Atom Transfer Radical Polymerisation
The synthesis of new homo- and copolymers with a precise composition, topology and architecture has grown to a broad field with the development of the controlled radical polymerisation methods (CRP). Control over radical polymerisation is based on an equilibrium between the free radical and a huge amount of a dormant species. Owing to the low concentration of the free radical the occurrence of termination reactions is reduced.The difference between the three CRP methods (stable free radical polymerisation, reversible addition fragmentation transferand atom transfer radical polymerisation) is the character of the dormant species and the radical generation reaction. In atom transfer radical polymerisation (ATRP) the dormant species is an alkyl- or arylhalide which is activated catalytically yielding the free radical through a halogene transfer. After first studies in 1995, ATRP developed rapidly to one of the most robust CRP methods which enables the polymerisation of many monomers.
The ATRP catalyst is a transition metal complex, which is subjected to a one-electron-oxidation and an expansion of the coordination sphere in the activation process. The reduced form of the catalyst serves as activator whereas the oxidised form acts as deactivator for the regeneration of the dormant species. Though many parts of the processes involved in ATRP are understood, the question for the mechanism of activation and deactivation is still open. Another target of further research is the development of very active catalysts for the polymerisation of less active monomers and the polymerisation with ppm amounts of catalyst. In most ATRP active systems, copper is coordinated by N donor ligands. The activity can be steered by denticity, donor strength, ligand bite and further factors.
We use polydentate guanidine ligands in ATRP as they offer a variable synthesis protocol, high donor strength, ability to stabilise metal centres in high and low oxidation states and tunable redox potentials of the guanidine complexes.