Theoretical prediction of pKa values for methacrylic acid oligomers using combined quantum mechanical and continuum solvation methods.

The acidity constant pKa for polymeric organic acid is expected to be different from its corresponding monomer value due to the change of chemical environment upon polymerization. Thermodynamic cycles were used to determine the free-energy changes for the proton dissociation processes in aqueous solution and the corresponding pKa values for monomer methacrylic acid and several similar carboxylic acids. First-principles calculations and continuum solvation model were used to determine the gas-phase and solvation free energies, respectively. A protocol was developed to use the efficient density functional calculations with B3LYP functional instead of the demanding CBS-QB3 method to determine the gas-phase free energies with relative high accuracy, thus enabling the determination of pKa values for the short oligomers of methacrylic acid. The predicted pKa values for the dimer and trimer of methacrylic acid are higher by about 0.8 pKa units than the predicted monomer value.