A New Generation of High-Efficiency Ziegler–Natta Catalyst for Synthesising Ultra-Clean Isotactic Polypropylene as Capacitor Film Substrate

A series of innovative electron donors (D1–D6) featured both internal and external electron donors in one molecular structure were designed and synthesised. The synthesised electron donors and traditional electron donors (DIBP) were effectively reacted with titanium-based catalysts supported on magnesium chloride to obtain a series of Ziegler–Natta catalysts (Cat 0–Cat 6). Cat 0 formed by electron donor DIBP. Similarly, Cat 1 by D1, Cat 2 by D2, Cat 3 by D3, Cat 4 by D4, Cat 5 by D5 and Cat 6 by D6, respectively, which were characterised by GC-MS, ¹H (¹³C)-NMR, XPS and SEM. It is interesting that the catalytic activity of the catalysts prepared by the newly synthesised electron donor was significantly higher than that of traditional DIBP for propylene polymerisation. Under atmospheric pressure condition of propylene, the catalytic activity of Cat 3 was 112.7 g PP/(g Cat · h) and Cat 6 reached 209.2 g PP/(g Cat · h) at the same conditions, both significantly higher than that of 34.1 g PP/(g Cat · h) of Cat 0. We adopted a prealkylation strategy for the catalysts preparation, reducing the optimal aluminium titanium ratio for propylene polymerisation from 50 to 30, decreasing the amount of cocatalyst used and thus reducing the ash content in the products. During the bulk polymerisation of propylene, the activity of prealkylated Cat 3 and Cat 6 is comparable, at 79.4 kg PP/(g Cat · h), which is about 2.3 times that of Cat 0 cooperated with external electron donor C during the polymerisation processes (35.2 kg PP/(g Cat · h)), and the obtained PP isotacticity reaches over 98%. Especially, Cat 1–Cat 6 were required no external electron donor for propylene polymerisation. In addition, the theoretical ash content of the products formed by Cat 3 and Cat 6 is only 22 ppm, which is significantly lower than that of Cat 0 (133 ppm). It is expected to be used in the industrial production of ultra-clean iPP powder for capacitor films.