Hydroxyl radical-initialized polymerization and degradation of N-vinylpyrrolidone in lipid and aqueous environments

Abstract

Poly(N-vinylpyrrolidone) (PVP) is an industrial polymer that has many uses in cosmetic, pharmaceutical, and biomedical formulations due to its low toxicity. Nonetheless, the destruction of PVPs by free radicals such as HO• under physiological conditions and in the environment may result in the formation of dangerous compounds, especially in case of the further reaction of the transition state of the monomer N-vinylpyrrolidone (NVP) with HO•. A comprehensive study of the mechanism and kinetics of the radical reactions of the monomer NVP has not yet been carried out. Here we report on quantum chemical computations of the radical polymerization and degradation of the NVP in reactions with the hydroxyl radical in aqueous and lipidic media. It was found that the reaction of NVP with HO• radical has an overall rate constant of koverall(app) = 3.64×109 and 6.16×109 M−1 s−1 in pentyl ethanoate and water, respectively. The good agreement between the calculated rate constant in koverall(f) = 1.86×109 M−1 s−1 at pH ≈ 7.0) and experimental data (k(NVP + HO•) = 1.2–2.0 × 109 M−1 s−1 validates the accuracy of the computational method. The propagation rate constant (kp) was 3.80×103 M−1 s−1 in water, which is also in good agreement with experimental data under the studied conditions: kp(calculated)/kp(experimental) = 0.22−0.42. The RAF(C7)-FHT(C5-H) reaction was the primary mechanism for the radical initiation of NVP in the lipid medium, whereas the RAF(C6)-SET reaction was responsible for the initiation in an aqueous solution. Secondary interactions of the intermediates with the hydroxyl radical yielded a range of products that were assessed and found to be less hazardous to the environment than NVP.