Well-defined iron complexes as efficient catalysts for "green" atom-transfer radical polymerization of styrene, methyl methacrylate, and butyl acrylate with low catalyst loadings and catalyst recycling

Environmentally friendly iron(II) catalysts for atom-transfer radical polymerization (ATRP) were synthesized by careful selection of the nitrogen substituents of N,N,N-trialkylated-1,4,9-triazacyclononane (R₃TACN) ligands. Two types of structures were confirmed by crystallography: "[(R₃TACN)FeX₂]" complexes with relatively small R groups have ionic and dinuclear structures including a [(R ₃TACN)Fe(μ-X)₃Fe(R₃TACN)]⁺ moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R₃TACN)FeX₂]_n complexes that were synthesized were subjected to bulk ATRP of styrene, methyl methacrylate (MMA), and butyl acrylate (BA). Among the iron complexes examined, [{(cyclopentyl) ₃TACN}FeBr₂] (4b) was the best catalyst for the well-controlled ATRP of all three monomers. This species allowed easy catalyst separation and recycling, a lowering of the catalyst concentration needed for the reaction, and the absence of additional reducing reagents. The lowest catalyst loading was accomplished in the ATRP of MMA with 4b (59ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4b (117ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined. It's easy being green: Structurally well-defined [(R₃TACN)FeX ₂] complexes realized green atom-transfer radical polymerization by judicious choice of the R group on the N,N,N-trialkylated-1,4,9- triazacyclononane (R₃TACN) ligands (see scheme). [{(Cyclopentyl) ₃TACN}FeBr₂] was the best catalyst for controlled polymerization of all three monomers.