Computational study of the kinetics and mechanism of radical polymerization of acrylic acid and derivatives in organic solvents

Abstract

Free-radical polymerization of acrylic acid derivatives (AADs), such as acrylic acid (AA), methyl acrylate (MA), acrylamide (AM), methacrylic acid (MAA), and methyl methacrylate (MMA), proceeds through radical addition of monomers during initiation and propagation. Despite the extensive experimental literature on this process from both a radical chemistry and an industrial point of view, there is little information on the mechanistic pathways of the radical addition process, particularly in organic solvents. In this work, quantum chemical methods were applied to explore the solvent and initiator effects on AAD polymerization. The reactivity of di-tert-butyl peroxide (TBO˙) and dicumyl peroxide (CMO˙) and 2-cyanoprop-2-yl radical (AI˙) as initiators was systematically studied in isopropanol (IP) and toluene (TL). Computational results predict that initiation is consistently faster in IP than in TL, with initiator efficiency ranked as TBO˙ > AI˙ > CMO˙. At 298 K, the predictions for propagation constants in IP (2.80 × 10¹ to 2.60 × 10⁴ M⁻¹ s⁻¹) are substantially higher compared to those in TL (2.60–1.50 × 10⁴ M⁻¹ s⁻¹). Additionally, solvent-derived radicals were predicted to participate actively in propagation. Temperature was found to significantly increase log(k_p) in both toluene and isopropanol, consistent with the Arrhenius kinetic model. The computed propagation rate constants for MA polymerization in toluene (1.00 × 10³ to 1.10 × 10⁵ M⁻¹ s⁻¹ at 320 K) and the activation energies of MA and MMA (3.6 and 3.4 kcal mol⁻¹, respectively) align well with experimental results, validating the accuracy and reliability of the computational approach. The effect is more pronounced in toluene due to the absence of hydrogen-bonding interactions, underscoring the key role of the solvent in controlling propagation kinetics. This study predicts that solvent polarity and the properties of the radical strongly govern the kinetics of AAD polymerization, thereby providing useful mechanistic insights for optimizing radical polymerization in non-aqueous media.