A comprehensive kinetic Monte Carlo (kMC) model is used to interpret and better understand the results of a systematic experimental investigation of activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP) of butyl methacrylate (BMA) using Sn(EH)(2) as reducing agent, ethyl 2-bromoisobutyrate (EBiB) as ATRP initiator, and CuBr2/TPMA (TPMA: tris[(2-pyridyl)methyl]amine) as deactivator. The model demonstrates the importance of slow initiation, with distinct activation and deactivation rate coefficients for the initiator and polymeric species required to match the experimental data. In addition, the model incorporates a second reduction step for the reducing agent and accounts for diffusional limitations on chain-length-dependent termination. The effect of temperature on the slow ATRP initiation is limited, and a sufficiently high initial reducing agent concentration is crucial to obtain a high conversion, although achieved at the expense of decreased end-group functionality.