Three clay samples (a kaolinite, a halloysite and an inseparable mixture of kaolinite-palygorskite-illite-montmorillonite) were, after a mild pretreatment, successfully tested as catalysts in the polymerization of styrene. The catalytic activity of the clay (measured by the conversion rate), the MW and the colour of the polystyrene obtained depended not only on the chemical composition and the structure of the clay catalyst but also on its concentration in the medium. The polystyrenes obtained on the clay catalyst were composed of two groups of macromolecules, one with low MW and the other with high MW.

The study of 13C NMR spectra of polystyrene thermally obtained on clays shows that thermal polymerization of styrene on these minerals proceeds by two parallel reaction mechanisms: the radical mechanism due to the thermal effect, and the cationic mechanism in which the initiatory cation H+ is provided by the clay. The latter mechanism, accelerated by increase in temperature, becomes more important as the clay content increases in the reaction medium. Otherwise, the thermal polymerization on clays is accompanied by the hydrogenation of some α carbons of the polystyrene molecule and the oxidation of certain aromatic carbons.