We report here on the polymerization of epoxide monomers on incipient aluminum nanoparticle cores and the effects of changing the epoxide-capping precursor and the metallic monomer ratio on the resultant stability and particle size of passivated and capped aluminum nanoparticles. When altering the ratio of aluminum to cap monomer precursor, nanoparticles capped with epoxydodecane, epoxyhexane, and epoxyisobutane show a clear decreasing trend in stability with decreasing alkane substituent length. The nanoparticle core size was unaffected by cap ratio or composition. PXRD (powder X-ray diffraction) and DSC/TGA (differential scanning calorimetry/thermal gravimetric analysis) confirm the presence of successfully passivated face-centered cubic (fcc) aluminum nanopartides. We also report preliminary results from ATR-FTIR (attenuated total reflectance-Fourier transform infrared), C-13 GPMAS (cross-polarization/magic-angle spinning), and Al-27 MAS solid-state NAIR (nuclear magnetic resonance) measurements. The most stable aluminum nanoparticle-polyether core-shell nanoparticles are found at an Al:monomer mole ratio of 10:1 with an active Al(o)content of 94%.