Upcycling Waste Polypropylene into Multifunctional Homogeneous Additives for Simultaneously Enhanced Mechanical Performance and Processability

Upcycling and incorporating waste polypropylene (PP) into a sustainable circular plastics economy remain a significant challenge. Herein, we report an efficient and cost-effective strategy to transform waste PP into advanced multifunctional homogeneous additives. Specifically, waste PP was converted into PP-vitrimers (PPv) through transesterification reactions and subsequently blended with commercial PP. Remarkably, the resulting PP/PPv blends exhibit substantial enhancements in mechanical properties, including elongation at break, tensile strength, Young’s modulus, and thermal creep resistance, as well as improved melt processability. Detailed investigations reveal that the insoluble fraction of PPv (PPv-insol) acts as an efficient nucleating agent, uniformly dispersed within the PP matrix, significantly influencing crystallization kinetics (increased nucleation density, elevated crystallization temperature, and accelerated crystallization rate), crystalline structure (coexistence of smaller, more uniform spherulites alongside randomly oriented crystals), and polymorphism (predominantly the α form with a small fraction of the β form). Meanwhile, the soluble fraction (PPv-sol) functions as a lubricant, reducing intermolecular friction, lowering melt viscosity, and enhancing melt flowability. This work not only provides fundamental insights into the role of dynamic cross-linked networks in regulating polymer crystallization behavior and viscoelastic properties, significantly advancing our understanding of polymer/vitrimer composites, but also presents a practical, scalable, and industrially viable strategy for the high-value upcycling and utilization of waste PP.