Diblock copolymers, micelles, and shell-crosslinked nanoparticles containing poly(4-fluorostyrene): Tools for detailed analyses of nanostructured materials
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abstract
Amphiphilic core-shell nanostructures containing 19F stable isotopic labels located regioselectively within the core domain were prepared by a combination of atom transfer radical polymerization (ATRP), supramolecular assembly, and condensation-based crosslinking. Homopolymers and diblock copolymers containing 4-fluorostyrene and methyl acrylate were prepared by ATRP, hydrolyzed, assembled into micelles, and converted into shell-crosslinked nanoparticles (SCKs) by covalent stabilization of the acrylic acid residues in the shell. The ATRP-based polymerizations, producing the homopolymers and diblock copolymers, were initiated by (1-bromoethyl)benzene in the presence of CuBr metal and employed N,N,N,N,N-pentamethyldiethylenetriamine as the coordinating ligand for controlled polymerizations at 75-90C for 1-3 h. Number-average molecular weights ranged from 2000 to 60,000 Da, and molecular weight distributions, generally less than 1.1 and 1.2, were achieved for the homopolymers and diblock copolymers, respectively. Methyl acrylate conversions as high as 70% were possible, without observable chain-chain coupling reactions or molecular weight distribution broadening, when bromoalkyl-terminated poly(4-fluorostyrene) was used as the macroinitiator. Poly(4-fluorostyrene), incorporated as the second segment in the diblock copolymer synthesis, was initiated from a bromoalkyl-terminated poly(methyl acrylate) macroinitiator. After hydrolysis of the poly(methyl acrylate) block segments, micelles were formed from the resulting amphiphilic block copolymers in aqueous solutions and were then stabilized by covalent intramicellar crosslinking throughout the poly(acrylic acid) shells to yield SCKs. The SCK nanostructures on solid substrates were visualized by atomic force microscopy and transmission electron microscopy. Dynamic light scattering was used to probe the effects of crosslinking on the resulting hydrodynamic diameters of nanoparticles in aqueous and buffered solutions. The presence of fluorine atoms in the diblock copolymers and resulting SCK nanostructures allowed for characterization by 19F NMR in addition to 1H NMR, 13C NMR, and IR spectroscopy.
