Simultaneous Reverse & Normal ATRP (SR&NI)
SR&NI, was developed to overcome the problems "reverse" ATRP has with:
- incorporation of an α-functionality into the chain end of linear copolymers,
- preparing polymers with targeted MW,
- preparation of polymeric materials with more complex architecture and,
- preparation of composite or hybrid materials.
These problems with "reverse" ATRP were amplified by a desire to use even more active catalyst systems in an ATRP reaction, since more active catalyst complexes are more easily oxidized by exposure to air.
In SR&NI a low concentration of an active catalyst complex is generated by decomposition of a standard free radical initiator, such as AIBN in a standard reverse ATRP procedure, while the majority of the polymer chains are initiated from an added alkyl (pseudo)halide via a normal ATRP process. This allows very active catalysts to be added to the reaction in their stable form and the bulk of the polymer to be formed from the added alkyl halide initiator.
The following schematic is a summary of the normal ATRP and reverse ATRP initiation mechanisms, shown above, illustrating how both initiation procedures are employed in SR&NI. The reagents shown in red are the reagents that are added to a SR&NI reaction.
The first formed radicals drive the reverse ATRP initiation reaction where an active catalyst complex in the higher oxidation state is reduced to the activator state by reaction with the formed radicals (kdeact) but the bulk of the polymer chains are initiated by the normal ATRP initiation mechanism from the added standard initiator (P-X) providing polymers with a high mole fraction of functional α-terminal functionality.
The degree of polymerization is predominately controlled by the concentration of initially added alkyl halide, as expressed in the following equation, where f is the initiation efficiency of the added free radical initiator.
SR&NI was initially developed for bulk polymerization using macroinitiators to prepare block copolymers.(1) However SR&NI was quickly adapted to miniemulsion systems where the ability to add an oxidatively stable catalyst precursor to the precursor of the emulsion prior to sonication simplifies the laboratory procedure(2-6) and allows the preparation of block, star, graft and hybrid copolymers in heterogeneous media.
(1) Gromada, J.; Matyjaszewski, K. Macromolecules 2001, 34, 7664-7671.
(2) Matyjaszewski, K.; Gromada, J.; Li, M. In PCT Int. Appl.; (Carnegie Mellon University, USA). WO 03/031481, 2003; p 46 pp.
(3) Li, M.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2003, 41, 3606-3614.
(4) Li, M.; Min, K.; Matyjaszewski, K. Macromolecules 2004, 37, 2106-2112.
(5) Li, M.; Jahed, N. M.; Min, K.; Matyjaszewski, K. Macromolecules 2004, 37, 2434-2441.
(6) Min, K. E.; Li, M.; Matyjaszewski, K. Journal of Polymer Science, Part A: Polymer Chemistry 2005, 43, 3616-3622.