Quantum-Chemical Study for Understanding the Low Incorporation of 2‑Methylen-1,3-Dioxepane (MDO) in Radical Ring-Opening Copolymerization with Vinyl Monomers.

Radical ring-opening polymerization (rROP) of cyclic ketene acetals (CKAs), particularly 2-methylene-1,3-dioxepane (MDO), offers a robust pathway to introduce degradable ester linkages into nondegradable vinyl backbones. However, the rational design of these materials is currently hindered by highly comonomer-dependent open-to-closed (ester vs acetal) incorporation ratios. In this study, a rigorous density functional theory (DFT) framework integrating systematic conformational sampling and implicit solvation is employed to establish a unified structure-reactivity map for MDO copolymerization across a diverse set of vinyl monomers. The computational results reveal that while MDO homopolymerization is thermodynamically driven toward quantitative ring opening due to the irreversibility of the open-chain radical, copolymerization is governed by a fine-tuned competition between the kinetic accessibility of vinyl addition and the stability of the resulting cyclic adduct. The closed-propagation barrier is primarily dictated by a SOMO-LUMO interaction, where the electronic nature and α-substitution of the comonomer modulate orbital overlap and energy. Three distinct reactivity regimes are identified: (1) high open incorporation (e.g., MDO, crotonates), where high vinyl barriers and adduct reversibility favor β-scission; (2) intermediate competition (e.g., vinyl acetate); and (3) ring-retention dominance (e.g., acrylates), where rapid vinyl addition and deep thermodynamic stabilization suppress the degradable pathway. This work provides a qualitative molecular basis for the rational selection of comonomers to maximize CKA incorporation in the open form for next-generation sustainable vinyl polymers.