Organometallic Intermediates in the Controlled Radical Polymerization of Styrene by α-Diimine Iron Catalysts

Abstract

The α-diimine iron complexes R‘,R‘‘[N,N]FeCl2 (R‘,R‘‘[N,N] = R‘−NCR‘ ‘−CR‘ ‘N−R‘) are efficient catalysts for the atom-transfer radical polymerization (ATRP) of styrene when R‘ and R‘ ‘ are electron-donating substituents and favor catalytic chain transfer (CCT) when electron-withdrawing substituents are employed. An organometallic pathway, alongside a halogen-atom-transfer equilibrium, is proposed to mediate the observed reactivity. The model alkyl complexes, R‘,R‘‘[N,N]FeCl2(R), where R = PhCH2 and Ph(Me)CH, were generated via treatment of R‘,R‘‘[N,N]FeCl3 with RMgCl. The alkyl derivatives obtained from intermediate spin-state R‘,R‘‘[N,N]FeCl3 complexes were found to be stable to ca. −30 °C and favor CCT, whereas the alkyl derivatives derived from high-spin-state trichloride precursors are unstable above −78 °C and favor ATRP. Azo-initiated polymerizations of styrene are moderately controlled by α-diimine iron catalysts. The role of organometallic-mediated radical polymerization (OMRP) in the controlled ...