Ultrasound-Mediated Atom Transfer Radical Polymerization for Polymer-Grafted Magnetic Nanoparticles with Controlled Packing Density

Surface-initiated atom transfer radical polymerization (SI-ATRP) is emerging as an innovative surface modification approach for the functionalization of magnetic nanoparticles (MNPs) to improve their colloidal stability, biocompatibility, or enhanced processability and dispersibility inside a polymer matrix. However, nanoparticle agglomeration challenges the applicability of ATRP, particularly of highly interacting systems where interparticle interactions, such as magnetic forces, exacerbate agglomeration and hinder the uniform grafting of polymer chains from individual nanoparticles. Here, we report ultrasound-mediated SI-ATRP (uSI-ATRP), as an innovative solution to address the challenges of agglomeration in strongly interacting MNPs. Through a systematic study of various reaction parameters, it is shown that uSI-ATRP follows first-order reaction kinetics, thus enabling precise control over the molecular weight and polydispersity index (PDI) of the grafted polymeric shell from the surface of MNPs by controlling the reaction time. It is demonstrated that ultrasound mediation prevents nanoparticle agglomeration very efficiently and yields uniform grafting of polymer chains from individual particles. Achieving agglomeration-free dispersion of polymer-grafted MNPs enables the realization of thin films of ferromagnetic nanoparticles with a packing density of >10¹¹ per cm² using large-area solution processing techniques. This work presents a highly reproducible route to creating well-defined polymer-grafted nanoparticles, even for systems prone to significant interparticle interaction, and opens avenues for the development of novel functional nanomaterials with tailored properties.