The effect of several types of ligands, i.e., 12-CE4 (1), K211 (2), LiOtBu (3), and LiOEEM (4), on the stereochemistry of MMA anionic polymerization at -78 degrees C in various solvents, i.e., toluene, THF, and a 9/1 toluene/THF mixture, has been studied. Similar to the LiCl-complexed system, the stereoregularity of MMA polymerization in the presence of these additives is seemingly controlled by association/complexation equilibria among associated, nonassociated, and ligand-complexed active species. Accordingly, complexation of active species by sigma-chelating ligands such as 12-CE4 and K211 simply shifts the association equilibrium toward the formation of ligand-complexed (nonassociated) species, giving preferably syndiotactic placements (rr) in a 9/1 toluene/THF mixture as well as in toluene. However, the effect of a mu-type ligand, i.e., LiOtBu, seems more complicated. In THF and toluene, similar to ligand-free systems, highly syndiotactic and isotactic PMMAs are produced, respectively, implying the existence of different types of stereoselective LiOtBu-complexed species. In a 9/1 toluene/THF mixture, moreover, addition of LiOtBu gives rise to an increase in isotactic placements (mm) and matches the values observed in toluene. This is in striking contrast to the LiCl-added system, for which it has been tentatively proposed that a meso or raceme placement might also critically depend on the aggregation degree of the living chain in the complexed species. Nevertheless, a mu/sigma dual ligand, i.e., LiOEEM, gives rise to an almost identical stereoregularity (i.e., highly syndiotactic) whatever the solvent used, well consistent with the involvement of the same type of active complex.