Spontaneous copolymerization of acrylic acid with 4-vinylpyridine and microscopic acid dissociation of the alternating copolymer

Abstract

A large amount of heat was generated by mixing concentrated acrylic acid and concentrated 4-vinylpyridine, which led to carbonized products. When they were mixed within the limited concentration range, however, the polymerization took place spontaneously, and gave the alternating copolymer a relatively low molecular weight. The copolymerization was accelerated by a cationic catalyst, and inhibited by butylamine although it was not accelerated by a radical initiator and was not inhibited by a radical inhibitor. These findings suggest that the spontaneous copolymerization proceeds via cationic mechanism. However, copolymerization is accelerated by water, which is a typical inhibitor of ionic polymerization. Macroscopic and microscopic acid dissociation constants of the alternating copolymer were determined by pH-titration method. The relative concentrations of respective microscopic forms were calculated using the relation between micro-constants and hydrogen ion concentration. The fraction of the protonated form, which exists exclusively in the lower pH range, decreases with increasing pH value, and fully vanishes at pH≈6, while the deprotonated form begins to appear from about pH=9. The zwitterionic and neutral molecule forms coexist over a wide range of pH, and the latter form is predominant regardless of the pH value. Macro- and micro-constants were also measured for the system of monomer mixtures, and homopolymer mixtures as well.