Prediction of pKa Values for Oligo-methacrylic Acids Using Combined Classical and Quantum Approaches

The pK(a) values for polymeric acids are very useful quantities for many applications including separations using ion-exchange membranes and catalysis using polymeric acids. However, the pK(a) value of a polymeric acid is typically different from the corresponding monomer value and is generally unknown. The pK(a) value of a particular acid group is strongly dependent on the local environment of the dissociating acid group and the broad dielectric medium. The pK(a) values for oligo-methacrylic acids were systematically determined as a function of degree of polymerization using combined classical and quantum mechanical approaches. The atomic charges were determined quantum mechanically, whereas the pK(a) values were determined solving the Poisson-Boltzmann equation. The pK(a) values were found to increase as the degree of polymerization (DP) increases and converge to a stable value after DP reaches 8. The predicted pK(a) values agree very well with experimental results.