Hydrolysis of alkoxysilanes

A method for controlling the chain length of silanol-stopped dialkylpolysiloxanes prepared by the catalytic hydrolysis of dialkyldialkoxysilanes comprising:

• (a) mixing a Group IIa metal oxide catalyst selected from the class consisting of magnesium oxide, calcium oxide and barium oxide with a dialkyldialkoxysilane and from about 3 to about 6 moles of water per mole of dialkyldialkoxysilane of about 0.degree. C. to about 60.degree. C., the catalyst being used in an amount from about 0.01 to about 1.5 parts by weight of metal oxide catalyst per 100 parts by weight of dialkyldialkoxysilane;
• (b) distilling the reaction mixture after vigorously mixing for about 10 minutes to about 24 hours to separate volatile components and water from the silanol-stopped dialkylpolysiloxane product;
• and
• (c) adding a solid filtering medium to the silanol-stopped dialkylpolysiloxane product and filtering said product, whereby the polymer chain length varies according to the specific metal oxide catalyst, the silanol-stopped dialkylpolysiloxane having an average chain length of about 2 to about 3 siloxane (--SiO--) units when magnesium oxide is used as a catalyst, an average chain length of about 4 to about 6 siloxane (--SiO--) units when calcium oxide is used as catalyst, and a longer average chain length when barium oxide is used as catalyst.

A process for the catalytic hydrolysis of dialkoxysilanes comprising:

• (a) mixing an alkoxysilane with a stoichiometric excess of water in the presence of an effective catalytic amount of solid acid catalyst to initiate hydrolysis of the alkoxysilane and thereby form a mixture comprising partially hydrolyzed alkoxysilane, solid acid catalyst, water and reaction by-products;
• (b) separating the volatile reaction byproducts and unreacted water from the partially hydrolyzed silane in the presence of the solid acid catalyst to condense alkoxy and silanol groups and thereby form silane diol having a low alkoxy concentration;
• (c) neutralizing the silane diol with a suitable solid neutralizing agent thereby obtaining a neutralized, low molecular weight, linear, silanol-stopped polysiloxane and condensation by-products;
• (d) optionally adding a solid condensation catalyst to the neutralized polysiloxane to increase the chain length of the polysiloxane;
• and
• (e) separating the volatile by-products and solid catalyst from the low molecular weight, linear, silanol-stopped polysiloxane.

A process for the catalytic hydrolysis of dialkyldialkoxysilanes comprising:

• (a) adding a dialkyldialkoxysilane to a mixture of about 3 to about 6 moles of water per mole of dialkyldialkoxysilane and about 0.5 to about 2.0 parts by weight of acid activated clay catalyst per 100 parts by weight of dialkyldialkoxysilane at a temperature of about -10.degree. C. to about 25.degree. C.;
• (b) mixing vigorously the mixture of step (a) at a temperature between about -10.degree. C. to about 50.degree. C. for the minimum time required to form a single phase to about 45 minutes to initiate hydrolysis of the dialkyldialkoxysilane and thereby form a mixture comprising partially hydrolyzed dialkylsilanes, acid solid catalyst, water and reaction by-products;
• (c) distilling the mixture at a temperature between about 15.degree. C. and about 50.degree. C. and at reduced pressure no less than about 15 mm Hg. pressure in the presence of the unneutralized solid acid catalyst to separate the volatile reaction by-products and water from the partially hydrolyzed dialkylsilane and to condense alkoxy and silanol groups, thereby forming low molecular weight, linear dialkylsilane diol product having a low alkoxy concentration;
• (d) adding about 0.05 to about 6.0 parts by weight solid magnesium oxide neutralizing agent and condensation catalyst per 1.0 part by weight of acid solid catalyst, and stirring for a sufficient time to evenly disperse the solid magnesium oxide, thereby obtaining a mixture containing neutralized, low molecular weight, linear, silanol-stopped dialkyl polysiloxane having an average chain length of about 3 to about 4 siloxane (--SiO--) units;
• (e) optionally adding up to about 6.0 parts by weight solid calcium oxide condensation catalyst per 1.0 part by weight of acid solid catalyst and stirring for a sufficient time to evenly disperse the solid calcium oxide in the mixture, thereby obtaining a mixture containing condensed, low molecular weight, linear, silanol-stopped dialkylpolysiloxane having an average chain length of about 5 to about 6 siloxane (--SiO--) units;
• (f) distilling the mixture containing low molecular weight, linear, silanol-stopped dialkylpolysiloxane until all volatile materials are removed;
• and
• (g) filtering the mixture remaining after removal of volatile materials to remove solid catalysts to obtain low molecular weight, linear, silanol-stopped dialkylpolysiloxane product.

A method for controlling the chain length of low molecular weight, linear, silanol-stopped polysiloxanes prepared by the catalytic hydrolysis of dialkoxysilanes comprising:

• (a) mixing a dialkoxysilane with a stoichiometric excess of water in the presence of an effective catalytic amount of solid acid catalyst to initiate hydrolysis of the dialkoxysilane and thereby form a mixture comprising partially hydrolyzed alkoxysilane, acid solid catalyst, water and reaction by-products;
• (b) separating the volatile reaction by-products and water from the partially hydrolyzed silane in the presence of the solid acid catalyst to condense alkoxy and silanol groups and thereby form silane diol having a low alkoxy concentration;
• (c) adding an effective amount of solid magnesium oxide catalyst to neutralize the acid solid catalyst and to condense the silane diol to form a mixture containing neutralized, low molecular weight, linear, silanol-stopped polysiloxane;
• (d) optionally adding an effective catalytic amount of solid calcium oxide catalyst to the mixture containing neutralized, low molecular weight, linear, silanol-stopped polysiloxane to condense the silanol-stopped polysiloxane and thereby increase the chain length of the polysiloxane;
• and
• (e) separating the volatile by-products and solid catalysts from the low molecular weight, linear, silanol-stopped polysiloxane, whereby the polymer chain length varies according to the specific catalyst or catalyst combination, the silanolstopped polysiloxane having an average chain length of about 2.5 to about 4 siloxane units when magnesium oxide is used as catalyst, and the silanol-stopped polysiloxane having an average chain length of about 5 to about 6 siloxane units when calcium oxide is used as catalyst after magnesium oxide is used as catalyst.

A method of making stabilized, crystalline tetramethyldisiloxanediol comprising:

• (a) mixing dimethyldimethoxysilane with a stoichiometric excess of water in the presence of an effective catalytic amount of solid acid catalyst for about 5 minutes to about 25 minutes at a temperature less than about 25.degree. C. to form a partially hydrolyzed dimethyldimethoxysilane;
• (b) distilling the reaction mixture in the presence of the solid acid catalyst at a temperature less than about 30.degree. C. and at a pressure less than atmospheric pressure until volatile components are removed to substantially react the remaining alkoxy groups in the dimethylsiloxanediol;
• (c) adding an effective catalytic amount of solid magnesium oxide catalyst to condense the hydrolyzed dimethylsiloxanediol and thereby form a reaction mixture containing a mixture of siloxanediols having chain lengths of 1, 2, 3 and 4, condensation by-products and solid catalysts;
• (d) filtering the reaction mixture to remove the solid catalysts from the filtrate;
• (e) distilling the filtrate at a temperature of about 25.degree. C. to about 65.degree. C. at a pressure less than atmospheric pressure until volatile components are removed therefrom to produce an oil;
• and,
• (f) cooling the oil to crystalize stable tetramethyldisiloxanediol.

A method of making crystalline dimethylsilanediol comprising:

• (a) mixing neutral dimethyldimethoxysilane with a stoichiometric excess of water in the presence of an effective catalytic amount of solid acid catalyst free of soluble acid for about 5 minutes to about 25 minutes at a temperature less than about 5.degree. C.;
• (b) filtering the reaction mixture cold to form a cold filtrate;
• (c) distilling the cold filtrate at a pressure less than atmospheric pressure until crystals of dimethyldisilanol form in the filtrate;
• and
• (d) isolating the dimethyldisilanol crystals from the filtrate.