Active, Versatile, and Removable Iron Catalysts with Phosphazenium Salts for Living Radical Polymerization of Methacrylates

Phosphazenium halides (PZN-X; X = Cl, Br, I), highly delocalized bulky salts, turned out to be excellent cocatalysts to be combined with iron halides (FeX2) 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 FeBr2/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 [FeBr2(PPh3)(2); Ph = C6H5] but also other hitherto known combinations of FeBr2 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.