Shape - memory polymers ( SMPs ) represent a very interesting class of stimuli - responsive materials . [ 1 ] The triggered change from a temporary shape to a preprogrammed permanent one ( i.e. , very well suited for the reversible ( switching ) phase ) . [ 25 ] The results of the ITC study are summarized in Table S1 and Figures S3 - S10 in the Supporting Information . Both , zinc(II ) trifluoromethane sulfonate ( Zn(TFMS ) 2 ) and nickel(II ) chloride , featured the desired complexation behavior , i.e. , simultaneous formation of stable terpyridine complexes and labile ones with the histidine ligands . Furthermore , it is important that at the minimum two ligands bind to one metal ion in order to form supramolecular crosslinks . Furthermore , to investigate the procedure of the complex formation when both ligands are present , like it is in the later desired polymers , a 1:1 mixture of both ligands was utilized in the ITC measurement ( ITC titration data in Figures S11 and S12 in the Supporting Information ) . Performing this experiment , it was found that the terpyridine is complexed by half an equivalent of the metal salt in the presence of the histidine . Thus , all the terpyridine ligands are complexed before a complexation of histidine ligands starts and , consequently , no mixed complexes can form , which was additionally confirmed via 1 H NMR measurements ( Figure S13 , Supporting Information ) .