Chapter 18
Living Radical Polymerization Mediated by Transition Metals: Recent Advances Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch018
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Mitsuo Sawamoto and Masami Kamigaito Department of Polymer Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-01, Japan This paper discusses the current scope, mechanism, and some exam ples of l i v i n g radical polymerizations mediated by transition metal -complexes. Combinations of alkyl and other halides (initiators; R-X) and metal complexes (MX ) lead to living polymerizations of meth acrylates, acrylates, and styrene at 60-100 °C; examples of the MX include homogeneous complexes of Ru(II), Ni(II), and Fe(II) such as RuCl2(PPh3)3 and NiBr2(P«Bu3)2. In addition to controlled molecular weights and their very narrow distributions of the product polymers, these processes are characterized by (a) tolerance towards water and alcohols as solvents and occurrence of living polymerizations therein, (b) quantitative attachment of halogens at the ω-terminals that are able to re-initiate living polymerization in the presence of RuCl2(PPh3)3, and (c) versatility towards various monomers as realized by modulat ing the structures of initiators and metal complexes. Some mechanistic aspects of the metal complex-mediated living radical polymerization are also discussed. n
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In the last few years, as vividly witnessed in several symposia attached to this A C S Meeting, many efforts have rapidly been devoted to development of living (or con trolled) radical polymerization, and promising systems have begun to emerge (7,2). In 1994-96, for example (eq 1), we first reported living radical polymerization of methyl methacrylate ( M M A ) mediated by a ruthenium(II) complex i n conjunction with carbon tetrachloride or related alkyl halides as initiators ( R - X ) (3-8). Similar l i v i n g radical polymerizations v i a transition metal catalysis have recently been reported by Matyjaszewski (9), Percec (10), and Teyssié (77). In the ruthenium-based systems the initiators R - X are mostly a l k y l halides and their derivatives (e.g., haloketones) (4,5) whose carbon-halogen bonds are homolytically or radically cleaved by RuCl2(PPh3)3 and other metal complexes to induce l i v ing and most likely radical propagation. A s we proposed (2,3), a key to the fine reaction control is the Ru(II)-assisted reversible and homolytic cleavage of the carbon-halogen bond, which w i l l reduce the instantaneous concentration of the active radicals and thereby suppress bimolecular termination reactions (radical recombination and disproportionation), which have been considered to be inherently unavoidable i n conventional "free radical" polymerizations. Another important aspect is the rapid interconversion between the dormant/covalent alkyl halide terminals and the 'Corresponding author
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© 1998 American Chemical Society
In Controlled Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.
Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on October 18, 2015 | http://pubs.acs.org Publication Date: January 8, 1998 | doi: 10.1021/bk-1998-0685.ch018
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growth-active radical ends generated v i a the assistance of the ruthenium and other metal complexes (eq 1), which is crucial to achieve controlled molecular weights and narrow molecular weight distributions ( M W D s ) . Since these findings have been reported, the use of transition metal complexes as a component of initiating systems has rapidly been expanded and, more important, generalized, to achieve hitherto difficult well-defined, living radical polymerizations of methacrylates, acrylates, and styrene derivatives (1-3; also see below). This paper discusses our recent results i n this area, particularly directed to the scope and design o f metal complexes and initiating systems, understanding o f the mechanism of the living radical polymerizations, and the development of novel living processes such as those operable in alcohols and even in water. Transition Metal-Mediated Living Radical Polymerization: Scope Figure 1 summarizes the current scope of our living radical processes that have been extended on the basis of the original ruthenium-based system (2,3,). Initiators. The initiators for the metal-mediated living radical polymerizations are mostly alkyl halides ( R - X ; X = C l , B r , I) and related compounds that have c a r b o n halogen bonds to be homolytically cleaved v i a the assistance o f transition metal complexes (2,3,7). A s summarized in Figure 1, reported examples include haloalkanes, haloketones, halonitriles, haloesters, and haloalkylbenezenes; note that the latter two groups bear "alkyl" moieties mimicking the structures of the growing ends to be generated from respective monomers. For example, esters of halogenated carboxylic acids are suited for (meth)acrylates and haloalkylbenzenes are preferentially employed for styrene and its derivatives. In addition, as shown below, haloketones are particularly suitable for M M A and related methacrylates to give polymers of controlled molecular weights and very narrow M W D s . Metal Complexes. A feature of the metal-mediated living radical polymerizations is of course the use of transition metal complexes that induce reversible and homolytic cleavage o f the initiator's carbon-halogen bond to give the initiating radical, along with the similar radical-forming processes in the propagation step. In this aspect, the metal complexes should be such that carry suitable metals, mostly group 8-10 transition metals, that can undergo one-electron oxidation to receive the halogen from the initiator i n the radical forming (forward) step. Equally important, they should be able to regenerate the dormant covalent species by releasing the once-captured halogen v i a a one-electron reduction; there should be no oxidative addition reactions. Thus, reported examples o f these metal complexes include those o f Ru(II) (46,8), Cu(I) (9,70
