When gem-dichlorocyclopropane (gDCC) copolymers derived from polybutadiene are subjected to ultrasonication, the gDCCs undergo ring opening to form 2,3-dichloroalkenes. The reactivity is not observed in low-molecular-weight (6.5 kDa) copolymers or side-chain gDCCs, consistent with mechanically induced reactivity due to the elongational strain of the polymers in the sonication flow fields. The ring openings occur several hundred times more frequently than polymer chain scission, and cis-coupled gDCCs are slightly more likely to react than their trans isomers. The ability to dramatically and specifically alter the structure of the polymer backbone through a coupled restoring force suggests new routes to postsynthetic polymer modification and motivates the design of easily scalable mechanophores for applications in stress-responsive polymers.