Influence des solvants sur les associations moleculaires et sur la cinetique de polymerisation de l'acrylonitrile. Precisions supplementaires sur l'“effet de matrice”

The polymerization of acrylonitrile is auto-accelerating in bulk and in solvents which do not dissolve the polymer. The “auto-acceleration indexes” remain almost constant in these systems. Trichloracetic acid, a very polar solvent, leads however to linear conversion curves. An earlier assumption is confirmed according to which auto-acceleration is not caused by non-stationary conditions but by an oriented association complex between the monomer and the polymer arising at the beginning of the reaction (“matrix effect”). DMF, a solvent for polyacrylonitrile, seems to produce a similar effect. The disappearance of auto-acceleration in DMF solutions was explained until now by the disappearance of the perturbation of the reaction connected with polymer precipitation (“occlusion effect”). Linear conversion curves were obtained, however, in 60% DMF solutions in which the polymer still precipitates. A detailed study of post-polymerization confirms the above interpretation. It is shown that auto-acceleration remains unaltered in the presence of solvents which swell polyacrylonitrile, such as acetonitrile or small amounts of DMF, whereas post-polymerization (caused by occluded chains) is strongly reduced. Moreover, a marked post-polymerization is observed in the presence of trichloracetic acid, demonstrating the presence of long-living chains in a system which gives rise to linear conversion curves. The association of acrylonitrile with numerous solvents is investigated; it is shown by swelling measurements that polyacrylonitrile associates with its monomer.