Designing a circular carbon and plastics economy for a sustainable future.

The linear production and consumption of plastics today is unsustainable. It creates large amounts of unnecessary and mismanaged waste, pollution and carbon dioxide emissions, undermining global climate targets and the Sustainable Development Goals. This Perspective provides an integrated technological, economic and legal view on how to deliver a circular carbon and plastics economy that minimizes carbon dioxide emissions. Different pathways that maximize recirculation of carbon (dioxide) between plastics waste and feedstocks are outlined, including mechanical, chemical and biological recycling, and those involving the use of biomass and carbon dioxide. Four future scenarios are described, only one of which achieves sufficient greenhouse gas savings in line with global climate targets. Such a bold system change requires 50% reduction in future plastic demand, complete phase-out of fossil-derived plastics, 95% recycling rates of retrievable plastics and use of renewable energy. It is hard to overstate the challenge of achieving this goal. We therefore present a roadmap outlining the scale and timing of the economic and legal interventions that could possibly support this. Assessing the service lifespan and recoverability of plastic products, along with considerations of sufficiency and smart design, can moreover provide design principles to guide future manufacturing, use and disposal of plastics.

Sequence Control from Mixtures: Switchable Polymerization Catalysis and Future Materials Applications.

There is an ever-increasing demand for higher-performing polymeric materials counterbalanced by the need for sustainability throughout the life cycle. Copolymers comprising ester, carbonate, or ether linkages could fulfill some of this demand as their monomer-polymer chemistry is closer to equilibrium, facilitating (bio)degradation and recycling; many monomers are or could be sourced from renewables or waste. Here, an efficient and broadly applicable route to make such copolymers is discussed, a form of switchable polymerization catalysis which exploits a single catalyst, switched between different catalytic cycles, to prepare block sequence selective copolymers from monomer mixtures. This perspective presents the principles of this catalysis, catalyst design criteria, the selectivity and structural copolymer characterization tools, and the properties of the resulting copolymers. Uses as thermoplastic elastomers, toughened plastics, adhesives, and self-assembled nanostructures, and for programmed degradation, among others, are discussed. The state-of-the-art research into both catalysis and products, as well as future challenges and directions, are presented.

Bio-based and Degradable Block Polyester Pressure-Sensitive Adhesives.

A new class of bio-based fully degradable block polyesters are pressure-sensitive adhesives. Bio-derived monomers are efficiently polymerized to make block polyesters with controlled compositions. They show moderate to high peel adhesions (4-13 N cm-1 ) and controllable storage and loss moduli, and they are removed by adhesive failure. Their properties compare favorably with commercial adhesives or bio-based polyester formulations but without the need for tackifier or additives.

Tolerant to air σ-alkane complexes by surface modification of single crystalline solid-state molecular organometallics using vapour-phase cationic polymerisation: SMOM@polymer.

Vapour-phase surface-initiated cationic polymerisation of ethylvinylether occurs at single-crystals of the σ-alkane complex [Rh(Cy2PCH2CH2PCy2)(NBA)][BArF4]. This new surface interface makes these normally very air sensitive materials tolerant to air, while also allowing for onward single-crystal to single-crystal reactivity at metal sites within the lattice.

Switchable Catalysis Improves the Properties of CO2-Derived Polymers: Poly(cyclohexene carbonate-b-ε-decalactone-b-cyclohexene carbonate) Adhesives, Elastomers, and Toughened Plastics.

Carbon dioxide/epoxide copolymerization is an efficient way to add value to waste CO2 and to reduce pollution in polymer manufacturing. Using this process to make low molar mass polycarbonate polyols is a commercially relevant route to new thermosets and polyurethanes. In contrast, high molar mass polycarbonates, produced from CO2, generally under-deliver in terms of properties, and one of the most widely investigated, poly(cyclohexene carbonate), is limited by its low elongation at break and high brittleness. Here, a new catalytic polymerization process is reported that selectively and efficiently yields degradable ABA-block polymers, incorporating 6-23 wt % CO2. The polymers are synthesized using a new, highly active organometallic heterodinuclear Zn(II)/Mg(II) catalyst applied in a one-pot procedure together with biobased ε-decalactone, cyclohexene oxide, and carbon dioxide to make a series of poly(cyclohexene carbonate-b-decalactone-b-cyclohexene carbonate) [PCHC-PDL-PCHC]. The process is highly selective (CO2 selectivity >99% of theoretical value), allows for high monomer conversions (>90%), and yields polymers with predictable compositions, molar mass (from 38-71 kg mol-1), and forms dihydroxyl telechelic chains. These new materials improve upon the properties of poly(cyclohexene carbonate) and, specifically, they show good thermal stability (Td,5 ∼ 280 °C), high toughness (112 MJ m-3), and very high elongation at break (>900%). Materials properties are improved by precisely controlling both the quantity and location of carbon dioxide in the polymer chain. Preliminary studies show that polymers are stable in aqueous environments at room temperature over months, but they are rapidly degraded upon gentle heating in an acidic environment (60 °C, toluene, p-toluene sulfonic acid). The process is likely generally applicable to many other lactones, lactides, anhydrides, epoxides, and heterocumulenes and sets the scene for a host of new applications for CO2-derived polymers.

Triblock polyester thermoplastic elastomers with semi-aromatic polymer end blocks by ring-opening copolymerization.

Thermoplastic elastomers benefit from high elasticity and straightforward (re)processability; they are widely used across a multitude of sectors. Currently, the majority derive from oil, do not degrade or undergo chemical recycling. Here a new series of ABA triblock polyesters are synthesized and show high-performances as degradable thermoplastic elastomers; their composition is poly(cyclohexene-alt-phthalate)-b-poly(ε-decalactone)-b-poly(cyclohexene-alt-phthalate) {PE-PDL-PE}. The synthesis is accomplished using a zinc(ii)/magnesium(ii) catalyst, in a one-pot procedure where ε-decalactone ring-opening polymerization yielding dihydroxyl telechelic poly(ε-decalatone) (PDL, soft-block) occurs first and, then, addition of phthalic anhydride/cyclohexene oxide ring-opening copolymerization delivers semi-aromatic polyester (PE, hard-block) end-blocks. The block compositions are straightforward to control, from the initial monomer stoichiometry, and conversions are high (85-98%). Two series of polyesters are prepared: (1) TBPE-1 to TBPE-5 feature an equivalent hard-block volume fraction (f hard = 0.4) and variable molar masses 40-100 kg mol-1; (2) TBPE-5 to TBPE-9 feature equivalent molar masses (∼100 kg mol-1) and variable hard-block volume fractions (0.12 < f hard < 0.4). Polymers are characterized using spectroscopies, size-exclusion chromatography (SEC), thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). They are amorphous, with two glass transition temperatures (∼-51 °C for PDL; +138 °C for PE), and block phase separation is confirmed using small angle X-ray scattering (SAXS). Tensile mechanical performances reveal thermoplastic elastomers (f hard < 0.4 and N > 1300) with linear stress-strain relationships, high ultimate tensile strengths (σ b = 1-5 MPa), very high elongations at break (ε b = 1000-1900%) and excellent elastic recoveries (98%). There is a wide operating temperature range (-51 to +138 °C), an operable processing temperature range (+100 to +200 °C) and excellent thermal stability (T d,5% ∼ 300 °C). The polymers are stable in aqueous environments, at room temperature, but are hydrolyzed upon gentle heating (60 °C) and treatment with an organic acid (para-toluene sulfonic acid) or a common lipase (Novozyme® 51032). The new block polyesters show significant potential as sustainable thermoplastic elastomers with better properties than well-known styrenic block copolymers or polylactide-derived elastomers. The straightforward synthesis allows for other commercially available and/or bio-derived lactones, epoxides and anhydrides to be developed in the future.

Orthogonal functionalization of alternating polyesters: selective patterning of (AB) n sequences.

Precision functionalized polyesters, with defined monomer sequences, are prepared using an orthogonal post-polymerization strategy. These polyesters can be synthesized from bio-derived monomers and are targeted to degrade, by hydrolysis processes, to biocompatible diols and diacids; the new structures enabled by this methodology would be very difficult to synthesize by alternative strategies. A series of 9 well-defined highly alternating AB-type copolyesters, containing terminal and internal alkene functionalities, are synthesized in high conversions by the ring-opening copolymerization of epoxides and cyclic anhydrides. Firstly, the polyesters are functionalized by a selective hydroboration-oxidation reaction to exclusively and quantitatively hydroxylate the terminal alkenes, leaving the alternating internal alkenes unreacted. Subsequently, the internal alkenes are quantitatively transformed into carboxylic acid, amine, alkyl and oligo-ether groups, by thiol-ene reactions, to afford AB polyesters with alternating functional substituents. Three polyesters showing alternating hydrophilic/hydrophobic side-chain sequences self-assemble in solution to form nanostructures that are characterized using transmission electron microscopy and dynamic light scattering methods (R h = 100-300 nm). The selective patterning methodology provides facile, efficient and orthogonal functionalization of alternating polyesters with near-quantitative (AB) n repeat sequences. The method is expected to be generalizable to other polymers and provides access to completely new AB alternating structures with the potential to exploit ligand multi-valency and adjacency to enhance properties.