The chain propogation mechanism of the constrained geometry catalysts (CGC) (SiH2(Cp)NH)MCH3+ (M= Ti, 1,Zr, 3, Hf, 4) and (SiH2(Cp)NH)TiCH3, 2, has been studied theoretically by density functional theory (DFT) and molecular mechanics. DFT energy profiles have been determined for the insertion of ethylene into the M-CH3 bonds of the aforementioned CGCs (L2M-CH3 + CH2=CH2 Æ L2M-CH2-CH2-CH3). One of the objectives of the study was to compare the insertion process involving the cationic Ti (IV) CGC, 1, and its neutral Ti(III) counterpart, 2. The insertion process for both oxidation states was found to be quite feasible with the Ti(IV) and Ti(III) complex possessing modest insertion barriers of 3.8 kcal/mol and 3.3 kcal/mol, respectively. The insertion process for the Ti(IV)-, Zr(IV)- and Hf(IV)-based CGCs were compared and it was found that the insertion barriers increased in the order Ti < Zr (a) Hf. The calculated insertion barriers were calculated to be 3.8, 5.1 and 5.8 kcal/mol for the Ti, Zr and Hf complexes, respectively. A possible chain rearrangement mechanism involving the rotation of the M-Ca bond was also examined by molecular mechanics. The results suggest that this process is sterically unhindered for the half-sandwich constrained geometry catalysts whereas it is significantly more hindered for the full sandwich bis-Cp metallocenes.