Revealing Mechanochemical Force Distributions with Polymechanophore Block Copolymers

Polymer mechanochemistry is most studied under ultrasonication conditions, where the force distribution along a polymer chain is typically modeled as a parabola centered at the chain midpoint. How far the forces required for mechanoactivation extend toward the chain ends remains a question of interest. Investigating the mechanochemical reactivity of mechanophores at defined locations off chain center and toward termini would provide valuable information regarding this, but preparing such polymers to study the effects of mechanophore location and chain length on mechanoactivation has been synthetically cumbersome. Using an operationally simple procedure, we synthesized a series of block copolymers containing a block of mechanophores by living ring opening metathesis polymerization of a ladderene-type mechanophore and a norbornene, with good control over block positions and lengths. We found that for polymers of initial degrees of polymerization (DP) ≈ 1000, terminal mechanophore blocks activated less than more centrally located ones. However, simply by extending the length of the mechanochemically inert block, terminal mechanophore blocks of DP ≈ 200 achieved surprisingly high degrees of mechanoactivation comparable to those of centrally located ones or mechanophore homopolymers, after an induction period. These findings revealed the broad range of high force coverage along a polymer chain under sonication conditions and the possibility to still achieve high degrees of productive mechanochemistry far away from chain centers.