Repurposing Diels-Alder Chemistry to Access Low-Strain Monomers for Chemically Recyclable ROMP Polymers

Diels-Alder reaction has been widely employed to access highly strained cyclic olefin monomers for efficient ring-opening metathesis polymerization (ROMP). However, the high ring strain energies (RSEs > 15 kcal/mol) of these traditional Diels-Alder monomers, while promoting efficient polymerization, compromise the depolymerizability of the resulting polymers, thereby limiting their chemical recyclability. Herein, we report a class of low-strain Diels-Alder monomers based on trans-succinimide-fused cyclohexenes (tSICHs), synthesized from readily available feedstocks, fumaric acid and 3-sulfolene. The relatively low RSEs of the tSICH monomers (6.1-6.2 kcal/mol) enable both efficient ROMP and ring-closing metathesis depolymerization (RCMD) through concentration-controlled metathesis equilibrium. All tSICH monomers achieved high conversions (>91%) during ROMP at an initial monomer concentration of 5 M, yielding polymers with high molecular weights (>100 kDa). Moreover, near-quantitative monomer regeneration (>95%) was achieved via RCMD at room temperature with an olefin concentration of 0.2 M. Critically, the resulting polymers exhibit highly tunable thermomechanical properties, affording a broad range of glass transition temperatures (30-78 °C) and tensile strains at break (3-550%). Overall, this work provides a new monomer design strategy that unlocks access to chemically recyclable polymers from Diels-Alder-derived ROMP monomers.