the following manner. Specifically it has 
been discovered that TNMAMS may be produced in satisfactory yield without 
loss of starting reactants or by-products in a manner which can be scaled 
up to acceptable quantities of production to permit commercial use of 
TNMAMS. 
The first step in the method of this invention is to form a solution of 
methylamine in an inert solvent, at a temperature of less than -20.degree. 
C. Any inert solvent which remains liquid at the operating temperatures of 
this method is acceptable. Particularly preferred are those organic 
solvents which can be maintained substantially free from water 
contamination, since water does enter into competing reactions which 
adversely affect the instant method. Most preferred is petroleum ether, 
and particularly petroleum ether which has been dehydrated or dried. As 
stated above, the reaction mixture is cooled to below -20.degree. C. and 
preferably to as low as -50.degree. C. Most preferred is a temperature of 
about -30.degree. C. 
After the liquid solution is prepared and cooled, a suitable quantity of 
dry methyltrichlorosilane is added preferably while maintaining the low 
temperature to prevent vaporization of the methylamine. The rate of 
addition should be slow enough to keep the temperature below -20.degree. 
C. and preferably below or about -30.degree. C. during the entire time of 
addition. Rapid addition of the halosilane could result in rapid 
production of the by-product hydrochloric acid, which will react with the 
TNMAMS being produced. With slow addition, the excess liquid methylamine 
reacts with the hydrochloric acid being produced, to form an insoluble 
salt. 
After the reaction mixture has been formed, cooling is stopped and heat is 
applied to raise the temperature to from about 35.degree. C. to about 
48.degree. C. It is held at this temperature for from as little as 15 
minutes to as long as 75 minutes. The preferred refulx temperature is 
about 40.degree. C. which is preferably maintained for about 60 minutes. 
After the heating step, the reaction mixture is filtered to remove the 
solids which have formed. Most of these solids are salts which have formed 
from reaction between hydrochloric acid as it is formed, and methylamine, 
although some salt is formed if the hydrochloric acid has the opportunity 
to react with the TNMAMS being produced. 
Finally, the liquid portion remaining after filtration, containing wash 
solvents if appropriate, is distilled to remove the solvents to obtain a 
clear liquid at room temperature which is the desired TNMAMS. 
As has been disclosed above, the reaction to produce TNMAMS of this 
invention is adversely affected by the presence of water or water vapor. 
Care should be taken to employ substantially dry reactants, and it is 
preferred that the entire reaction be carried out in a dry inert 
atmosphere. Any of the normal inert gasses may be used, such as argon or 
the like, but nitrogen is quite suitable and preferred from a convenience 
and cost standpoint. 
The resulting product may be used in the production of various fibers, such 
as by total reflux distillation at 520.degree. C. for about 3 hours.

EXAMPLES 
The following examples are presented to illustrate the preferred embodiment 
of this invention. They are not intended nor should it be inferred that 
they limit the scope of the invention as claimed. 
EXAMPLE I 
A first operation of the method of this invention was performed in the 
laboratory. 
A 3 liter three-necked flask was fitted with a nitrogen inlet, a condenser, 
and a dropping funnel. To the flask was added 1500 ml. of dry petroleum 
ether. The temperature was then lowered to -30.degree. C. and liquified 
monomethylamine (150 g., 5 moles) was added to the reaction vessel to form 
a liquid solution. While stirring and maintaining a dry nitrogen 
atmosphere, methyltrichlorosilane (75 g., 0.5 mole) was added to the flask 
dropwise to form a reaction mixture. The reaction vessel was then modified 
with a Friedrich's condenser fitted with a drying tube. The other two 
necks of the flask were sealed with glass stoppers while maintaining the 
dry nitrogen atmosphere. The entire reaction apparatus was then 
transferred to a hood and the reaction mixture heated at 40.degree. C. for 
one hour. The contents of the flask were then filtered through a medium 
fritted glass filter into a 3 liter three-necked flask to remove solids 
while maintaining a dry inert atmosphere. The flask containing the 
filtrate was fitted with distillation apparatus and tris (N-methylamino) 
methylsilane was collected at 149.degree.-151.degree. C. The product was a 
clear liquid at room temperature. Quantities of this TNMAMS was 
polymerized to form a useful polycarbosilizane resin. 
EXAMPLE II 
A large scale operation of the method of this invention was then performed 
to demonstrate the suitability for commercial quantity production. 
A 22.5 liter three neck flask was placed in section, fitted with a stirrer 
and calcium chloride drying tube and filled with 13 liter of petroleum 
ether. A temperature bath constructed from a 30 gallon container and 
having 40 feet of 1/2 inch diameter coiled copper tubing placed therein 
was filled with 15 gallons of ethanol and cooled to -30.degree. C. in one 
hour by passing liquid nitrogen through the copper coil placed in the 
bath. 
The system was then purged with nitrogen and 6 liters of methylamine was 
allowed to condense into the cold petroleum ether. This was done by 
passing a line from the amine gas tank first through the liquid 
nitrogen-cooled ethanol contained in the bath and then into the cold 
petroleum ether. 
After addition of the amine to the petroleum ether to form a liquid 
solution, a dropping funnel containing methyltrichlorosilane (900 ml., 
1150 g) was installed into one of the necks of the flask. The 
methyltrichlorosilane was slowly added with stirring to form a reaction 
mixture. Upon completion, the dropping funnel was replaced with a 
Friedrich's condenser equipped with a calcium chloride drying tube. The 
temperature was maintained at -30.degree. C. 
The liquid nitrogen flow through the copper coil was stopped after the 
formation of the reaction mixture. After a short period where ambient air 
was passed through the tubing until the temperature of the reaction 
mixture reached 40.degree. C. This temperature was held for one hour by 
alternately heating and cooling the coil with hot or cold water. 
After the heating of the reaction mixture was completed, the condenser was 
removed and the reaction flask connected to a filtration system as herein 
described. Adequate care was taken to insure that the glass tubing vessel 
did not contact the salt produced by the reaction. 
The filtration operation is accomplished by the establishment of a pressure 
differential in the system that causes the reaction mixture to flow from 
the reaction pot, through the filtration device, and into a 22.5 liter 
three neck filtration flask. The pressure differential was established by 
connecting the filtration pot to a water aspirator. 
Glass tubing inserted through a rubber stopper was placed into one of the 
necks of the filtration flask and connected to the water aspirator with 
rubber tubing. Next a calcium chloride drying tube and stopcock was fitted 
into the connection between the filtration flask and water aspirator. 
Flow of the reaction mixture from the reaction vessel into the filtration 
flask was accomplished by starting the water aspirator. The rate of flow 
was regulated by opening and closing the stopcock. This operation was 
stopped once the petroleum ether level was barely above the surface of the 
salt. The stopcock was then closed and dry air pumped into the system. 
After attaining atmospheric pressure in the reaction pot, the port through 
which the air had been pumped was opened and a dry nitrogen inlet line was 
quickly installed in the opening. A funnel was also placed in the opening 
with the dry nitrogen line, and about six liters dry petroleum ether was 
poured through the funnel into the reaction pot. The stirrer was then 
activated for five minutes to rinse product from the salt. The salt was 
allowed to settle to the pot bottom after stopping the stirrer and the 
filtration device. 
After the filtration operation was completed, the filtration pot contained 
about 14 liters of ether-product solution which was completely salt free. 
At this point, the filtration device and water aspirator connections were 
both removed from the outside port of the filtration pot. One pot was 
plugged with a glass stopper and the other with a drying tube under 
nitrogen. 
The pot containing the ether-product solution was then connected to a 
distillation apparatus. 
After making the proper connections and installations, the water aspirator 
was started. About 500 ml of ether-product solution was allowed to flow 
into the distillation pot by opening the stopcock located between the 
petroleum ether-product containing vessel and distillation pot. The water 
aspirator remained on as a heating mantel was placed under the 
distillation flask. The heating was started and the stopcock between the 
ether-product pot and distillation pot adjusted to allow solution to 
trickle into the distillation pot at about the rate which the condensate 
was collected in the recovery pot. 
The distillation operation was continued until less than 100 mls of 
ether-product remained in the filtration pot. At this time, the heating 
mantel was turned off and removed from the distillation pot. The water 
aspirator remained on until the temperature of the vapors was 40.degree. 
C. Then the isolation valve between the recovery pot and aspirator was 
closed. Shortly afterward the vacuum system connection to the recovery pot 
was replaced by a drying tube. 
The solution in the pot was now highly concentrated in product and extreme 
care was exercised to prevent moisture from entering the system at any 
time. 
Although the distillation pot solution was mostly product containing a 
little dissolved ether, small amounts of other impurities such as side 
reaction products were also present. Therefore further purification of the 
product was necessary. 
To begin the purification operation, the previously removed heating mantel 
was placed around the distillation pot and turned on. This distillation 
proceeded at laboratory atmospheric pressure, not under a vacuum as in the 
previous distillation operation. 
Shortly after the vapor temperature reached 146.degree. C., the heating 
mantel was turned off and removed from the distillation pot. When the 
distillation pot temperature reached 50.degree. C., the recovery pots were 
removed while a dry nitrogen stream was purging the distillation system. 
One port of a clean, flame dried, 1 liter -3 port distillation pot was 
immediately connected to the takeoff and the other two ports were plugged 
with drying tubes. 
The heating mantel was again placed around the distillation pot and turned 
on. The temperature of the vapors quickly reached 146.degree. C. and the 
product was collected until the temperature began to increase. 
Immediately, when the temperature moved above 146.degree. C., the heating 
mantel was turned off and removed from the distillation pot. 
In this particular experiment, 673 mls of the product TNMAMS were 
recovered. Material remaining after the 146.degree. C. distillation 
separation was considered impurities and discarded after the product pot 
was removed from the system. Yield was calculated to be approximately 
58.6% by weight. 
Infra-red spectra of tris (N-methylamino) methylsilane were taken using a 
Perkin-Elmer 137 Sodium Chloride Spectrophotometer. The liquid monomer was 
placed in sodium chloride cells under anhydrous conditions. 
NMR spectra were prepared using a Varian EM-300 system with deuterated 
chloroform as the solvent and tetramethylsilane as an external reference. 
A Waters Associated HPLC with 46 K injector and Model 440 UV absorbance 
detector was used for chromatographic analysis. The weight percentage of 
tris (N-methylamino) methylsilane in the final product mixture was 
estimated by comparison of the areas under the peaks of the chromatogram 
using the cutting and weighing technique. The product was polymerized to 
yield a useful polycarbosilizane resin. 
The above example is merely illustrative and is not to be construed as 
limiting the scope of our invention, which is limited only as indicated by 
the appended claims.