This article discussed the root causes of the interesting differences between rac-Et(Ind)(2)ZrCl2 and dimethyl (pyridyl-amido)hafnium in catalyzing the propylene/-halo--alkene copolymerization. Confirmed by density functional theory (DFT) calculations, the larger spacial opening around the active center of rac-Et(Ind)(2)ZrCl2 contributes to the coordination and insertion of the monomers, resulting in the higher catalytic activity, while the narrow spacial opening around the Hf center retards the chain transfer reaction, leading to the much higher molecular weights (M(w)s) of the copolymers. The superior tolerability of Zr catalyst toward halogen groups might be attributed to that the dormant species generated from halogen coordination could be promptly reactivated. DFT calculations indicated the higher probability for the -halo--alkene vinyl to coordinate with the Hf catalyst leading to the better ability to incorporate halogenated monomers. The high M(w)s and the outstanding isotacticity achieved by the Hf catalyst determined the higher melting temperature values of the copolymers with a certain amount of halogen groups. In addition, the chain transfer schemes were employed to analyze why the presence of halogenated monomers greatly decreased the M(w)s of the copolymers when rac-Et(Ind)(2)ZrCl2 was used, while had no or little effect upon the M(w)s in the copolymerization by the Hf catalyst. (c) 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014, 52, 3421-3428