[en] Full text: The study of polymer/clay nanocomposites are of fundamental importance due to a better understanding of the structural properties which optimizes its applications. The morphology of these nanocomposites depends on strongly of the synthesis conditions such as clay content and the synthetic method. The most common method is in situ polymerization involving intercalation of the monomer, initiator and/or catalyst within the silicate layers. In situ free radical polymerization has many obvious advantages compared to, for instance, thermal polymerization. The use of octanethiol (OCT) as free-radical chain transfer agent can promote the increase of the conversion degree of polymers in a process so call thiolation. In this study, OCT was used as chain transfer agent to prepare nanocomposites based on PMMA/Montmorillonite protonated (MMT H⁺ ) and PMMA- co- methacryloxy propyltrimethoxysilane (MPTS)/MMT H⁺ . Structural features of the nanocomposites were studied by small Angle X-Ray Scattering (SAXS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, gravimetric conversion degree, thermogravimetric analysis (TG) and sulfur elemental analysis (SEA). The results showed that OCT did not behave as effective chain transfer agent since SEA suggested that the conversion degree was not increased by thiolation mechanism, but mainly due to catalytic action of the clay. MMA H⁺ provides active sites allowing an increase in this conversion acting as catalyst [1]. Thermal stability of nanocomposites in OCT presence was increased, probably due to some interaction between the inorganic phase and OCT functional group. (author)