TRANSITION-METAL COORDINATION IN POLYMER BLENDS AND MODEL SYSTEMS

Several metal-containing polymer blends which form coordination complexes have been investigated in the solid state. The d-block model compounds are zinc salts of acetic acid, lauric acid, and stearic acid. The ligand is poly(vinylpyridine) with nitrogen in either the 2- or 4-position. Thermal analysis via differential scanning calorimetry (DSC) is used to probe the phase behavior of these binary mixtures at the macroscopic level. DSC thermograms identify the melting transition of the small-molecule-rich phase and the glass transition of the amorphous polymer-rich phase. High-resolution carbon-13 solid-state NMR spectroscopy is used in an indirect detection mode to probe microenvironmental factors that influence mixing. The interaction-sensitive carboxyl carbon resonance of the zinc salts is perturbed in blends with poly(vinylpyridine) when the nitrogen ligand is structurally accessible. The NMR data suggest that all of the small molecules form coordination complexes with poly(4-vinylpyridine) except magnesium acetate. The divalent magnesium cation is classified as a hard acid, and it favors the acetate anion and the waters of hydration, which are hard bases, instead of the pyridine ligand, which is a borderline base. d-Metal complexation is observed in polymer-ionomer blends representing a direct extension of the above-mentioned studies on model systems, particularly those for zinc stearate with either poly(2-vinylpyridine) or poly(4-vinylpyridine). As expected, carbon-13 NMR spectroscopic detection of solid-state coordination in polymer-ionomer blends yields positive results when the ionic copolymer is neutralized with zinc and when the nitrogen ligand is in the 4-position of the pyridine ring. These phenomenological observations suggest that zinc coordinates to the pyridine ring via nitrogen's lone pair and infrared spectroscopic data support the concept that the pyridine group participates in metal-ligand pi-bonding. The stress-strain properties of poly(4-vinylpyridine)/zinc ionomer blends exhibit a synergistic mechanical performance when the nitrogen/zinc molar ratio is optimized.