Tentative Mechanisms for Acrylate Intercalation and in Situ Polymerization in Nickel-Based Layered Double Hydroxides C. Vaysse, L. Guerlou-Demourgues, C. Delmas, and E. Duguet* Institut de Chimie de la Matie ` re Condense ´ e de Bordeaux-CNRS and Ecole Nationale Supe ´ rieure de Chimie et Physique de Bordeaux, Universite ´ Bordeaux I, 87 Avenue du Dr Albert Schweitzer, 33608 Pessac Cedex, France Received November 27, 2002; Revised Manuscript Received October 23, 2003 ABSTRACT: 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. Introduction Research efforts in the field of hybrid organic- inorganic materials (HOIM) have been continuously increasing for 15 years, in particular because these materials are attractive candidates for optical devices, separation media, catalysts, sensor coatings, structural materials, etc. 1 The promising family of layered HOIM results basically from the intercalation of organic species into the interslab space of inorganic lamellar lattices. Strategies for intercalating organic macromolecules between the slabs of host materials are mainly as follows: (i) direct intercalation of preformed macro- molecules, 2-8 (ii) precipitation of mineral slabs (or restacking after exfoliation) in a macromolecular solu- tion, 9,10 (iii) interslab polymerization of intercalated monomers, 11-15 and (iv) redox intercalative polymeri- zation (RIP). 16-19 In this last case, the intercalation process is accompanied by a spontaneous reduction of the mineral component (Cu 2+ -fluorohectrorite, 16 FeOCl, 17 V 2 O 5 , 18 VOPO 4 , etc. 19 ) and a concomitant oxidative polymerization of the monomers, which are pyrrole, 16,17 aniline, etc. 18,19 Layered double hydroxides (LDHs), known as anionic clays, have also received attention as precursors of HOIM materials. 20 The structure of these materials consists of stacked brucite-type [M 1-y II L y III (OH) 2 ] slabs. The excess positive charge, due to the partial substitu- tion of trivalent cations (L) for divalent cations (M), is compensated by X n- anions, co-intercalated with water molecules within the interslab space. The X n- inter- lamellar anions amount is thus theoretically directly related to their own negative charge (n) and to the concentration of L trivalent cations within the slabs (y). This leads to the following general formula 21,22 [M 1-y II L y III (OH) 2 ] y+ X y/n n- [H 2 O] z , designed in this paper by the shortened formula LDH(M 1-y L y -X). The struc- ture is stabilized by the electrostatic interactions be- tween the slabs and the X n- anions, as well as by a strong hydrogen bond network between the water molecules, the anions and the slab hydroxyls. Whereas the intercalation of many organic anions in LDHs, such as carboxylate for example, have been reported in the literature, 20,23 and references given in those references, only some studies deal with the intercalation of macromolecules. 24-36 Their most com- mon preparation routes are the coprecipitation method or the anionic exchange route. 20,37 The coprecipitation method was successfully used for synthesizing inter- calate based on LDH(CaAl) and poly(vinyl alcohol), 27-30 poly(acrylic acid), poly(vinylsulfonate) or poly(styrene- sulfonate). 32,33 The swelling by acrylonitrile of LDH- (MgAl-dodecyl sulfate) and its subsequent polymeri- zation, initiated by benzoyl peroxide was also reported. 25 The anionic exchange route was investigated by Kato and co-workers for intercalating acrylate anions in LDH(MgAl-nitrate), 24 and by Schwarz and co-workers for intercalating 4-styrenesulfonate anions in recon- structed LDH(MgAl). 26 Recently, the synthesis of poly- (2-hydroxyethyl methacrylate)/LDH(MgAl-dodecyl sul- fate) nanocomposite materials was reported. 38 In such a case, LDH(MgAl-dodecyl sulfate) was delaminated under high mechanical shear in 2-hydroxyethyl meth- acrylate, and after the free-radical polymerization of the monomer the inorganic component was still in the delaminated form. A new preparation route of LDH(Ni 1-y L y ) (with L ) Co or Fe), decoupling the building of the Ni 1-y L y O 2 slabs from the anionic intercalation, was developed in our laboratory a few years ago and applied to the intercala- * To whom correspondence should be addressed. Tel: +33-540- 002-651. Fax: +33-540-002-761. E-mail: duguet@icmcb.u-bor- deaux.fr. 45 Macromolecules 2004, 37, 45-51 10.1021/ma025882w CCC: $27.50 © 2004 American Chemical Society Published on Web 12/05/2003