A "grafting onto" method for the synthesis of protein-based grafted copolymers is here described. Monofunctional oligo(oxypropylene) chains carrying a terminal isocyanate group were prepared and coupled with gelatin likely by reaction with hydroxyl or primary amino groups. A grafting yield higher than 85% was obtained by using an amount of isocyanate terminated oligo(oxypropylene) of 0.2 mmol per gram of gelatin. A tailored purification procedure based on fractionation with selective solvents was set up to remove homo-coupling products, affording water soluble gelatin-oligo(oxypropylene) graft copolymers having up to 19% wt of grafted oligo(oxypropylene) chains, as indicated by elemental analysis. The composition of all sample fractions were accomplished through NMR, FT-IR and elemental analysis. A negligible insoluble fraction and a water soluble gelatin fraction having a higher modification degree were obtained when the unreacted 1,6-diisocyanatehexane was removed by distillation from the intermediate reagents. The solid films of the graft copolymers showed single-phase morphology by scanning electron microscopy (SEM) observation, confirming the covalent bond formation between oligo(oxypropylene) and gelatin. Simple oligo(oxypropylene)/gelatin blends were prepared and characterized by SEM and stress-strain measurements for comparison. (C) 2012 Elsevier Ltd. All rights reserved.