The mechanism of stereoselectivity in methyl methacrylate (MMA) monometallic polymerization promoted by the C-1-symmetric ansa-Me2C(CP)(Ind)Zr-based zirconocene, presenting two diastereotopic coordination sites, was investigated within density functional theory. At variance from the case of propene polymerization, our results suggest that MMA addition is stereoselective when the growing chain is in the more hindered as well as in the more open diastereotopic coordination site. In both cases the same enantioface of the enolate bond of the ester enolate growing chain is selected, although selectivity is higher when the growing chain is in the more hindered diastereotopic coordination site. Further, there is no substantial energy difference between these two situations, which indicates that the moderate isotacticity of the produced PMMA is consistent with an almost regular chain-migratory mechanism. The different stereoselectivity of the two diastereotopic sites is due to different steric interaction between the growing chain (chain effect) or MMA (monomer effect) with the indenyl ligand. This decomposition of stereoselectivity into chain and monomer effects is used to reinvestigate the stereoselectivity of strictly related C-2- and C-s-symmetric zirconocenes.