Process for preparation of cyclic organohydrosiloxanes

Abstract: The present invention is a process for preparation of cyclic organohydrosiloxanes. The process comprises contacting an organohydrodihalosilane with a near stoichiometric quantity of water to form a mixture containing cyclic and short-chain linear organohydrosiloxanes. This mixture is distilled to separate the cyclic and short-chain linear organohydrosiloxanes from other components of the mixture. The separated cyclic and short-chain linear organohydrosiloxanes are then contacted with an additional quantity of water to effect hydrolysis and condensation of the short-chain linear organohydrosiloxanes to higher molecular weight linear organohydrosiloxanes which are easily separated from the cyclic organohydrosiloxanes by a second distillation step.

Description: The present invention is a process for preparation of cyclic organohydrosiloxanes. The process comprises contacting an organohydrodihalosilane with a near stoichiometric quantity of water to form a mixture containing cyclic and short-chain linear organohydrosiloxanes. This mixture is distilled to separate the cyclic and linear organohydrosiloxanes from other components of the mixture. The separated cyclic and linear organohydrosiloxanes are then contacted with an additional quantity of water to effect hydrolysis and condensation of the linear organohydrosiloxanes to higher molecular weight linear organohydrosiloxanes which are easily separated from the cyclic organohydrosiloxanes by a second distillation step. The present process provides cyclic organohydrosiloxanes that are essentially free of linear organohydrosiloxanes. The cyclic organohydrosiloxanes formed by the present process are useful as reactive intermediates that can be reacted with, for example, polydimethylsiloxanes to form block copolymers and with unsaturated polyethers to form surfactants, compatibilizing agents and stabilizing agents. The cyclic organohydrosiloxanes formed by the present process are also useful as a crosslinker for other siloxanes to form silicone elastomers and gels. Sauer et al., J. Am. Chem. Soc. 68 :962 (1946), observed that when methyldichlorosilane was reacted with water a highly viscous oil was formed which within a few minutes was transformed into a rubbery gel. Sauer et al. teach that the formation of this high polymer can be avoided by running the hydrolysis process at low temperature in an ether solution. This process suffers both from the need for a low temperature to be maintained and from the use of a highly flammable solvent. Curry, U.S. Patent No. 3,484,468, issued December 16, 1969, teaches making cyclic methylhydrosiloxanes by reacting a tertiary alcohol with an organohydrodihalosilane in the present of a solvent such as benzene. This process is less than optimal because it generates a chlorinated hydrocarbon as a by-product that has little value. Furthermore, the use of a tertiary alcohol as the source of oxygen is far more expensive than the water required in the present process. Miller et al., U.S. Patent No. 3,714,213, issued January 30, 1973, teaches a process for making cyclic organohydrosiloxanes by catalytically cracking and cyclizing linear polysiloxanes containing methyl substituents, hydrogen substituents and high molecular weight chain terminal groups, such as hexyldimethysilyl groups. A major drawback to this process is the synthesis of the linear polysiloxanes with the high molecular weight chain terminal groups. The present invention provides a process for high yields of essentially pure cyclic organohydrosiloxanes without the use of low temperatures, hazardous or flammable solvents, catalysts and chain terminators, as required by processes taught in the cited art. Our process comprises: (A) contacting an organohydrodihalosilane described by formula RHSiX 2 with a near stoichiometric quantity of water at a temperature within a range of 0°C. to 50°C. to form a first mixture; (B) distilling the first mixture to recover a second mixture comprising cyclic organohydrosiloxanes and short-chained polyorganohydrosiloxanes; (C) contacting the second mixture with water to effect polymerization of the short-chained polyorganohydrosiloxane to long-chain polyorganohydrosiloxanes and forming a third mixture; and (D) distilling the third mixture to recover cyclic organohydrosiloxanes described by formula (RHSiO) n , where each R is a radical independently selected from alkyls comprising one to 20 carbon atoms, alkenyls comprising one to 20 carbon atoms and aryls; X is a halogen atom; and n =3 to 12. The contacting of the organohydrodihalosilanes with a near stoichiometric quantity of water is effected in any standard reactor suitable for the hydrolysis of halosilanes. The process is conducted as a continuous process or as a batch process. The reactor is either a plug-flow type reactor or a continuous-stir type reactor. The organohydrodihalosilanes useful in the present invention are described by the formula RHSiX 2 , where each R is a radical independently selected from alkyls comprising one to 20 carbon atoms, alkenyls comprising one to 20 carbon atoms and aryls. The radical R is methyl, ethyl, propyl, butyl, tert-butyl, heptyl, hexyl, vinyl, allyl, hexenyl, phenyl, tolyl, xylyl and naphthyl. The preferred radical is when R is methyl. The organohydrodihalosilane also has two substituents X, were each X is a halogen atom. The halogen atom X is selected from chlorine, iodine, fluorine or bromine. The preferred substituent is when X is chlorine. The organohydrodihalosilane is contacted with a near stoichiometric quantity of water. In the present process, a stoichiometric quantity of water is considered to be one mole of water per mole of organohydrodihalosilane. By "near stoichiometric", it is meant that there is present in the process from 0.8 mole of water to 1.2 moles of water per mole of organohydrodihalosilane. The preferred quantity is when the process is run from 0.9 to 1.0 mole of water per mole of organohydrodihalosilane. Contact of the organohydrodihalosilane with a near stoichiometric quantity of water is effected at a temperature of 0°C. to 50°C. The preferred temperature is when the process is run within a range of 10°C. to 30°C. The temperature of the process is maintained by standard means, for example, a heating mantle or jacket surrounding the exterior of the reactor. The product of contacting the organohydrodihalosilane with the near stoichiometric quantity of water constitutes a first mixture comprising cyclic organohydrosiloxanes, short-chained linear chlorine end-terminated polyorganohydrosiloxanes and, typically, long-chained linear chlorine end-terminated polyorganohydrosiloxanes and any excess water. By the term "short-chained", it is meant those linear chlorine end-terminated polyorganohydrosiloxanes having a molecular weight sufficiently close to the cyclic organohydrosiloxanes to make separation by distillation difficult. The chlorine end-terminated linear polyorganohydrosiloxanes for purpose of brevity are hereafter referred to as short-chain or long-chain polyorganohydrosiloxanes. Hydrogen chloride gas is generated as a by-product of contacting the organohydrodihalosilane with a near stoichiometric quantity of water. If desired, this hydrogen chloride gas is recovered from the process by standard methods and used as a feed to other processes. The first mixture is distilled by standard methods to recover a second mixture comprising cyclic organohydrosiloxanes and short-chained polyorganohydrosiloxanes. Distillation is effected by standard methods for separating liquid mixtures, whereby the cyclic organohydrosiloxanes and short-chained polyorganohydrosiloxanes are recovered from the vapor phase exiting the distillation apparatus. The preferred technique for distillation is flash distillation. The optimal temperature and pressure for distilling the first mixture will depend upon the particular organohydrodihalosilane employed in the process and the resultant products. The second mixture containing the cyclic organohydrosiloxanes and short-chained polyorganohydrosiloxanes is contacted with additional water to effect polymerization of the short-chained polyorganohydrosiloxanes to long-chained polyorganohydrosiloxanes and form a third mixture. The term "long-chained" polyorganohydrosiloxanes refer to linear polyorganohydrosiloxanes having a molecular weight sufficiently different from the molecular weight of the cyclic organohydrosiloxanes to allow for separation by distillation. The amount of water contacted with the second mixture to create the third mixture is any amount sufficient to cause conversion of the short-chained polyorganohydrosiloxanes to long-chained polyorganohydrosiloxanes. Generally, from 0.01 to 10 volumes of water per volume of the second mixture is considered effective. The preferred volume of water is from 0.05 to 1.0 volumes of water per volume of the second mixture. The third mixture is distilled to recover cyclic organohydrosiloxanes. In a preferred process, before the third mixture is distilled, the third mixture is separated from any water present. Separation of the water from the third mixture is effected by standard methods for effecting phase separation of siloxanes and water, for example, gravity separation of the phases followed by decanting of a phase. Distillation of the third mixture is accomplished by standard methods as described for distilling of the first mixture. Distillation of the third mixture also results in a component containing long-chained polyorganohydrosiloxanes. These long-chained polyorganohydrosiloxanes are recovered and used as is or end-cap by standard methods to form stable end-capped long-chained polyorganohydrosiloxanes. The cyclic organohydrosiloxanes recovered from the present process are described by the formula (RHSiO) n , where R is as previously described and n is an integer from 3 to 12. Preferred is when n is an integer from 4 to 8. Most preferred is when R is methyl and n is an integer from 4 to 8. Generally, it is preferred that the cyclic organohydrosiloxanes recovered by distillation of the third mixture be washed with one or more additional quantities of water to remove any residual chlorine. The number of washes and volume of water used will depend upon the residual chlorine present in the cyclic organohydrosiloxanes and the required stability of the cyclic organohydrosiloxanes. The following example is provided to illustrate the present invention. Methyldichlorosilane was contacted with water and cyclic methylhydrosiloxanes isolated from the resultant hydrolysate. The process was conducted in a 1.1 cm x 33.5 cm Teflon® reactor operated in a plug-flow mode. About 10.5 mL/min. of methyldichlorosilane (MeHSiCl 2 ) and 1.79 mL/min. of water were fed into the reactor. The temperature of the reactor was maintained at 20°C. The product from the reactor was collected and flash distilled at a temperature of 170°C. and at a pressure of about 6.65kPa (50 mm Hg) to separated a vapor fraction containing cyclic methylhydrosiloxanes and short-chained linear polymethylhydrosiloxanes. The vapor fraction was condensed in a standard laboratory condenser and collected. The resultant liquid mixture comprising the cyclic methylhydrosiloxanes and short-chained linear polymethylhydrosiloxanes was mixed with water at a ratio of one volume of water per ten volumes of siloxanes for one minute. The water phase was allowed to gravity separate and then separated from the siloxane phase. The siloxane phase was flash distilled at temperature of 125°C. and at a pressure of 6.65kPa (50 mm Hg) to separate a vapor fraction containing the cyclic methylhydrosiloxanes. The vapor fraction was condensed in a standard laboratory condenser and analyzed by gas chromatography (GC) using a mass spectrometer (MS) as detector. This condensed vapor fraction was determined by GC/MS to be a pure mixture of cyclic methylhydrosiloxanes of formula (MeHSiO) n , where n =3 to 8.

Claims: A process for preparation of cyclic organohydrosiloxanes comprising: (A) contacting an organohydrodihalosilane described by formula RHSiX 2 with a near stoichiometric quantity of water at a temperature within a range of 0°C. to 50°C. to form a first mixture; (B) distilling the first mixture to recover a second mixture comprising cyclic organohydrosiloxanes and short-chained polyorganohydrosiloxanes; (C) contacting the second mixture with water to effect polymerization of the short-chained polyorganohydrosiloxane to long-chain polyorganohydrosiloxanes and forming a third mixture; and (D) distilling the third mixture to recover cyclic organohydrosiloxanes described by formula (RHSiO) n , where each R is a radical independently selected from alkyls comprising one to 20 carbon atoms, alkenyls comprising one to 20 carbon atoms and aryls; X is halogen atom and n =3 to 12. A process according to claim 1, where a near stoichiometric quantity of water is 0.8 mole of water to 1.2 mole of water per mole of organohydrodihalosilane. A process according to claim 1 where the second mixture is contacted with about 0.01 to 10 volumes of water per volume of the second mixture. The process according claim 2 wherein the temperature of (A) is within a range of 10°C. to 30°C., the volume of water per volume of the second mixture of (C) is from 0.05 to 1.0, R is methyl, X is chlorine and n=4 to 8 in the formula of the organohydrosiloxane of (D).

Inventors: FREEBURNE STEVEN KERRY, PUCKETT DAVID ERIC, FREEBURNE, STEVEN KERRY, PUCKETT, DAVID ERIC
Applicants: DOW CORNING, DOW CORNING CORPORATION
Application year: 1993
Main classification: C07F7
IPC codes: C07F7, C08G77