Group association equilibria in isolated polymer coils

Abstract

AbstractWhen two or more groups attached to a single chain molecule participate in the formation of an association complex, the equilibrium for complex formation in dilute polymer solution depends on the local concentrations in the swollen polymer coil rather than on the stoichiometric concentration in the bulk of the solution. The theory of intramolecular group association is developed using two models. In the first, the interacting groups are evenly spaced along the polymer chain and the probabilities of their association are governed by Kuhn's statistics of chain configuration. In the second model, every segment of the chain has the same probability of carrying an interacting group and the Kuhn statistics are applied only to large rings, the probability of small ring formation being governed by steric factors. The predictions of the theory are in agreement with the extent of carboxyl association in dilute solutions of seven styrene‐methacrylic acid copolymers, measured by infrared spectroscopy. The chelation of cations in dilute polyelectrolyte solution also involves the participation of groups attached to a single macromolecule, but the interpretation of experimental data requires that the electrostatic free energy of complex formation be taken into account. Spectroscopic evidence shows that in copper(II) binding to poly(methacrylic acid) or poly(N, ε‐methacrylyllysine) higher complexes are formed than in solutions of simple carboxylic acids or α‐amino acids, respectively. Dialysis equilibrium studies indicate that, in the concentration range 4.7 × 10−3 to 15.6 × 10−3 N poly(acrylic acid), the polymer coils act independently of each other in chelating small amounts of copper(II), while the copper (II) affinity of poly(methacrylic acid) molecules drops off with rising polyacid concentration.