Cooperative Interactions of Unlike Macromolecules:  3. NMR and Theoretical Study of the Electrostatic Coupling of Sodium Polyphosphates with Diallyl(dimethyl)ammonium Chloride−Acrylamide Copolymers

Electrostatic coupling of sodium polyphosphates (polymerization degrees 15 and 65) with statistical copolymers of diallyl(dimethyl)ammonium chloride (DADMAC) and acrylamide (AA) of variable charge densities was studied using theoretical models, quantum chemical calculations, 1H and 23Na NMR spectra, relaxations, and pulsed-gradient-stimulated-echo (PGSE) experimental methods. Sodium polyphosphates with polymerization degrees 15−65 are readily coupled with DADMAC−AA statistical copolymers, containing 8−73% mol of the DADMAC ionic groups. The quantum-chemical prediction that even polycations with low density of ionic groups can be linked to densely charged polyphosphates in a ladder-like fashion is in accordance with the experimental results. Despite the predicted decrease in stabilization energy with increasing coupled sequence, the systems show a definitely cooperative behavior with the polyphosphate sequence of about 15 monomeric units, already sufficient for the full effect. Longer polyphosphate chains and higher charge densities on the DADMAC−AA copolymer cause a theoretically predicted pseudo-irreversible coupling in less advantageous positions (parking problem) leading to larger complex particles and slightly lower coupling degrees. As predicted by theory, this effect can be alleviated by a slight increase in the ionic strength. This effect is a further evidence of a true (or second-order, nonlinear) cooperativity in the system. Electrostatic energy is clearly an important factor in the over-all Gibbs energy balance but the driving force of cooperative coupling is probably the entropy gain caused by liberation of the small Na+ and Cl- counterions and a part of the hydrating water molecules.