This study investigates the efficiency of a novel trifunctional alkoxyamine (1) based on N-tert-butyl-1-diethylphosphono-2,2-dimethylpropyl nitroxide (SG1) (2) as initiator for the controlled radical polymerization of styrene (S) and n-butyl acrylate (nBuA). Three factors, namely the concentration of SG1 initially introduced in excess, the monomer conversion, and-in the specific case of PS samples the monomer autopolymerization, were found to control the quality of the star samples obtained. Well-defined T-(PBuA-b-PS)(3) star block copolymers could also be synthesized by sequential copolymerization using the same trifunctional initiator.