New Iron(II) Complexes for Atom‐Transfer Radical Polymerization: The Ligand Design for Triazacyclononane Results in High Reactivity and Catalyst Performance

Abstract

Mononuclear cordinatively unsaturated iron(II) complexes having a triazacyclononane ligand were developed as highly efficient and environmentally friendly catalysts for the atom-transfer radical polymerization (ATRP). These iron catalysts showed high performance in the well-controlled ATRP of styrene, methacrylates, and acrylates. The high reactivity of these catalysts led to well-controlled polymerization and block copolymerization even with lower catalyst concentrations.Keywords: atom transfer radical polymerization; environmentally friendly catalyst; iron; ligand design; polymerization Transition metal-catalyzed atom-transfer radical polymerization (ATRP) is a representative example of controlled radical polymerization (CRP), which is an important methodology to construct well-defined polymers on both laboratory and industrial scales. [1,2] In ideal cases, good catalysts for ATRP realize: 1) access to polymers with the desired molecular weight and narrow molecular weight distribution, 2) high reaction rate and durability to achieve complete monomer conversion in the construction of block co-polymers, 3) versatile applicability to several monomers, and 4) minimum residual heavy metal catalysts in the product. While the first three points are general requirements for CRP, the fourth point is a special problem for transition metal-catalyzed reactions. It is known that residual metals make the properties of the formed polymers worse and can be potentially harmful. Facile removal of the catalyst from the polymer has thus been investigated using biphasic systems, solid-supported catalysts, and solubility control of catalysts. [3][4][5][6][7][8] Recent results by Matyjaszewski and coworkers showed that reduction of the catalyst concentration to be a solution for this problem as well. [9]

Iron‐Catalyzed Alkyl–Alkyl Suzuki–Miyaura Coupling

“…R 1 , R 2 , R 3 = alkyl groups carrying functional groups as shown in Tables 2 and 3. [20,21] The alkyl radical (R 1 C) may be added to the iron center of B to form the Fe n+2 intermediate C, which can undergo reductive elimination to give D. Transmetalation with magnesium tetraalkylborate regenerates the reactive species A to complete the catalytic cycle. TMS = trimethylsilyl, dppe = 1,2-bis(diphenylphosphanyl)ethane, dppf = 1,1'-bis(diphenylphosphanyl)ferrocene, dppp = l,3bis(diphenylphosphanyl)propane, acac = acetylacetonate.…”

“…[19] Thus, the catalytic cycle begins with the homolytic cleavage of the C sp 3-halogen bond of the alkyl halide by A to form an alkyl radical (R 1 C) and Fe n+1 species B; this reaction is known as a fundamental step in metal-catalyzed living radical polymerization. [20,21] The alkyl radical (R 1 C) may be added to the iron center of B to form the Fe n+2 intermediate C, which can undergo reductive elimination to give D. Transmetalation with magnesium tetraalkylborate regenerates the reactive species A to complete the catalytic cycle. We have proposed the direct addition/substitution of the alkyl radical (R 1 C) to an aryl ligand on the iron(II) center (an ipso substitution) in the cross-coupling of alkyl halides and aryl metal reagents in the presence of bisphosphine ligands with a small bite angle (cf.…”

Iron-catalyzed atom transfer radical polymerization

“…However, the 80 polymerization was too slow to complete the reaction even after days, when [(Me 3 tcan) 2 Fe 2 Br 3 ][(Me 3 tcan)FeBr 3 ] was used as catalyst. 294 Notably, Mononuclear iron(II) complexes [(i-Pr) 3 tcan]FeX 2 (X = Br or Cl) bearing a triazacyclononane ligand were also developed as highly efficient catalysts for ATRPs of 85 styrene, MMA, and BA. The well-controlled polymerization was able to be performed even in the presence of lower concentration of catalyst.…”

Iron-based catalytic systems in atom-transfer controlled-radical-polymerization processes

“…However, the men tioned catalytic systems were less effective for control over molecular weight of polymers than the phos phine complexes of iron. ( catalysts for the synthesis of polymers with narrow molecular weight distributions [26,27]. These com plexes are useful for the polymerization of a wide range of monomers, including styrene, acrylates, and meth acrylates.…”