 Solution

A plausible explanation is a stronger, less labile Hf–C interaction, reflecting the different bonding of the heavier metal. Breaking and reorganizing that interaction in migratory insertion can raise the propagation barrier and reduce the insertion rate. At the same time, stronger retention of the metal-bound alkyl can suppress beta-hydride elimination or other chain transfer in chain polymerization processes even more strongly.

The chain length depends on competing rates, not on propagation speed alone. In a simple stationary competition,where the denominator includes the operative chain-transfer or chain-stopping rates per active chain. The Hf catalyst can propagate more slowly yet make longer chains if transfer is suppressed by a still larger factor. Slower insertion alone would not explain longer polymers. Distinguishing particular transfer pathways would require polymer end-group analysis and the concentration dependence of molecular mass and polymerization rate.