Isotactic (type II)polymers of methyl methacrylate are obtained from anionic polymerizations initiated by organolithium compounds at low temperature ( −70°)in toluene solution. The presence of small amounts of various ethers leads to polymers of reduced isotacticity; the effectiveness of these ethers in disrupting stereospecificity can be related semiquantitatively to their Lewis base strength. Certain Grignard reagents also produce highly isotactic polymer of methyl methacrylate, while others lead to less stereospecific results. The nature of the inorganic component of such initiators appears to determine the steric course of the addition reaction. The formation of a magnesium halide–monomer complex may be involved.