Multi-functional coordination crosslinks in poly(vinylamine) complexes with cobalt chloride

Poly(vinylamine) complexes with cobalt chloride hexahydrate exhibit the largest glass transition temperature enhancements that have been measured in this laboratory. T-g increases from 56 degrees C in the undiluted polymer to 193 degrees C when the molar concentration of cobalt is only 3%, based on the moles of polymeric repeat units. This translates to a 45 degrees C enhancement in T-g per mol% cobalt. Higher glass transition temperatures have been measured for other polymeric complexes with d-block salts, but this study reports the largest Delta T-g=T-g(complex)-T-g(undiluted polymer) when transition-metal catalyzed chemical crosslinking reactions do not occur. A plausible explanation for this effect is based on the fact that cobalt chloride hexahydrate maintains pseudo-octahedral symmetry in the undiluted crystalline state and in an amorphous glassy complex with the polymer. The lone pair on nitrogen in the amino sidegroup is a strong base and, most likely, displaces all four waters of hydration that coordinate directly to the metal center. In fact, estimates of the ligand field splitting and the ligand field stabilization energy for octahedral cobalt complexes that contain methyl amine (CH3NH2) model ligands suggest that five or six amino sidegroups from several different chains might coordinate to each metal center, drastically reducing the mobility of the polymer. Both chloride anions remain near each divalent Co2+ center, but either one or two Cl- might not reside in the first-shell coordination sphere of the transition metal. This bonding picture suggests that octahedral cobalt acts as a multi-functional bridge between five or six amino sidegroups. Empirical ligand field stabilization calculations support the concept that octahedral Co2+ is a multi-functional coordination crosslinking agent, and the inorganic literature provides experimental verification that similar small-molecule complexes (i.e., [Co(NH3)(6)](2+)) exist. It is proposed that the onset of T-g occurs when sufficient thermal energy is supplied to remove either x or (x-1) amino sidegroup ligands from the coordination sphere of cobalt, where x is either 5 or 6. This is an endothermic process based on bond energies, as well as calculations which focus on the relative energies of the metal d-electrons.