A side-armed bis(oxazoline) (SaBOX)/copper catalyst, possessing a low equilibrium constant (K-ATRP = k(act)/k(deact)) and bearing large side-arm groups, was employed for supplemental activation and reducing agent atom transfer radical polymerization (SARA ATRP) of methyl methacrylate (MMA), which suppressed radical-radical termination events and successfully achieved very fast and controlled polymerizations, high end group fidelity, and ultrahigh molecular weight (up to 741 kg/mol) under common conditions. Compared with the copper complex bearing pentamethyldiethylenetri-amine or tris[2-(dimethylamino)ethyl]amine (Me6TREN) as the ligand, the SaBOX/copper catalyst performed much better in terms of polymerization rate and controllability under the same conditions via lowering the growing radical concentration and prolonging the time scale of living polymerization, an indicative of its unprecedentedly high efficiency in ATRP of MMA. The versatility and robustness of the SaBOX/copper system was further demonstrated by the controlled polymerization of various monomers, such as methacrylates, acrylates, styrenes, acrylamide, and even methacrylic acid. In situ chain extension experiments confirmed the very high end group fidelity, allowing the convenient preparation of well-defined block copolymers.