Plastic waste is currently entering the natural environment at a rate of over 300 Mt year^-1. Coupled with the long lifetimes of common plastics, this input is already resulting in rapid accumulation, yet our understanding of plastic persistence in the environment is very limited. This review summarizes the existing literature on environmental degradation rates and pathways for the major types of thermoplastic polymers. It also highlights the need for better experimental studies of polymer degradation under well-defined reaction conditions, and standardized reporting of rates. A metric to harmonize disparate types of measurements, the specific surface degradation rate (SSDR), is implemented and used to extrapolate half-lives. SSDR values cover a very wide range, with some variability due to degradation studies conducted in different natural environments. For example, SSDRs for HDPE in the marine environment range from essentially 0 to ca. 11 μm year^-1. Using a mean SSDR for HDPE in the marine environment and linear extrapolation of the extent of degradation with time leads to estimated half-lives ranging from 58 years (bottles) to 1,200 years (pipes). SSDRs for HDPE and PLA are surprisingly similar in the marine environment, although PLA degrades ca. 20x faster than HDPE on land.

With these guidelines, we aim to unite the lignin-first biorefining research field around best practices for performing or reporting feedstock analysis, reactor design, catalyst performance, and product yields.

A new future for polyethylene: Most current plastic recycling involves chopping up the waste and repurposing it in materials with less stringent engineering requirements than the original application. Chemical decomposition at the molecular level could, in principle, lead to higher-value products. However, the carbon-carbon bonds in polyethylene, the most common plastic, tend to resist such approaches without exposure to high-pressure hydrogen. F. Zhang et al. now report that a platinum/alumina catalyst can transform waste polyethylene directly into long-chain alkylbenzenes, a feedstock for detergent manufacture, with no need for external hydrogen (see the Perspective by Weckhuysen). Science , this issue p. 437 ; see also p. 400

Lithium is widely used in contemporary energy applications, but its isolation from natural reserves is plagued by time-consuming and costly processes. While polymer membranes could, in principle, circumvent these challenges by efficiently extracting lithium from aqueous solutions, they usually exhibit poor ion-specific selectivity. Toward this end, we have incorporated host–guest interactions into a tunable polynorbornene network by copolymerizing 1) 12-crown-4 ligands to impart ion selectivity, 2) poly(ethylene oxide) side chains to control water content, and 3) a crosslinker to form robust solids at room temperature. Single salt transport measurements indicate these materials exhibit unprecedented reverse permeability selectivity (∼2.3) for LiCl over NaCl—the highest documented to date for a dense, water-swollen polymer. As demonstrated by molecular dynamics simulations, this behavior originates from the ability of 12-crown-4 to bind Na⁺ ions more strongly than Li⁺ in an aqueous environment, which reduces Na⁺ mobility (relative to Li⁺) and offsets the increase in Na⁺ solubility due to binding with crown ethers. Under mixed salt conditions, 12-crown-4 functionalized membranes showed identical solubility selectivity relative to single salt conditions; however, the permeability and diffusivity selectivity of LiCl over NaCl decreased, presumably due to flux coupling. These results reveal insights for designing advanced membranes with solute-specific selectivity by utilizing host–guest interactions. Significance Lithium is a key ingredient in batteries, which are integral components of next-generation automobiles, airplanes, grid energy storage, and electronic devices. Unfortunately, lithium extraction from natural sources is laborious, slow, and costly, motivating the search for more efficient isolation techniques. While polymeric membranes could reduce the cost of lithium recovery, current membrane materials lack sufficient lithium-ion selectivity. To address this challenge, we introduce a class of polymeric membranes that incorporate ion binding sites, which significantly increases the transport selectivity of LiCl over NaCl. These studies provide guidelines and practical considerations for incorporating specific interactants into polymers that mediate selective ion transport.