Tough recyclable polyacetals
Cyclic acetals such as dioxolane are appealing building blocks for recyclable plastics but have proven to be difficult to polymerize controllably. Abel
et al
. show that ...optimal pairing of a bromomethyl ether and indium or zinc Lewis acid produces polydioxolane with high tensile strength that may be advantageous for packaging applications. Heating this plastic in strong acid easily breaks it back down to its acetal monomer, which can then be recovered by distillation from mixed plastic waste streams in high yield. —JSY
Optimal pairing of an initiator and a Lewis acid catalyst produces tough polyacetals that are easily recyclable to monomer.
Identifying plastics capable of chemical recycling to monomer (CRM) is the foremost challenge in creating a sustainable circular plastic economy. Polyacetals are promising candidates for CRM but lack useful tensile strengths owing to the low molecular weights produced using current uncontrolled cationic ring-opening polymerization (CROP) methods. Here, we present reversible-deactivation CROP of cyclic acetals using a commercial halomethyl ether initiator and an indium(III) bromide catalyst. Using this method, we synthesize poly(1,3-dioxolane) (PDXL), which demonstrates tensile strength comparable to some commodity polyolefins. Depolymerization of PDXL using strong acid catalysts returns monomer in near-quantitative yield and even proceeds from a commodity plastic waste mixture. Our efficient polymerization method affords a tough thermoplastic that can undergo selective depolymerization to monomer.
The majority of post-consumer plastic waste is not recycled. Impediments to the recycling of commodity polymers include separation, impurities and degradation of the macromolecular structures, all of ...which can negatively affect the properties of recycled materials. An attractive alternative is to transform polymers back into monomers and purify them for repolymerization — a form of chemical recycling we term chemical recycling to monomer (CRM). Material recycled in this way exhibits no loss in properties, creating an ideal, circular polymer economy. This Review presents our vision for realizing a circular polymer economy based on CRM. We examine the energetics of polymerization and other challenges in developing practical and scalable CRM processes. We briefly review attempts to achieve CRM with commodity polymers, including through polyolefin thermolysis and nylon 6 ring-closing depolymerization, and closely examine the recent flourishing of CRM with new-to-the-world polymers. The benefits of heterocycle ring-opening polymerization are discussed in terms of synthetic control and kinetically accessible polymer-backbone functionality. Common chemical and structural characteristics of CRM-compatible ring-opening-polymerization monomers are identified, and the properties, benefits and liabilities of these recyclable polymers are discussed. We conclude with our perspective on the ideals and opportunities for the field.Unrecycled post-consumer plastic waste is an enormous, growing problem. Chemical recycling to monomer (CRM) delivers recycled material without degradation in properties. This Review assesses the viability of commercial polymer CRM, the flourishing of CRM with new polymers and opportunities for the field.
Polyesters synthesized through the alternating copolymerization of epoxides and cyclic anhydrides compose a growing class of polymers that exhibit an impressive array of chemical and physical ...properties. Because they are synthesized through the chain-growth polymerization of two variable monomers, their syntheses can be controlled by discrete metal complexes, and the resulting materials vary widely in their functionality and physical properties. This polymer-focused review gives a perspective on the current state of the field of epoxide/anhydride copolymerization mediated by discrete catalysts and the relationships between the structures and properties of these polyesters.
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In honor of the 100th anniversary of Staudinger’s “Polymer Hypothesis”, we write this trend review to describe some key developments in the synthesis of cationic polymers. Cationic ...polymers provided early evidence in support of the Polymer Hypothesis and have been recognized for their tremendous potential in various sub-disciplines of chemistry and biology. This article is focused on a specific class of cationic polymers that conduct hydroxide anions, namely alkaline anion exchange membranes (AAEMs). These polymers are essential components in anion exchange membrane fuel cell (AEMFC) energy conversion devices that can operate with zero-emission fuels (H2 and O2) and do not require precious metals (e.g. platinum) as electrocatalysts for oxygen reduction. One of the critical considerations for AAEMs is their long-term stability to alkaline media, and as such, a wide range of polymer backbones and appended cations have been evaluated as potential constructs for this purpose. Herein, an overview of the different classes of cations is presented, along with a description of the typical synthetic methods for the preparation of AAEMs. By organizing the review according to the synthetic strategies, we hope to inspire the design and preparation of new cationic materials that are suitable for AEMFCs.
Solid polymer electrolyte (SPE) membranes are a critical component of high specific energy rechargeable Li-metal polymer (LMP) batteries. SPEs exhibit low volatility and thus increase the safety of ...Li-based batteries compared to current state-of-the-art Li-ion batteries that use flammable small-molecule electrolytes. However, most SPEs exhibit low ionic conductivity at room temperature, and often allow the growth of lithium dendrites that short-circuit the batteries. Both of these deficiencies are significant barriers to the commercialization of LMP batteries. Herein we report a cross-linked polyethylene/poly(ethylene oxide) SPE with both high ionic conductivity (>1.0 × 10–4 S/cm at 25 °C) and excellent resistance to dendrite growth. It has been proposed that SPEs with shear moduli of the same order of magnitude as lithium could be used to suppress dendrite growth, leading to increased lifetime and safety for LMP batteries. In contrast to the theoretical predictions, the low-modulus (G′ ≈ 1.0 × 105 Pa at 90 °C) cross-linked SPEs reported herein exhibit remarkable dendrite growth resistance. These results suggest that a high-modulus SPE is not a requirement for the control of dendrite proliferation.
Advances in catalysis have enabled the ring-opening copolymerization of epoxides and cyclic anhydrides to afford structurally and functionally diverse polyesters with controlled molecular weights and ...dispersities. However, the most common systems employ binary catalyst/cocatalyst pairs which suffer from slow polymerization rates at low loadings. Inspired by new mechanistic insight into the function of binary metal salen/nucleophilic cocatalyst systems at low concentrations, we report a bifunctional complex in which the salen catalyst and an aminocyclopropenium cocatalyst are covalently tethered. A modular ligand design circumvents the extended linear syntheses typical of bifunctional catalysts, enabling systematic variation to understand and enhance catalytic activity. The optimized bifunctional aluminum salen catalyst maintains excellent activity for the ring-opening copolymerization of epoxides and cyclic anhydrides at low concentrations (≥0.025 mol %), and the aminocyclopropenium cocatalyst suppresses undesirable transesterification and epimerization side reactions, preserving the integrity of the polymer backbone.
A dibenzobarrelene‐bridged, α‐diimine NiII catalyst (rac‐3) was synthesized and shown to have exceptional behavior for the polymerization of ethylene. The catalyst afforded high molecular weight ...polyethylenes with narrow dispersities and degrees of branching much lower than those made by related α‐diimine nickel catalysts. Catalyst rac‐3 demonstrated living behavior at room temperature, produced linear polyethylene (Tm=135 °C) at −20 °C, and, most importantly, was able to copolymerize ethylene with the biorenewable polar monomer methyl 10‐undecenoate to yield highly linear ester‐functionalized polyethylene.
Polarity and crystallinity: A dibenzobarrelene‐derived nickel diamine catalyst copolymerizes ethylene and a renewable ester to afford semi‐crystalline linear polyethylene with pendant functional groups. The catalyst polymerizes ethylene in a living fashion with high activity and exhibits an important combination of functional group tolerance and decreased chain walking.
Poly(lactic-co-glycolic acid) (PLGA) is used in vivo for various biomedical applications. Due to its biodegradability and biocompatibility, PLGA is uniquely suited for controlled drug delivery with ...parenteral administration. Previously, we established the synthesis of isotactic, alternating PLGA from enantiopure starting materials. Here, to fill in the gap of the current field, we have developed the synthesis of syndioenriched, alternating PLGA from racemic methyl-glycolide (rac-MeG). The synthesis of alternating PLGA is accomplished by a highly regioselective ring-opening polymerization of rac-MeG with an optimized racemic aluminum catalyst. Mechanistic studies are carried out to elucidate the pairing-enhanced catalyst regio- and stereocontrol. Polymer sequence fidelity has been established by NMR investigations, confirming a high degree of alternation of the comonomer sequence and moderate syndiotacticity within the backbone stereoconfiguration. The resulting syndioenriched material is amorphous, which will facilitate the drug complexation behavior.
The alternating copolymerization of epoxides and cyclic anhydrides is an increasingly popular route to aliphatic polyesters that are of interest as biodegradable replacements for petroleum-based ...polymers and for use in the biomedical field. However, broad and bimodal molecular weight distributions in these polymerizations continues to be an issue, limiting synthesis of multiblock copolymers. By use of a bifunctional catalytic system, the reversible-deactivation anionic alternating ring-opening copolymerization of epoxides and cyclic anhydrides gives unimodal polymers with
values generally less than 1.07. This allowed for the formation of well-defined triblock copolymers. Additionally, by incorporating both aldehyde and alkene functionalities into the polymer, orthogonal post-polymerization modification was achieved, giving access to well-defined highly modifiable aliphatic polyesters.
In order to prepare base-stable, mechanically strong, and synthetically feasible alkaline anion exchange membranes (AAEMs) for applications in alkaline fuel cells, an imidazolium-fused cyclooctene ...monomer was prepared and subjected to ring-opening metathesis polymerization (ROMP) conditions. Surprisingly, macrocyclic oligomers were obtained instead of high molecular weight polymers. High-performance AAEMs were synthesized by using a bifunctional monomer to cross-link the macrocycles. The resultant AAEMs showed high ionic conductivities (σOH– = 59 mS/cm at 50 °C), robust mechanical properties, and excellent alkaline stabilities.
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