title: 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.
abstract: 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 (R3 TACN) ligands. Two types of structures were confirmed by crystallography: "[(R3 TACN)FeX2 ]" complexes with relatively small R groups have ionic and dinuclear structures including a [(R3 TACN)Fe(μ-X)3 Fe(R3 TACN)](+) moiety, whereas those with more bulky R groups are neutral and mononuclear. The twelve [(R3 TACN)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)3 TACN}FeBr2 ] (4 b) 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 4 b (59 ppm of Fe based on the charged monomer). Catalyst recycling in ATRP with low catalyst loadings was also successful. The ATRP of styrene with 4 b (117 ppm Fe atom) was followed by precipitation from methanol to give polystyrene that contained residual iron below the calculated detection limit (0.28 ppm). Mechanisms that involve equilibria between the multinuclear and mononuclear species were also examined.
authors.affiliation: Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 816-8580 (Japan).
authors.forename: So-Ichiro
authors.initials: S
authors.lastname: Nakanishi
authors.forename: Mitsunobu
authors.initials: M
authors.lastname: Kawamura
authors.forename: Hidetomo
authors.initials: H
authors.lastname: Kai
authors.forename: Ren-Hua
authors.initials: RH
authors.lastname: Jin
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Sunada
authors.forename: Hideo
authors.initials: H
authors.lastname: Nagashima
keywords: catalyst recycling; iron; polymerization; radical reactions; redox chemistry