A process for preparing a polymer containing a structural element involves: (a) providing an uretdione of formula (I), and (b) opening a ring of the uretdione at a temperature greater than 50° C. and in the presence of a catalyst composition containing at least one catalyst, to obtain the polymer. In the element —N—C(═O)—X2, X2 is a derivative of the group X1 reactive towards isocyanates: The polymer containing the element —N—C(═O)—X2—obtained from the process is useful. A method for modifying polymers which have reactive groups towards isocyanate groups involves: (i) providing a polymer having at least one functional group reactive towards isocyanate groups; (ii) providing an uretdione, of formula (II); and (iii) reacting the polymer with the uretdione at a temperature of more than 50° C. in the presence of a catalyst composition containing at least one catalyst. The modified polymer obtained according to the method is useful.