Understanding of the governing parameters that control the interaction of bio-sourced fillers with synthetic polymer materials is a long-standing challenge for their exploitation as a platform for material engineering. For the case of graft-lignin embedded in poly (methyl methacrylate) (PMMA) it is demonstrated that tethering of polymeric chains with appropriate chain length to the surface of lignin-fillers dramatically increases the mechanical properties of PMMA/lignin composites, suggesting the PMMA grafts significantly enhanced filler-matrix interactions. Most metrics were maximized at 1% loading, with a 3-fold increase in yield stress, a 4-fold increase in tensile strength, and a 7-fold increase in toughness, with a combination of properties that compare favorably to high-performance engineering polymers and polymer nanocomposites based on inorganic nanoparticles. The versatility of the surface-initiated controlled radical polymerization used for polymer graft modification suggests that the approach should be broadly applicable to a wide range of commodity and engineering polymers. (C) 2015 Elsevier Ltd. All rights reserved.