Effect of pH on the reactivity ratios in aqueous solution copolymerization of acrylic acid and N‐vinylpyrrolidone

Acrylic acid (AA) and N‐vinylpyrrolidone (NVP) were copolymerized in aqueous solution at 30°C in the pH range 4–9 and the monomer reactivity ratios (r1 for AA and r2 for NVP) were determined as a function of pH from the high conversion data by using both the differential (YBR) and the integrated copolymerization equations. The value of r1 decreased from 5.2 at pH 4 to a minimum of 1.3 at pH 5 and then increased to 8.1, 6.6, and 7.2 at pH 7, 8, and 9, respectively. Addition of 1M NaCl at pH 6.5 restored the r1 nearly to that at pH 4, the predominantly un‐ionized acid. The r2 values for NVP were nearly zero at all pH values except at pH 5. The variation of the reactivity ratios with pH was examined in terms of the electrostatic interactions between the ionized monomer molecules, the rate of homopolymerization of acrylic acid, and the cation binding to the poly(acrylic acid) molecules. The r2 values for NVP compared favorably with the literature values reported in bulk and organic solvent systems, although r1 values are much higher.