Polymerisation de l'acide methacrylique en masse et en solution

In spite of the fact that the bulk polymerization of methacrylic acid proceeds under precipitating conditions, all conversion curves are linear and start from the origin. The overall activation energy of the gamma ray initiated reaction is very small: 1.3 kcal/mol. Methanol and water are solvents for the polymer but also form monomer-solvent complexes through H-bonds. It was found that, over a limited concentration range in these solvents, the reaction becomes auto-accelerating both in precipitating and homogeneous reaction media. Non-polar solvents (hydrocarbons) lead to a significant reduction in the polymerization rate but this effect is not as pronounced as for acrylic acid. Chlorinated derivatives reduce the polymerization rate of acrylic acid to the same extent as hydrocarbons but, for methacrylic acid, chlorinated derivatives lead to sensitization. By analogy with earlier results for acrylic acid, it is assumed that the auto-acceleration observed in water and methanol solutions is caused by a “matrix effect”. In bulk, the monomer undoubtly also associates with the polymer but, in view of the bulky methyl groups, the regularly oriented structure which favours propagation presumably never arises. The very small activation energy of the polymerization suggests that chain termination requires a significant activation energy. The mechanism of this process is not clear.