Process for the preparation of polyisocyanates

A process for the production of a polyisocyanate from a carbamate corresponding to the formula:

• R.sup.1 (NH--CO--O--R.sup.2).sub.n
• wherein
• R.sup.1 represents an aliphatic hydrocarbon radical which may have inert substituents and/or be olefinically unsaturated with a total of from 2 to 18 carbon atoms;
• a cycloaliphatic hydrocarbon radical which may contain inert substituents and/or be olefinically unsaturated with a total of from 3 to 18 carbon atoms;
• an araliphatic hydrocarbon radical which may contain inert substituents with a total of from 7 to 18 carbon atoms;
• or an aromatic hydrocarbon radical which may have inert substituents with a total of from 6 to 27 carbon atoms;
• R.sup.2 represents a radical of the type obtained by removing the hydroxyl group from a primary or secondary aliphatic, cycloaliphatic or araliphatic alcohol having a boiling point which is at least 10.degree. C. higher or lower than the boiling point of the polyisocyanate;
• and
• n represents an integer greater than or equal to 2
• comprising:
• (a) thermally splitting the carbamate at a temperature of from 150.degree. to 350.degree. C. and pressure of from 0.001 to 20 bar in the presence of an auxiliary compound selected from the group consisting of hydrogen chloride, organic acid chlorides, compounds having an alkylating effect, organotin (IV) chlorides and mixtures thereof;
• and if necessary,
• (b) separating the thus-formed polyisocyanate and alcohol.