Dense polymer brush growth from gold nanorod surfaces enabled by a lipoic acid-derived RAFT agent

Polymer functionalization of gold nanorods (AuNRs) offers a means to tailor their colloidal stability, interfacial interactions, and integration into advanced functional nanocomposites - yet achieving dense polymer brushes without compromising nanorod structure or surface chemistry remains challenging. Here, we develop a mild, room temperature surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) strategy that enables direct polymer growth from AuNR surfaces under visible light. A lipoic acid-derived chain transfer agent (LA-CTA) was immobilized onto AuNRs and subsequently activated through blue-light photoiniferter polymerization (λ = 460 nm), eliminating the need for thermal initiators or added metal catalysts. Using methyl acrylate as a model monomer, this approach produced dense poly(methyl acrylate) brushes while largely preserving the anisotropic plasmonic response of the AuNRs and imparting excellent colloidal stability in organic solvents. Surface-grown polymers were cleaved from the nanoparticles and characterized directly, yielding number-average molecular weights of 48.9-176.1 kg mol^-1 and dispersities as low as Đ = 1.20. Thermogravimetric analysis indicated high grafting densities of σ ≈ 0.73-0.85 chains per nm². Time-resolved ¹H NMR further showed sustained monomer conversion under visible-light irradiation, confirming continued polymer growth from the surface-bound RAFT agents. Beyond providing a route to polymer-coated AuNRs, this work establishes photoiniferter SI polymerization as a platform for independently controlling and quantitatively interrogating polymer growth from anisotropic nanoparticle surfaces. The combination of mild reaction conditions, high brush density, tunable chain length, and direct characterization of the grafted polymer population provides a foundation for designing polymer-nanoparticle hybrids in which interfacial polymer structure can be systematically connected to colloidal, optical, and materials properties.