Different protonation behaviour of two poly(methacrylic acid) derivatives containing N-acyl-glycine and N-acyl-b-alanine residues: thermodynamic and FT-IR studies

Two poly(methacrylic acid) derivatives containing amide and carboxyl groups in the side chain of the vinyl structure, namely poly(N-methacryloylglycine) (PMP1) and poly(N-methacryloyl-β-alanine) (PMP2), were studied in aqueous solution at 25 °C by thermodynamic (potentiometry, calorirnetry, and viscometry) and spectroscopic (FT-IR) techniques together with the corresponding monomers. The basicity constants (log K) of both polymers were compared with those of other polymers with a vinyl backbone, and the anomalous behavior of PMP2 was revealed by the n value of the modified Henderson-Hasselbalch equation. The slight exothermic enthalpy changes were pH-dependent and the -ΔH° increased with the degree of protonation α for PMP1 but decreased for PMP2. The extension of hydration of the amide group revealed by FT-IR spectra was the main factor controlling the different protonation behavior of the two polyelec-trolytes. © 1991, American Chemical Society. All rights reserved.