Tentative mechanisms for acrylate intercalation and in situ polymerization in nickel-based layered double hydroxides

New hybrid organic-inorganic materials, based on polyacrylate macromolecules intercalated into Layered Double Hydroxides (LDHs) deriving from Ni(OH)(2), were prepared through an original route made of successive redox exchange reactions (chimie douce synthesis). The nature of the substituting metallic cation in the slabs appears to strongly influence monomer intercalation and in situ polymerization mechanisms. So, the phase containing intercalated acrylate monomer anions has been isolated in the case of iron-based LDH and then interslab free-radical polymerization of acrylate anions has been successfully initiated by potassium persulfate. On the other hand, a one-step process occurs in cobalt-and manganese-based LDHs, leading straight to LDHs containing polyacrylate anions. In these last few cases, a free-radical polymerization mechanism is also suggested, but involving a redox initiation step where a few amount of acrylate anions would be oxidized during the intercalation reaction and would lead to the formation of radical carbonium ions. For the first time, in situ polymerized macromolecules have been extracted from interslab spaces by anionic exchange, derivatized into poly(methyl acrylate) and analyzed by size exclusion chromatography. As expected from the above mechanisms, only oligomers have been detected, with weight-average molar masses ranging from 300 to 2900 g/mol vs polystyrene standards.