Universal Confinement Length Governs Radical Depolymerization Kinetics in Nanoconfined Polymers

Thermal depolymerization is typically studied in bulk, yet many systems involve polymers confined within nanoscale geometries. Understanding how nanoconfinement alters radical depolymerization kinetics is essential for interpreting polymer stability in nanostructured environments. Here we show that thermal depolymerization of poly(methyl methacrylate) (PMMA) is governed by a universal confinement length scale that suppresses radical unzipping and increases the onset depolymerization temperature by up to ∼150°C. We systematically introduce confinement in two geometries: 1D thin films and 3D interstitial pores within SiO₂ nanoparticle packings. Nanoconfinement progressively suppresses chain-end-initiated unzipping and shifts the onset of random-chain-scission-initiated depolymerization to higher temperatures. This behavior stems from restricted segmental mobility near polymer-SiO₂ interfaces, imposing kinetic constraints on radical propagation. By defining an effective confinement length unifying 1D and 3D architectures, data from both geometries collapse onto a single master curve, revealing a critical threshold at ∼7.2 R_g of the PMMA chains. Below this scale, depolymerization enters a confinement-saturated regime where further dimensional reduction no longer increases the depolymerization temperature. This collapse demonstrates that depolymerization kinetics are governed by a universal length scale independent of specific architecture, establishing nanoconfinement as a fundamental physical parameter governing radical reaction dynamics in nanostructured environments.