Halide anions as ligands in iron-mediated atom transfer radical polymerization

Halide anions have been used as complexing ligands in iron-mediated ATRP, in both direct and reverse ATRP. In direct ATRP, iron(II) bromide complexed with ammonium and phosphonium chloride, bromide, or iodide salts has been shown to catalyze the polymerization of both styrene and (meth)acrylates in a controlled manner under appropriate conditions. The experimental molecular weights increased linearly with monomer conversion and were close to the calculated values. The polymerization rates and polydispersities (M-w/M-n = 1.1-1.4) were dependent on the monomer employed. Reverse ATRP, initiated by AIBN/FeBr3/onium salts, led to a controlled polymerization of both methyl methacrylate and methyl acrylate, while for styrene uncontrolled molecular weights and high polydispersities were obtained, presumably due to the involvement of the cationic polymerization. Tt is suggested that different iron complexes may be involved in ATRP, depending on the onium salt/FeBr3 ratio. Because of their ionic nature, the iron complexes could be removed easily from the reaction mixture by washing the polymerization mixture with water.