New evidence on the polymerization kinetics of methyl methacrylate and molar mass distributions is presented which together with earlier work on the stereochemistry of the polymerization of α,β-dideuteroacrylates shows that these are not typical anionic polymerizations. The persistence and stability of the active centers, together with evidence that the α-carbon of the monomer assumes the tetrahedral configuration it takes in the polymer chain as it becomes the active chain end, suggests that the active centers are covalent magnesium-carbon bonds. Existing hypotheses about the mechanism of steric control in homogeneous polymerizations are examined. Kinetic evidence confirms that complexing of the monomer to the active center does occur. However, intramolecular complexing of the penultimate or antepenultimate chain carbonyl group to the metal is not a prerequisite for isotactic polymerization though it may be involved in controlling the in-chain configuration of the β-carbon. Gel-permeation chromatography shows that in some systems more than one active center operates independently. These centers are established in the initiation stages and persist. The proportions of active centers of different reactivity and stereo-specificity are determined by conditions prevailing during initiation and it is these that primarily determine the structure of the polymers formed. Evidence is reported that confirms this.