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.

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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.

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produce all-polymer composite by means of flow-induced crystallization during classical injection molding employed in the state-of-the-art processing of commodity and engineering plastics. Today, progress made in all-polymer composite technology holds great promise for upgrading and diversifying existing thermoplastics. Especially, with respect to converting commodity polymer such as polyolefins into high performance materials. As reviewed by Karger-Kocsis and coworkers, one-step (in situ) and multistep (ex situ) processes have been pioneered to produce all-polymer composites via selfreinforcement resulting from oriented polymer crystallization during polymer processing. [1,2] In one-step strategies, flow-induced crystallization is achieved in extrusion and injection molding processes either by redesigning machines or by employing special precisely controlled processing conditions such as high pressure, processing temperature close to polymer melting temperature, and high frequency. [3,4] However, most one-step processes prolong cycle times, require high investment costs, and are highly incompatible with the existing cost-efficient processing technology established for molding of commodity plastics. In multistep processes, oriented polymer crystallization results from either drawing of fibers or stretching of tapes, respectively, followed by lamination such as hot compaction in the subsequent process step. Again, such processes involving lamination of fibers, stacking of stretched films massively impair both throughput and cost efficiency typical for classical injection molding. [5,6] Flow-induced oriented polymer crystallization and in situ formation of extended-chain polymer microand nanofibers represent the key prerequisite for producing all-polymer composites in one-step processes. The concept of flow-induced polymer coil-stretch transition was pioneered by de Gennes for dilute solutions and by Keller and Kolnaar for elongation flow in polymer melts. [7,8] Only when exceeding a critical polymer molar mass flow-induced crystallization produces shish-kebab structures, in which extended-chain polymers form shish, which nucleate the crystallization of low molecular weight polymer forming kebab. Below the critical mass polymer, chains rapidly relax to the coiled state and fail Nanostructure Composites All-polyethylene composites exhibiting substantially improved toughness/ stiffness balance are readily produced during conventional injection molding of high density polyethylene (HDPE) in the presence of bimodal polyethylene reactor blends (RB40) containing 40 wt% ultrahigh molar mass polyethylene (UHMWPE) dispersed in HDPE wax. Scanning electron microscopy (SEM) and differential scanning calorimetry (DSC) analyses shows that flow-induced crystallization affords extended-chain UHMWPE nanofibers forming shish which nucleates HDPE crystallization producing shish-kebab structures as reinforcing phases. This is unparalleled by melt compounding micron-sized UHMWPE. Injection molding of HDPE with 30 wt% RB40 at 165 °C aff...

Multimodal molar mass distributions (MWD) of high-density polyethylene (HDPE) were tailored either by reactor cascade technology using a chain transfer agent or by multisite polymerization catalysis combining different single-site catalysts on the same support in a single reactor. Herein,2,6dimethylphenylimino)ethyl]pyridine chromium(III) (CrBIP) is supported on methylaluminoxane (MAO)-tethered ultrathin γ-Al(OH) 3 (gibbsite) single crystal nanoplatelets to produce reactor blends of HDPE wax and higher molar mass HDPE in a single reactor without adding either a second catalysts or a chain transfer agents. Lowering the MAO/gibbsite weight ratio enables unique switching from single-site to multisite nature of this catalyst system. In sharp contrast, ethylene polymerization on both homogeneous MAO/CrBIP and state-of-the-art heterogeneous CrBIP@MAO@SiO 2 catalysts exclusively produce HDPE wax (1000 g/mol) with narrow MWD (1.5). Both the MAO/gibbsite weight ratio of the CrBIP@MAO@gibbsite catalyst system and the polymerization time govern the HDPE wax/HDPE weight ratio. In addition, gibbsite single crystal nanoplatelets are calcinated at different temperatures prior to MAO tethering to establish the correlations between calcination temperature, MWDs, and catalyst activity. Upon calcination at 600 °C, highly active catalysts are obtained, but the gibbsite single crystal structure is destroyed, and the resulting catalyst fails to produce higher molar mass HDPE. Cosupporting CrBIP together with quinolylsilylcyclopentadienylchromium(III) (CrQCp), which produces ultrahigh molar mass HDPE (UHMWPE), on the same gibbsite support yielded CrQCp&CrBIP@MAO@gibbsite dual-site catalysts producing trimodal MWDs. Here the UHMWPE content is increased by increasing the CrQCp/CrBIP molar ratio. Moreover, the nanophase separation of UHMWPE during polymerization and melt-flow processing accounted for the formation of all-hydrocarbon nanocomposites self-reinforced by unentangled extended chain UHMWPE 1D nanostructures.

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Nacre-mimicking layered organic/inorganic hybrid materials exhibiting ultrahigh stiffness and strength frequently require multistep processing that is restricted to polar and even water-soluble polymers. Herein, nacre-mimetic hydrocarbon composites were fabricated by single-step injection molding. The key intermediates are organophilic ultrathin γ-Al­(OH)3 (O-gibbsite) single-crystal nanoplatelets and all-hydrocarbon composites (All-PE) containing aligned, extended-chain ultrahigh-molecular-weight polyethylene (UHMWPE) as one-dimensional (1D) nanostructures embedded in a polyethylene (PE) matrix. This formation of flow-induced UHMWPE 1D nanostructures mimics chitin nanofibers in nacre and drives the alignment of O-gibbsite nanoplatelets to assemble bricks. Unprecedented high contents of up to 70 wt % O-gibbsite nanoplatelets are tolerated in injection molding. As verified by focused ion beam-scanning electron microscopy (FIB-SEM), the resulting brick-and-mortar architectures contain aligned O-gibbsite as bricks and UHMWPE/high-density polyethylene (HDPE) shish-kebab structures as mortar. The resulting nacre-mimetic hydrocarbon/O-gibbsite composites exhibit substantially improved mechanical properties, as evidenced by high tensile strength of 200 MPa and a superior notched Izod impact strength (28 kJ/m2). In contrast to other nacre-mimetic composites, these superb mechanical properties are retained after immersing the composites in water for several days. As γ-Al­(OH)3 splits off the water at elevated temperature, nacre-inspired hydrocarbon composites are flame retardant despite the high flammability of hydrocarbons.