ROMP-Derived Pyridylborate Block Copolymers: Self-Assembly, pH-Responsive Properties, and Metal-Containing Nanostructures

The functionalization of organic polymers with polydentate ligands offers opportunities in areas ranging from supported catalysts to materials with desirable magnetic, redox-active, stimuli-responsive, and self-healing properties. Herein, we present the synthesis and self-assembly of tris(2-pyridyl)borate (Tpyb)-functionalized homo and block copolymers, prepared via ring-opening metathesis polymerization (ROMP) of (bicyclo[2.2.1]hept-5-en-2-yl)-4-phenyl) (pyridin-1-ium-2-yl)di(pyridin-2-yl)borate (M1) and dimethyl-7-oxabicyclo[2.2.1]hept-5-ene-exo,exo-2,3-dicarboxylate (M2) using Grubbs third-generation catalyst. Controlled polymerization was confirmed by gel permeation chromatography (GPC; also referred to as size exclusion chromatography, SEC) and multinuclear NMR spectroscopy. The solution self-assembly in block-selective solvents (MeOH, THF) was investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), scanning tunneling electron microscopy (STEM), and transmission electron microscopy (TEM), and the microphase separation in a thin film was imaged by atomic force microscopy (AFM). Hydrolysis of the ester-substituted oxanorbornene block with NaOH led to a new copolymer with carboxylate functionalities that can be dispersed in water. The latter displays multiresponsive properties as each of the individual blocks can be reversibly switched between hydrophobic and hydrophilic states by simple adjustment of pH. Cross-linking of the block copolymer aggregates via metal ion complexation was accomplished, and the feasibility of metal ion exchange was demonstrated by energy-dispersive X-ray spectroscopy (EDX).