トリス[2-(ジメチルアミノ)エチル]アミン(Me₆TREN) (1)は原子移動ラジカル重合(ATRP)に有用な多座配位子です。ATRPは金属触媒を用いる重合反応であり，高分子の分子量，分子量分布など精密制御を可能にします。例えば一価銅に多座配位子1を配位させた銅(I)錯体は高い活性を示します。続いてハロゲン化アルキルの一電子還元によりラジカル種が生成すると，モノマーとの重合が開始します。一方，一電子酸化を受けた二価銅と1からなる銅(II)錯体は，ハロゲン原子移動を伴う一電子還元により，ポリマー成長末端を休止種(ドーマント種)にします。この一連の反応の繰り返しにより，ATRPは完結します。
この重合では，わずか数ppmの銅触媒で十分な触媒効果が得られます（従来のATRPの約1/1000～1/10000量）。そのため，研究分野のみならず，工業分野への応用が非常に期待されています。

 

[1] “Activators Regenerated by Electron Transfer for Atom Transfer Radical Polymerization of Styrene”

W. Jakubowski, K. Min, K. Matyjaszewski, Macromolecules 2006, 39, 39. DOI: 10.1021/ma0522716

The amount of Cu-based catalysts in atom transfer radical polymerization (ATRP) of styrene has been reduced to a few ppm in the presence of the appropriate reducing agents such as FDA approved tin(II) 2-ethylhexanoate (Sn(EH)2) or glucose. The reducing agents constantly regenerate ATRP activator, the Cu(I) species, from the Cu(II) species, formed during termination process, without directly or indirectly producing initiating species that generate new chains. Moreover, the reducing agents allow starting an ATRP with the oxidatively stable Cu(II) species. The reducing/reactivating cycle may also eliminate air or some other radical traps in the system. This new catalytic system is based on regeneration of the activators for an ATRP by electron transfer and therefore was named activators regenerated by electron transfer (ARGET) ATRP. The optimum amount of reducing agent and minimal amount of ATRP Cu catalyst depend on the particular system. For example, styrene was polymerized with 10 ppm of CuCl2/Me6TREN and 100 ppm of Sn(EH)2 resulting in a polystyrene with Mn = 63 000 (Mn,th = 64 000) and Mw/Mn= 1.17.

 

[2] “Synthesis of High Molecular Weight Poly(styrene-co-acrylonitrile) Copolymers with Controlled Architecture”

J. Pietrasilk, H. Dong, K. Matyjaszewski, Macromolecules 2006, 39, 6384.　DOI:10.1021/ma0611927

High molecular weight styrene−acrylonitrile (SAN) copolymers were prepared under azeotropic conditions (60 mol % of styrene) by ARGET (activators regenerated by electron transfer) ATRP (atom transfer radical polymerization) at 80 °C in anisole. When a normal ATRP of styrene and acrylonitrile was conducted, the molecular weight of the resulting SAN copolymers was limited due to outer-sphere electron-transfer reactions. This was due to oxidation of polystyryl radicals to carbocations or reduction of polyacrylonitirile radicals to carbanions via reactions with Cu(II) and Cu(I) species, respectively. Since ARGET ATRP employs much lower concentrations of copper catalyst, the contributions of these side reactions are reduced, enabling formation of high molecular weight SAN copolymers (Mn  200 000) with low polydispersity (Mw/Mn < 1.3). Additionally, SAN copolymers with controlled chain architecture were prepared including block copolymers and starlike copolymers.

[3]　”Use of Ascorbic Acid as Reducing Agent for Synthesis of Well-Defined Polymers by ARGET ATRP”

K. Min, H. Gao, K. Matyjaszewski, Macromolecules 2007, 40, 1789. DOI: 10.1021/ma0702041
[4] “Diminishing catalyst concentration in atom transfer radical polymerization with reducing agents”

K. Matyjaszewski, W. Jakubowski, K. Min, W. Tang, J. Huang, W. A. Braunecker, N. V. Tsarevsky, Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15309. DOI: 10.1073/pnas.0602675103

The concept of initiators for continuous activator regeneration (ICAR) in atom transfer radical polymerization (ATRP) is introduced, whereby a constant source of organic free radicals works to regenerate the CuIactivator, which is otherwise consumed in termination reactions when used at very low concentrations. With this technique, controlled synthesis of polystyrene and poly(methyl methacrylate) (Mw/Mn < 1.2) can be implemented with catalyst concentrations between 10 and 50 ppm, where its removal or recycling would be unwarranted for many applications. Additionally, various organic reducing agents (derivatives of hydrazine and phenol) are used to continuously regenerate the CuI activator in activators regenerated by electron transfer (ARGET) ATRP. Controlled polymer synthesis of acrylates (Mw/Mn < 1.2) is realized with catalyst concentrations as low as 50 ppm. The rational selection of suitable Cu complexing ligands {tris[2-(dimethylamino)ethyl]amine (Me6TREN) and tris[(2-pyridyl)methyl]amine (TPMA)} is discussed in regards to specific side reactions in each technique (i.e., complex dissociation, acid evolution, and reducing agent complexation). Additionally, mechanistic studies and kinetic modeling are used to optimize each system. The performance of the selected catalysts/reducing agents in homo and block (co)polymerizations is evaluated.

 

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