Monomolecular layers of methyl methacrylate inserted between the lamellae of montmorillonite clay were polymerized. Finely divided complexes of poly(methyl methacrylate)–montmorillonite were suspended in boiling hydrocarbons and subjected to thermal degradation at 215°C. and above, under air and nitrogen, with constant vigorous stirring. Aliquots were regularly withdrawn, centrifuged, and extracted with appropriate solvents. The weight losses upon calcination at 1000°C. were determined, and from them the rates of reversal to monomer were followed. Complexes containing inserted and external polymer were studied. The inserted polymer was found to resist thermal degradation under conditions at which complete degradation of the external polymer was found. This effect seems to be due to steric factors hindering the thermal motion of the segments of the polymer sandwiched between the lamellae of the crystallites. The insertion polymer isolated from the polymer–clay complex, had a greater thermal stability than poly(methyl methacrylate) made in a conventional free radical system without the presence of clay.

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Monomolecular layers of methyl methacrylate adsorbed on the surface of sodium montmorillonite clay were prepared by saturation of the clay with the monomer, followed by desorption of the excess monomer by means of repeated washings with nonpolar hydrocarbons. No spontaneous polymerization was observed. The polymerization of these monolayers was initiated with γ‐ray irradiation or by free radical catalysts. The doses of γ‐rays and the concentration of free radical catalysts required to initiate polymerization are much higher than for the polymerization of methyl methacrylate in bulk. Complexes containing 25% MMA gave inserted polymer only; complexes containing more than 25% of MMA gave inserted and external polymer. The yields of polymerization of the inserted polymer were found to be below 50%. The inserted polymer is strongly retained by the surface of the clay. X‐ray data show that its segments are oriented horizontally (lying flat) on the surface.

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