The invention relates to polyurethane block copolymers of general formula (I) to (IV), in which the variables are defined as follows: A represents a polyurethane block, which contains at least one hydrophilic group, B is a hydrophobic polyurethane block, X.sup.1, X.sup.2 are hydrophilic end groups and n is a whole number from 1 to 20. The invention also relates to the production of cross-linkable dispersion additives from the inventive polyurethane block copolymers and to the use of said cross-linkable dispersion additives for colorant preparations, in particular inks for ink-jet printing.