Nanoparticle Surface Curvature Impacts the Synthesis of Polymer Brushes

Surface-initiated ring-opening metathesis polymerization (SI-ROMP) is a powerful method for grafting polymers with different macromolecular architectures and chemical compositions onto surfaces, offering the potential to design hybrid materials with tailored properties. However, inherent challenges remain, especially regarding secondary metathesis reactions, resulting in lower graft polymer conversion, broader molecular weight distributions, and the formation of free polymer. Here, we report that surface curvature plays a critical role in the graft-from polymerization of poly(cyclooctadiene) (PCOD)-grafted silica nanoparticles, where nanoparticles with high curvature (i.e., smaller diameters) reach targeted molecular weights and graft densities, and calculated dry brush heights, which is ascribed to fewer interchain cross-metathesis occurrences. SI-ROMP was conducted on a series of silica particles with diameters ranging from 13 to 128 nm, representing surfaces that are highly curved to essentially flat, respectively. The nanoparticle curvature was quantified by the ratio of the nanoparticle radius (R) to the dry brush height (h), where R/h values much greater than 1 signify a "flat" surface. Smaller particles (e.g., 13 nm diameter), with higher surface curvature (R/h ≈ 1), demonstrated more successful grafting and higher polymer conversion compared to larger particles with flatter surfaces (R/h ≈ 10). ReaxFF-based reactive molecular dynamics simulations revealed that polymer chain grafting density, which controls neighboring chain proximity, directly affects the propensity of secondary metathesis. Fewer secondary metathesis events occur as the spacing between neighboring polymer chains increases. The findings highlight the role of propagating chain proximity in facilitating or deterring secondary metathesis, which influences the uniformity of graft chains and polymerization efficiency. The effects of particle surface curvature in surface-initiated polymerizations, particularly with respect to SI-ROMP, are an overlooked parameter, yet provide a path forward for improving graft quality on lower curvature surfaces.