Peculiar aspects of the free-radical polymerization of 1-vinylimidazole

Abstract

The polymerization of 1-vinylimidazole (VI) exhibits several unusual features. The reaction is auto-accelerated in bulk and in solution, an observation attributed to a “matrix” effect. Abnormal orders are found with respect to monomer concentration and to rate of initiation and these are dependent on the monomer content in the reaction mixture. Molecular monomer-monomer and monomer-solvent associations were characterized in these systems, suggesting a gradual change in the reactivity of the monomer upon dilution. A cross-linked gel arises in systems which generate a concentrated polymer solution in the monomer. It is assumed that growing polymer chains can add onto double bonds of the imidazole rings in the polymer but this reaction is slow and only occurs significantly under the conditions stated above. A complicated process takes place when benzoyl peroxide is used to initiate the reaction. A rapid polymerization takes place at very low temperatures leading to low molecular weight polymers and to other addition products. G(R) values were derived from experiments with gamma-ray initiation by combining polymerization rate and molecular weight data. The estimated value for pure VI is G(R)=2.2. The monomer was found to “protect” water and methanol in their radiolysis, being itself “sensitized” by CCL₄.