Roles of Proton in the Formation of Particles: Soap‐free Emulsion Polymerization of Styrene using AIBN and Potassium Persulfate as Initiators

Abstract

AbstractIn this paper, the hydrophobic effect or hydration of hydrophobe was firstly introduced into the mechanism of polystyrene particles formation. Negatively charged surface of hydration shell was proposed due to the ordered structure of water molecules. Consequentially the concentration of hydrophobic chains depended on pH because protons destroyed the hydration shell, thereby extruded hydrophobic chains from the H‐bonds networks. The soap‐free emulsion polymerization of styrene by using AIBN (azodiisobutyronitrile) and KPS (Potassium Persulfate), respectively was performed at various pHs. It was proved that protons played a crucial role on the formation of PSt (polystyrene) particles. By using AIBN, particles were not prepared at pH 2.0 (HAc), pH 4.0 (HCl) and pH 2.0 (HCl), whereas by using KPS, particles were obtained regardless to pH. It indicated that anions of sulfate greatly enhanced the tolerance of hydration shell to the suffering from protons. Meanwhile, the roles of polymerization in the monomer phase, aqueous phase and growing particles were elucidated. By using AIBN, ca. 30 % PSt with Mn (Numeral average molecular weight) =300‐1500 formed in the monomer phase at pH 6.8, which dissolved in water at pH 8.0 and pH 10.0. By using KPS, ca. 20 % and 30 % conversion of St were found in the aqueous phase at pH 2.0 (HCl) and pH 10.0 (NaOH), respectively. At pH 2.0 (HCl), oligomers generated in the aqueous phase were protonated, whereas at pH 10.0 (NaOH), they were soluble due to the seriously suppressed dissociation of water. At other pHs, oligomers captured protons, thereby relieved particles and interface from the suffering of protons. As the concentration of oligomers increased, oligomers precipitated to form clusters due to the increase of average number of protons on the surface of oligomers hydration shell, which successively incorporated into the growing particles, thus enlarged PDI (Polydispersity index) of MW (Weight‐average molecular weight) distribution in particles. By using AIBN, owing to the limited partition of AIBN in the interfacial layer of St phase, the flat‐roof peaks of GPC curves were observed, namely that the weight fraction of Mi was constant. It implies that the free‐radicals are living in the growing particles.