Process for the preparation of a functionalized polymer containing an additive, in which process there is formed a compound that contains, besides at least one blocked isocyanate group, a free amino, hydroxy or carboxy group; this compound is linked to an additive via the free amino, hydroxy or carboxy group in the additive; this additive linked to the compound mentioned is contacted with a polymer that contains at least one free amino or hydroxyl group, at a temperature above the polymer's melting point and at least above 150.degree. C., such that the blocked isocyanate group reacts with the free amino or hydroxy group of the polymer to form the functionalized polymer. The invention also relates to the intermediate products formed in the process and in the preparation thereof. Lastly, the invention relates to functionalized polymers and polymer compositions containing the functionalized polymer.