Phosphazenium halides (PZN-X; X ) Cl, Br, I), highly delocalized bulky salts, turned out to be excellent cocatalysts to be combined with iron halides (FeX 2 ) to form in situ active anionic Fe(II) complexes that effectively catalyze living radical polymerization of alkyl and functionalized methacrylates with improved catalytic activity and tolerance to polar functionalities. For example, equimolar combinations of FeBr 2 /PZN-Br efficiently induced a living radical polymerization of methyl methacrylate (MMA) with a bromide initiator [H-(MMA) 2 -Br] to give polymers with controlled molecular weights and narrow molecular weight distributions (M w /M n < 1.2). Polymer molecular weight could be extended upon addition of second feeds of monomer or at lower initiator dose, while retaining narrow distributions. In terms of activity and controllability, the PZN-based catalysts were thus predominantly distinguished from not only a conventional iron complex [FeBr 2 (PPh 3 ) 2 ; Ph ) C 6 H 5 ] but also other hitherto known combinations of FeBr 2 with such an onium salt as tetrabutylammonium or phosphonium bromide, as further demonstrated by their reversible and hysteresis-free redox cycles with lower oxidation and reduction potentials (cyclic voltammetry). The new iron catalysts could be readily removed from as-prepared polymer solutions by simple washing with water to give virtually colorless products with the metal residue below 5 ppm.

A series of pentamethylcyclopentadienyliron(II) complexes, ligated by one carbonyl (CO) and one phosphine [Cp*Fe(CO)(L phos )Br; Cp* = C 5 Me 5 ; L phos = PPh 3 , PMePh 2 , PMe 2 Ph, P(m-tol) 3 , and P(p-tol) 3 ], were employed for living radical polymerization. In conjunction with a bromide initiator [H-(MMA) 2 -Br], these Cp*Fe complexes catalyzed living radical polymerization of methyl methacrylate (MMA) better controlled than those with the corresponding cyclopentadienyl (Cp) complexes [CpFe(CO)(L phos )Br; Cp = C 5 H 5 ]. The finer control was demonstrated by successful monomer-addition experiments, a wider range of controllable molecular weight (M n = 10 4 -10 5 or DP n = 100-1000), and narrower molecular weight distributions (M w /M n ∼ 1.2). FT-IR analysis of initiator-catalyst model reactions showed that an efficient carbonyl release from the original coordinatively saturated 18e complex into the unsaturated 16e form is important in the catalysis to generate a growing radical from the initiator. The higher catalytic activity allowed controlled polymerizations of other monomers that are not available for the Cp catalysts, such as methyl acrylate and a functional methacrylate with poly(ethylene glycol) pendent group.

Two neutral ligands, carbonyl (CO) and phosphine, were cooperatively incorporated into half-metallocene iron(II) complexes (CpFeBr(CO)(PR 3 ); Cp¼C 5 H 5 ; PR 3 ¼PPh 3 , P(OPh) 3 , PMePh 2 , PMe 2 Ph, P(n-Bu) 3 ) for more active and versatile systems in transition metal-catalyzed living radical polymerization. For methyl methacrylate (MMA) with a bromide initiator [Me 2 C(CO 2 Me)CH 2 C(Me)-(CO 2 Me)Br; Me¼CH 3 ] [H-(MMA) 2 -Br], these hetero-ligated catalysts are superior, in terms of catalytic activity and molecular weight control, to similar homo-ligated half-metallocenes carrying two identical ligands such as CpFeBr(CO) 2 and CpFeBr(PR 3 ) 2 . Among the CpFeBr(CO)(PR 3 ) complexes examined, CpFeBr(CO)(PMePh 2 ) showed the highest activity and the best controllability (490% conversion within 24 h; M w /M n ¼1.29), and the 'living' character of the polymerizations therewith was proved by sequential monomer addition experiments. In spite of the high activity, the Fe(II) complex is stable and robust enough to be handled under air, rendering it suitable for practical use. The concomitant high activity and high stability were attributed to the in situ generation of a real active catalyst with a 16-electron configuration by the irreversible release of the CO group from CpFeBr(CO)(PR 3 ) on the activation of a terminal C-Br bond, as confirmed by the Keywords: carbonyl ligand; half-metallocene; iron catalyst; living radical polymerization; metal catalysis; methacrylate; phosphine ligand INTRODUCTION A key component of transition metal-catalyzed chemical reactions is obviously a metal complex catalyst, which determines and controls critical parameters including rate, efficiency, selectivity, versatility and so on, 1 and it is particularly true for metal-mediated living radical polymerization (Scheme 1), which we have been pursuing for over a decade (for recent reviews on transition metal catalyzed living radical polymerization, see Kamigaito et al., 2,3 Ouchi et al. 4 and Matyjaszewski and Xja 5 ). In general, a metal complex consists of a transition metal center and ligands, and the two components are connected through coordination and sometimes through metal-carbon bonds formed from a vacant d-orbital of the former and s-, p-or n-electrons of the latter. The ligands thereby affect the electronic as well as steric environments of complexes and, in turn, their catalytic performance.Living radical polymerizations are now powerful tools to synthesize controlled polymeric architectures, because, unlike the ionic counterparts, they are simple and easy to execute, robust and reproducible under varying conditions, and, above all, versatile and applicable to a wide range of monomers including functional derivatives that are often required in biochemistry, materials science and other disciplines

Quite recently, we have found that carbonyl-phosphine heteroligated cyclopentadienyl complexes [(Cp 0 )Fe(CO)(L phos )Br; Cp 0 ¼ g-C 5 H 5 (Cp) 23 or g-C 5 (CH 3 ) 5 (Cp*): 24 L phos ¼ phosphine], prepared via the ligand exchange for (Cp 0 )Fe(CO) 2 Br in the Correspondence to: M. Ouchi IRON CATALYZED LIVING RADICAL POLYMERIZATION, ISHIO, OUCHI, AND SAWAMOTO 537 SCHEME 2 Preparation of (Cp 0 )Fe(CO)(L phos )Br and living radical polymerization with (Cp 0 )Fe(CO)(L phos )Br. SCHEME 3 Living radical polymerization with (Cp Ph )Fe(CO) 2 Br. SCHEME 1 Transition metal-catalyzed living radical polymerization.