Previously, nitroxide-mediated controlled copolymerization (NMP) of methyl methacrylate (MMA) using BlocBuilder unimolecular initiator was found to require much less controlling comonomer when using 9-(4-vinylbenzyl)-9H-carbazole (VBK) compared to styrene (S) as a comonomer (minimum similar to 1 mol % VBK versus 4.4 mol % S). Here, we explored why this was the case. Initially, the use of dimethylformamide (DMF) solvent in the copolymerization of MMA/S was studied as the MMA/VBK copolymerizations were done in DMF. The results confirmed that the increased effectiveness of VBK as a controlling comonomer was not due to the solvent or other experimental conditions. Second, the propagation rate constant k(P,VBK) and various < k(P)>(MMA/VBK) for MMA/VBK copolymerizations were determined using pulsed laser polymerization-size exclusion chromatography (PLP-SEC), and the dissociation rate constants k(d,VBK) for the VBK-BlocBuilder adduct and PVBK chains were determined using electron paramagnetic resonance (EPR) spectroscopy, showing that k(d) for VBK was very similar to S and < k(P)>(MMA/VBK) was very similar to k(P,MMA). Finally, modeling of the system using PREDICI was done and illustrated that the difference in reactivity ratios between MMA/S and MMA/VBK was ultimately one of the major reasons for the increased control of VBK versus S. This study showed that propagation rate of the copolymerization and equilibrium constant for the dormant/active species are not the only parameters that govern controlled NMP, and the effect of reactivity ratios between the methacrylate and the controlling comonomer must also be considered for the controlled nitroxide-mediated copolymerization of methacrylate-rich mixtures.