The present invention provides, among other things, compositions comprising nanofibrils, at least one maleic-anhydride (MA) copolymer and at least one matrix polymer, and methods of making such compositions. The provided methods and compositions allow for the production of composites with unexpectedly superior properties including improved impact resistance, tensile modulus of elasticity, tensile strength, and flexural modulus of elasticity as compared to previously known composites. In some embodiments, the present invention provides methods including the steps of providing cellulose nanofibrils, associating the cellulose nanofibrils with a maleic-anhydride (MA) copolymer to form a nanofibril-MA copolymer blend, preparing the nanofibril-MA copolymer blend for addition to a matrix polymer, and forming a composite by associating the nanofibril-MA copolymer blend with the matrix polymer, wherein the amount of cellulose nanofibrils in the composite is between 3% and 50% by weight of the composite.