Preparation of isocyanatosilanes

A method for producing a high purity isocyanatoorganosilane of the general formula (I)

• R2R3R4Si—R1—N═C═O (I),
• comprising:
• (A) feeding a carbamatoorganosilane of the general formula (II)
• R2R3R4Si—R1—NH—CO—OR (II),
• where
• R is a monovalent alkyl radical containing from 1 to 10 carbon atoms or an aromatic group of from 6 to 12 carbon atoms,
• R1 is a divalent hydrocarbon radical containing from 1 to about 6 carbon atoms, and R2, R3 and R4 are independently selected from the group consisting of methyl, ethyl, n-propyl, isopropyl, methoxy, ethoxy, n-propoxy and isopropoxy radicals, to a cracking device;
• (B) heating the fed carbamatoorganosilane to a temperature and pressure effective to thermally dissociate the carbamatoorganosilane to a reaction product mixture comprising isocyanatoorganosilane, alcohol, unreacted carbamatoorganosilane, and heavies,
• (C) separating isocyanatoorganosilane having a purity greater than about 90% from the rest of the reaction product mixture in a distillation column having a bottom part and a top part, and
• (D) collecting the isocyanatoorganosilane having greater than about 90% purity through a side stream which is located between the bottom part of the distillation column and the top part of the distillation column and
• wherein the ratio of the length of the bottom part of the distillation column to the length of the top part of the distillation column is from about 2:1 to about 10:1.