One-Pot Synthesis of High-Melt-Strength Isotactic Polypropylene Ionomers

High melt strength (HMS), shear thinning, and extensional strain hardening (SH) are highly desirable properties in commercial polypropylene, which are typically achieved by the incorporation of long-chain branching (LCB). The current state-of-the-art approach to produce LCB involves post-reactor modification steps, which are not only costly but also generate undesirable side products as a result of polymer chain scission. We report a novel one-pot synthetic route to produce HMS isotactic polypropylene (iPP) ionomers bearing aluminum carboxylate groups. The synthesis of iPP ionomers is achieved by the direct copolymerization of an alkenyl aluminum comonomer and is facilitated by a novel C1-symmetric metallocene catalyst, producing highly isospecific iPP ionomers (Tm > 157 °C) with high activity (>200 000 g-polymer mmol-Zr–1 h–1). X-ray scattering experiments conducted in the solid and melt states confirm the presence of ion clusters as independent entities from the crystalline lamellae. The ion content in the iPP ionomers is very low (<0.1 mol %), which results in insignificant effects on the crystallinity, melting point, and mechanical properties when compared to the iPP homopolymer. Remarkably, such a low level of ion content is sufficient to drastically improve the processability of the ionomers, as indicated by the increase in melt strength, shear thinning, and extensional SH.