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.

Iron Catalysts in Atom Transfer Radical Polymerization

“…Alkyl amines with mono or multidentate coordinating sites or bipyridine derivatives were used for conducting iron ATRP [19,[44][45][46][47]. A series of Fe II complexes with N,N,N-trialkyl-1,4,9triazacyclononane (TACN) ligands were investigated in ATRP [48][49][50][51]. The complexes formed either a mononuclear iron species in the presence of TACN ligands substituted with bulky groups, or trinuclear ionic species with a cationic dinuclear and an anionic mononuclear complex in the presence of less bulky substitution groups to form coordinatively saturated compounds.…”

PNIPAM grafted surfaces through ATRP and RAFT polymerization: Chemistry and bioadhesion

“…Iron is also the most abundant metal on earth making it relatively cheap; these characteristics have initiated a lot of research and interest in Fe catalyzed organic chemistry, including ATRP, in line with the perspectives of "green chemistry" [66,67]. The choice of the ligand is a complex question; it depends on the nature of the polymer and the catalyst used [68]: for example, pentamethyldiethylenetriamine (PMDETA) can be used in combination with Cu [69], and tris(3,6-dioxaheptyl) amine (TDA) with Fe [70]. Furthermore, the activators regenerated by electron transfer (ARGET) ATRP, developed by Matyjaszewski et al diminish the amount of catalyst needed (< 50 ppm) [71,72].…”

Iron-catalyzed atom transfer radical polymerization

“…However, the controllability was poor when [sadim]FeCl 2 60 complex was used as catalyst and AIBN as initiator for reverse ATRP of styrene (M w /M n ~ 2.0). Nagashima et al 48, 293, 294 reported a variety of reusable and environmentally friendly "green" ionic trinuclear iron catalysts bearing nitrogen substituents of triazacyclononane (tcan) ligands 65 for…”