(421f) Theoretical Prediction of Pka Values for Polymeric Methacrylic Acids Using Combined Ab Initio Molecular Dynamics Simulations and the Continuum Dielectric Model

The acidity constants of methacrylic acid polymers are expected to be different from the monomer due to the changes in the dielectric environment surrounding the carboxylic acid groups. This variation provides a great opportunity in designing new bioseparation membranes coated with polymer brushes by tuning the polymer's pKa values to meet the application needs. To understand the pKa dependence on polymer chain length, chain conformation, and the dielectric medium, we used the ab initio molecular dynamics modeling to derive accurate conformations and partial charges for the methacrylic acid oligomers (n = 2 to 20). The reaction field energies of the carboxylic acid groups were calculated by using the quantum-based polarizable continuum model (PCM). For large oligomers, we used the multiconformation continuum electrostatic (MCCE) program that involves solving numerically the Poisson-Boltzmann equation. The advantage of this program is that the variance in conformations is taken into account by utilizing a molecular mechanics force filed in Monte Carlo sampling. The predicted pKa values of the methacrylic acid oligomers show an increase as the chain length grows in agreement with experimental observations