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The world population will rapidly grow from 7 to 9 billion by 2050 and this will parallel a surging annual plastics consumption from today's 350 million tons to well beyond 1 billion tons. The switch from a linear economy with its throwaway culture to a circular economy with efficient reuse of waste plastics is therefore mandatory. Hydrocarbon polymers, accounting for more than half the world's plastics production, enable closed‐loop recycling and effective product‐stewardship systems. High‐molar‐mass hydrocarbons serve as highly versatile, cost‐, resource‐, eco‐ and energy‐efficient, durable lightweight materials produced by solvent‐free, environmentally benign catalytic olefin polymerization. Nanophase separation and alignment of unentangled hydrocarbon polymers afford 100% recyclable self‐reinforcing all‐hydrocarbon composites without requiring the addition of either alien fibers or hazardous nanoparticles. Recycling of durable hydrocarbons is far superior to biodegradation. The facile thermal degradation enables liquefaction and quantitative recovery of low molar mass hydrocarbon oil and gas. Teamed up with biomass‐to‐liquid and carbon dioxide‐to‐fuel conversions, powered by renewable energy, waste hydrocarbons serve as renewable hydrocarbon feedstocks for the synthesis of high molar mass hydrocarbon materials. Herein, an overview is given on how innovations in catalyst and process technology enable tailoring of advanced recyclable hydrocarbon materials meeting the needs of sustainable development and a circular economy.

Tailored polyethylene reactor blend additives (RB) with ultrabroad bimodal molar mass distributions comprise nanophase-separated ultrahigh molar mass polyethylene (UHMWPE) uniformly dispersed in polyethylene wax. During injection molding of high-density polyethylene (HDPE) together with variable amounts of the nanophase-separated HDPE wax/ UHMWPE (70/30) additive (RB30) flow-induced oriented crystallization affords shish-kebab fiber-like UHMWPE nanostructures accounting for efficient HDPE self-reinforcement. RB additives are readily prepared by ethylene polymerization on silica-supported two-site chromium catalysts which simultaneously produce HDPE wax together with disentangled nanoplateletlike UHMWPE. The presence of HDPE wax is essential for lowering melt viscosity at high UHMWPE content. Since HDPE wax crystallizes onto extended-chain UHMWPE shish to form kebab structures, high HDPE wax content is tolerated without encountering emission problems and impairing mechanical properties as observed in the absence of UHMWPE. This in situ reinforcement substantially improves HDPE toughness/stiffness/strength balance as reflected by simultaneously increased Young's modulus (+365%), tensile strength (+392%), and impact resistance (+197%). The performance of self-reinforced polyethylene (PE-SRC) is far superior to that of melt-blended UHMWPE/HDPE and the majority of PE nanocomposites. Neither hazardous UHMWPE nanoparticles nor alien inorganic nanofillers are required.

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Tailoring trimodal polyethylene (PE) molar mass distributions by means of ethylene polymerization on three-site catalysts, supported on functionalized graphene (FG), enables nanophase separation during polymerization and melt processing, paralleled by PE self-reinforcement. Typically, FG/MAO-supported three-site catalysts combine bis­(iminopyridyl)­chromium trichloride (CrBIP), producing PE wax having high crystallization rate, and quinolylcyclo­pentadienyl­chromium dichloride (CrQCp), forming in situ ultrahigh molecular weight PE (UHMWPE) nanostructures, with bis­(iminopyridyl)iron dichloride (FeBIP) or bis­(tert-butyl cyclo­pentadienyl)­zirconium (ZrCp), respectively, producing HDPE with variable intermediate molar mass. During injection molding, the formation of shish-kebab fiber-like extended-chain UHMWPE structures, as verified by SEM, AFM, and DSC, account for effective self-reinforcement. Only in the presence of high UHMWPE content, PE wax, usually an unwanted byproduct in HDPE synthesis, functions as a built-in processing aid and enables the incorporation of much higher UHMWPE contents (30 wt %) than previously thought to be tolerable in injection molding. Whereas the incorporation of UHMWPE/PE wax blends improves stiffness and strength, the simultaneous FG dispersion accounts for substantially higher impact strength.

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Scheme 1. Freeze-drying of CNF hydrogels (identified by atomic force microscopy (AFM) height image; z-scale = 4 nm) leading to 2D sheet-like NC aerogels (image taken by scanning electron microscopy (SEM)) due to lamellar ice-templating.

Fused filament fabrication (FFF) of polyethylene (PE) reactor blends containing high amounts of nanophaseseparated disentangled ultrahigh molar mass polyethylene (UHMWPE) generates self-reinforced all-PE composites that exhibit superior mechanical properties. Scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and small/ wide-angle X-ray scattering (SAXS/WAXS) analyses reveal that flow-induced crystallization accounts for the in situ formation of fiberlike extended-chain one-dimensional (1D) nanostructures that nucleate high-density PE (HDPE) crystallization, resulting in "shish-kebab" structures as a reinforcing phase. Both the orientation and the content of the 1D nanostructures are controlled by varying the three-dimensional (3D) printing parameters like nozzle temperature, printing pathway, and printing speed. Compared with conventional HDPE, this selfreinforcement simultaneously improves the stiffness (+100%), tensile strength (+200%), and impact strength (+230%) of the material. For the first time, the limited scope of conventional melt processing is advanced and 3D printing, guided by computer design, is applied to enable the fabrication of both unidirectional and digitally tuned multidirectional all-PE composites with quasiisotropic properties as a function of the 3D printing pathway.