Stereocontrol energy (DELTAE(O)) is investigated as a measure of enantioselectivity of ansazirconocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using pi complex (piC) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-eta5-indenyl)- (1) and rac-ethylenebis (1-eta5-4,5,7,8-tetrahydroindenyl) (2) zirconocenium species are isospecific in either the pi-complexes or the transition states. The stereoselectivity is greater if there is alpha-agostic interaction; it is lowered in the cases of beta and gamma agostic interactions. The C-13-NMR steric pentad distribution indicates the poly (propylene) to be much less stereoregular than that predicted by DELTAE(O). Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss.