The role of polymer dynamics on nanocrystal growth in the synthesis of composites is studied using a well-controlled Mg nanocrystal–poly(methyl methacrylate) (PMMA) system. The in situ formation of Mg nanocrystals in PMMA is examined under different conditions, varying polymer conformation in solution. The change of hydrogen sorption properties of Mg is investigated to assess functional impacts of morphological changes.