AbstractPolymer architecture plays an important role in morphology and mechanochemical response. Here, graft copolymers containing mechanophores at the backbone‐arm junction are synthesized through a combination of post‐polymerization modification of butyl rubber and controlled radical polymerization. A series of graft copolymers with 70:30 or 50:50 polyisobutylene:polystyrene (PIB:PS) volume fractions were synthesized by changing the grafting density and arm length to keep the volume fractions constant. The morphologies for these polymers are disordered but show a decreasing domain size with shorter arm lengths, and the samples with a greater number of PS side chains show greater domain mixing. Using pulsed ultrasound experiments, we found greater backbone scission occurs for polymers with a greater number of short grafts per chain than the long grafts with fewer grafting points. Finally, mechanophore activation is greater for samples with arm molecular weights ≥13 kg/mol, attributed to exceeding the cutoff molecular weight for PS.