Preparation of polyisocyanates of the trimer type

1. A process for preparing polyisocyanates from aliphatic and/or cycloaliphatic isocyanates, wherein the fraction of isocyanurate structures and iminooxadiazinedione structures in the polyisocyanate formed is more than 60 mol %, the fraction of uretdione structures is less than 25 mol %, and the fraction of carbodiimide structures and uretonimine structures is less than 5 mol %, based on the sum of all the types of structure formed, comprising reacting at least one aliphatic and/or cycloaliphatic isocyanate in the presence of a heterocyclic catalyst of formula (I)

wherein

R 1 is a non-cyclic or cyclic aliphatic C 1 -C 50 radical; and

R 2 and R 3

together with the phosphorus atom, and optionally with R 1 , define a cycloaliphatic C 2 -C 20 radical.

2. The process of claim 1 , wherein in formula (I)

R 1 is a non-cyclic or cyclic, aliphatic C 1 -C 12 radical; and

R 2 and R 3

together with the phosphorus atom define a saturated cycloaliphatic C 4 -C 6 radical.

3. The process of claim 1 , wherein said catalyst of formula (I) is selected from the group consisting of 1-methyl-1-phospholane, 1-ethyl-1-phospholane, 1-propyl-1-phospholane, 1-butyl-1-phospholane, 1-pentyl-1-phospholane, 1-hexyl-1-phospholane, 1-octyl-1-phospholane, 9-methyl-9-phosphabicyclononane, 9-ethyl-9-phosphabicyclononane, 9-propyl-9-phosphabicyclononane, 9-butyl-9-phosphabicyclononane, 9-pentyl-9-phosphabicyclononane, 9-hexyl-9-phosphabicyclononane, 9-octyl-9-phosphabicyclononane, 9-dodecyl-9-phosphabicyclononane, 9-eicosyl-9-phosphabicyclononane, and mixtures thereof.

4. The process of claim 1 , wherein said at least one aliphatic and/or cycloaliphatic isocyanate is reacted in the presence of the heterocyclic catalyst of formula (I) at a temperature in the range of from 0 to 150° C.

5. The process of claim 1 , wherein said at least one aliphatic and/or cycloaliphatic isocyanate is reacted in the presence of the heterocyclic catalyst of formula (I) under an inert gas atmosphere.