The association/dissociation equilibrium between vinyl monomers (M) and the complex [Cu-I-(PMDETA)]BPh4 (where PMDETA = N,N,N',N",N"-pentamethyldiethylenetriamine) to form [Cu-I-(PMDETA)(π-M)]BPh4 was studied by variable temperature H-1 NMR. The enthalpy and entropy of coordination and the relative binding constants of methyl acrylate (MA), 1-octene (Oct), styrene (Sty), and methyl methacrylate (MMA) were determined. The affinity of the monomers to the Cu-I center decreased in the order MA > Oct > Sty > MMA with formation constants (at room temperature) equal to 760, 320, 250, and 6 M-1, respectively. Quantification of this equilibrium process allowed the study of the effect that coordination has on the reactivity of the monomer in free radical polymerization. During these studies, an additional side reaction between free radical azo-initiators (AIBN and V-70) and the Cu-I complex was identified and analyzed. Additionally, the consequence of monomer coordination to Cu-I in atom transfer radical polymerization (ATRP) was examined. The reactivity of the monomers in conventional free and ATRP was not significantly affected by the coordination.