Abstract: The present invention relates to a process for the functionalization of a rubber polymer comprising reacting the rubber polymer with at least one source of nitrene or carbene, in the presence of a catalyst of formula TpxAg, wherein Tpx represents a hydrotris(pyrazolyl)borate ligand. This process leads to selective rubber functionalization without secondary reactions such as chain scission or cross linking and produces functionalized rubbers with improved compatibilization properties. The invention also refers to the polymer produced therefrom and to uses of said polymer.