High-density polyethylene composites are now able to go beyond previously realized storage moduli at 6-7% v/v loading using cellulose nanocrystals (CNC)s as the filler material. Using a custom catalyst, polymerization is carried out directly from the surfaces of the CNCs in a heterogeneous (slurry) fashion. Mechanical testing showed that this technology affords composites having storage moduli that are three times the moduli observed for unfilled ultra-high-molecular-weight linear polyethylene. The modulus of our polyethylene composites reaches 4000±500 MPa. This new technology seeks to utilize a bio-renewable support for nanocomposites, potentially making clear, recyclable materials with superior mechanical properties. The polymerization is easily performed, the composite material is processed at low temperatures (121 °C or 250 °F), and the material exhibits good clarity. The supported catalyst system, once attached, is resistant to poisoning from air, and exhibits a reasonable activity of 500 kgpE"atm- 1moI-1catalyst at 300 K.