The N, N, N', N', N"-pentamethyldiethylenetriamine (PMDETA) ligand, commonly used in copper-catalyzed atom transfer radical polymerization (ATRP), is known to have an inhibitive effect on the kinetics of the ATRP of alkyl acrylate monomers due to the chain transfer reaction involving the ligand species. However, the exact chemical nature of the ligand radicals resulting from the chain transfer reaction and other reactions that the ligand radicals may undergo subsequently are presently unknown, making it difficult to establish the precise reaction mechanism of the copper/PMDETA-based ATRP of alkyl acrylate monomers. In an effort to gain a better understanding of the chemistries and reactions of the PMDETA species in the ATRP of n-butyl acrylate, we conducted (i) experimental measurements to determine how changes in PMDETA concentration influence the ATRP kinetics and the molecular properties of the product, (ii) electron density functional theory (DFT) calculations to identify probable pathways for reactions involving the PMDETA ligand, and (iii) fitting of the experimental data obtained from (i) to a kinetic model derived on the basis of the reaction feasibility analyses from (ii) to test the validity of the proposed ATRP mechanism. The results suggest that (a) the proton abstraction from the PMDETA ligand occurs predominantly at a specific site of the molecule, (b) once generated, the ligand radical is unlikely to be involved in any further reaction, and (c) there appears to exist a disparity in the reactivity of PMDETA toward chain transfer reactions between the free and copper-bound states.