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

Abstract

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 C 1 -symmetric metallocene catalyst, producing highly isospecific iPP ionomers (T m > 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.

Foaming Performance of Linear Polypropylene Ionomers

“…As expected, both iPP homopolymers (iPP and iPP-LCB) do not reveal any signs of ion cluster peak. The inset in Figure a presents the normalized ion peak intensity parameter, defined in ref as the ratio between the scattering intensities of the ionomers and iPP (nonionomer). This plot confirms that a higher ionomer content results in stronger ion interactions.…”

“…I iPP ( q ) represents all the nonionomeric structural features in the ionomers. The integrated intensity of the ion peak, termed the total ion strength (TIS), was then calculated by using the equation where the integration interval is chosen to cover the ion peak …”

“…Their study reported a series of modified PP ionomers with different strengths of ionic associations that led to different degrees of melt elasticity, demonstrating convenient tunability of rheological properties and foamability. Despite the valuable contributions it adds to the research community, the synthesis process described in their work entails a costly two-step process, as opposed to the single-step process presented in ref which is used to produce the iPP ionomers tested in this study.…”

“…For these reasons, some research efforts have been directed toward a single-step process for synthesizing HMS-PP. In a recent study that overcomes the drawbacks of PRM, López-Barrón et al reported a one-pot synthetic route to produce high melt strength linear PP ionomers with an unprecedented combination of superior processability and mechanical properties . It was shown that minute ionic content (<0.1 mol %) is enough to produce remarkable improvements to the melt strength, strain hardening, and shear thinning responses compared to those of homopolymer PP.…”

Potential of Functionalized Polyolefins in a Sustainable Polymer Economy: Synthetic Strategies and Applications

“…17 Transforming these functionalized polyolefins into their corresponding ionomers causes significant changes in properties such as the modulus, glass-transition temperature, melting point, viscosity, melt strength, and fatigue. 28,29 As the semicrystalline nature of most polyolefins complicates the investigation of the effect of hydrogen bonding or ionic interactions in functionalized polyolefins and their corresponding ionomers, we started studying the properties of randomly hydroxyl-and acid-functionalized atactic polypropylene (aPP). The copolymerization of propylene with TiBA-passivated 10undecen-1-ol or 10-undecenoic acid using 3 as a catalyst produced amorphous functionalized polymers.…”