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-polydimethyl-siloxane (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 C-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 polydimethylsi- loxane unit length (M-n: 850-900, 5000, or 25000 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).