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Cellulose nanocrystal (CNC)-reinforced composites are gaining commercial attention on account of their high strength and sustainable sourcing. Grafting polymers to the CNCs in these composites has the potential to improve their properties, but current solution-based synthesis methods limit their production at scale. Utilizing dynamic hindered urea chemistry, a new method for the melt-functionalization of cellulose nanocrystals has been developed. This method does not require toxic solvents during the grafting step and can achieve grafting densities competitive with state-of-the-art solution-based grafting methods. Using cotton-sourced, TEMPO-oxidized CNCs, multiple molecular weights of poly(ethylene glycol) (PEG) as well as dodecane, polycaprolactone, and poly(butyl acrylate) were grafted to the CNC surface. With PEG-grafted nanoparticles, grafting densities of 0.47 chains nm−2 and 0.10 chains nm−2 were achieved with 2000 and 10,000 g mol−1 polymer chains respectively, both of which represent significant improvements over previous reports for solution-based PEG grafting onto CNCs.