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Cellulose nanocrystals (CNCs) have attracted growing interest as a reinforcement agent in polymer matrices thanks to their biodegradability and excellent mechanical properties. However, due to the polar nature and strong interactions among them, CNCs form aggregates which are difficult to disperse in hydrophobic matrices. To improve CNCs’ dispersion in apolar matrices, we have studied modifications via surface grafting with biodegradable polymers. In this work, CNCs were successfully modified with the P(CL-b-LLA) copolymer via a “grafting onto” method. First, the P(CL-b-LLA) copolymers (Mn molecular weight of 6190 and 10,500 Da) were synthesized by sequential copolymerization of ε-caprolactone (CL) and l-lactide (LLA) using stannous octoate and 1-butanol as initiator and co-initiator, respectively. Then, the grafted CNCs, P(CL-b-LLA)-g-CNC, were obtained through the functionalization of copolymers with toluene-2,4-diisocyanate and subsequent covalent linkage to CNCs’ surface in dry toluene. Surface grafting was confirmed using Fourier transform infrared, proton nuclear magnetic resonance (1H NMR), transmission electron microscopy (TEM), and thermogravimetry (TGA). TEM and TGA analyses indicated that the surface modification of the CNCs occurred to a greater extent with the use of the lower molecular weight copolymer. The resulting modified nanoparticles can find applications as filler and compatibilizer in the field of bionanocomposite materials