= 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,
$$
\overline X_n\ \sim\ \frac{k_p[\mathrm{propene}]}{k_{\mathrm{tr,eff}}},
$$
where the denominator includes the operative chain-transfer or chain-stopping rates per active chain. \b[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.
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