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, H-1 and Na-23 NMR spectr a, relaxations, and pulsed-gradient-stimulated-echo (PGSE) experimental met hods. Sodium polyphosphates with polymerization degrees 15-65 are readily c oupled with DADMAC-AA statistical copolymers, containing 8-73% mol of the D ADMAC ionic groups. The quantum-chemical prediction that even polycations w ith low density of ionic groups can be linked to densely charged polyphosph ates in a ladder-like fashion is in accordance with the experimental result s. Despite the predicted decrease in stabilization energy with increasing c oupled sequence, the systems show a definitely cooperative behavior with th e polyphosphate sequence of about 15 monomeric units, already sufficient fo r the full effect. Longer polyphosphate chains and higher charge densities on the DADMAC-AA copolymer cause a theoretically predicted pseudo-irreversi ble coupling in less advantageous positions (parking problem) leading to la rger complex particles and slightly lower coupling degrees. As predicted by theory, this effect can be alleviated by a slight increase in the ionic st rength. This effect is a further evidence of a true (or second-order, nonli near) cooperativity in the system. Electrostatic energy is clearly an impor tant factor in the over-all Gibbs energy balance but the driving force of c ooperative coupling is probably the entropy gain caused by liberation of th e small Na+ and Cl- counterions and a part of the hydrating water molecules .