Computational mechanistic studies on persulfate assisted p-phenylenediamine polymerization

p-Phenylenediamine (p-PDA) is a monomer of many important polymers such as kevlar, twaron, poly-p-PDA. Most of the noticed polymers formation is initiated by a free-radical, but their polymerization mechanism is not elucidated computationally. The proposed study helps to fully understand the frequently utilized initiator/oxidant, potassium persulfate (K₂S₂O₈) role in the aromatic diamines polymerization, which support experimental protocols, and a polymer scope. The formation of the poly-p-PDA is studied with the density functional theory (DFT) B3LYP-D3 functional using experimental polymerization parameters (0°C and aqueous media). K₂S₂O₈ initiated free-radical polymerization of p-PDA is studied in detail, taking into account sulfate free-radical (SO₄⁻)^·, SFR, persulfate anion (S₂O₈)²⁻, PA and K₂S₂O₈ cluster, PP. The reaction mechanism is calculated as the conversion of p-PDA to free-radical, the p-PDA free-radical attack to the next p-PDA (dimerization), ammonia extrusion from the dimer adduct, the dimer adduct conversion to the free-radical (completion of p-PDA polymerization cycle) for the polymer chain elongation. Calculations show that the dimerization step is the rate-limiting step with a 29.2 kcal/mol energy barrier when SFR initiates polymerization. In contrast, the PA-assisted dimerization energy barrier is only 12.7 kcal/mol. PP supported polymerization is calculated to have very shallow energy barriers completing the polymerization cycle, i.e., dimerization (TS2K, ∆G^‡ = 11.6 kcal/mol) and ammonia extrusion (TS3K, ∆G^‡ = 6.7 kcal/mol).