The Effects of Substitution and Degree of Polymerization on the Pka Values of Polymeric Polystyrene Sulfonic Acids

The acidity of a polymeric acid has a complex relationship with its structure since it is dictated by the underlying interactions including electronic, van der Waals, electrostatic and hydrogen bonding interactions. The acidity of a polymeric acid will depend not only on its monomer unit, but also on its conformation and interactions between the monomers, and the degree of polymerization. We use a combinatorial approach to screen computationally the acidity of a monomer styrene sulfonic acid by ring substitution using first-principles calculations, and for various substituted dimer structures. Ring substitution with electron withdrawing elements or groups such as F, Cl and Br are found to increase the acidity of the styrene sulfonic acid. For polymers with degree of polymerization (DP) larger than 2, we solve the Poisson-Boltzmann equation coupled with Monte Carlo simulations. The acidity of polymeric polystyrene sulfonic acid with DP varying from 2 to 20 is also determined. The acid activity is found to decrease with increased polymer chain length.