Polymer nanocomposites continue to receive considerable attention as multifunctional hybrid materials, with most nanocomposites fabricated by physical dispersion of surface-functionalized nanoscale objects. In this study, we explore the viability of growing Pd-containing nanoparticles from Na2PdCl4 in two different polymers: hypercrosslinked polystyrene (HPS) and an aromatic polyimide (PIm). In HPS, single Pd-containing nanoparticles possessing a relatively narrow size distribution (ca. 1-4 nm) form upon reduction of the divalent PdCl42-ions. Single nanoparticles with a broad size distribution ranging from approximate to 2 to 16 nm develop in PIm, which simultaneously undergoes chemical crosslinking during ion reduction. Such hybrid materials hold promise in molecular catalysis and gas separation.