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

Abstract

ABSTRACT 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‐H 2 ). 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.