Preparation of uretdione polyisocyanates

Process for the dimerization of isocyanates comprising reacting:

• a) at least one organic isocyanate,
• b) a catalyst containing at least one pyridine derivative corresponding to the formula (II)
• where
• R1, R6 are, independently of one another, identical or different, substituted or unsubstituted and/or branched hydrocarbon radicals,
• R2, R4 are, independently of one another, hydrogen or identical or different hydrocarbon radicals which may be substituted by heteroatoms or functional groups and/or be branched and
• R3, R5 are, independently of one another, radicals having the definition of the radicals R2, R4 or together form a cyclic, 4- to 7-membered hydrocarbon segment which bridges the two N atoms and may be substituted by hydrocarbon radicals, heteroatoms or functional groups and/or be unsaturated
• c) optionally solvents and
• d) optionally additives are reacted.