WO2008074710A1

WO2008074710A1 - Method for the production of tetramethylcyclotetrasiloxane

Info

Publication number: WO2008074710A1
Application number: PCT/EP2007/063817
Authority: WO
Inventors: Wolfgang Knies; Karin Bögershausen
Original assignee: Wacker Chemie Ag
Priority date: 2006-12-20
Filing date: 2007-12-12
Publication date: 2008-06-26
Also published as: DE102006060355A1

Abstract

The invention relates to a method for the production of 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS) by the treatment of the process waste gas produced during the conversion of dichloromethylsilane in the presence of water into linear, terminated polymethylpolysiloxanes.

Description

Process for the preparation of tetramethylcyclotetrasiloxane

The invention relates to a process for the preparation (isolation) of 2,4,6,8-tetramethylcyclotetrasiloxane, to the process products obtainable therewith and to their use in the electronics industry, in particular for the production of layers with a low dielectric constant.

2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS) is used, for example, as a precursor in CVD processes for the deposition of low-dielectric-constant layers on electronic components. However, a problem here is that TMCTS is easily decomposable and prone to gel, especially at elevated temperatures. As a result of these conversion processes, changes occur during the execution of the

CVD method, the composition and thus the Abschei- tion characteristic of a Siloxancharge, so that available electronic components no longer have the required quality. If the gagging process progresses, this eventually leads to clogging of the equipment.

In order to prevent these adverse decomposition or gelation processes, stabilizers can be added to the TiMCTS. For this purpose, a variety of stabilizers are known. The

Stabilizers are added to the TMCTS in a separate step. One group of conventional stabilizers includes polymethyl polysiloxanes. For example, JP-A 7145179 teaches the addition of 1 to 20% by weight of linear polymethylpolysiloxanes, such as 1, 1, 1, 3, 5, 5, 5-heptamethyltrisiloxane

(BTMS), as a stabilizer for low molecular weight polymethylcyclo polysiloxanes (4 to 8 Si atoms), such as TMCTS. It is emphasized that the stabilizing effect of polymethylpolysiloxanes decreases if their proportion, based on polymethylcyclopolysiloxane, drops below 1% by weight. The GB-A

2405404 describes on the basis of stability investigations that batches of TMCTS containing 1% by weight of BTMS as the sole state to a considerable extent tend to decompose and gel.

For the stabilization of polymethylcyclopolysiloxanes, a multiplicity of further additives are known from US Pat. No. 2,858,697, GB Pat. No. 2,405,404 and WO Pat. No. 2,004,027,110, in addition to polymethylpolysiloxanes such as BTMS. These are, for example, weakly acidic or neutral representatives from the class of β-diketones, carboxylic acids, phenols, polyols or anhydrides, or compounds known as radical scavengers, such as 2,6-di-tert. ~ Butyl-4-methylphenol (BHT).

TMCTS is accessible via different synthetic routes. Thus TMCTS is obtainable by the process described in JP-A 6080680 by hydrolysis of monomeric silanes such as dichloromethylsilane in the presence of a solvent mixture consisting of tetrahydrofuran, a hydrocarbon and water. In JP-11322934 A2 are reactions of dihalosilanes of the general formula H _n R ₂ - _I1 SiXa (with n = 0 to 2; R = organic group (alkyl, aryl, alkenyl, amino group); X = halogen (F, Cl, Br)) with carbonates (eg D ₂ CO ₃ , MgCO ₃ ) or metal oxides such as alkali metal oxides (eg Li ₂ O, NaO) for the preparation of organosiloxanes of the general formula H _n R ₂ - _n Si (-O-) ₂ described ,

In an alternative process route, the synthesis of TMCTS is effected by depolymerization of polysiloxanes. As is described in JP-A 2000169487 the synthesis of Polymethylcyclopolysiloxanen such as TMCTS from depolymerization of linear or cyclisehen polymethylpolysiloxanes under heating and in Ge ^¬ genwart of metal oxides such as alumina. JP-A2 2000086766 describes the depolymerization of polysiloxanes with heating in the presence of an acidic solid catalyst (eg acidic zeolite) and addition of a certain amount of water. Distillation in vacuo gives mixtures of different cyclosiloxanes. Also in US 4,895,967 linear polyorganosiloxanes on a fixed bed catalyst (eg, acidic zeolite, modified with sulfuric acid molecular sieves) in a temperature range between 200 to SOO ⁰ C and optionally depolymerized in vacuo, wherein mixtures of zycli- polysiloxanes are distilled off comprising TMCTS.

No. 5,395,956 describes a two-step process in which first dichloroorganosilane is hydrolyzed with a stoichiometric amount of water and then in the presence of an acidic rearrangement catalyst (e.g., protic acids such as hydrogen chloride, sulfuric acid, chlorosulfonic acid or Lewis acids such as acidic zeolites) in an inert solvent

(e.g., alkanes) is converted to a mixture of different cyclic polyorganopolysiloxanes.

The known methods for obtaining stabilized TMCTS are associated with economic disadvantages. The preparation of TMCTS starting from dihalogenorganosilanes, such as, for example, dichloromethylsilane, has the disadvantage that, after the synthesis of TMCTS, it is necessary to mix with one or more stabilizers. The stabilizers must be prepared and purified in a separate step. Depolymerizations of polysiloxanes lead to complex mixtures of different cyclosiloxanes and low yield of TMCTS. The isolation of TMCTS from such complex mixtures is very expensive.

Against this background, the task was to provide stabilized, high-purity TMCTS in an efficient manner.

Surprisingly, it has been found that the process exhaust gas obtained in the synthesis of linear, end-stopped polymethylpolysiloxane contains TMCTS and polymethylpolysiloxanes which have stabilizing effects on TMCTS.

The invention relates to a process for the preparation of 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS) by working up the process exhaust gas formed in the reaction of dichloromethylsilane in the presence of water to form linear, end-stopped polymethylpolysiloxanes. The process for the reaction of dichloromethylsilane in the presence of water for the preparation of linear, end-blocked polymethylpolysiloxanes is known to the person skilled in the art and is analogous to W. NoIl (Chemie und Technologie der Silicones, page 163 to page 168, 2nd edition Weinheim, 1968). and the references cited therein.

The process can be carried out in water or a mixture of water and one or more organic solvents. Both water-soluble (solubility j> 10 g / l) and water-insoluble (solubility 1 1 g / l) organic solvents (solubility under normal conditions 23/50 in accordance with DIN 50014) are suitable here. Preferred organic solvents are ethers, such as tetrahydrofuran, dioxane, diethyl ether, dibutyl ether, aliphatic or aromatic hydrocarbons, such as toluene, xylene, or halogen-containing organic solvents, such as trichlorethylene. Particularly preferred are mixtures containing toluene and water.

The reaction of dichloromethylsilane with water is preferably carried out at temperatures of -25 ° C to 120 ⁰ G, more preferably at temperatures of 0 ⁰ C to 50 ⁰ C, and most preferably at temperatures of 0 ⁰ C to 35 ⁰ C.

Preferably, dichloromethylsilane is added to an excess of water and then one or more organic solvents are added. Dichloromethylsilane is hydrolyzed. The or, where appropriate, the organic solvents are able to dissolve the hydrolysis products of dichloromethylsilane and, when water-insoluble organic solvents are used, to remove the hydrochloric acid formed during the hydrolysis and predominantly in the aqueous phase. Under these conditions, predominantly the formation of linear polymethylpolysiloxanes occurs. The hydrolysis of dichloromethylsilane can be carried out continuously or batchwise.

The composition of the hydrolysis products of di-chloromethylsilane can be controlled by the choice of the reaction conditions. The proportion of low-polymer cyclopolysiloxanes (4 to 8 Si atoms) is usually between 3 and 75% by weight, based on the total weight of the hydrolysis products.

Finally, the endstopperation takes place in the manner known to those skilled in the art, for example by reaction of the hydrolysis products with compounds of the formula Hal-Si (R ') ₃ , where Hal is a halogen atom and R' is preferably a linear, branched or cyclic radical with 1 to 10 C atoms, particularly preferably having 1 to 5 C atoms. Preferably, Hal is fluorine, chlorine, bromine or iodine, particularly preferably chlorine. Examples of preferred radicals R 'are methyl, ethyl, n-propyl, i-propyl, n-butyl, s-butyl or t-butyl. Particularly preferred radicals R 'are methyl and ethyl. A preferred compound of the formula Hal-Si (R ') ₃ is trimethylchlorosilane.

The volatile hydrolysis products obtained during the hydrolysis are contained in the process exhaust gas. In addition to 2,4,6,8-tetramethylcylotetrasiloxane (TMCTS), the process offgas may contain further cyclic polymethylpolysiloxanes such as, for example, pentamethylcyclopentasiloxane, hexamethylcyclohexasiloxane, heptamethylcycloheptoxysiloxane, octamethylcyclooctasiloxane and also higher methylsiloxane cycles C≥9Si atoms). Furthermore, the process exhaust gas contains linear polysiloxanes, such as those of the general formula R ₃ Si (O-SiR ₂ J _n -OSiR ₃ , where R is selected from the group comprising hydrogen and methyl groups and n in particular can assume values from 0 to 8 ,

The condensate of the process offgas generally contains from 3 to 75% by weight of 2, 4, 6, 8-tetramethylcyclotetrasiloxane (TMCTS), 0.1 to 5% by weight 1, 1, 1, 3, 5, 5, 5 Heptamethyltrisiloxan (BTMS) and other low molecular weight, linear polysiloxanes of the above formula R ₃ Si (0-SiR ₂ ) _n -OSiR ₃ , in particular hexamethyldi siloxane, and further of the abovementioned polymethylcyclopolysiloxanes, but in particular pentamethylcyclopentasiloxane, and also solvents, the details in% by weight being based on the total weight of the condensate of the process offgas.

Preferably, the proportion of BTMS 0.01 to 10 wt .-%, based on the total weight of the condensate of the process exhaust gas.

The volatile hydrolysis products (process offgas) are preferably distilled off in vacuo from the linear, end-blocked polymethyl polysiloxanes and then condensed. The distillation of the process off-gas is preferably carried out immediately after the end-stop of the hydrolysis products of dichloromethylsilane, i. preferably without any further chemical transformation being carried out. The condensate of the process exhaust gas is purified by one or more further, preferably 1 to 3, distillations to high purity 2,4,6,8-tetramethylcyclotetrasiloxane (TMCTS). The distillation is conducted such that the proportion of TMCTS in the distillate is at least 98% by weight, preferably at least 99% by weight, more preferably at least 99.9% by weight.

The distillation can be carried out under atmospheric pressure or in vacuo, batchwise or continuously. Preferably, distilled at atmospheric pressure (DIN 50014). In this case, the columns known to those skilled in the art can be used, such as, for example, packed columns, packed columns (for example Montz packing) or columns with structured bottoms (for example Oldershaw column, bubble-capped columns). The

Temperature of the distillation template (bottom temperature) is preferably ≤ 170 ⁰ C.

The distillation can also be conducted in such a way that in addition to TMCTS in the abovementioned amounts, stabilizing polymethyl polysiloxanes are also isolated for TMCTS. The proportion of stabilizing Polymethylpolysi- Loxanes in the distillate <2 wt .-%, particularly preferably <0.1 wt -.%.

The proportion of one or more stabilizers in the composition according to the invention can be adjusted via the distillation parameters. Preferably, columns having at least 8 theoretical plates, more preferably columns having at least 10 theoretical plates, and most preferably, columns having at least 12 theoretical plates are used. By reducing the number of theoretical plates in the columns used, the proportion of one or more stabilizers in the composition according to the invention can be increased.

The distillations are preferably carried out under protective gas, preferably under a nitrogen or inert gas atmosphere such as argon atmosphere.

The distillatively separated linear and cyclic polymethyl polysiloxanes can be recycled to the process according to the invention and thus recycled.

Another object of the invention is a composition comprising 2, 4, 6, 8-tetramethylcyclotetrasiloxane (THCTS) and optionally one or more stabilizers obtainable by working up in the reaction of dichloromethylsilane in the presence of water to linear, endstopper Polymethylpolysiloxanen resulting process exhaust gas.

Preferred stabilizers are one or more linear polymethylpolysiloxanes of the general formula R ₃ Si (O-SiRa) _n -OSiR ₃ , where 0 ≦ _n ≦ 8 and R is selected from the group comprising hydrogen and methyl groups. A particularly preferred stabilizer is 1,1,1,3,5,5,5-heptamethyltrisiloxane (BTMS).

The proportion of the stabilizers in the composition according to the invention is preferably 0.01 to 2.0 wt .-%, especially preferably 0.01 to 0.1 wt .-%, each based on the total weight of the composition according to the invention.

2, 4, 6, 8-tetramethylcyclotetrasiloxane (TMCTS) can be obtained in the form of a very stable composition by the process according to the invention. It is not necessary to add a stabilizer to the TMCTS in a separate step. In addition, the composition according to the invention is accessible in a very economical manner, since process gases from the production of linear, end-blocked polymethyl polysiloxanes are recycled for its production. So far, the process exhaust gas has been recycled to the reactor for the synthesis of linear end-capped polymethyl polysiloxanes or disposed of as expensive waste.

The composition according to the invention is suitable, for example, as a precursor for CVD processes for depositing silicon oxide layers on substrates. Since layers with low dielectric constants are obtained in this way, the composition according to the invention is suitable in particular for the production of electronic components.

The following example provides a detailed explanation of the invention and is in no way to be interpreted as a restriction.

Example:

A condensate of the process offgases from the preparation of linear, end-blocked polymethylpolysiloxanes by hydrolysis of methyldichlorosilane and endstopper was distilled at atmospheric pressure (DIN50014) and a bottom temperature of 139 to 145 ⁰ C via a Füllkδrperkolonne (height: 1 m) with 10 theoretical plates. The distillate was removed at a head temperature of 133 to 134 ⁰ C. In this first distillation, the composition comprising TMCTS was freed in particular of higher boilers. Table 1: Composition of the process waste gas and the first distillate:

a) The composition was determined by gas chromatography

The distillate of the first distillation was subjected to a second distillation with a sieve tray column with 12 theoretical plates under otherwise identical distillation conditions as in the first distillation.

Table 2: Composition of the distillate of the second distillation:

a) The composition was determined by gas chromatography

To obtain high purity TMCTS, the distillate of the second distillation was fed to a third distillation, again using the distillation conditions of the second distillation. The distillate of the third distillation contained a very small amount of BTMS and no other components. Table 3: Composition of the distillate of the third distillation:

a) The composition was determined by gas chromatography

Stability studies:

For stability testing, a composition prepared according to the invention was kept in a screwed steel tube for 7 days at 110 ⁰ C. At the end of the stability study, the composition still had the same consistency and accordingly had only very low levels of decomposition products in the GC.

Table 4: Stability study:

a) The composition was determined by gas chromatography

It is described in JP-A 7145179 that the effectiveness of the stabilizing effect of, for example, BTMS on a composition containing TMCTS and BTMS becomes lower when the proportion of the stabilizer based on the weight of the TMCTS is ≦ 1 wt%. It is also described in GB-A 2405404 that batches of TMCTS containing 1 wt .-% BTMS are already decomposed or gelelt after 72 hours at 80 ⁰ C to a considerable extent. Surprisingly, it could be observed in the compositions according to the invention containing TCMTS and very low proportions of BTMS as stabilizer that no or no significant decomposition reactions occurred during storage at room temperature (DIN 50014) in air over the entire observation period (Table 5, lot 1 and lot 2). Contrary to the observation described in JP-A 7145179, the batch proved to have the lower proportion of BTMS, during which no decomposition or change in composition (Table 5, lot 2) was found to be more stable.

Table 5: Long-term stability study:

a) The composition was determined by gas chromatography b) Storage under standard conditions according to DIN 50014.

Claims

Claims:

l. A process for the preparation of 2, 4, 6, 8-tetramethylcyclo tetrasiloxane (TMCTS) by working up the resulting process in the implementation of dichloroethylsilane in the presence of water to linear, endgestopperten Polyπaethylpolysiloxanen.

2. The method according to claim 1, characterized in that the workup is carried out by one or more distillations.

3. The method according to claim 2, characterized in that the work-up is performed so that the proportion of TMCTS in the distillate is at least 98% by weight.

4. The method according to claim 2 to 3, characterized in that the work-up is performed so that in addition to TMCTS stabilizing Polysilethylpolysilo- xanes are still isolated for TMCTS.

5. The method according to claim 2 to 4, characterized in that columns are used with at least 8 theoretical plates for distillation.

6. Composition comprising 2, 4, 6, 8-tetramethylcyclotetrasiloxane (TMCTS) and optionally one or more stabilizers obtainable by working up the resulting in the implementation of dichloromethylsilane in the presence of water to linear, end-stopped polymethylpolysiloxanes process exhaust gas.

7. The composition according to claim 6, characterized in that as stabilizers one or more linear polymethyl polysiloxanes of the general formula R ₃ Si (0-SiR ₂ ) _n - OSiR _{3 are} included, wherein n = 0 to 8 and R is selected from the group comprising hydrogen and methyl groups. Use of a composition according to claim 6 or 7 as a precursor for CVD processes.