Method for Preparing Asymmetrical Disiloxanes

The present invention relates to a method for preparing disiloxanes and more particularly to a method for preparing asymmetrical disiloxanes.

In the preparation of asymmetrical disiloxanes by the cohydrolysis of, for example, trimethylchlorosilane and dimethylchlorosilane, it is impossible to produce asymmetrical disiloxanes in any quantity which substantially exceeds the amount of available symmetrical disiloxane. (See Journal fur Praktische Chemie, Vol. 11, 1960, pp. 336-340).

German Patent Application No. 30 02 683 (published Aug. 7, 1980, Inventor: N. N. Novichy) describes a method for preparing pentamethyldisiloxane by reacting trimethylsilane with dimethylchlorosilane at a low temperature in the presence of pyridine. The use of pyridine in large scale production creates problems, particularly since it is toxic and presents a health hazard. In addition, the expense of recovering the pyridine from the resultant product makes the process very unattractive on a commercial scale.

Therefore, it is an object of the present invention to provide a method for preparing asymmetrical disiloxanes. Another object of the present invention is to provide a method for preparing improved yields of asymmetrical disiloxanes. A further object of the present invention is to provide a method for preparing asymmetrical disiloxanes in the absence of pyridine.

The foregoing objects and others which will become apparent from the following description are accomplished in accordance with this invention, generally speaking, by providing a method for preparing asymmetrical disiloxanes which comprises reacting (a) a silane containing three Si-bonded monovalent organic radicals of which at least one may be substituted with Si-bonded hydrogen and an SiOC-bonded monovalent organic radical with (b) a symmetrical disiloxane which is different from the disiloxane obtained from the hydrolysis of silane (a) and contains at least two identical substituents in the presence of water and an acid.

In accordance with the method of this invention, a silane (a) of the formula

where the R radicals, which may be the same or different, represent hydrogen or monovalent SiC-bonded organic radicals and R.sup.1 represents an SiOC-bonded monovalent organic radical, is reacted with a symmetrical disiloxane of the formula

where R is the same as above, R' may be the same as R, but differs from R, and n is 1, 2 or 3, preferably 1, in the presence of water and an acid. The symmetrical disiloxane (b) is prepared prior to the addition of the silane (a).

Preferred examples of organic radicals represented by R and R' are alkyl radicals having from 1 to 3 carbon atoms per radical, such as the methyl, ethyl and n-propyl radical, alkenyl radicals having from 1 to 3 carbon atoms per radical, such as the vinyl and the allyl radical, and substituted hydrocarbon radicals having from 1 to 6 carbon atoms per radical, such as the chloromethyl, gamma-chloropropyl and the gamma-methacryloxy-propyl radical.

It is preferred that R.sup.1, which represents an SiOC-bonded monovalent organic radical, consist of carbon, hydrogen and optionally oxygen. Preferred examples of R.sup.1 are methyl, ethyl, propyl, isopropyl, tert-butyl and the methoxyethylene radical, as well as the acetyl radical.

In accordance with a preferred embodiment of this invention, the silane (a) is a silane of the formula

wherein R.sup.1 is the same as above, and the disiloxane (b) is a tetramethyldisiloxane, i.e., a disiloxane of the formula

An example of an asymmetrical disiloxane obtained from the method of this invention is pentamethyldisiloxane, i.e., a disiloxane of the formula

Another example of silicon-containing reactants which may be employed in the method of this invention is the reaction of vinyldimethylethoxysilane with hexamethyldisiloxane in the presence of acid and water.

Simple experiments will determine whether it is better to use allyldimethoxysilane and hexamethyldisiloxane as silicon-containing reactants, or whether trimethylethoxysilane and 1, 3-diallyl-1,1,3,3-tetramethyldisiloxane should be used.

Preferably the molar ratio of the silane (a), i.e., trimethylethoxysilane, to the disiloxane (b), i.e., the tetramethyldisiloxane, is in the range of from 2:1 to 4:1. If the molar ratio of the silane (a) to the disiloxane (b) is smaller than 2:1, optimal yields cannot be obtained. An increase in molar ratio of the silane (a) to the disiloxane (b) does not produce any additional advantages.

The amount of water is preferably between about 0.5 to about 1.5 moles per mole of silane (a). If the molar ratio of water to the silane (a) is less than about 0.5:1, the yields are no longer optimal. Molar ratios of water to silane (a) greater than 1.5:1 do not yield any additional advantages.

It is preferred that the acid employed in the method of this invention be a protonic acid having a negative logarithm of the acid constant of no more than 1 (pK.sub.s or pK.sub.a, see H. R. Christen, "Grundlagen der allgemeinen und anorganischen Chemie", Frankfurt/Main, 1973, page 354). Examples of such acids are hydrochloric acid, sulfuric acid and phosphoric acid, with hydrochloric acid being the preferred acid because in the presence of water it has only a slight equilibrating effect and because it is readily available.

The acid is preferably used in an amount of from 0.01 to 2 percent by weight and more preferably from about 0.1 to 1 percent by weight, based on the total weight of the silane, disiloxane, water and acid.

Preferably the method of this invention is carried out at temperatures between -10.degree. C. and 50.degree. C., and more preferably between 0.degree. and 25.degree. C. in order to prevent equilibration.

Furthermore, it is preferred that the process of this invention be performed in stages and that the silane (a) be added to a mixture containing water, disiloxane (b) and acid.

Also, it is preferred that the acid be neutralized by the addition of, for example, sodium bicarbonate as soon as the reaction is completed. The mixture can then be washed with water and the disiloxane can be dried and purified by distillation.

The method of this invention can provide yields of the order of from 80 to 95 percent by weight based on theoretical.

To a mixture having an initial temperature of 0.degree. C. and consisting of 1340 gm (10 mole) of tetramethyldisiloxane, 220 gm of 1N HCl and 140 gm of water, i.e., containing 0.47 percent by weight of HCl, are added dropwise, 2360 gm (20 mole) of trimethylethoxysilane and at a rate such that the temperature does not exceed 5.degree. C. As soon as the addition of the trimethylethoxysilane is complete, the acid is neutralized by the addition of 40 gm of sodium bicarbonate. The resultant mixture is then washed 5 times with 500 gm of water each and dried with sodium sulfate. According to the .sup.1 H-NMR spectrum, the mixture consists of 90 mole percent of pentamethyldisiloxane and 10 mole percent of hexamethyldisiloxane. The resultant product is distilled in a packed column and 2350 gm of pentamethyldisiloxane is recovered (80 percent by weight of theoretical).