Hybrid Nanospheres Formed by Intermixed Double-Hydrophilic Block Copolymer Poly(ethylene oxide)-block-poly(2-ethyloxazoline) with High Content of Metallacarboranes

In search for biocompatible hydrophilic polymers suitable for preparation of delivery systems of boron cluster compounds with high loading capacity, we studied the interaction of metallacarborane sodium [3-cobalt(III) bis(1,2-dicarbollide)] with poly(2-ethyloxazoline) (PEOX) and with a double-hydrophilic block copolymer poly(ethylene oxide)-block-poly(2-ethyloxazoline) (PEO-PEOX) in aqueous solutions by a combination of scattering, microscopy, spectroscopy, and thermochemistry techniques. The paper is a contribution to our long-time study of novel hybrid nanostructures based on hydrophilic polymer-metallacarborane complexes. PEOX homopolymer interacts with metallacarborane, resulting in a water-soluble, negatively charged complex. In the case of diblock copolymer PEO-PEOX, both blocks interact with metallacarborane via dihydrogen bonds and participate in the formation of hybrid gel-like nanostructures in 0.1 M NaCl aqueous solutions, which are unique as compared to other boron cluster-containing polymeric systems. The stable spherical nanoparticles with high metallacarborane content do not adopt core/shell structure, which has been observed for other PEO-containing double hydrophilic block copolymers [Macromolecules 2009, 42, 4829], but the nanospheres are homogeneous. They contain intermixed PEO and PEOX blocks, which are cross-linked by metallacarborane molecules. The size of the nanospheres depends on a preparation protocol, while their inner structure does not. Besides the detailed study on PEO-PEOX/metallacarborane system, a high application potential of PEO-PEOX complexes with several metallacarborane-based drugs is also shown. The study clearly demonstrates that PEOX is suitable polymer for designing novel hybrid nanostructures.