Abstract: The present invention relates to a process for the production of oligomeric polyisocyanates by the dimerization or trimerization of a portion of the isocyanate groups in organic polyisocyanates in the presence of tertiary phosphines or in the presence of a peralkylated phosphorus acid triamides as catalyst, followed by termination of the dimerization and/or trimerization reaction at the required degree of oligomerization by the addition of a catalyst poison, characterized in that a sulfonyl isocyanate is used as the catalyst poison.The present invention also relates to the use of the oligomeric polyisocyanates obtained by this process, optionally in admixture with unreacted starting polyisocyanate or optionally blocked by blocking agents for isocyanate groups, as the polyisocyanate component in the production of polyurethane plastics.

Abstract: Polyisocyanates are prepared by thermally splitting a carbamate corresponding to the formula:R.sup.1 (NH--CO--O--R.sup.2).sub.nin which R.sup.1, R.sup.2 and n are as defined herein into the corresponding isocyanate and alcohol. The splitting reaction is carried out at a temperature of from 150.degree. to 350.degree. C., at a pressure of from 0.001 to 20 bar and in the presence of an auxiliary agent. Suitable auxiliary agents are hydrogen chloride, organic acid chlorides, compounds having an alkylating effect and organotin (IV) chlorides. Inert solvents and catalysts may also be employed in the splitting products. The thus-produced isocyanate and alcohol are then separated, if necessary.