The copolymerization of butyl methacrylate and butyl acrylate initiated by CuBr 2 /tris[(2-pyridyl)methyl]amine/ethyl 2-bromoisobutyrate, with tin(II) 2-ethylhexanoate as reducing agent, is studied experimentally and using kinetic Monte Carlo simulations at 70 and 90• C under batch activator regenerated by electron transfer atom transfer radical polymerization conditions in anisole at low Cu levels (down to 35 ppm Cu on a molar basis with respect to monomer). With increasing initial butyl acrylate content, the initiator efficiency is improved with an accompanying increase in polymer dispersity due to oligomer formation. The addition of reducing agent during the polymerization, i.e. a semi-batch approach, allows an increase in polymerization rate and initiator efficiency, driving the polymerization towards full conversion and allowing for better initiator consumption.