High Mechanophore Content, Stress-Relieving Copolymers Synthesized via RAFT Polymerization

We report the mechanochemical activity of copolymers formed through the radical addition polymerization of cyclobutene carboxylates that are fused to larger rings. The fused ring repeats act as stored-length mechanophores along the polymer backbone, and so these polymers are a rare example of high mechanophore content polymers formed by addition polymerization. The monomers are amenable to controlled radical polymerization techniques, specifically reversible addition-fragmentation chain transfer (RAFT) copolymerization with butyl acrylate, which enables comparative studies of mechanochemical activity under ultrasonication of polymer solutions, relative to that of previous high mechanophore content polymers. Analysis of mechanophore activation as a function of the polymer scission cycle reveals that these cyclobutane-based mechanophores operate in high force regimes. In addition, the kinetics and "controllability" of the RAFT polymerizations are investigated as a function of (co)monomer composition, providing a foundation for the design of bulk polymer networks that contain a tunable amount of these stored-length mechanophores.