In this study we examine the Atom Transfer Radical Polymerization (ATRP) of vinyl monomers mediated by a new class of ruthenium alkylidene catalysts containing a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and a Schiff base ligand. A controlled radical polymerization can also be achieved with the corresponding cationic complexes using both toluene and a water/toluene mixture as solvent. Our results point out that with these cationic complexes the solvent is crucial for the activity and controllability of the metal-catalysed polymerization. In aqueous/organic biphasic conditions these cationic systems are active in the radical polymerization of hydrophobic monomers such as methyl methacrylate, methyl acrylate and styrene and polymers with controlled molecular weights and narrow molecular weight distributions are obtained.