Distillation of polyisocyanates

A process for producing polyisocyanates, comprising the following steps:

• (i) modifying at least one monomeric diisocyanate to obtain a mixture comprising at least one polyisocyanate and unreacted monomeric diisocyanate,
• (ii) separating the mixture obtained in step (i) into at least one gaseous stream containing monomeric diisocyanate and a liquid stream depleted in monomeric diisocyanate,
• (iii) partially condensing the gaseous stream from (ii) in at least one condenser, affording a liquid condensate and an uncondensed vapor stream, wherein the partially condensing in step (iii) takes place at a condenser temperature within a range from 16 to 135° C.,
• (iv) performing a secondary condensation of the uncondensed vapor stream obtained in step (iii) in at least one secondary condenser, affording a secondary condensate and an uncondensed offgas, wherein the secondary condensation in step (iv) takes place at a secondary condenser temperature within a range from −33 to 130° C., and
• (v) feeding the uncondensed offgas from step (iv) to the suction side of a vacuum pump, wherein the at least one secondary condenser in step (iv) is operated at a secondary condenser temperature and the at least one condenser in step (iii) is operated at a condenser temperature wherein the secondary condenser temperature is 5 to 50 K lower than the condenser temperature.