Structural Features of Polymer Ligand Environments Dramatically Affect the Mechanical and Room-Temperature Self-Healing Properties of Cobalt(II)-Incorporating Polysiloxanes

Cobalt­(II)-pyridinedicarboxamide-co-polydimethylsiloxane
(Co-Py-PDMSs) and cobalt­(II)-bipyridinedicarboxamide-co-polydimethylsiloxane (Co-Bipy-PDMSs) polymer–metal complexes
were prepared by complexation between Py-PDMSs or Bipy-PDMSs ligands
and cobalt­(II); the metal content in these complexes varied from 0.09
to 2.41 wt %. The CoII binding patterns (the Co–NPy and Co–O coordination in Co-Py-PDMSs and Co–NBipy in Co-Bipy-PDMSs) were established by UV–vis and
IR methods and by comparison with model CoII complexes
exhibiting relevant O,N,O- and N,N-coordination
environments, respectively. The mechanical properties of the polymer–metal
complexes were controlled by the coordination of Py-PDMSs or Bipy-PDMSs
to CoII at various metal-to-ligand molar ratios (1:(1–6))
and by the variation of the polydimethylsiloxane unit length (Mn: 850–900, 5000, or 25 000 g·mol–1). Utilization of the chelated Py-PDMSs and Bipy-PDMSs
polymer ligands, which are capable of tri- or bidentate binding of
CoII, led to (2–4)-fold increases in tensile strength
(up to 1.75 MPa) and much higher elongation at break ((2–3)-fold
increase up to 2100%) compared with the previously reported CoII-based polymer–ligand systems featuring monodentate
ligation entities. Changing the main-chain ligand from Py-PDMSs to
Bipy-PDMSs led to an increase in tensile strength of (2–4)-fold
in comparison with Py-PDMS and a lower hysteresis (4%). The room temperature
self-healing efficiency was up to 96% for Co-Py-PDMSs and 40% for
Co-Bipy-PDMSs, as measured for a polydimethylsiloxane unit with Mn = 25 000 g·mol–1.