Ultrafast and Reversible Multiblock Formation by the SET-Nitroxide Radical Coupling Reaction

The single electron transfer-nitroxide radical coupling (SET-NRC) reaction has been used to produce multiblock polymers with high molecular weights in under 3 min at 50 degrees C by coupling a difunctional telechelic polystyrene (Br-PSTY-Br) with a dinitroxide. The well known combination of dimethyl sulfoxide as solvent and Me(6)TREN as ligand facilitated the in situ disproportionation of Cu(I)Br to the highly active nascent Cu(0) species. This SET reaction allowed polymeric radicals to be rapidly formed from their corresponding halide end-groups. Trapping of these carbon-centred radicals at close to diffusion controlled rates by dinitroxides resulted in high-molecular-weight multiblock polymers. Our results showed that the disproportionation of Cu(I) was critical in obtaining these ultrafast reactions, and confirmed that activation was primarily through Cu(0). We took advantage of the reversibility of the NRC reaction at elevated temperatures to decouple the multiblock back to the original PSTY building block through capping the chain-ends with mono-functional nitroxides. These alkoxyamine end-groups were further exchanged with an alkyne mono-functional nitroxide (TEMPO-equivalent to) and 'clicked' by a Cu(I)-catalyzed azide/alkyne cycloaddition (CuAAC) reaction with N(3)-PSTY-N(3) to reform the multiblocks. This final 'click' reaction, even after the consecutive decoupling and nitroxide-exchange reactions, still produced high-molecular-weight multiblocks efficiently. These SET-NRC reactions would have ideal applications in re-usable plastics and possibly as self-healing materials.