Transition metal catalyzed atom transfer radical polymerization (ATRP) is one of the most extensively studied reversible deactivation radical polymerization (RDRP) methods nowadays. It presents a versatile technique for exerting precise control over polymer molecular weights, molecular weight distributions, and complex architectures. Polymers prepared by ATRP are highly chain end-functionalized and therefore can participate in various post-polymerization modifications. ATRP is based on the reversible reaction of a low oxidation-state metal complex Mt(z)L (e.g., Mt = Cu, Ru, Fe, z = oxidation state, L = Ligand) with an alkyl halide (RX) yielding radicals and the corresponding metal complex, X-Mt(z+1)L. The development of new ATRP catalysts and novel polymerization systems based on rational ligand design was the aim during the stay at CMU. Active copper catalysts were developed, which enable to work with only ppm amount of metal complex, and achieve excellent control over the resulting polymers. The novel Cu/tris((4-methoxy-3,5-dimethylpyridin-2-yl)methyl)amine (TPMA*) system is currently the most reducing and the most active catalyst for acrylates employing atom transfer radical polymerizations (ATRP). A variety of derivates of the tris(2-pyridylmethyl)amine) scaffold could be synthesized, characterized, studied and successfully applied in polymerizations of methyl methacrylate and acrylates. A concept for using these active catalysts was developed and their participation in organometallic-mediated radical polymerization (OMRP) under certain conditions investigated. In addition, various iron complexes were studied and applied in ATRP reactions, but a state-of-the-art system remains to be a challenge.