Synthesis of single chain polystyrene nanoparticles via reversible ionic interactions

Advances in polymer science have enabled the synthesis of complex macromolecular structures, inspired by the defined sequences found in natural molecules, such as proteins. In this light, the development of single-chain nanoparticle (SCNP) polymeric systems is currently being intensively investigated. These nanoparticles are formed through self-folding of the polymer chain at specified sites and depending on the nature of the bonds, a dynamic system can be developed. Intramolecular networking through dynamic bonds, such as interactions between ionic dipoles, allows for a change in nanoparticle configuration in response to external stimuli. In this study, polystyrene-based SCNPs synthesized through the development of ionic interactions are presented. Controlled radical RAFT polymerization was carried out for the synthesis of linear polystyrene chains containing 4-vinylpyridine monomer units at a 1-3% molar ratio. Through reaction with 1,3-propanesultone, the 4-vinylpyridine monomer serves as the site for the introduction of ionic dipoles into the polymer chain via the quaternization of pyridine nitrogen. In the presence of a nonpolar solvent, the ionic dipoles subsequently attract each other, forming reversible intramolecular cross-links along the polymer chain. The molecular weight and polydispersity of the polymeric samples were evaluated by size exclusion chromatography, while the percentage of 4-vinylpyridine and ion pairs incorporated into the polymer chain was determined by ¹H-NMR spectroscopy measurements. Differential scanning calorimetry (DSC) was employed to examine the thermal properties of the linear polymer chain compared to the forming network. Additionally, viscosity and dynamic light scattering (DLS) measurements were performed to verify network formation, by comparing the intrinsic viscosity and hydrodynamic radius of the linear and cross-linked samples.