Iron-mediated reversible deactivation controlled radical polymerization

Metal-mediated reversible deactivation radical polymerization (RDRP) is now a cornerstone of functional polymer synthesis, dominated by copper complexes and the Atom Transfer Radical Polymerization (ATRP) moniker. A limitation of this approach is the contamination of the resultant polymers by the coloured copper complexes, thus requiring further purification, although protocols to reduce the amount of copper catalyst have been developed. Iron is an interesting alternative because of its low cost, low toxicity and reduced intensity of its optical absorption spectrum. Use of this metal in RDRP began in the late 90s and has continuously intensified. This review comprehensively covers all the work reported so far on RDRP mediated by iron complexes, organized according to ligand type, and discusses the specificities of this metal in terms of the multitude of accessible spin states and the interplay of different equilibria: atom transfer vs. direct radical trapping, associative vs. dissociative exchange, chain transfer by direct β-H atom transfer vs. β-H elimination from the dormant alkyl species.