Synthesis of Ethylene–Styrene Copolymers by N ‑Heterocycle‑Fused Scandium Catalysts: Tuning Mechanical Properties and Enhancing LDPE/PS Compatibility

Abstract Ethylene–styrene (E–St) copolymers with long polyethylene (PE) and polystyrene (PS) sequences have attracted considerable interest owing to their distinctive physical and mechanical properties, as well as their potential to facilitate the recycling of incompatible PE/PS waste streams. Herein, we synthesized N-heterocycle-fused cyclopentadienyl scandium complexes 1–4 and evaluated their performance in E–St copolymerization. Complex 2 exhibited the highest activity and precise sequence control, affording high-molecular-weight copolymers (Mn up to 40.2 × 104 g·mol–1) across a wide styrene composition range (5.2–73.1 mol %). The mechanical behavior of these copolymers was strongly composition-dependent: the 73.1 mol % St copolymer displayed hard, brittle fracture (σB = 57.0 MPa, εB = 6.2%), surpassing syndiotactic polystyrene (σB = 38.1 MPa, εB = 3.4%), whereas below 62.9 mol % St, a ductile transition occurred, with 28.8 mol % copolymer reaching an impact resistance of 125 kJ/m2 (vs 98 kJ/m2 for HDPE). This superior toughness enabled effective compatibilization of LDPE/PS blends; addition of 5–20 wt % of the 33.2 mol % St copolymer substantially improved tensile strength and elongation at break.