The Mechanism and Kinetics of Hydroxyl Radical Polymerization of Nonionized Acrylic Acid and Its Derivatives in Aqueous Solution

In the free-radical polymerization of acrylic acid derivatives (AAD), including acrylic acid (AA), methyl acrylate (MA), acrylamide (AM), methacrylic acid (MAA), and methyl methacrylate (MMA), initiation and propagation occur via radical addition to AAD. Although extensive experimental data exist about this industrial process, many mechanistic aspects of the reactions themselves remain unclear, largely because of the challenge that the characterization of reaction mechanisms poses to experimental methodology. Computational methods offer an alternative avenue to deliver fast and accurate results on the mechanistic details, as evidenced by various theoretical studies in the literature. Here, we report on quantum chemical computations of the radical polymerization of AAD initiated by the HO^• radical in an aqueous environment (that is used regularly in the polymerization of these species) across a temperature range of 273-373 K. It was determined that the computed rate constants for HO^•-initiated AAD reactions, ranging from 7.95 × 10⁸ to 3.17 × 10⁹ M^-1 s^-1, show strong agreement with experimental values further confirming the accuracy of the method. The calculated propagation rates of AAD range from 1.40 × 10³ to 3.90 × 10⁵ M^-1 s^-1 and also closely align with experimental findings. In terms of the mechanism, the propagation reaction occurs predominantly at the C3 position of the double bond, resulting in polymers containing COOH, CONH₂, or COMe groups situated on each side of the polymeric backbone. The propagation rate increases with temperature within the studied range of 273-373 K.