Ferrocene is classically regarded as being highly inert owing to the large dissociation energy of metal-cyclopentadienyl (Cp) bonds. We show that the Fe-Cp bond in ferrocene is the preferential site of mechanochemical scission in the pulsed ultrasonication of main-chain ferrocene-containing polybutadiene-derived polymers. Quantitative studies reveal that the Fe-Cp bond is similar in strength to the carbon-nitrogen bond of an azobisdialkylnitrile (bond dissociation energy < −0 kcal/mol), despite the significantly higher Fe-Cp bond dissociation energy (approximately 90 kcal/mol). Mechanistic studies are consistent with a predominately heterolytic mechanism of chain scission. DFT calculations provide insights into the origins of ferrocene’s mechanical lability.

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Cyclic polymers containing multiple gem-dichlorocyclopropane (gDCC) mechanophores along their backbone were prepared using ring expansion metathesis polymerization. The mechanochemistry of the cyclic polymers was investigated using pulsed ultrasonication. The fraction of gDCC mechanophores that are activated per chain halving event (Φ) was compared to that of linear analogs. For 167 kDa cyclic polymer, Φ = 0.38, vs Φ = 0.62 for 158 kDa linear polymers analogs, even though cyclic chain fragmentation necessarily proceeds through a linear intermediate of comparable composition to the initially linear systems. Ozonolysis of the mechanochemical products further shows that the mechanochemical “activation zone” in the cyclic polymer is less continuous than in the linear polymer. These results suggest that the linear intermediate in cyclic polymer fragmentation undergoes subsequent scission during the same high strain rate extensional event in which it is formed and furthermore retains at least a partial memory of its original cyclic conformation at the time of fragmentation.