Process for the Preparation of Alkyl and Aryl Substituted Oligosiloxanes Suitable for Use as Diffusion Pump Oils

1. Field of the Invention

The present invention is directed to an improved process for preparing alkyl and aryl substituted oligosiloxanes suitable for use as diffusion pump oils. More particularly, the present invention is directed to employing Grignard reactions for the preparation of 1,3,5-trimethyl, 1,1,3,5,5-pentaphenyltrisiloxane which is eminently suited for use as a diffusion pump oil.

2. Brief Description of the Prior Art

Organic liquids or oils have been in use for a long time as diffusion pump oils. In order to be suitable for use as a diffusion pump oil, an oil must have low volatility and yet be vacuum distillable. Perhaps even more importantly, the oil must have a very high degree of thermal stability.

It was recognized a long time ago in the prior art that certain organo-siloxane compounds readily meet the above-noted requirements and therefore are well suited for use as diffusion pump oils. More particularly, it was recognized in the prior art that certain methyl and phenyl substituted trisiloxanes serve well as diffusion pump oils. Still more particularly, British Pat. No. 787,800 describes the synthesis and use of 1,3,3,5-tetramethyl,-1,1,5,5-tetraphenyltrisiloxane as a diffusion pump oil. British Pat. No. 808,613 describes the preparation and use of 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane as a diffusion pump oil.

Although the application of the above-noted specific compounds as diffusion pump oils has been sustantially universally accepted in the art, the use of these compounds still suffers from the disadvantage of relatively high costs. The high costs are principally occasioned by the general inadequacy, and more particularly by the low yields of the state-of-the-art synthetic processes for preparing these compounds.

For example, 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane has hitherto been prepared by co-hydrolysis of the corresponding alkyl and aryl substituted chlorosilanes, by catalytic co-polymerization of tetraphenyldimethyldisiloxane and phenylmethylsiloxane, or by the reaction of the sodium salt of diphenylmethylsilanol with phenylmethyldichlorosilane. British Pat. No. 808,613 states that the "best" process for preparing 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane involves the reaction of diphenylmethylsilanol with phenylmethyldichlorosilane. As is stated in British Pat. No. 787,800, the "best" process for preparing 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane involves the reaction of diphenylmethylsilanol with dimethyldichlorosilane.

Because experience has shown that the above-noted two processes are relatively uneconomical due to their low yield and excessive side product formation, the principal object of the present invention was to provide an alternative synthetic route to alkyl and aryl substituted oligosiloxanes in general, and to 1,3,5-trimethyl,-1,1,3,5,5-pentaphenyltrisiloxane in particular. The desired synthetic route was expected to be more economical principally because of lesser degree of side product formation and hence better yields.

The present invention utilizes aryl Grignard reagents to replace alkoxy leaving groups either in already formed alkyl, alkoxy substituted oligosiloxanes or in alkyl-alkoxy substituted silane precursors of the desired oligosiloxanes. In order to properly illuminate the background chemistry of the present invention, attention is directed to the book "Chemistry and Technology of Silicones", written by Walter Noll, Academic Press, New York, San Francisco, London, 1968 and particularly to pages 41-49 thereof. The above-noted pages discuss the use of organometallic reagents, such as Grignard reagents in the chemistry of silicones. Other references of interest to the present invention are the following: "Organosilicon Compounds" by C. Eaborn, Butterworth Scientific Publications, 1960 (London), pages 307-312, and "The Partial Hydrolysis of Methyltrimethoxysilane" by M. M. Sprung and F. O. Guenther, J. Am. Chem. Soc., 77, 4173 (1955).

It is an object of the present invention to provide a relatively economical synthetic process for the preparation of alkyl and aryl substituted oligosiloxanes, and particularly for the preparation of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane.

It is another object of the present invention to provide a synthetic process for the preparation of alkyl and aryl substituted oligosiloxanes wherein the formation of undesirable halogenated biphenyls is avoided.

In accordance with the present invention, an oligosiloxane or a silane containing alkyl and alkoxy substituents is reacted with an aryl Grignard reagent to provide, respectively, an oligosiloxane or silane having alkyl and aryl substituents.

According to one aspect of the present invention, a compound having the general formula of Alkyl.sub.1 --Si--(OAlkyl.sub.2).sub.3 wherein Alkyl.sub.1 has 1 or 2 carbons and Alkyl.sub.2 has 1-6 carbons, is allowed to undergo partial hydrolysis and condensation to provide the oligosiloxane having alkyl and alkoxy substituents. The oligosiloxane is then reacted with the aryl Gignard reagent to replace all of its remaining alkoxy substituents with aryl groups.

In accordance with another aspect of the present invention, the compound Alkyl.sub.1 --Si--(OAlkyl.sub.2).sub.3 is reacted with an aryl Gignard reagent in a manner so as not to replace all of the alkoxy substituents with the aryl group. The resulting alkyl-alkoxy-aryl-silanes are subjected to hydrolysis and substantially simultaneous condensation to give the desired alkyl-aryl-oligosiloxanes having no residual alkoxy groups. Trisiloxanes, and to a lesser extent disiloxanes, prepared in accordance with the present invention are useful as diffusion pump oils.

Further features and aspects of the present invention can be best understood from the ensuing detailed description and specific examples.

The following description sets forth the preferred embodiments of the present invention in such a manner that any person skilled in the chemistry of silicon compounds can use the invention. The embodiments of the invention disclosed herein are the best modes contemplated by the inventors for carrying out their invention in a commercial environment, although it should be understood that various modifications can be accomplished within the parameters of the present invention.

The present invention is directed to the relatively economical preparation of compounds having the General Formula I which are useful as diffusion pump oils. ##STR1## wherein n equals 0 to 1, Alkyl.sub.1 is an alkyl group having 1-2 carbons and R is an aryl or alkylaryl group, with the alklaryl group having 1 or 2 carbons in the alkyl chain.

As it was stated briefly in the introductory section of the present application for patent, a diffusion pump oil must have low volatility and yet be distillable in high vacuum. It also must have a very high degree of thermal stability. The limitations regarding the number of siloxane moieties, and the nature of the alkyl and aryl or alkylaryl side chains in the compounds corresponding to General Formula I are principally imposed by the above stated requirements.

Thus, it was found in practice that certain trisiloxanes (n=1) and to a somewhat lesser extent disiloxanes (n=0) have the requisite properties regarding volatility in order to serve as diffusion pump oils. Furthermore, only short alkyl substituents on the silicon atom and on the alkyl side chain of the alkylaryl group have sufficient thermal stability to withstand gradual degradation, under the high temperatures of an operating vacuum diffusion pump. Therefore, the presence of the aryl or alkylaryl substituents in the compounds of General Formula I is required to confer the requisite low volatility, while the aryl or alkylaryl groups, particularly mononuclear aryl groups such as the phenyl group, possess the requisite thermal stability.

Most preferred compounds which are prepared in accordance with the present invention are in decreasing order of preference:

1,3,5-trimethyl, 1,1,3,5,5-pentaphenyltrisiloxane (Formula II); 1,3,3,5-tetramethyl, 1,1,5,5-tetraphenyltrisiloxane (Formula III), and 1,3, dimethyl, 1,1,3,3-tetraphenyldisiloxane (Formula IV). ##STR2##

According to the process of the present invention, the compounds of General Formula I are prepared by the use of a Grignard reagent which is characterized in General Formula V,

wherein X is Cl, Br or I and R is as defined as above for General Formula I. The Grignard reaction with the reagent R--Mg--X is preferably performed in tetrahydrofurane, although other solvents conventionally used for carrying out Grignard reactions may also be used. It was found desirable in some instances to add xylene or like high boiling inert solvent to the medium wherein the Grignard reaction is conducted in order to raise the reaction temperature.

According to one aspect of the novel process of the present invention, the Grignard reaction is performed on a compound having the General Formula VI in such a manner that not all of the alkoxy substituents of the compound are replaced by the R group of the Grignard reagent.

In the compounds of General Formula VI, Alkyl.sub.1 is defined as above, and Alkyl.sub.2 is an alkyl group having 1-6 carbons.

In order to eventually obtain trisiloxanes of the type set forth in General Formula I, it is necessary to obtain in the Grignard reaction of R--Mg--X with Alkyl.sub.1 --Si--(OAlkyl.sub.2).sub.3 monaaryl and diaryl substituted alkyl-alkoxysilanes which respectively correspond to a General Formulae VII and VIII. ##STR3##

Generally speaking, both the desired monoaryl and diaryl substituted alkylalkoxysilanes are formed in the Grignard reaction, although usually some undesired triarylalkylsilane is also formed. The ratio of the several products in this Grignard reaction is influenced to some extent by the molar ratio of the Grignard reagent to the alkyltrialkoxysilane. For example when CH.sub.3 --Si--(OCH.sub.3).sub.3 is used, phenyl magnesium bromide is employed in a 1.5 to 1.75 molar ratio relative to CH.sub.3 --Si--(OCH.sub.3).sub.3.

The monoaryl and diaryl substituted alkyl-alkoxysilanes, the compounds of General Formulae VII and VIII, are usually separable from one another and from other contaminants by fractional distillation and/or fractional vacuum distillation. In some cases it is, however, advantageous to employ fractional distillation, and/or vacuum distillation merely to remove some of the more volatile monoaryl-alkylalkoxysilane (compound of General Formula VII), so as to adjust in the remaining mixture the molar ratio of the monoaryl-alkyl-dialkoxysilane (General Formula VII) to diaryl-alkylalkoxysilane (General Formula VIII) to approximately 1:2.

In accordance with one preferred embodiment and specific example of the process of the present invention wherein CH.sub.3 --Si--(OCH.sub.3).sub.3 is reacted with phenyl magnesium bromide, methylphenyldimethoxysilane is formed in excess of its desired ratio. The excess is removed by distillation so that the remaining intermediate product mixture is adjusted to have an approximate ratio of methyl-phenyldimethoxysilane to methyl-diphenylmethoxysilane of 1.2:2. The removed excess of methyl-phenyldimethoxysilane may be added to the Grignard reaction wherein CH.sub.3 --Si--(OCH.sub.3).sub.3 is reacted with phenyl magnesium bromide to increase the yield of the desired methyl-diphenylmethoxysilane.

In the next step of the process of the present invention as it is practiced in accordance with the first aspect thereof, the compounds of the General Formulae VII and VIII (present preferably in approximately 1:2 molar ratios) are heated with strong aqueous mineral base to affect hydrolysis of the remaining alkoxy groups. Substantially simultaneous condensation of the siloxane moieties yields a product mixture containing the desired compounds in accordance with General Formula I. Potassium hydroxide is well suited as the strong mineral base for affecting the hydrolysis and condensation.

Although the product mixture of the hydrolysis and condensation reaction contains other components such as cyclic tri-, and higher siloxanes as well as linear tetra-, and higher siloxanes, the desired products may be readily isolated by fractional vacuum distillation. In accordance with established procedures in the prior art, the cyclic and linear siloxane side products may be treated with strong mineral base such as KOH to cause equilibration and formation of more of the desired products corresponding to General Formula I.

In accordance with another aspect of the present invention, the alkyl-trialkoxysilane compounds of General Formula VI are partially hydrolyzed and simultaneously condensed to give intermediate alkyl-alkoxyoligosiloxanes, compounds corresponding to General Formula IX, wherein the symbols Alkyl.sub.1, Alkyl.sub.2 and n have the same meaning as above. ##STR4##

The partial hydrolysis and condensation step is carried out in analogy to the procedure described in the reference article, J. Am. Chem. Soc. 77, 4173 (1955). It is affected by heat in the presence of strong and weak aqueous mineral bases, strong and weak acids and acid anhydrides. In essence, 2/3 molar ratio is required to yield from the alkyltrialkoxysilane a trisiloxane compound corresponding to General Formula IX, wherein n=1.

In practical experience it was found that optimum results are obtained when the hydrolysis reaction is not allowed to consume all of the starting alkyltrialkoxysilane compound. Instead the hydrolysis reaction is interrupted at an earlier stage. For reasons of economy, the unreacted alkyltrialkoxysilane is recovered from the reaction mixture. Products of the hydrolysis and condensation reaction include the desired intermediate di-, and trisiloxanes corresponding to the General Formula IX, as well as undesired cyclic tri-, and higher siloxanes, and linear tetra-, and higher siloxanes.

The desired intermediate alkyl-alkoxydisiloxanes and alkyl-alkoxy-trisiloxanes may be respectively isolated in substantially pure states by distillation in vacuum. In practice however, it is sufficient to obtain a fraction which contains the desired alkyl-alkoxydisiloxanes and alkyl-alkoxytrisiloxanes substantially free of starting alkyl-trialkoxysilane and methanol.

In the next step of the process of the present invention as practiced in accordance with the herein described second aspect thereof, the compounds of General Formula IX are reacted with a Grignard reagent R--Mg--X to yield the desired products corresponding to the General Formula I.

As it was mentioned hereinbefore, the Grignard reaction is preferably performed in hot THF, containing some xylene. The Grignard reaction of the compounds of the General Formula IX forms substantially no undesirable side products, and gives a substantially 100% yield. The final products are readily obtained in substantially pure state from the Grignard reaction by fractional vacuum distillation.

In accordance with a third aspect of the process of the present invention, 1,3,3,5-tetraalkyl-1,1,5,5-tetraalkoxytrisiloxanes, the compounds corresponding to General Formula X wherein Alkyl.sub.1 and Alkyl.sub.2 are defined as above, and Z is an alkyl group of 1-2carbons, are reacted with the Grignard reagent R--Mg--X to give, in excellent yields, 1,3,3,5-tetraalkyl-1,1,5,5-tetraaryltrisiloxanes. The letter compounds are represented by General Formula XI wherein Y has the same definition as Z above. ##STR5##

As an example, 1,3,3,5-tetramethyl-1,1,5,5-tetraphenyltrisiloxane (Formula III) is obtained in this manner from 1,3,3,5-tetramethyl-1,1,5,5-tetramethoxytrisiloxane with the use of phenyl magnesium bromide. The starting material 1,3,3,5-tetramethyl-1,1,5,5 tetramethoxytrisiloxane may be obtained in accordance with the teachings of the prior art by reaction of dimethoxymethylsilanol with dimethyldicholorosilane.

Generally speaking, the use of phenyl magnesium bromide as the Grignard reagent, methoxy group as the alkoxy substituents, and methyl group as the alkyl substituents is preferred in the several embodiments of the process of the present invention.

Important advantages of the process of the present invention are improved yields and relatively low degree of side product formation as compared to the prior art. Another advantage of replacing alkoxy rather than halo groups in a Grignard reaction is the complete avoidance of the formation of halogenated biphenyls.

The following specific examples further illustrate the novel process of the present invention. It should be specifically understood that the specific parameters of the specific examples are for the purpose of fully disclosing the present invention, and not for the purpose of limiting the same. Accordingly, the scope of the present invention should be limited only by the appended claims.

CH.sub.3 --Si--(OCH.sub.3).sub.3 and H.sub.2 O containing 1.25 g/l NaOH are continuously added to an agitated 5 L reaction vessel at a ratio of one mole CH.sub.3 --Si--(OCH.sub.3).sub.3 to 0.44 moles of H.sub.2 O. The combined rate of addition of CH.sub.3 --Si--(OCH.sub.3).sub.3 and H.sub.2 O is approximately 0.5 liters/minute. The reaction vessel is heated so as to maintain an internal temperature of approximately 65.degree. C. The contents of the reaction vessel comprising the product of hydrolysis are allowed to exit by overflow into a storage container.

The hydrolyzate is subjected to distillation to remove methanol and unreacted CH.sub.3 --Si--(OCH.sub.3).sub.3 (b.p 103.degree. C. at 760 Hgmm) which is recovered in approximately 35% yield.

The remainder of the hydrolyzate is subjected to vacuum distillation to isolate 1,1,3,5-tetramethoxy-1,3,-dimethyldisiloxane (b.p 44.5.degree. C./3 mm) and 1,1,3,5,5-pentamethoxy-1,3,5-trimethyltrisiloxane (b.p 94.degree. C./5 mm) either in respectively pure states or as a mixture with one another. From 100 parts by weight of starting CH.sub.3 --Si--(OCH.sub.3).sub.3 approximately 10 parts of 1,1,3,5,5-pentamethoxy-1,3,5-trimethyl-trisiloxane and 20 parts of 1,1,3,3-tetramethoxy-1,1,-dimethyldisiloxane is obtained. The hydrolysis reaction also yields approximately 4 parts of the corresponding linear tetrasiloxane, and 2 parts of the corresponding linear pentasiloxane as well as other side products. Traces of sn, Pb and Fe ions interfere with the hydrolysis reaction. Therefore, special care must be taken to avoid these impurities. The reaction is best conducted in a glass vessel.

Into a 100 gallon capacity reaction vessel suitable for running a Grignard reaction, 12.8 kg of dry magnesium chips are added, followed by addition of 28 kg of dry THF. Subsequently, 4 kg of dry bromobenzene are added. After the formation of the Grignard reagent has initiated as evidenced by significant rise in temperature, simultaneous addition of additional bromobenzene and THF-xylene mixture (in the ratio of THF:xylene 1.0:1.3) is commenced. The bromobenzene is added at the rate of approximately 1.2 kg/min. and the THF-xylene is added at the rate of 3.0 kg/min. Cooling of the reaction vessel, and if necessary, the rate of addition of bromobenzene, are adjusted so as to maintain an internal temperature between 65.degree.-140.degree. C. The addition of THF-xylene is discontinued when a total of approximately 155 kg of the mixture has been added. When approximately 40.8 kg of bromobenzene has been added, addition of 1,3,5-trimethyl-1,1,3,5,5-trimethoxytrisiloxane is commenced at a rate of 0.8 kg/min. Alternatively, a mixture of 1,3,5-trimethyl-1,1,3,5,5-trimethoxytrisiloxane and 1,3 -dimethyl-1,1,3,3-tetramethoxydisiloxane may be added at the same rate. A total of 19.0 kg of the above-noted trisiloxane or mixed trisiloxane and disiloxane are added in this manner. During the addition of the siloxanes, the addition of bromobenzene is continued until a total of 81.6 kg of bromobenzene has been added. After the addition of bromobenzene is completed, heating is applied to the reaction vessel as necessary to maintain an internal temperature of approximately 94.degree. C. for approximately 11/2 hours. After heating for 11/2 hours, cooling is applied and 33 kg of H.sub.2 O is added. An aqueous (lower) and an organic (upper) phase separates. The organic phase contains THF, xylene and product. The THF, xylene and any residual bromobenzene are removed from the upper phase by strip distillation. The residue is subjected to fractional vacuum distillation to obtain 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane (b.p 245.degree. C. at 0.5 Hgmm), and, if applicable, 1,3,-dimethyl-1,1,3,3-tetraphenyldisiloxane (b.p. 187.degree. C. at approximately 0.5 Hgmm). The yield of the methylphenyl siloxanes is 95% or higher, based on the added methylmethoxy siloxanes.

12.4 kg of dry magnesium chips are added, into a 100 gallon capacity reaction vessel suitable for running a Grignard reaction, followed by 28 kg of THF and 4 kg of bromobenzene. When formation of the Grignard reagent has commenced, CH.sub.3 --Si--(OCH.sub.3).sub.3 and additional bromobenzene are added simultaneously at the respective rates of 1.1 Kg/min. and 0.7 kg/min. CH.sub.3 --Si--(OCH.sub.3).sub.3 is always maintained at a molar excess relative to bromobenzene in order to avoid formation of undesirable biphenyl. After a total of 42 kg of CH.sub.3 --Si--(OCH.sub.3).sub.3 and a total of 78.3 kg of bromobenzene are added, heat is applied as necessary in order to maintain an internal temperature of approximately 94.degree. C. for approximately 2 hours. Thereafter the reaction mixture is cooled, and 33 kg of H.sub.2 O is added to form two phase. The upper organic phase contains THF, methylphenyldimethoxysilane, methyldiphenylmethoxysilane, and some methyltriphenylsilane. The organic phase is strip distilled to remove THF, and the residue is subjected to fractional distillation. In the fractional distillation methylphenyldimethoxysilane (b.p. 199.degree. C. at 750 Hgmm) and methyldiphenylmethoxysilane (b.p. 150.degree. C. at 5 Hgmm) may be obtained in substantially pure states. These compounds are typically formed in approximately 1:1 ratio in the reaction. Alternatively and preferably, methylphenyldimethoxysilane is distilled out until the molar ratio of methyldiphenylmethoxysilane and methylphenyldimethoxysilane is adjusted in the residue to approximately 2.0:1.2. The concentration of these compounds is readily monitored by gas chromatography. In a typical reaction, from 100 parts by weight of CH.sub.3 --Si--(OCH.sub.3).sub.3 35 parts of methylphenyldimethoxysilane, and 51 parts of methyldiphenylmethoxysilane and 6 parts of methyltriphenylsilane are obtained.

A mixture of methyldiphenylmethoxysilane and methylphenyldimethoxysilane in the approximate molar ratio of 2.0:1.2 is refluxed with H.sub.2 O and KOH for one hour. 10 moles of H.sub.2 O and 0.08 moles of KOH are present for each 2 moles of methyldiphenylmethoxysilane. After reflux, the H.sub.2 O and CH.sub.3 OH which is formed in the reaction is distilled and trapped in a Dean-Stark-type trap, and the temperature is raised to 200.degree. C. for one hour. Thereafter, 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane and 1,3-dimethyl-1,1,3,3-tetraphenyldisiloxane are obtained by fractional vacuum distillation. In a typical reaction, from 100 parts by weight of mixed methyldiphenylmethoxysilane and methylphenyldimethoxysilane 15 parts of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane and 10 parts of 1,3,-dimethyl-1,1,3,3-tetraphenyldisiloxane are obtained.