The use of tert-, sec-, iso-and normal-butylmagnesium bromides and chlorides in toluene, tetrahydrofuran and mixed solvents to prepare Bernoullian, stereoblock and isotactic poly(methyl methacrylate), PMMA, and the factors controlling the stereochemistry are described. By careful control of solvent composition, highly isotactic polymer can be prepared with all the butylmagnesium bromides, but none of the corresponding dibutylmagnesiums. The halide content does not influence stereospecificity when it is in excess of the Grignard stoichiometry but it influences the mol. wt distribution and retards the polymerization. The mol. wt distributions are usually polymodal and dependent on the same factors that control stereospecificity. The effect of THF concentration on stereospecificity operates in a manner quite distinct from its effect on complex concentration and rate. The concentration of residual THF in toluene-rich solution determines the type of initiation and propagating species that prevail. Structures are proposed for those responsible for stereospecific initiation and propagation. It is concluded that tert-butylmagnesium bromide is the most reliable and robust initiator for preparing isotactic PMMA.