Microscopic Acid Dissociation of the 1:1 Copolymer of Methacrylic Acid and 2-Vinylpyridine

Abstract

Copolymerizations of methacrylic acid (MA) and methyl methacrylate (MMA) with 2-vinylpyridine (2VP) were carried out at 60°C with 2,2′-azobis(isobutyronitrile) (AIBN) as the initiator. Monomer reactivity ratios were obtained as r1=0.24 and r2=0.46 for the MA (M1)-2VP (M2) system, and r1=0.46 and r2=1.13 for the MMA (M1)-2VP (M2) system, respectively. The 1:1 copolymers of MA-2VP and MMA-2VP were prepared on the basis of monomer reactivity ratios obtained. Macroscopic and microscopic acid dissociation constants of the 1:1 copolymer of MA and 2VP 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 of the 1:1 copolymer of MA and 2VP, 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 at pH=5. The zwitterionic and neutral molecule forms coexist in the pH 2 to 10 region, and the latter form is predominant regardless of pH value. Macro- and micro-constants were also measured for the systems of the monomer mixture and the homopolymer mixture. Similar pH-distribution curves are obtained for three systems (the 1:1 copolymer, the 1:1 homopolymer mixture, and the 1:1 monomer mixture)