Abstract: Certain clay minerals have the ability to catalyze the polymerization of some unsaturated organic compounds (styrene, hydroxyethyl methacrylate) and yet to inhibit polymer formation from other closely related monomers (e.g. methyl methacrylate). This apparently contradictory behaviour of the clay minerals can be rationalized in terms of electron accepting and electron donating sites in the silicate layers. The electron acceptor sites are aluminium at crystal edges and transition metals in the higher valency state in the silicate layers; the electron donor sites are transition metals in the lower valency state.
The catalyzed polymerizations involve the conversion of the organic molecule to a reactive intermediate; thus where the clay mineral accepts an electron from the vinyl monomer a radical-cation is formed, where the organic compound gains an electron it forms a radical-anion. Examples of these reactions are discussed.
The inhibition of polymerization processes involves the conversion of reactive organic intermediates, such as free radicals, which have been formed by heat or radical initiators, to non-reactive entities. For example, loss of an electron from the free radical gives a carbonium ion; in some cases this will not undergo polymerization. An example of this type is the thermal polymerization of methyl methacrylate.
The color reactions on clay minerals are useful in predicting the electron accepting or electron donating behaviour of the clay minerals because they proceed by similar mechanisms to the polymerization reactions. For example, the benzidine blue reaction is a one electron transfer from the organic molecule to the electron accepting sites in the mineral (aluminium edges, transition metals in higher valency state).
Masking of the crystal edge with a polyphosphate destroys the electron accepting properties of the crystal edge; this technique can be used to control the reactivity of the mineral and to distinguish between the crystal edge and transition metal sites as electron-acceptor sites in the clay minerals.