Nitrogen based ligands, copper

The most common catalysts for ATRP are complexes based on a copper(T) halide and nitrogen based ligand(s). Various ligands have been employed and those most frequently encountered are summarized in Table 9.5. Typically, four nitrogens coordinate to copper. The bidentate bipyridyl (bpy) ligands 132-133 are known to form a 2 1 complex. The tetradentate ligands are expected to form a 1 1 complex. 

Abstract During the past decade, atom transfer radical polymerization (ATRP) has had a tremendous impact on the synthesis of macromolecules with well-defined compositions, architectures, and functionalities. Structural features of copper and copper(II) complexes with bidentate, tridentate, tetradentate, and multidentate nitrogen-based ligands commonly utilized in ATRP are reviewed and discussed. Additionally, recent advances in mechanistic understanding of copper-mediated ATRP are outlined. 

Scheme 7 Nitrogen-based ligands commonly used in copper-catalyzed ATRA and ATRP...

In our seminal report, [Cu (TPMA)X][X] (TPMA = tris(2-pyridylmethyl)amine), X= Br and Cl) complexes were found to be very active in ATRA reactions of polyhalogenated compounds to alkenes in the presence of reducing agent (AIBN). Tetradentate nitrogen-based ligand TPMA was chosen for this study because its complexation to Cu X (X=Br or Cl) results in a formation of one of the most active catalysts in copper mediated ATRP (vide... 

A majority of the literature on ATRP focuses on the synthesis of styrene and its derivatives with copper-based catalysts. One of the most extensively studied systems is the polymerization of styrene conducted with CuBr, nitrogen-based ligands, and alkyl bromides as initiator. Better molecular weight control is obtained at low temperatures, presumably due to a lower contribution of thermal self-initiation during the early stage of the polymerization. For example, the reaction temperature can be lowered to 80-90 °C when efficient catalysts, such as CuBr/PMDETA, are used [19-22, 30]. 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

As indicated above, the majority of work on ATRP has been conducted using copper as the transition metal. An enormous variety of nitrogen-based ligands have been used in conjimction with Cu(I) halide. The most popular hgands are... 

ATRP of styrene and methyl methacrylate (MMA) was successfully conducted with various alkyl diethyldithiocarbamate (DC) initiators in the presence of copper catalysts with nitrogen-based ligands. Scheme 5. " Well-controlled polymers with narrow MWD were achieved, M /Mn< 1.1 (St) and Mw/Mn<1.2 (MMA). 

The equilibrium constants for copper-based ATRP were determined for a wide range of nitrogen-based ligands and initiators in MeCN at 22 °C. Values of Katrp were measured with CuBr/L complexes and EBriB (constant Kbh and Kea). The results displayed a linear correlation between Katrp and 1/2 (i.e., ffEx) for the series of Cu complexes formed with the different ligands. Figure g xhe correlation should be... 

The important study of the kinetics of the reaction of benzyl azide with phenylacetylene, mentioned above, was carried out using the copper(II) sulfate-sodium ascorbate protocol in the absence of added nitrogen-based ligands [92]. The data obtained for the reaction shown in Scheme 12 gave rate orders at low... 

Various metals and ligands have been used as catalysts in ATRP. However, the most commonly employed are copper-based catalysts (Cu(I) and Cu(II)) and nitrogen-based ligands (Figure 1.5). Simple initiators have been used in ATRP for the synthesis of glycopolymers (Figure 1.6). 

Copper-catalyzed ATRP is a multicomponent system, consisting of a monomer, an initiator with a transferable (pseudo)halogen, and a copper complex (composed of a copper (pseudo)halide and nitrogen-based complexing ligand). For a successful... 

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