Compositions are disclosed herein comprising a graft copolymer that comprises : (i) a backbone comprising dextran that has been modified with about 1%-25% alpha-1, 2 branches, and (ii) one or more alpha-1, 3-glucan side chains comprising at least about 50% alpha-1, 3 glycosidic linkages. Further disclosed are reactions for producing such graft copolymers, as well as their use in derivatives, films and various other applications.