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 Co II binding patterns (the Co–N Py and Co–O coordination in Co-Py-PDMSs and Co–N Bipy in Co-Bipy-PDMSs) were established by UV–vis and IR methods and by comparison with model Co II 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 Co II at various metal-to-ligand molar ratios (1:(1–6)) and by the variation of the polydimethylsiloxane unit length ( M n: 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 Co II, 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 Co II -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 M n = 25 000 g·mol –1 .