Patent ID: 7884224

Claim:
A process for preparing phosphonatosilanes of the formula (II) R 1 n (X) 3-n —Si—R 3 —PO(OR 4 )(OR 5 ) (II) comprising reacting halogenated silanes of the formula (III) R 1 n (X) 3-n —Si—R 3 —Hal (III) with phosphites of the formula (IV) P(OR 4 )(OR 5 )(OR 6 ) (IV) in a reaction vessel, where R 1 is an optionally halogen-substituted hydrocarbon radical having 1-20 carbon atoms or hydrogen, X is a hydrolyzable group or OH, R 3 is an optionally halogen-substituted alkylene radical having 1-10 carbon atoms, R 4 , R 5 and R 6 are each optionally halogen-substituted hydrocarbon radicals having 1-20 carbon atoms, Hal is a halogen atom and n is 0, 1, 2 or 3, in which a portion of the reaction mixture is withdrawn continuously or repeatedly during the reaction, separating the product formed from the reactants by applying the reaction mixture to a thin-film evaporator, separating a bottoms high boiler fraction containing product, and recycling distillate to the reaction vessel.