Self-healing mechanism of metallopolymers investigated by QM/MM simulations and Raman spectroscopy

The thermally induced self-healing mechanisms in metallopolymers based on bisterpyridine complexes of iron(II) sulfate and cadmium(II) bromide, respectively, were studied by means of combined quantum mechanical/molecular mechanical (QM/MM) simulations and Raman spectroscopy. Two possible healing schemes, one based on a decomplexation of the cross-linking complexes and a second one relying on the dissociation of ionic clusters, have been addressed. Temperature-dependent Raman spectroscopy displayed bathochromic shifts of the Raman intensity pattern upon heating. QM/MM simulations on the polymer models assign these alterations to a partial decomplexation of the metal terpyridine complexes, i.e. signals originating from free terpyridine ligands increase upon heating. Thus, a healing mechanisms based on partial decomplexation of the cross-linking complexes is suggested. The possibility that the dissociation of ionic clusters, which are assumed to be present in this class of self-healing polymers, is also responsible for the self-healing process was investigated as well. However, such calculations on model clusters revealed relatively strong binding of the clusters, which renders reversible cluster breaking and reformation upon temperature cycling in the range up to 100 degrees C unlikely.