We report on the synthesis of air-stable aluminum nanoparticles (Al NPs) capped with 1,2-epoxy-9-decene. Long-chain epoxides have proven to be effective capping agents for Al NPs as the epoxide ring is highly susceptible to ring-opening polymerization, leading to the formation of putative polyether loops on the nascent Al NP surface. However, these materials are observed to degrade within several hours to days following exposure to ambient air. By inducing polymerization of the additional terminal alkene functionality on the epoxide, we have produced Al NPs that exhibit both a shelf life of ~6 weeks and a high active Al content. Transmission electron microscopy confirms that these spherical nanostructures, ~25 nm in diameter, are embedded in a covalently bound polymer matrix that serves as a prophylactic barrier against water/air (H2O/O2) degradation, and 27Al solid-state NMR is used to nondestructively confirm the presence of both metallic Al0 and oxidized Al3+. In addition, we have induced polymerization of the epoxide terminal alkene functionality with a long-chain diene monomer, 1,13-tetradecadiene, leading to the formation of Al NPs protected by an extremely hydrophobic polymer matrix. These core–shell nanomaterials also have high active Al contents along with extremely long shelf lives (up to 6 months upon air exposure).