Over the past decade, synthetic methods have been harnessed to make new classes of stress-responsive polymers that translate macroscopic deformations into tailored, molecular-level, covalent chemical responses along the polymer main chain through the introduction of mechanically responsive functional groups (mechanophores). The most common technique to date for synthesizing mechanophore containing polymers has been controlled radical polymerization (CRP) from a difunctional initiator. Although synthetically useful, CRP has largely been limited to polymer systems where a single mechanophore resides at or near the center of the mainchain, thereby limiting the magnitude of the desired mechanochemical response. Inspired by the recently demonstrated ability to polymerize cyclobutene carboxylates that are fused to larger macrocycles via conventional free radical polymerization, in this work we report the reversible addition-fragmentation chain transfer (RAFT)-based random copolymerization of these monomers, leading to high content of stored-length mechanophores along the polymer backbone that are mechanically active. In addition, the kinetics and “controllability” of the polymerization are monitored as a function of (co)monomer composition, which we envision will be useful information for designing bulk polymer networks that contain a tunable amount of these stored-length mechanophores. We also present the ring-opening metathesis polymerization (ROMP) of fused-ring monomers with a tunable “loop size”, which leads to polymers with adjustable stored-length mechanophore content. We characterize the mechanochemical reactivity of these polymers via single-molecule force spectroscopy (SMFS), the data of which highlights the power of these mechanophores for energy dissipation within overstretched polymers.

Stress-dissipating polymers synthesized via the RAFT or ROMP of bicyclic, mechanoresponsive monomers

ACS Fall 2020 Virtual Meeting

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