In order to assess the role of the solvent mediating the disproportionation of Cu(I)X into Cu(0) and CU(II)X-2 during SET-LRP, a series of polymerizations of methyl acrylate initiated with methyl 2-bromopropionate and catalyzed by Cu(0)/Me-6-TREN at 25 degrees C in DMSO, MeCN, and their mixtures were investigated. These polymerizations proceed in a dissimilar manner. SET-LRP of MA in DMSO, a solvent that mediates disproportionation, was exemplary of "ultrafast living radical polymerization" with complete conversion in 45 min following the expected first-order kinetics and providing perfect fidelity of the structure of the polymer chain ends. When the polymerization was performed in MeCN, a solvent in which Cu(I)X species do not disproportionate, it displayed nonlinear first-order kinetics with a poor retention of the bromine chain end functionality. The lack of first-order kinetics and decrease of chain end functionality with conversion indicate a nonliving polymerization in MeCN. Increasing the concentration of DMSO in DMSO/MeCN mixtures provided a continuous transition from a nonliving to a living polymerization. The incompatibility of MeCN with SET-LRP lends support that disproportionation of Cu(I)X/N ligand to Cu(0) and Cu(II)X-2/N ligand is the key step in SET-LRP.