In chapter 1, a thorough review of the literature identifies key shortcomings in prior cellulose nanocrystal (CNC) composite research. In chapter 2, poly(ethylene glycol)-grafted CNCs are used as a model system for understanding the effects of polymer molecular weight and grafting density on the resulting mechanical properties of polymer-grafted CNC composites. In chapter 3, a method of functionalizing CNCs in the melt is developed that takes advantage of dynamic hindered urea chemistry to generate reactive isocyanate moieties \textit{in-situ}, which can subsequently react with surface hydroxyl groups on the CNCs to attach a wide variety of polymer chains. Chapter 4 utilizes the fundamental lessons learned in chapters 2 and 3 to synthesize block copolymer-grafted CNCs in the melt and investigate their effect on composite mechanical properties, with a thorough analysis of grafting density with bulkier polymer chains. Finally, chapter 5 summarizes the work in more detail with personal perspectives and outlook for future research.