Tuning of ruthenium N-heterocyclic carbene catalysts for ATRPElectronic supplementary information (ESI) available: plots of ln([M]0/[M]) vs. time and of Mn and Mw/Mn vs. conversion. See http://www.rsc.org/suppdata/cc/b3/b301733h

Depending on the substituents, R1 and R2, ruthenium(II)–p-cymene complexes bearing N-heterocyclic carbene ligands are either efficient catalysts for the well-controlled atom transfer radical polymerisation of methyl methacrylate and styrene, or promote a redox-initiated free-radical process.