The relevance of penultimate monomer unit (PMU) effects and the selection of the correct initiator species under typical reversible deactivation radical copolymerization conditions is illustrated, using matrix-based kinetic Monte Carlo simulations allowing the visualization of all monomer sequences along individual chains. Initiators for continuous activator regeneration atom transfer radical polymerization (ICAR ATRP) is selected as illustrative polymerization technique with n-butyl acrylate and methyl methacrylate as comonomers, aiming at the synthesis of well-defined gradient copolymers. Using literature based model parameters, in particular temperature dependent monomer and radical reactivity ratios, it is demonstrated that PMU effects on propagation and ATRP (de)activation cannot be ignored to identify the most suited ICAR ATRP reactants (e.g., tertiary ATRP initiator) and reaction conditions (e.g., feeding rates under fed-batch conditions). The formulated insights highlight the need for further research on PMU effects on all reaction steps in radical polymerization. (C) 2017 American Institute of Chemical Engineers.