Revealing the Role of High‐Molecular‐Weight EPR and HIPP in Balancing Toughness and Stiffness of Polypropylene

ABSTRACT Toughness is one of the most crucial properties of polypropylene (PP) for demanding industrial applications. While the ethylene propylene rubber (EPR) content is widely considered a primary contributor to PP toughness, the role of EPR molecular weight in enhancing toughness remains insufficiently understood. Herein, we compare two impact PP samples with contrasting EPR: R1 (high content, low molecular weight) and R2 (lower content, much higher molecular weight) to isolate molecular‐weight influence. TREF and complementary characterization confirmed their EPR contents (31.54 vs. 24.15 wt%) and molecular weights (293.0 vs. 474.9 kg/mol). Despite lower EPR content, R2's much higher EPR molecular weight yielded markedly greater toughness than R1 (62 vs. 17 kJ/m 2 ), indicating that, for the resins examined here, the toughness difference correlates more strongly with EPR molecular weight than with EPR content. Chain‐structure analysis further showed a larger high‐molecular‐weight isotactic polypropylene (HIPP) fraction in R2 (50.88 vs. 36.73 wt%), consistent with its slightly higher stiffness. Overall, these results suggest that increasing the EPR molecular weight can be an important lever for improving PP toughness in the resins examined here, while maintaining stiffness through an appropriate PP‐rich backbone.