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

Abstract

Poly(vinylamme) complexes with cobalt chloride hexahydrate exhibit the largest glass transition temperature enhancements that have been measured in this laboratory. Tg increases from 56°C in the undiluted polymer to 193°C when the molar concentration of cobalt is only 3%, based on the moles of polymeric repeat units. This translates to a 45°C enhancement in Tg per mol% cobalt. Higher glass transition temperatures have been measured for other polymeric complexes with d‐block salts, but this study reports the largest ΔTg=Tg(complex)‐Tg(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 Tg 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.

The Self‐Healing Potential of Triazole‐Pyridine‐Based Metallopolymers

“…This network formation resulted in an increase of the T g for all copolymers 4a to 4l to −40 °C (Table ). An increase of the glass transition temperature was also reported for other metallo­polymer systems, e.g., cross‐linked poly(vinylamines) or the self‐healing metallopolymers described by Rowan and Weder. These systems, showed an increase from −51 °C to −23 °C.…”

Transition metal compatibilization of poly(vinylamine) and poly(ethylene imine)

“…Co 2ϩ produces a synergistic T g enhancement of 45°C per mol % salt, up to 3 mol % Co 2ϩ . This extraordinary enhancement in T g is attributed to multifunctional coordination crosslinks 14 where amino side groups in the polymer could displace two or more waters of hydration in the coordination sphere of Co 2ϩ , as well as an anionic chloride ligand, which is displaced to the second shell. A "microclustering" effect of several amino side groups around a single metal center could reduce chain mobility significantly.…”

“…8 It seems reasonable that the glass-transition temperature should experience larger enhancements when a single metal center coordinates to more polymeric sidegroups, due to microclustering of the ligands and a reduction in their mobility in the vicinity of the metal cation. 14 …”

Microstructure of N-Picolylpolyurethane Transition Metal Complexes

“…Belfiore has described a molecular thermodynamic interpretation of macroscopic thermal properties of macromolecular metal complexes. 26 According to this method, disruption of cross-links in PUPYCo 2+ can be described as a process that one picolyl ligand is removed from the coordination sphere of cobalt(II) ion. Metal complex species for PUPYCo 2+ before and after dissocia-tion are CoL 2 Cl 2 and CoLCl 2 Z, respectively, where L is the picolyl group and Z is the water of hydration.…”

“…Taking the concentration dependence of the coordination interaction into consideration, a simple energetic model is formulated which accounts for the disruption of coordination cross-links and includes the ligand field stabilization energy: 26 where x is the molar fraction of metal chloride in the hard domain. The empirical parameter β is included to account for the existence of imperfections that arise from geometric distortions of central metal ions to lower symmetry, intramolecular loops, coordination pendent groups, etc.…”