Engineering Free Volume within Frontal Ring‐Opening Metathesis Polymerization via Pendant Plasticization

Frontal ring-opening metathesis polymerization (FROMP) enables rapid, energy-efficient access to high-performance thermosets and thermoplastics, but the range of accessible properties remains constrained by the rigidity of norbornene-type backbones. Here we introduce a side-chain plasticization strategy for FROMP, wherein norbornene esters bearing n-alkyl groups of varying length (n = 8, 12, 16) are copolymerized with dicyclopentadiene (DCPD) or hydrogenated DCPD (DCPD-H₂). Systematic incorporation of these pendants tunes free volume, resulting in predictable reductions in glass transition temperature (T_g), decreased moduli, and a transition from rigid thermosets to elastomers exceeding 800% elongation at break. Free-volume analysis via dynamic mechanical analysis and solvent swelling ratios confirms pendant length and distribution as key parameters governing network porosity and mobility. Moreover, high-alkyl-content formulations exhibit nonlinear front propagation (spin modes) and strain-induced whitening-features that highlight opportunities for spatial patterning and cooperative molecular alignment under load. Collectively, these results establish side-chain engineering as a versatile design principle for expanding FROMP into elastomeric regimes, providing a scalable pathway to soft, tunable, and structurally programmable materials.