DE102017214382B4 - Process for preparing siloxane compositions with low silanol and hydrocarbyloxy group content - Google Patents

Abstract

A process for the preparation of a mixture (A) of siloxanes which contains siloxanes of the general formula V(R63SiO1/2)o(R62SiO2/2)p(R6SiO3/2)q(SiO4/2)r,(V) in which o are integers Values from 2 to 60,p are integer values from 0 to 4000,q are integer values from 0 to 50,r are integer values from 0 to 30 and R6 is a radical R1, in which a mixture (B) of siloxanes, the siloxanes of the general formula I( R13SiO1/2)a(R12SiO2/2)b(R1SiO3/2)c(SiO4/2)d,(I)in dera integer values from 2 to 60,b integer values from 0 to 4000,c integer values from 0 to 50,d are integer values from 0 to 30 and R1 is a monovalent C1-C20 hydrocarbon radical or halogen, and siloxanes of the general formula II(R23SiO1/2)e(R22SiO2/2)f(R2SiO3/2)g(SiO4/2) h,(II)in dere are integer values from 2 to 60,f integer values from 0 to 4000,g integer values from 0 to 50,h integer values from 0 to 30,R2 is a radical R1 or OR3 and R3 is a radical H or C1-C20 hydrocarbon radical mean, the siloxanes of a General formula II have at least one radical OR3per molecule, wherein the sum of the proportions of all siloxanes of general formula I is at least 1% by mass, based on the entire mixture (B), wherein in the mixture (B) the molar ratio of all radicals R1 is OR310000 to 5 for all radicals, and the sum of the proportions of all siloxanes of the general formula I is at least 50% by mass, based on the entire mixture (A), is reacted with silicon compounds selected from silanes of the general formula III and siloxanes of the general formula IV and mixtures thereofR4(3-n)Si-Hn(III) in which the values 1 or 2 and R4 are a radical R1,(R53SiO1/2)i(R52SiO2/2)j(R5SiO3/2)k( SiO4/2)l,(IV)in whichi are integers from 2 to 60,j is integers from 0 to 4000,k is integers from 0 to 50,l is integers from 0 to 30, andR5 is a radical R1 or H, where in the Siloxanes of the general formula IV at least one radical R5 per molecule, the B has the meaning H, in the presence of a Lewis acid selected from A1C13 and a boron compound of the general formula BR6R7R8, with the radicals R6, R7, R8 being independently halogen or unsubstituted or halogen-substituted C4-C16 hydrocarbyl radical, and mixtures thereof .

Description

The invention relates to a process for preparing a mixture (A) of siloxanes, in which a mixture (B) comprising siloxanes containing alkoxy groups and containing silanol groups is treated with H-sil(ox)anes and a catalyst to reduce the alkoxy and silanol groups becomes.

Alkoxy groups in siloxanes are labile groups, since they form silanol groups in connection with water, where chain degradation processes, usually with the formation of siloxane cycles, start. The release of volatile compounds (alcohols, siloxane cycles) is also undesirable. In the case of alkoxy groups with more than one carbon atom, alkene elimination and silanol formation also take place at higher temperatures as a further process.
Alkoxy groups and also silanol groups (resulting from alkoxy groups or present as a result of production) therefore generally reduce the stability and thus also the quality of siloxanes.

In particular when siloxanes are used as heat transfer oils, siloxanes containing alkoxy groups are unsuitable because of the high thermal stress, since the alkoxy groups decompose to form silanols with elimination of alkenes. Due to their high vapor pressure, the alkenes that are split off increase the pressure in the system and the silanols formed react further by condensation with elimination of water or by attacking the Si-C or Si-O bonds. The water split off can in turn attack Si-C or Si-O bonds, producing an alkane (H-R) and a silanol (Si-OH) in the case of a Si-R bond and a silanol (Si-OH) in the case of a Si-OR' bond Silanol (Si-OH) and a HOR` (R': C1-C20 or Si) are formed. In any case, the reactions described lead to an undesirable change in the original organosiloxane, which has negative effects on the intended use of the organosiloxane.

In order to avoid these problems, it is essential to produce the desired organosiloxanes with very low alkoxy contents.

For example describes DE602004001641 a process for preparing low-alkoxy and low-silanol silicone oils (organopolysiloxanes) by converting unwanted alkoxy- and silanol-containing siloxanes, which are present as impurities in alkoxy- and silanol-free organosiloxanes, by means of an acid-catalyzed reaction in the presence of disiloxanes. As a result, organopolysiloxanes with low alkoxysilane or silanol contents are obtained. The disadvantage here, however, is that the reaction is acid-catalyzed, especially with sulfuric acid. It is known that organopolysiloxanes equilibrate under acid catalysis and thereby rearrange into low-molecular and higher-molecular organosiloxanes. Another disadvantage of the process is that the sulfuric acid has to be removed in a complicated manner. However, the removal of the acid is essential, especially when used as a heat transfer oil, since traces of acid cause the organosiloxane to decompose (equilibrate) at high temperatures.

If the required acid is used in the form of acidic resins (e.g. (poly)acrylic acid resin, (poly)methacrylic acid resin, (poly)phosphoric acid resin, sulfonic acid resin, sulfuric acid resin) [J. organomet. Chem. 340, 31-36, 1988], which eliminates the time-consuming removal of the acid by extraction and neutralization, and other disadvantages result. The acid-catalyzed reaction of the alkoxy-containing to alkoxy-free organosiloxanes releases alcohol, which reacts with the acidic resins to form the corresponding esters. This deactivates the acidic resin, which makes protons (H ⁺ ) available catalytically. The resin then has to be regenerated or discarded before it can be used again. As a result, the method is cumbersome and expensive and is therefore not economically feasible. In addition, the phenomenon already described above also occurs here, namely that organosiloxanes equilibrate under acid catalysis and rearrange in the process into low-molecular and higher-molecular organosiloxanes, which is not desired.

Alkoxy- and silanol-containing organosiloxanes (prepared by acid-catalyzed equilibration) can also, as in EP1398338 described, be treated with a base. However, it is not possible to reduce the alkoxy and silanol content to below 0.2 mol %. In addition, the basic post-treatment leads to the formation of high-molecular compounds.

Alkoxy- and silanol-containing organosiloxanes (produced by acid-catalyzed equilibration) can also be post-treated thermally (heating to 250-1000°C under inert conditions). It is known that alkoxy groups are decomposed under thermal stress (e.g.: Si-OCH ₂ CH ₃ → Si-OH+CH ₂ =CH ₂ ). The disadvantage of this process is that silanol groups are formed, which then react further by condensation with elimination of water or by attacking Si—C or Si—O bonds. The water split off can in turn attack Si-C or Si-O bonds, producing an alkane (HR) and a silanol (Si-OH) in the case of a Si-R bond and a silanol in the case of a Si-OR bond (Si-OH) and an alcohol (HOR) are formed. The alcohol thus released can attack Si-O-Si bonds under the prevailing conditions, whereby, for example, Si-OH and RO-Si groups result, which in turn can continue to react as described above. In any case, the reactions described lead to an undesirable change in the original organosiloxane, which has negative effects on the intended use of the organosiloxane.

Furthermore, it is possible to obtain the desired organosiloxanes by a Piers-Rubinsztajn reaction ( US7241851 ) starting from alkoxysil(ox)anes and H-sil(ox)anes in one step (rkt.: Si-OR + H-Si → Si-O-Si + RH). The disadvantage of this process is that alkoxysil(ox)anes and in particular H-sil(ox)anes are generally significantly more expensive and less available than the starting materials (water glass, chlorosilanes, alkoxysilanes) usually used for the production of siloxanes. Another disadvantage when siloxanes are exclusively prepared by the Piers-Rubinsztajn reaction is that the reaction has a very high exotherm and allows for poor reaction control. In addition, large amounts of combustible gases are formed, which also leads to foaming of the reaction mixture.

Alkoxy-free silicone resins can be DE102015200704 be produced because alcohol is not used, however, silanol groups cannot be avoided and production is significantly more difficult in terms of process technology compared to production with alcohol as a reaction moderator.

US2003/0139287A1 discloses a process for reacting (A) polysiloxane monomers, oligomers or polymers which have at least one Si-H group per molecule with (B) polysiloxane monomers, oligomers or polymers which have at least one Si-OH group per molecule group.

CHOJNOWSKI, Julian [et al.]: Mechanism of the B(C 6 F 5) 3 - catalyzed reaction of silyl hydrides with alkoxysilanes: kinetic and spectroscopic studies. In: Organometallics, Vol. 24, 2005, no. 25, pp. 6077-6084. - ISSN 0276-7333 discloses the reaction of triorganoalkoxysilanes with triorganohydrosilanes to obtain disiloxanes.

The object was therefore to provide a simple and inexpensive process which makes it possible to produce organopolysiloxanes with a very low alkoxy and silanol content without the disadvantages described above.

The subject matter of the invention is the method described in patent claim 1.

In the process according to the invention, undesired siloxanes of the general formula II containing alkoxy groups and present as an impurity in organosiloxanes of the general formula I react with H-sil(ox)anes of the general formulas III or IV in the presence of a Lewis acid catalyst defined above . Surprisingly, it was also found that the silanol groups that interfere in other applications also react under the conditions, which represents an additional particular advantage.

This reaction is used for the aftertreatment of organosiloxanes produced on an industrial scale according to the current state of the art (for example by acid hydrolysis/condensation from chlorosilanes, alkoxysilanes, water glass) which contain alkoxy-containing and silanol-containing siloxanes as impurities.

The advantage of this method is that no Bronsted acid, no base and no greatly increased temperature has to be used. In addition, the process enables the production of almost alkoxy-free and silanol-free siloxanes in the mixture (A). After application of the process, the content of alkoxy and silanol groups in the mixture (A) is preferably below 0.1% by weight (NMR) or area % (GC), particularly preferably below 0.01% by weight ( NMR) or area % (GC), very particularly preferably below 0.001% by weight (NMR) or area % (GC). In particular, after the method has been used, the alkoxy and silanol groups cannot be detected by gas chromatography, ¹ H nuclear magnetic resonance and ²⁹ Si nuclear magnetic resonance spectrometry.

The reaction is very rapid, the by-products formed (alkanes) can be easily removed, as can excess H-sil(ox)anes of the general formulas III or IV (starting material), and the Lewis acid used can be easily removed, e.g. by filtration , or can remain in the product, as it does not negatively affect many application properties.

The hydrocarbon radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ are preferably selected independently of one another from alkyl radicals, aryl radicals and olefinically unsaturated hydrocarbon radicals. The hydrocarbon radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ are preferably, independently of one another, C1-C12 hydrocarbon radicals, in particular C1-C6 hydrocarbon radicals. Examples of the hydrocarbon radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ are methyl, ethyl, vinyl, n-propyl, i-propyl, allyl, n-butyl, i-butyl and n-pentyl -, i-pentyl, n-hexyl, i-hexyl, cyclohexyl, n-heptyl, n-octyl, i-octyl, phenyl and tolyl radicals. Methyl, ethyl, vinyl, n-propyl, n-butyl, i-octyl and phenyl radicals are particularly preferred.

The hydrocarbon radicals R ³ are preferably C1-C6 hydrocarbon radicals, in particular methyl or ethyl radicals.

The halogen radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ are preferably selected independently of one another from fluorine, chlorine and bromine, in particular from fluorine and chlorine.

The sum of a + b + c + d is preferably on average 3 to 50, particularly preferably 4 to 40, in particular 5 to 20.

The sum of e+f+g+h is preferably on average 3 to 50, particularly preferably 4 to 40, in particular 5 to 20.

The sum of i + j + k + 1 is preferably on average 3 to 50, particularly preferably 4 to 40, in particular 5 to 20.

In the general formulas I, II and IV, a, e and i are dimensioned such that the siloxanes on groups (R ¹ ₃ SiO _1/2 ) (R ² ₃ SiO _1/2 ) or (R ⁵ ₃ SiO _{1/ 2} ) are saturated.

The molar ratio of the H or C1-C20 hydrocarbon radicals in R ³ is preferably from 0.01 to 5, in particular from 0.05 to 2.

The molar ratio of all radicals R ¹ to all radicals OR ³ in the mixture (B) is preferably from 1000 to 10, in particular from 100 to 15.

The mixture (A) of siloxanes preferably consists of siloxanes of the general formula V.

The sum of o + p + q + r is preferably on average 3 to 50, particularly preferably 4 to 40, in particular 5 to 20.

The sum of the proportions of all siloxanes of the general formula I is preferably at least 95% by mass, based in each case on the mixture (A) as a whole.

The halogens of the boron compound of the general formula BR ⁶ R ⁷ R ⁸ are preferably selected independently from fluorine, chlorine and bromine, in particular from fluorine and chlorine.

The process is preferably carried out within 10 minutes to 10 hours, particularly preferably within 30 minutes to 5 hours. The process is particularly preferably carried out continuously.

The process is preferably carried out at a temperature of 0°C to 100°C, particularly preferably 5°C to 80°C, in particular 10°C to 50°C.

The process is preferably carried out at a pressure of 0.01 to 30 MPa (abs.), particularly preferably at 0.8 to 10 MPa (abs.), in particular at 1.0 to 1.2 MPa (abs.).

After the reaction of the siloxanes of the general formula II with the silicon compounds selected from silanes of the general formula III and siloxanes of the general formula IV and mixtures thereof, volatile reaction products are preferably removed by distillation, in particular under reduced pressure.

All of the above symbols of the above formulas each have their meanings independently of one another. The radicals in the formulas can be the same or different. In all formulas the silicon atom is tetravalent. The sum of all components of the siloxane mixtures is 100% by weight.

In the following examples, unless otherwise stated, all amounts and percentages are by weight, all pressures are 0.10 MPa (abs.) and all temperatures are 20.degree.

Explanation of abbreviations:

• Q = SiO _4/2
• T = _MeSiO3 /2
• D = _{Me2SiO2/ 2}
• M = _{Me3SiO1/ 2}
• M ^Vi = Me ₂ ViSiO _1/2 Et = ethyl radical
• Vi = vinyl residue
• EtO-Si = ethoxy group
• HO-Si = silanol residue

analytics

gas chromatography (GC)

The silicone oils were analyzed by gas chromatography (Agilent 7890A, DB-1701 (30 m, 0.32 mm, 1.00 µm, detector (FID; GC area %/area %)) with regard to their composition.

Measurement of the content of M, M ^Vi , D, T and Q groups ( ²⁹ Si NMR)

The proportion of M, M ^Vi , D, T and Q groups (SiO _4/2 ) in the silicone resins was determined by means of nuclear magnetic resonance spectroscopy (the ²⁹ Si nuclear magnetic resonance spectra were recorded on a Bruker Avance 300 spectrometer ( ²⁹ Si : 59.6 MHz) with a 10 mm Quadruple Resonance (QNP) probe included. Recording was performed with the INEPT pulse sequence or with the inverse gated pulse sequence (quartz glass tube, NS=1024; 2000 mg siloxane in 1000 μl of a 1% solution of Cr(acac) ₃ in C ₆ D ₆ /toluene). For this purpose, the integral of the M, M ^Vi , D, T and Q values was compared with the total sum of the integral values for M groups, D groups, T groups, Q groups, ethoxy and silanol groups (for measurement conditions, see below) (%M, % ^MVi , %D, %T, %Q, %EtO- and %HO-).

Measurement of ethoxy and silanol content ( ¹ H and ²⁹ Si NMR)

The ethoxy and silanol content was determined by means of nuclear magnetic resonance spectroscopy (the ¹ H nuclear magnetic resonance spectra were recorded on a Bruker Avance 500 spectrometer ( ¹ H: 500.1 MHz) with a 5 mm broadband (BBO) probe head (NS = 128; 50 mg siloxane in 1000 µl CDCl ₃ ).The ²⁹ Si NMR spectra were recorded on a Bruker Avance 300 spectrometer ( ²⁹ Si: 59.6 MHz) with a 10 mm quadruple resonance (QNP) probe the inverse gated pulse sequence (quartz glass tube, NS=1024; 2000 mg siloxane in 1000 μl of a 1% solution of Cr(acac) ₃ in C ₆ D ₆ /toluene).The detection of the ethoxy and silanol groups was carried out via ¹ H- NMR The respective integral was converted into ^wt . CH ₃ CH ₂ O _1/2 , Mw = 37 g/mol; silanol: HO _1/2 , Mw = 9 g/mol).

Example 1:

An alkoxy and silanol-containing silicone oil (5 g, composition: 1.24 area % QM ₂ (OEt) ₂ , 15.56 area % QM ₃ OEt, 0.61 area % QM ₃ OH, 64.0 area % % QM ₄ , 0.41 area % Q ₂ M ₄ (OEt) ₂ , 1.81 area % Q ₂ M ₅ OEt, 4.55 area % Q ₂ M ₆ ) prepared by acid catalyzed hydrolysis and condensation of Tetraethoxysilane and ethoxytrimethylsilane in toluene were dissolved in dichloromethane (3.5g) followed by the addition of pentamethyldisiloxane (384mg). 120 mg of a 0.18% strength by weight solution of B(C ₆ F _s ) ₃ in toluene were added dropwise to the stirred solution, whereupon moderate evolution of gas was discernible about 2 minutes after the addition. The reaction mixture was stirred for a further 30 min, freed from volatile constituents (among others dichloromethane, toluene, pentamethyldisiloxane) under vacuum, to remove 5.3 g of a silicone oil which has no detectable alkoxy and silanol groups (composition [0 area %=below the detection limit] : 0 area % QM ₂ (OEt) ₂ , 0 area % QM ₃ OEt, 0 area % QM ₃ OH, 0 area % Q ₂ M ₄ (OEt) ₂ , 0 area % Q ₂ M ₅ OEt ;69.8 area % QM ₄ , 19.9 area % QDM ₄ , 2.0 area % QD ₂ M ₄ , 0 area % Q ₂ M ₄ (OEt) ₂ , 0 area % Q ₂ M ₅ OEt, 5.1 area % Q ₂ M ₆ , 2.4 area % Q ₂ DM ₆ , 0.7 area % Q ₂ D ₂ M ₆ ). The example shows that it is possible to post-treat an alkoxy- and silanol-containing organosiloxane in quantitative yield, so that an organosiloxane mixture is obtained which has no detectable alkoxy and silanol groups.

Example 2:

An alkoxy and silanol-containing silicone oil (5 g, composition: 0.07 area % QM ₂ (OEt) ₂ , 4.65 area % QM ₃ OEt, 0.51 area % QM ₃ OH, 84, 95 area % QM ₄ , 0.24 area % Q ₂ M ₄ (OEt) ₂ , 2.35 area % Q ₂ M ₅ OEt, 7.73 area % Q ₂ M ₆ ) prepared by acid catalyzed hydrolysis and condensation of Tetraethoxysilane and ethoxytrimethylsilane in toluene were dissolved in dichloromethane (20 g), cooled to 0°C and then trimethylsilane was added (bubbled through) until the solution was saturated. 105 mg of a 0.18% strength by weight solution of B(C ₆ F _s ) ₃ in toluene were added dropwise to the stirred solution, whereupon moderate evolution of gas was discernible about 2 minutes after the addition. The reaction mixture was stirred for a further 30 min, freed from volatile constituents (among others dichloromethane, toluene, pentamethyldisiloxane) under vacuum, to remove 5.1 g of a silicone oil which has no detectable alkoxy and silanol groups (composition [0 area %=below the detection limit] : 0 area % QM ₂ (OEt) ₂ , 0 area % QM ₃ OEt, 0 area % QM ₃ OH, 0 area % Q ₂ M ₄ (OEt) ₂ , 0 area % Q ₂ M _s OEt 89.45 area % QM ₄ , 0 area % Q ₂ M ₄ (OEt) ₂ , 0 area % Q ₂ M ₅ OEt, 10.55 area % Q ₂ M ₆ ). The example shows that it is possible to post-treat an alkoxy- and silanol-containing organosiloxane in quantitative yield, so that an organosiloxane mixture is obtained which has no detectable alkoxy and silanol groups.

Example 3:

An alkoxy and silanol containing T silicone resin (5 g, composition: 28.93 wt% EtO-Si, 0.03 wt% HO-Si, 0.74 D and 70.30 wt% T) prepared by acid-catalyzed hydrolysis and condensation of methyltriethoxysilane (contaminated with dimethyldiethoxysilane) in toluene was dissolved in toluene (25 g), cooled to 0°C and then trimethylsilane was added until the solution was saturated (bubble through). 500 mg of a 0.18% strength by weight solution of B(C ₆ F _s ) ₃ in toluene were added dropwise to the stirred solution, whereupon vigorous evolution of gas was discernible about 2 minutes after the addition. The reaction mixture was stirred for a further 30 minutes and freed of volatile constituents (including toluene and trimethylsilane) in vacuo to give 6.8 g of a silicone resin which had no detectable alkoxy and has silanol groups (composition [0 wt% = below detection limit]: 0.00 wt% EtO-Si, 0.00 wt% HO-Si; 0.77 wt% D, 55.40 wt % T and 43.83% by weight M). The example shows that it is possible to post-treat an alkoxy- and silanol-containing silicone resin in quantitative yield, so that an organosiloxane mixture is obtained which has no detectable alkoxy and silanol groups.

Example 4:

An alkoxy and silanol containing MQ silicone resin (5 g, composition: 2.67 wt% EtO-Si, 0.27 wt% HO-Si, 47.48 wt% M and 49.58 wt% -% Q) prepared by acidic and then base-catalyzed hydrolysis and condensation of tetraethoxysilane and ethoxytrimethylsilane in toluene was dissolved in toluene (25 g), cooled to 0° C. and trimethylsilane was then added until the solution was saturated (bubble through). 500 mg of a 0.18% strength by weight solution of B(C ₆ F ₅ ) ₃ in toluene were added dropwise to the stirred solution, whereupon vigorous evolution of gas was discernible about 2 minutes after the addition. The reaction mixture was stirred for a further 30 minutes and freed of volatile constituents (including toluene, trimethylsilane) under reduced pressure, to give 5.2 g of a silicone resin which has a significantly reduced alkoxy and silanol group concentration (composition: 0.19% by weight EtO-Si , 0.02 wt% HO-Si, 56.62 wt% M and 43.17 wt% Q). The example shows that the alkoxy and silanol content can be significantly reduced even in MQ resins.

Example 5:

An alkoxy and silanol containing MQ silicone resin (5 g, composition: 2.37 wt% EtO-Si, 0.23 wt% HO-Si, 42.2 wt% M, 6.63 wt% -% M ^Vi and 48.65 wt Added saturation of the solution (bubble through). 500 mg of a 0.18% strength by weight solution of B(C ₆ F _s ) ₃ in toluene were added dropwise to the stirred solution, whereupon vigorous evolution of gas was discernible about 2 minutes after the addition. The reaction mixture was stirred for a further 30 minutes and freed from volatile constituents (including toluene, trimethylsilane) under reduced pressure, to give 5.1 g of a silicone resin which has a significantly reduced alkoxy and silanol group concentration (composition: 0.15% by weight EtO-Si , 0.02 wt% HO-Si, 47.90 wt% M, 6.43 wt% ^MVi and 45.18 wt% Q). The example shows that the alkoxy and silanol content can be significantly reduced even in MQ resins.

Claims (7)

Process for preparing a mixture (A) of siloxanes, the siloxanes of the general formula V (R ⁶ ₃ SiO _1/2 ) _o (R ⁶ ₂ SiO _2/2 ) _p (R ⁶ SiO _3/2 ) _q (SiO _4/2 ) _r , (V) contains, in which o is an integer from 2 to 60, p is an integer from 0 to 4000, q is an integer from 0 to 50, r is an integer from 0 to 30 and R ⁶ is a radical R ¹ in which a mixture (B ) of siloxanes, the siloxanes of general formula I (R ¹ ₃ SiO _1/2 ) _a (R ¹ ₂ SiO _2/2 ) _b (R ¹ SiO _3/2 ) _c (SiO _4/2 ) _d , (I) in which a is an integer from 2 to 60, b is an integer from 0 to 4000, c is an integer from 0 to 50, d is an integer from 0 to 30 and R ¹ is a monovalent C1-C20 hydrocarbon radical or halogen, and siloxanes of the general formula II (R ² ₃ SiO _1/2 ) _e (R ² ₂ SiO _2/2 ) _f (R ² SiO _3/2 ) _g (SiO _4/2 ) _h , (II) in which e is an integer from 2 to 60, f is an integer from 0 to 4000, g is an integer from 0 to 50, h is an integer from 0 to 30, R ² is an R ¹ or OR ³ radical and R ³ is an H or radical C1-C20 hydrocarbon radical, where the siloxanes of the general formula II have at least one OR ³ radical per molecule, the sum of the proportions of all siloxanes of the general formula I being at least 1% by mass, based on the entire mixture (B) is, where in the mixture (B) the molar ratio of all radicals R ¹ to all radicals OR ³ is 10000 to 5, and the sum of the proportions of all siloxanes of the general formula I is at least 50% by mass, based on the entire mixture (A ) is reacted with silicon compounds selected from silanes of general formula III and siloxanes of general formula IV and mixtures thereof R ⁴ _(3-n) Si-H _n (III) in which n is 1 or 2 and R ⁴ is a radical R ¹ , ( _R ⁵ ₃ SiO _1/2 ) _i ( _R ⁵ ₂ SiO _2/2 ) _j (R ⁵ SiO _3/2 ) _k (SiO _4/2 ) _l , (IV) in which i is an integer from 2 to 60, j is an integer from 0 to 4000, k is an integer from 0 to 50, l is an integer from 0 to 30 and R ⁵ is a radical R ¹ or H, where in the siloxanes of the general Formula IV at least one radical R ⁵ per molecule has the meaning H, in the presence of a Lewis acid which is selected from A1C1 ₃ and a boron compound of the general formula BR ⁶ R ⁷ R ⁸ , with the radicals R ⁶ , R ⁷ , R ⁸ which are independently halogen or unsubstituted or halogen-substituted C4-C16 hydrocarbyl, and mixtures thereof. procedure after claim 1 , in which the hydrocarbon radicals R ¹ , R ² , R ³ , R ⁴ and R ⁵ are independently C1-C6 hydrocarbon radicals. Process according to one or more of the preceding claims, in which, in the case of R ³ , the molar ratio of the radicals H or C1-C20 hydrocarbyl radicals is from 0.01 to 5. Process according to one or more of the preceding claims, in which the mixture (A) of siloxanes consists of siloxanes of the general formula V. Process according to one or more of the preceding claims, which process is carried out continuously. Process according to one or more of the preceding claims, which is carried out at a temperature of from 0°C to 100°C. Process according to one or more of the preceding claims, wherein the Lewis acid is selected from B (C ₆ F ₅ ) ₃ , BF ₃ , BCl ₃ and B (C ₆ F _x H _(5-x) , where x are integer values of 1 to 5, and mixtures thereof.