A linear polyethylene precisely functionalized with geminal phosphonic acid pendants on every 21st carbon atom exhibits face-centered cubic (FCC) packing of acid aggregates. X-ray scattering from isotropic films result in higher-order scattering peaks used to determine the lattice parameter at room temperature (aFCC = 4.19 nm) and above the melting temperature of the polyethylene matrix (aFCC = 4.06 nm). Upon stretching the precise acid copolymer, an anisotropic scattering pattern featuring two coexisting crystalline orientations results, both having the ⟨110⟩ direction of the FCC lattice along the stretching direction. This is the first report of cubic ordering of aggregates in an acid copolymer and it is the direct consequence of the molecular precision of the polymer.