Covalent vs metallo-supramolecular block copolymer micelles

An amphiphilic metallo-supramolecular polystyrene-block-poly(ethylene oxide) diblock copolymer containing a bis(2,2':6',2"-terpyridine)ruthenium(II) complex (PS20-[Ru]-PEO70) as a supramolecular connection between the two constituting blocks has been compared to the covalently bonded counterpart (PS22-b-PEO70). The two different copolymers have been used to prepare kinetically frozen aqueous micelles that consist of a glassy polystyrene core surrounded by a poly(ethylene oxide) corona. The micelles have been characterized by dynamic light scattering (DLS). In the case of the PS22-b-PEO70 copolymer, micelles with a hydrodynamic diameter (D-h) of 18 nm are observed while stable micelles with a D-h of 65 nm and larger aggregates are formed for the PS20-[Ru]-PEO70 sample. Addition of different salts during the initial stage of micelle formation has a deep effect on the final micellar characteristic features, which are now similar to the ones of the covalent system. This effect is attributed to a decreased electrostatic repulsion between the charged bis(2,2':6',2"-terpyridine)ruthenium(II) complexes which are present at the interface between the immiscible polystyrene and poly(ethylene oxide) blocks. Furthermore, addition of different salts to solutions of the PS20-[Ru]-PEO70 micelles, initially prepared in the absence of salt, caused a decrease of their size, although the packing of the charged bis(2,2':6',2"-terpyridine)ruthenium(II) complexes could not be changed in these kinetically frozen micelles. A decrease in micelle size has also been observed as temperature increases. These effects have not been observed in the covalently connected counterpart and are thought to originate from stretched poly(ethylene oxide) chain segments. A model for metallo-supramolecular micelles is tentatively proposed that takes into account the experimental observations.