Recent developments in metal-catalyzed living radical polymerization

Polymer Journal (2011) 43, 105–120 & The Society of Polymer Science, Japan (SPSJ) All rights reserved 0032-3896/11 $32.00 www.nature.com/pj INVITED REVIEW Recent developments in metal-catalyzed living radical polymerization Masami Kamigaito This review presents a short overview of recent developments in metal-catalyzed living radical polymerization, mainly focusing on our recent research studies related to the subject. Metal-catalyzed living radical polymerization or atom transfer radical polymerization, which was originally developed via evolution of the metal-catalyzed Kharasch or atom transfer radical addition to chain-growth polymerization via reversible activation, has now been widely developed in many aspects. The effective metal catalysts include various transition metals, such as ruthenium, copper, iron and nickel, and highly active and versatile catalytic systems have been developed by designing ligands, applying lower oxidation metal species and using additives to widen the scope of controllable monomers and to minimize the amount of metal catalysts and the residual metals in the products. The development of the initiating systems has enabled the synthesis of a wide variety of novel, well-defined polymers, including end-functionalized, block, graft and star polymers, but also more complicated polymers possessing multiple controlled structures. Furthermore, metal-catalyzed living radical polymerization has been judiciously combined with stereospecific radical polymerization based on the use of polar solvents or Lewis acid additives, resulting in the dual control of the molecular weight and the tacticity of the resulting polymers and enabling the preparation of stereoblock and stereogradient polymers. Polymer Journal (2011) 43, 105–120; doi:10.1038/pj.2010.113; published online 17 November 2010 Keywords: block polymer; living radical polymerization; precision polymer synthesis; transition metal catalyst; star polymer; stereospecific polymerization INTRODUCTION Since the discovery of metal-catalyzed living radical polymerization or atom transfer radical polymerization (ATRP), there have been many developments in this research area, including active and versatile metal catalytic systems; the scope of controllable monomers; well-defined polymers with various controlled architectures; hybridization of the controlled polymers with inorganic, metal and biomolecular compounds; and attempts or real applications to a variety of industrial materials.1–17 Besides these metal catalytic systems, there have also been substantial developments in various living radical polymerizations, such as nitroxide-mediated polymerizations, reversible addition fragmentation chain-transfer polymerizations and others, and in all of these, there are characteristic features of the mechanisms and components.18–34 The metal-catalyzed living radical polymerization was originally discovered via evolution of the metal-catalyzed Kharasch or atom transfer radical addition reaction35 to the chain-growth polymerization of vinyl monomers (Figure 1).1 The initiating system generally consists of a transition metal complex in a lower oxidation state and an organic halide, in which the carbon–halogen bond of the halogen compound is activated by the metal catalyst to generate the carbon radical species upon a one-electron oxidation of the metal complex associated with the abstraction of the halogen by the metal species. The carbon radical species generated then adds to the monomer to generate the adduct radical, which is eventually capped with the halogen on the higher oxidation state metal complex or may add to another monomer molecule. The halogen-capping reaction of the radical species occurs faster than monomer addition and slows the very fast propagation of a particular growing radical chain end to suppress the formation of very long polymer chains, particularly during the initial stage of the polymerization. Furthermore, the newly formed carbon–halogen bond of the adduct or at the oligomeror polymer-chain end can be activated again by the metal catalyst to reversibly generate the growing radical species. Herein, the covalent species with the carbon–halogen terminal is called the ‘dormant’ species as in the other living anionic and cationic polymerizations via similarly reversible, but heterolytic activation of the covalent terminal group into the active species.36,37 Such a metal-catalyzed reversible activation of the carbon–halogen terminal gives an almost equal opportunity of propagation to each polymer chain to enable control of the chain length of the resulting polymer chains. In addition, the equilibrium between the radical and dormant species can diminish the radical concentration, which contributes to the suppression of the bimolecular termination between the growing radical species. A wide variety of transition metals have now been used as effective catalysts in the presence of the appropriate ligands. Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, Japan Correspondence: Professor M Kamigaito, Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan. E-mail: Received 8 August 2010; revised 1 October 2010; accepted 8 October 2010; published online 17 November 2010 Metal-catalyzed living radical polymerization M Kamigaito 106 End-Functionalized Polymer MnXnLm R R X Block Polymer R1 Kharasch Addition Star Polymer Mn+1Xn+1Lm CH2 C R2 Gradient Polymer R1 Graft Polymer Block-Arm R1 MnXnLm R CH2 C X R R2 R2 Metal-Catalyzed Living Radical Polymerization R1 R CH2 C X R2 Dormant Species Mn+1Xn+1Lm CH2 C Block-Graft Polymer R1 CH2 C Surface-Func R2 R1 MnXnLm R CH2 C Mn+1Xn+1Lm R2 Growing Radical Species Figure 1 Evolution of the Kharasch addition reaction into metal-catalyzed living radical polymerization. We originally discovered the ruthenium catalytic systems1 and then expanded the systems to other metals, such as iron, nickel, rhenium and manganese. The details on how we discovered the catalytic systems can be found in our previous reviews.7,9 In contrast, the most extensively used catalysts are based on copper with nitrogen ligands, which were separately developed by many research groups, including Matyjaszewski, Percec, Haddleton and others2–4,12–17 (The reaction mechanisms for the metal-catalyzed living radical polymerization and ATRP are supposed to be the same. Although the former name is mostly used in this review, it never intends to exclude the latter. Thus, the metal-catalyzed living radical polymerization also covers ATRP in the contexts.). As suggested by the reaction mechanism, the polymerizations are affected by various factors or parameters such as the central metal atom, its ligands, the halogen originating from the initiator, the initiating radical species derived from the initiator, the monomer, the solvent and (...truncated)