Patent Publication Number: US-11021575-B2

Title: Process for producing acetoxy-bearing siloxanes

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a 35 U.S.C. § 119 patent application which claims the benefit of European Application No. 18189073.2 filed Aug. 15, 2018, which is incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to a process for producing trifluoromethanesulfonic acid-acidified, end-equilibrated, acetoxy-bearing siloxanes. 
     BACKGROUND 
     As reactive starting materials for producing SiOC-based silicone derivatives, in particular SiOC-based polyether siloxanes, acetoxy-bearing siloxanes having linear or branched structure type are important compound classes. 
     The as yet unpublished European patent applications having the application numbers EP18172882.5, EP18172876.7, EP17195510.7 and EP17204277.2 are concerned with the production of SiOC-bonded polyether siloxanes, wherein trifluoromethanesulfonic acid-acidified, equilibrated acetoxysiloxanes having linear or branched structure type are used as reactive intermediates. 
     On closer investigation of the modes of production recited in these documents for producing the acetoxysiloxanes used therein the inventors have found that immediately after their production the trifluoromethanesulfonic acid-acidified acetoxysiloxanes still comprise relatively large proportions of siloxane cycles (see to this end examples 3 and 6 of the present invention) which, however, upon storage of the siloxanes at 23° C. over the course of about 2 to 3 weeks become incorporated in the respective siloxane matrix in such a way that the content of remaining siloxane cycles (D 4 +D 5 +D 6 ) corresponds to the end equilibrium established at this temperature and a pressure of 1013.25 hPa. 
     The trifluoromethanesulfonic acid-acidified, equilibrated acetoxysiloxanes resulting therefrom thus meet all quality demands that may be placed on these reactive intermediates. However, it remains desirable to achieve improved kinetics of equilibration to allow immediate further processing of the intermediates. 
     SUMMARY 
     Against this background the present invention has for its object to provide a process for producing end-equilibrated acetoxy-bearing siloxanes which advantageously accelerates the kinetics of equilibration such that reactive siloxanes capable of immediate further processing are already isolated at the end of the production process. 
    
    
     DETAILED DESCRIPTION 
     It has now been found in the context of the present invention that, surprisingly, end-equilibrated acetoxysiloxanes of both linear and branched structure type are obtainable when the reactions described in the as yet unpublished European patent applications having the application numbers EP18172882.5, EP18172876.7, EP17195510.7 and EP17204277.2 are implemented with addition of acetic acid. 
     The present invention provides a process for producing trifluoromethanesulfonic acid-acidified, end-equilibrated, acetoxy-bearing siloxanes which comprises reacting cyclic siloxanes, in particular comprising D 4  and/or D 5 , and/or mixtures of cyclic branched siloxanes of the D/T type with acetic anhydride using trifluoromethanesulfonic acid as catalyst and with addition of acetic acid. 
     In terms of the usability of DT cycles the inventors have found that not only mixtures of cyclic branched siloxanes of the D/T type which consist exclusively of siloxanes comprising D and T units and whose  29 Si NMR spectroscopy-determinable cumulative proportion of D and T units comprising Si-alkoxy and/or SiOH groups present in the siloxane matrix is less than 2 mole percent, preferably less than 1 mole percent, and which advantageously further contain at least 5% by weight of siloxane cycles, such as preferably octamethylcyclotetrasiloxane (D 4 ), decamethylcyclopentasiloxane (D 5 ) and/or mixtures thereof, but also mixtures of cyclic branched siloxanes comprising exclusively D and T units whose  29 Si NMR spectroscopy-determinable cumulative proportion of D and T units comprising Si-alkoxy and/or SiOH groups present in the siloxane matrix is greater than 2 and less than 10 mole percent are particularly suitable for use according to the invention. 
     The cyclic branched siloxanes of the D/T type usable according to the invention are not only described by way of example in the experimental part but also described in detail in the European patent application EP3321304A1 and in the as yet unpublished patent application EP 17169876.4. Both documents are accordingly incorporated in their entirety into the disclosure content of this invention. 
     Suitable for obtaining the mixtures of cyclic branched siloxanes comprising exclusively D and T units whose  29 Si NMR spectroscopy-determinable cumulative proportion of D and T units comprising Si-alkoxy and/or SiOH groups present in the siloxane matrix is greater than 2 and less than 10 mole percent is a process comprising the steps of
         (a) an acid-catalysed equilibration of trialkoxysilanes with siloxane cycles and/or α,ω□□-dihydroxypolydimethylsiloxane in the presence of at least one acidic catalyst and then   (b) a hydrolysis and condensation reaction initiated by addition of water, and addition of a silicon-containing solvent, followed by   (c) a distillative removal of the alcohol released, of water present in the system and of silicon-containing solvent, and a neutralization or removal of the acidic catalyst and optionally removal of salts that have possibly formed,   wherein the silicon-containing solvent preferably comprises the isomeric siloxane cycles octamethylcyclotetrasiloxane (D 4 ), decamethylcyclotetrasiloxane (D 5 ) and/or mixtures thereof and mass ratios of silicon-containing solvent to the siloxane comprising D and T units of 1:1 to 5:1 are advantageously employed
 
as described in the as yet unpublished patent application EP 17169876.4.
       

     Shown hereinbelow by way of example for the sake of enablement is the production of these abovementioned mixtures of cyclic branched siloxanes comprising exclusively D and T units: In a 500 ml four-necked round-bottomed flask with a KPG stirrer and fitted with a reflux cooler, 52.2 g (0.293 mol) of methyltriethoxysilane are heated to 60° C. together with 130.3 g (0.351 mol) of decamethylcyclopentasiloxane while stirring, 0.400 g of trifluoromethanesulfonic acid is added and the mixture is equilibrated for 4 hours. Then 15.8 g of water and 4.0 g of ethanol are added and the mixture is heated to reflux temperature (about 80° C.) for a further 4 hours. 10.6 g of water and 200 ml of decamethylcyclopentasiloxane (D5) are added and the reflux cooler is exchanged for a distillation bridge, and the constituents that are volatile up to 90° C. are distilled off within the next hour. The reaction mixture is left at 90° C. for a further 2 hours, then allowed to cool down to 50° C., and 5 ml of a 25% aqueous ammonia solution are added and the mixture is stirred for a further hour to complete the neutralization. At 100° C. and with an auxiliary vacuum of &lt;1 mbar applied, water and the decamethylcyclopentasiloxane (D 5 ) used as solvent are distilled off. After cooling the distillation bottoms, with the aid of a pleated filter, the precipitated ammonium triflate is removed. The filtrate is a colorless mobile liquid, whose  29 Si NMR spectrum shows a D/T ratio of 6.1:1 (target 6.0:1). Based on the sum of the Si units detected by spectroscopy, the D and T units bearing Si-alkoxy and SiOH groups respectively have a proportion of 4.1 mole percent. 
     Suitable for obtaining the mixtures of cyclic branched siloxanes of the D/T type which consist exclusively of siloxanes comprising D and T units and whose  29 Si NMR spectroscopy-determinable cumulative proportion of D and T units comprising Si-alkoxy and/or SiOH groups present in the siloxane matrix is ≤2 mole percent, preferably less than 1 mole percent, and which advantageously further contain at least 5% by weight of siloxane cycles, such as preferably octamethylcyclotetrasiloxane (D 4 ), decamethylcyclopentasiloxane (D 5 ) and/or mixtures thereof, is a process in which a trialkoxysilane is reacted in a solvent with siloxane cycles and/or α,ω-dihydroxypolydimethylsiloxane with addition of water and in the presence of at least one acidic catalyst as described in European patent application EP3321304A1. Example 1 of the present invention shows by way of example the production of corresponding mixtures of cyclic branched siloxanes of the D/T type. 
     A preferred embodiment of the invention comprises adding acetic acid in amounts of 0.4 to 3.5 percent by weight, by preference 0.5 to 3 percent by weight, preferably 0.8 to 1.8 percent by weight, particularly preferably in amounts of 1.0 to 1.5 percent by weight, based on the reaction matrix consisting of acetic anhydride and cyclic siloxanes comprising D 4  and/or D 5  or consisting of acetic anhydride and mixtures of cyclic branched siloxanes of the D/T type or consisting of cyclic siloxanes comprising D 4  and/or D 5  and mixtures of cyclic branched siloxanes of the D/T type. 
     In a preferred embodiment of the invention the catalyst trifluoromethanesulfonic acid is employed in amounts of 0.1 to 1.0 percent by mass, preferably 0.1 to 0.3 percent by mass, based on the reaction matrix consisting of acetic anhydride and cyclic siloxanes, in particular comprising D 4  and/or D 5 , and/or cyclic branched siloxanes of the D/T type. 
     The term “end-equilibrated” is to be understood as meaning that the equilibrium established at a temperature of 23° C. and a pressure of 1013.25 hPa has been reached. Employable as an indicator for reaching the equilibrium is the total cycles content determined by gas chromatography and defined as the sum of the D 4 -, D 5 -, D 6 -contents based on the siloxane matrix and ascertained after derivatization of the α,ω-diacetoxypolydimethylsiloxanes to the corresponding α,ω-diisopropoxypolydimethylsiloxanes or after derivatization of the branched acetoxysiloxanes to the corresponding branched isopropoxysiloxanes. The inventive use of acetic acid here makes it possible to readily undershoot otherwise customary equilibrium proportions of about 13 percent by weight of total cycles content for the linear α,ω-diacetoxypolydimethylsiloxanes and of about 8 percent by weight of total cycles content for the branched acetoxysiloxanes. It is therefore in accordance with a preferred embodiment when equilibrium proportions of the total cycles content of less than 13, preferably less than 12, percent by weight for the linear α,ω-diacetoxypolydimethylsiloxanes and equilibrium proportions of the total cycles content of less than 8, preferably less than 7, percent by weight for the branched acetoxysiloxanes are undershot. The derivatization to afford the α,ω-diisopropoxypolydimethylsiloxanes or to afford the branched isopropoxysiloxanes is intentionally chosen in order to prevent a thermally induced retrocleavage reaction of the α,ω-diacetoxypolydimethylsiloxanes or of the branched acetoxysiloxanes which may take place under the conditions of analysis by gas chromatography (regarding the retrocleavage reaction see inter alia J. Pola et al., Collect. Czech. Chem. Commun. 1974, 39(5), 1169-1176 and also W. Simmler, Houben-Weyl, Methods of Organic Chemistry, Vol. VI/2, 4th Edition, 0-Metal Derivates of Organic Hydroxy Compounds p. 162 ff)). 
     In a preferred embodiment the reaction is carried out in a temperature range of 140° C. to 160° C. and over a duration of 4 to 8 hours. 
     The present invention further relates to trifluoromethanesulfonic acid-acidified, end-equilibrated linear α,ω-acetoxy-bearing siloxanes having total cycles contents defined as the sum of the content fractions of the cyclic siloxanes comprising D 4 , D 5  and D 6  based on the siloxane matrix and determined by gas chromatography after their derivatization to afford the corresponding linear α,ω-isopropoxysiloxanes of less than 13, preferably less than 12, percent by weight, preferably produced by a process according to the invention as described above. 
     The present invention further relates to trifluoromethanesulfonic acid-acidified, end-equilibrated branched acetoxy-bearing siloxanes having total cycles contents defined as the sum of the content fractions of the cyclic siloxanes comprising D 4 , D 5  and D 6  based on the siloxane matrix and determined by gas chromatography after their derivatization to afford the corresponding branched isopropoxysiloxanes of less than 8, preferably less than 7, percent by weight, preferably produced by a process according to the invention as described above. 
     The present invention further relates to the use of the end-equilibrated trifluoromethanesulfonic acid-acidified acetoxy-bearing siloxanes as described above as starting materials for the production of SiOC-bonded polyether siloxanes for use thereof in PU foam stabilizers, in defoamers, in demulsifiers, in emulsifiers and in paint and flow control additives. 
     EXAMPLES 
     The examples which follow are provided merely to elucidate this invention to those skilled in the art and do not constitute any limitation of the claimed subject matter whatsoever. The determination of water contents is in principle performed by the Karl Fischer method based on DIN 51777, DGF E-III 10 and DGF C-III 13a.  29 Si-NMR spectroscopy was used for reaction monitoring in all examples. 
     In the context of the present invention the  29 Si NMR samples are analysed at a measurement frequency of 79.49 MHz in a Bruker Avance III spectrometer equipped with a 287430 sample head with gap width of 10 mm, dissolved at 22° C. in CDCl 3  and against a tetramethylsilane (TMS) external standard [δ( 29 Si)=0.0 ppm]. 
     GPCs (gel permeation chromatography) are recorded using THF as the mobile phase on an SDV 1000/10000A column combination having a length of 65 cm, ID 0.80, at a temperature of 30° C. using a SECcurity 2  GPC System 1260 (PSS Polymer Standards Service GmbH). 
     The gas chromatograms are recorded on an Agilent Technologies GC 7890B GC instrument fitted with an HP-1 column having dimensions of 30 m×0.32 mm ID×0.25 μm dF (Agilent Technologies No. 19091Z-413E) using hydrogen as a carrier gas and employing the following parameters: 
     Detector: FID; 310° C. 
     Injector: Split; 290° C. 
     Mode: constant flow, 2 ml/min 
     Temperature programme: 60° C. at 8° C./min-150° C. at 40° C./min-300° C. 10 min. 
     Employed as an indicator for reaching the equilibrium is the total cycles content determined by gas chromatography and defined as the sum of the D 4 -, D 5 -, D 6 -contents based on the siloxane matrix and ascertained after derivatization of the α,ω-diacetoxypolydimethylsiloxanes to the corresponding α,ω-diisopropoxypolydimethylsiloxanes. The derivatization to afford the α,ω-diisopropoxypolydimethylsiloxanes is intentionally chosen in order to prevent a thermally induced retrocleavage reaction of the α,ω-diacetoxypolydimethylsiloxanes which may take place under the conditions of analysis by gas chromatography (regarding the retrocleavage reaction see inter alia J. Pola et al., Collect. Czech. Chem. Commun. 1974, 39(5), 1169-1176 and also W. Simmler, Houben-Weyl, Methods of Organic Chemistry, Vol. VI/2, 4th Edition, 0-Metal Derivates of Organic Hydroxy Compounds p. 162 ff)). 
     Example 1 
     Production of a Cyclic Branched Siloxane Having a Target D/T Ratio of 6:1 
     In a 101 four-necked round-bottomed flask with a KPG stirrer and fitted with a reflux cooler 783 g (4.39 mol) of methyltriethoxysilane together with 978.7 g (2.64 mol) of decamethylcyclopentasiloxane are heated to 60° C. with stirring, admixed with 2.98 g of trifluoromethanesulfonic acid and the mixture is equilibrated for 4 hours. 237 g of water and 59.3 g of ethanol are then added and the batch is heated to reflux temperature for a further 2 hours. 159.0 g of water and 978.8 g (2.64 mol) of decamethylcyclopentasiloxane (D 5 ) are added and the reflux cooler is exchanged for a distillation bridge and the constituents that are volatile up to 90° C. are distilled off over the next hour. 3000 ml of toluene are then added to the reaction batch and the water still present in the system is removed by distillation up to a bottoms temperature of 100° C. at the water separator. The reaction mixture is allowed to cool to about 60° C., the acid is neutralized by addition of 60.0 g of solid sodium hydrogencarbonate, and the mixture is then stirred for a further 30 minutes to achieve complete neutralization. After cooling to 25° C. the salts are removed with the aid of a pleated filter. 
     At 70° C. and with an auxiliary vacuum of &lt;1 mbar applied, the toluene used as solvent is distilled off. The distillation bottoms are a colorless mobile liquid, whose  29 Si NMR spectrum shows a D/T ratio of 6.2:1 (target 6.0:1). Based on the sum of the Si units detected by spectroscopy, the D and T units bearing Si-alkoxy and SiOH groups respectively, have a proportion of 0.52 mole percent. The gas chromatography analysis of the liquid also shows a proportion of about 15 percent by weight of simple siloxane cycles in the form of D 4 , D 5  and D 6 . The GPC has a broad molar mass distribution, characterized by Mw=55 258 g/mol; Mn: 1693 g/mol and Mw/Mn=32.63. 
     Example 2 (Inventive) 
     Production of an Acetoxy-Terminated, Branched Siloxane with 1.5% Acetic Acid Addition 
     In a 1000 ml four-necked flask with a KPG stirrer, internal thermometer and fitted with a reflux cooler 49.9 g (0.489 mol) of acetic anhydride together with 268.1 g of the DT cycles produced in example 1 (D/T ratio according to  29 Si-NMR spectrum=6.18:1, M=525.42 g/mol and a proportion of SiOH/SiOEt moieties of 0.52 mol %) and 188.5 g of decamethylcyclopentasiloxane (D 5 ) are initially charged with stirring and admixed with 1.03 g (0.56 ml) of trifluoromethanesulfonic acid (0.2% by mass based on the total batch) and 7.6 g of acetic acid (1.5% based on the mass of the reactants) and swiftly heated to 150° C. The initially slightly cloudy reaction mixture is held at this temperature for 6 hours with continued stirring. 
     After cooling of the batch a colorless clear mobile liquid whose  29 Si-NMR spectrum demonstrates the presence of Si-acetoxy groups in a yield of about 88.2% based on the employed acetic anhydride and the complete disappearance of spectroscopically detectable proportions of Si-alkoxy and SiOH groups is isolated. 
     Conversion of the Branched Acetoxysiloxane into the Corresponding Branched Isopropoxysiloxane for Analytical Characterization 
     Immediately after the synthesis in a 250 ml four-necked round-bottomed flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 100.0 g of this trifluoromethanesulfonic acid-acidified, equilibrated branched □acetoxysiloxane are mixed together with 23.2 g of a molecular sieve-dried isopropanol by stirring at 22° C. Gaseous ammonia (NH 3 ) is then introduced to the reaction mixture until alkaline reaction (moist universal indicator paper) and the mixture is then stirred at this temperature for a further 45 minutes. The precipitated salts are separated using a pleated filter. 
     A colorless, clear liquid is isolated, whose accompanying  29 Si-NMR spectrum demonstrates the quantitative conversion of the branched acetoxysiloxane into a branched □isopropoxysiloxane. 
     An aliquot of this branched □isopropoxysiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents (reported in percent by mass): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Total 
                 Isopropanol 
               
               
                 D 4   
                 D 5   
                 D 6   
                 (D 4 -D 6 ) 
                 content 
               
               
                   
               
             
            
               
                 2.3% 
                 1.4% 
                 0.4% 
                 4.1% 
                 7.1% 
               
               
                   
               
            
           
         
       
     
     Taking account of the isopropanol excess the contents of siloxane cycles (D 4 , D 5  and D 6 ) are calculated solely based on the siloxane proportion. 
     Example 3 (Noninventive) 
     Production of an Acetoxy-Terminated Branched Siloxane 
     In a 1000 ml four-necked flask with a KPG stirrer, internal thermometer and fitted with a reflux cooler 49.9 g (0.489 mol) of acetic anhydride together with 268.1 g of the DT cycles produced in example 1 (D/T ratio according to  29 Si-NMR spectrum=6.18:1, M=525.42 g/mol and a proportion of SiOH/SiOEt moieties of 0.52 mol %) and 188.5 g of decamethylcyclopentasiloxane (D 5 ) are initially charged with stirring and admixed with 1.03 g (0.56 ml) of trifluoromethanesulfonic acid (0.2 mass % based on the total batch) and rapidly heated to 150° C. The initially slightly cloudy reaction mixture is held at this temperature for 6 hours with continued stirring. 
     After cooling of the batch a colorless clear mobile liquid whose  29 Si-NMR spectrum demonstrates the presence of Si-acetoxy groups in a yield of about 92.3% based on the employed acetic anhydride and the complete disappearance of spectroscopically detectable proportions of Si-alkoxy and SiOH groups is isolated. 
     Conversion of the Branched Acetoxysiloxane into the Corresponding Branched Isopropoxysiloxane for Analytical Characterization 
     Immediately after the synthesis in a 250 ml four-necked round-bottomed flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 100.0 g of this trifluoromethanesulfonic acid-acidified, equilibrated branched acetoxysiloxane are mixed together with 23.2 g of a molecular sieve-dried isopropanol by stirring at 22° C. Gaseous ammonia (NH 3 ) is then introduced to the reaction mixture until alkaline reaction (moist universal indicator paper) and the mixture is then stirred at this temperature for a further 45 minutes. The precipitated salts are separated using a pleated filter. 
     A colorless, clear liquid is isolated, whose accompanying  29 Si-NMR spectrum demonstrates the quantitative conversion of the branched acetoxysiloxane into a branched □isopropoxysiloxane. 
     An aliquot of this branched □isopropoxysiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents (reported in percent by mass): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Total 
                 Isopropanol 
               
               
                 D 4   
                 D 5   
                 D 6   
                 (D 4 -D 6 ) 
                 content 
               
               
                   
               
             
            
               
                 2.03% 
                 21.85% 
                 0.83% 
                 24.71% 
                 11.7% 
               
               
                   
               
            
           
         
       
     
     Taking account of the isopropanol excess the contents of siloxane cycles (D 4 , D 5  and D 6 ) are calculated solely based on the siloxane proportion. 
     Example 4 (Inventive) 
     Production of an Acetoxy-Terminated, Linear Polydimethylsiloxane with 1.5% Acetic Acid Addition 
     In a 1000 ml four-necked flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 77.3 g (0.757 mol) of acetic anhydride together with 732.8 g (1.98 mol) of decamethylcyclopentasiloxane (D 5 ) and 12.2 g of acetic acid (1.5% by weight based on the total mass of the reactants) are initially charged with stirring and admixed with 1.62 g (0.88 ml) of trifluoromethanesulfonic acid (0.2 percent by mass based on the total batch) and swiftly heated to 150° C. The initially slightly cloudy reaction mixture is held at this temperature for 6 hours with continued stirring. 
     After cooling of the batch a colorless, clear, mobile liquid is isolated, whose  29 Si-NMR spectrum demonstrates the presence of Si-acetoxy groups in a yield of about 93% based on employed acetic anhydride corresponding to an α,ω-diacetoxypolydimethylsiloxane having an average total chain length of about 14. 
     Conversion of the α,ω-Diacetoxypolydimethylsiloxane into the Corresponding α,ω-Diisopropoxypolydimethylsiloxane for Analytical Characterization 
     Immediately after the synthesis in a 250 ml four-necked round-bottomed flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 50.0 g of this trifluoromethanesulfonic acid-acidified, equilibrated α,ω-diacetoxypolydimethylsiloxane are mixed together with 11.3 g of a molecular sieve-dried isopropanol by stirring at 22° C. Gaseous ammonia (NH 3 ) is then introduced to the reaction mixture until alkaline reaction (moist universal indicator paper) and the mixture is then stirred at this temperature for a further 45 minutes. The precipitated salts are separated using a pleated filter. 
     A colorless, clear liquid is isolated, whose accompanying  29 Si-NMR spectrum demonstrates the quantitative conversion of the α,ω-diacetoxypolydimethylsiloxane into an α,ω-diisopropoxypolydimethylsiloxane. 
     An aliquot of this α,ω-diisopropoxypolydimethylsiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents (reported in percent by mass): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Total 
                 Isopropanol 
               
               
                 D 4   
                 D 5   
                 D 6   
                 (D 4 -D 6 ) 
                 content 
               
               
                   
               
             
            
               
                 4.94% 
                 4.04% 
                 1.07% 
                 10.06% 
                 11.00% 
               
               
                   
               
            
           
         
       
     
     Taking account of the isopropanol excess the contents of siloxane cycles (D 4 , D 5  and D 6 ) are calculated solely based on the siloxane proportion. 
     Example 5 (Inventive) 
     Production of an Acetoxy-Terminated, Linear Polydimethylsiloxane with 3.0% Acetic Acid Addition 
     In a 1000 ml four-necked flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 77.3 g (0.757 mol) of acetic anhydride together with 732.8 g (1.98 mol) of decamethylcyclopentasiloxane (D 5 ) and 24.3 g of acetic acid (3.0% by weight based on the total mass of the reactants) are initially charged with stirring and admixed with 1.62 g (0.88 ml) of trifluoromethanesulfonic acid (0.2 percent by mass based on the total batch) and swiftly heated to 150° C. The initially slightly cloudy reaction mixture is held at this temperature for 4 hours with continued stirring. 
     After cooling of the batch a colorless, clear, mobile liquid is isolated, whose  29 Si-NMR spectrum demonstrates the presence of Si-acetoxy groups in a yield of about 93% based on employed acetic anhydride corresponding to an α,ω-diacetoxypolydimethylsiloxane having an average total chain length of about 14. 
     Conversion of the α,ω-Diacetoxypolydimethylsiloxane into the Corresponding α,ω-Diisopropoxypolydimethylsiloxane for Analytical Characterization 
     Immediately after the synthesis in a 250 ml four-necked round-bottomed flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 50.0 g of this trifluoromethanesulfonic acid-acidified, equilibrated α,ω-diacetoxypolydimethylsiloxane are mixed together with 11.3 g of a molecular sieve-dried isopropanol by stirring at 22° C. Gaseous ammonia (NH 3 ) is then introduced to the reaction mixture until alkaline reaction (moist universal indicator paper) and the mixture is then stirred at this temperature for a further 45 minutes. The precipitated salts are separated using a pleated filter. 
     A colorless, clear liquid is isolated, whose accompanying  29 Si-NMR spectrum demonstrates the quantitative conversion of the α,ω-diacetoxypolydimethylsiloxane into an α,ω-diisopropoxypolydimethylsiloxane. 
     An aliquot of this α,ω-diisopropoxypolydimethylsiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents (reported in percent by mass): 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Total 
                 Isopropanol 
               
               
                 D 4   
                 D 5   
                 D 6   
                 (D 4 -D 6 ) 
                 content 
               
               
                   
               
             
            
               
                 4.09% 
                 2.62% 
                 0.86% 
                 7.57% 
                 4.60% 
               
               
                   
               
            
           
         
       
     
     Taking account of the isopropanol excess the contents of siloxane cycles (D 4 , D 5  and D 6 ) are calculated solely based on the siloxane proportion. 
     Example 6 (Noninventive) 
     Production of an Acetoxy-Terminated, Linear Polydimethylsiloxane 
     In a 1000 ml four-necked flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 77.3 g (0.757 mol) of acetic anhydride together with 732.8 g (1.98 mol) of decamethylcyclopentasiloxane (D 5 ) are initially charged with stirring and admixed with 1.62 g (0.88 ml) of trifluoromethanesulfonic acid (0.2 percent by mass based on the total batch) and swiftly heated to 150° C. The initially slightly cloudy reaction mixture is held at this temperature for 6 hours with continued stirring. 
     After cooling of the batch a colorless, clear, mobile liquid is isolated, whose  29 Si-NMR spectrum demonstrates the presence of Si-acetoxy groups in a yield of about 90% based on employed acetic anhydride corresponding to an α,ω-diacetoxypolydimethylsiloxane having an average total chain length of about 14. 
     Conversion of the α,ω-Diacetoxypolydimethylsiloxane into the Corresponding α,ω-Diisopropoxypolydimethylsiloxane for Analytical Characterization 
     Immediately after the synthesis in a 250 ml four-necked round-bottomed flask fitted with a KPG stirrer, internal thermometer and a reflux cooler 50 g of this trifluoromethanesulfonic acid-acidified, equilibrated α,ω-diacetoxypolydimethylsiloxane are mixed together with 11.3 g of a molecular sieve-dried isopropanol by stirring at 22° C. Gaseous ammonia (NH 3 ) is then introduced to the reaction mixture until alkaline reaction (moist universal indicator paper) and the mixture is then stirred at this temperature for a further 45 minutes. The precipitated salts are separated using a pleated filter. 
     A colorless, clear liquid is isolated, whose accompanying  29 Si-NMR spectrum demonstrates the quantitative conversion of the α,ω-diacetoxypolydimethylsiloxane into an α,ω-diisopropoxypolydimethylsiloxane. 
     An aliquot of this α,ω-diisopropoxypolydimethylsiloxane is withdrawn and analysed by gas chromatography. The gas chromatogram shows the following contents: 
     
       
         
           
               
               
               
               
               
             
               
                   
               
               
                   
                   
                   
                 Total 
                 Isopropanol 
               
               
                 D 4   
                 D 5   
                 D 6   
                 (D 4 -D 6 ) 
                 content 
               
               
                   
               
             
            
               
                 1.72% 
                 39.12% 
                 0.88 
                 41.72 
                 9.50 
               
               
                   
               
            
           
         
       
     
     Taking account of the isopropanol excess the contents of siloxane cycles (D 4 , D 5  and D 6 ) are calculated solely based on the siloxane proportion.