Organocatalysis: Opportunities and Challenges for Polymer Synthesis

Advancing H-Bonding Organocatalysis for Ring-Opening Polymerization: Intramolecular Activation of Initiator/Chain End

Organocatalytic ring-opening polymerization (ROP) of lactones is a green method for accessing renewable and biodegradable polyesters. Developing new organocatalysts with high activity and controllability is a major and challenging research topic in this field. Here, we report a series of organocatalysts to achieve a fast and controlled ROP of lactones. These catalysts incorporate (thio)urea and alkoxide in one molecule and act as initiators in the ROP. Such catalysts enable an effective intramolecular activation of initiator/chain end, as revealed by computational studies, resulting in higher activity and fewer (thio)urea loads than existing (thio)urea/alkoxide binary systems. These organocatalysts exhibit ultrahigh activity comparable to metal complexes, i.e., turnover number up to 900 and turnover of frequency up to 4860 min-1, affording polyesters with tailor-made structure, predicted molecular weights, narrow dispersity, less epimerization, and minimal transesterification. The catalyst synthesis is simple and scalable, allowing widely tuned activities of the ROP.

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket

Organocatalyzed ring-opening polymerization (ROP) is a versatile technique for synthesizing biodegradable polymers, including polyesters and polycarbonates. We introduce o-phenylene bisurea (OPBU) (di)anions as a novel class of organocatalysts that are fast, easily tunable, mildly basic, and exceptionally selective. These catalysts surpass previous generations, such as thiourea, urea, and TBD, in selectivity (kp/ktr) by 8 to 120 times. OPBU catalysts facilitate the ROP of various monomers, achieving high conversions (>95%) in seconds to minutes, producing polymers with precise molecular weights and very low dispersities (Đ ≈ 1.01). This performance nearly matches the ideal distribution expected from living polymerization (Poisson distribution). Density functional theory (DFT) calculations reveal that the catalysts stabilize the oxyanion transition state via a hydrogen bond pocket similar to the "oxyanion hole" in enzymatic catalysis. Both experimental and theoretical analyses highlight the critical role of the semirigid o-phenylene linker in creating a hydrogen bond pocket that is tight yet flexible enough to accommodate the oxyanion transition state effectively. These new insights have provided a new class of organic catalysts whose accessibility, moderate basicity, excellent solubility, and unparalleled selectivity and tunability open up new opportunities for controlled polymer synthesis.

O/S Exchange Reaction in Synthesizing Sulfur-Containing Polymers

Using carbon disulfide (CS2) and carbonyl sulfide (COS) as sulfur-containing and one-carbon feedstocks to make value-added products is paramount for both pure and applied chemistry and environmental science. One of the practical strategies is to copolymerize these bulk chemicals with epoxides to produce sulfur-containing polymers. This approach contributes to improving the sustainability of polymer manufacturing, provides highly desired functional polymer materials, and has attracted much attention. However, these copolymerizations invariably exhibit the intensely complicated chemistry of O/S exchange reaction, leading to sulfur-containing polymers with diverse architectures. As the understanding of O/S exchange continues to deepen, recent efforts have guided significant advances in the synthesis of CS2- and COS-based polymers. This review examines the O/S exchange chemistry and summarizes the recent progress in this field to promote the further advance of synthesizing sulfur-containing polymers from CS2 and COS.

Leveraging Electron Push-Pull Effect for Catalytic Polymerization and Degradation of a Cyclobutane Monomer System

Ring-opening polymerization (ROP) offers a striking solution to solve problems encountered in step-growth condensation polymerization, including precise control over molecular weight, molecular weight distribution, and topology. This has inspired our interest in ROP of cycloalkanes with an ultimate goal to rethink polyolefins, which clearly poses a number of challenges. Practicality of ROP of cycloalkanes is actually limited by their low polymerizability and elusive mechanisms which arise from significantly varied ring size and non-polar C-C bonds in monomers. In this work, by using Lewis acid/Brønsted base/C(sp3)-H initiator system previously developed in our laboratory, we focus on cyclobutanes and explore the positional and electronic effects of substituents on the ring, namely electron push-pull effect, in promoting controlled polymerization to afford densely functionalized poly(cyclobutanes), as well as catalytic degradation of obtained polymers for upcycling. More importantly, experiments and DFT calculations unveil considerable population of Lewis-acid-induced thermostabilized 1,4-zwitterions, which distinguish cyclobutanes from cyclopropanes and others. All these findings would shed light on catalytic synthesis and degradation of saturated all-carbon main-chain polymers, as well as small molecule transformations of cyclobutanes.

Preparation and Characterization of H-Shaped Polylactide

An H-polymer has an architecture that consists of four branches symmetrically attached to the ends of a polymer backbone, similar in shape to the letter "H". Here, a renewable H-polymer efficiently synthesized using only ring-opening transesterification is demonstrated. The strategy relies on a tetrafunctional poly(±-lactide) macroinitiator, from which four poly(±-lactide) branches are grown simultaneously. 1H NMR spectroscopy, size exclusion chromatography (SEC), and matrix-assisted laser desorption/ionization (MALDI) spectrometry were used to verify the macroinitiator purity. Branch growth was probed using 1H NMR spectroscopy and SEC to reveal unique transesterification phenomena that can be controlled to yield architecturally pure or more complex materials. H-shaped PLA was prepared at the multigram scale with a weight-average molar mass Mw > 100 kg/mol and low dispersity Đ < 1.15. Purification involved routine precipitations steps, which yielded products that were architecturally relatively pure (∼93%). Small-amplitude oscillatory shear and extensional rheology measurements demonstrate the unique viscoelastic behavior associated with the H-shaped architecture.

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Organoboron-mediated polymerizations

The scientific community has witnessed extensive developments and applications of organoboron compounds as synthetic elements and metal-free catalysts for the construction of small molecules, macromolecules, and functional materials over the last two decades. This review highlights the achievements of organoboron-mediated polymerizations in the past several decades alongside the mechanisms underlying these transformations from the standpoint of the polymerization mode. Emphasis is placed on free radical polymerization, Lewis pair polymerization, ionic (cationic and anionic) polymerization, and polyhomologation. Herein, alkylborane/O2 initiating systems mediate the radical polymerization under ambient conditions in a controlled/living manner by careful optimization of the alkylborane structure or additives; when combined with Lewis bases, the selected organoboron compounds can mediate the Lewis pair polymerization of polar monomers; the bicomponent organoboron-based Lewis pairs and bifunctional organoboron-onium catalysts catalyze ring opening (co)polymerization of cyclic monomers (with heteroallenes, such as epoxides, CO2, CO, COS, CS2, episulfides, anhydrides, and isocyanates) with well-defined structures and high reactivities; and organoboranes initiate the polyhomologation of sulfur ylides and arsonium ylides providing functional polyethylene with different topologies. The topological structures of the produced polymers via these organoboron-mediated polymerizations are also presented in this review mainly including linear polymers, block copolymers, cyclic polymers, and graft polymers. We hope the summary and understanding of how organoboron compounds mediate polymerizations can inspire chemists to apply these principles in the design of more advanced organoboron compounds, which may be beneficial for the polymer chemistry community and organometallics/organocatalysis community.

A Single Amino Acid Able to Promote High-Temperature Ring-Opening Polymerization by Dual Activation

Amino acids are indispensable compounds in the body, performing several biological processes that enable proper functioning. In this work, it is demonstrated that a single amino acid, taurine, is also able to promote the ring-opening polymerization (ROP) of several cyclic monomers under industrially relevant conditions. It is shown that the unique zwitterionic structure of taurine, where the negatively charged sulfonic acid group and the protonated amine group are separated by two methylene groups, not only provides high thermal stability but also leads to a dual activation mechanism, which is corroborated by quantum mechanical calculations. This unique mechanism allows for the synthesis of polylactide of up to 50 kDa in bulk at 180 °C with good end-group fidelity using a highly abundant catalyst. Furthermore, cytotoxicity tests confirm that PLLA synthesized with taurine is non-toxic. Moreover, it is demonstrated that the presence of taurine does not have any detrimental effect on the thermal stability of polylactide, and therefore polymers can be used directly without any post-polymerization purification. It is believed that the demonstration that a simple structure composed of a single amino acid can promote polymerization can bring a paradigm shift in the preparation of polymers.

Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems

Organocatalyzed ring-opening polymerization is a powerful tool for the synthesis of a variety of functional, readily degradable polyesters and polycarbonates. We report the use of (thio)ureas in combination with cyclopropenimine bases as a unique catalyst for the polymerization of cyclic esters and carbonates with a large span of reactivities. Methodologies of exceptionally effective and selective cocatalyst combinations were devised to produce polyesters and polycarbonates with narrow dispersities (Đ = 1.01-1.10). Correlations of the pKa of the various ureas and cyclopropenimine bases revealed the critical importance of matching the pKa of the two cocatalysts to achieve the most efficient polymerization conditions. It was found that promoting strong H-bonding interactions with a noncompetitive organic solvent, such as CH2Cl2, enabled greatly increased polymerization rates. The stereoselective polymerization of rac-lactide afforded stereoblock poly(lactides) that crystallize as stereocomplexes, as confirmed by wide-angle X-ray scattering.

Catalytic Ring-Opening Polymerisation of Cyclic Ethylene Carbonate: Importance of Elementary Steps for Determining Polymer Properties Revealed via DFT-MTD Simulations Validated Using Kinetic Measurements

The production of CO2-containing polymers is still very demanding in terms of controlling the synthesis of products with pre-defined CO2 content and molecular weight. An elegant way of synthesising these polymers is via CO2-containing building blocks, such as cyclic ethylene carbonate (cEC), via catalytic ring-opening polymerisation. However, to date, the mechanism of this reaction and control parameters have not been elucidated. In this work, using DFT-metadynamics simulations for exploiting the potential of the polymerisation process, we aim to shed more light on the mechanisms of the interaction between catalysts (in particular, the catalysts K3VO4, K3PO4, and Na2SnO3) and the cEC monomer in the propagation step of the polymeric chain and the occurring CO2 release. Confirming the simulation results via subsequent kinetics measurements indicates that, depending on the catalyst's characteristics, it can be attached reversibly to the polymeric chain during polymerisation, resulting in a defined lifetime of the activated polymer chain. The second anionic oxygen of the catalyst can promote the catalyst's transfer to another electrophilic cEC monomer, terminating the growth of the first chain and initiating the propagation of the new polymer chain. This transfer reaction is an essential step in controlling the molecular weight of the products.

The Nature of the Spark Is a Pivotal Element in the Design of a Miller-Urey Experiment

Miller and Urey applied electric sparks to a reducive mixture of CH4, NH3, and water to obtain a complex organic mixture including biomolecules. In this study, we examined the impact of temperature, initial pressure, ammonia concentration, and the spark generator on the chemical profile of a Miller-Urey-type prebiotic broth. We analyzed the broth composition using Gas Chromatography combined with Mass Spectroscopy (GC/MS). The results point towards strong compositional changes with the nature of the spark. Ammonia exhibited catalytic properties even with non-nitrogen-containing compounds. A more elevated temperature led to a higher variety of substances. We conclude that to reproduce such a broth as well as possible, all the studied parameters need to be tightly controlled, the most difficult and important being spark generation.

Systematic Order-Made Synthesis of Sequence-Defined Polyurethanes with Length, Types, and Topologies

Polyurethanes are industrially and academically important as soft materials. They are conventionally synthesized by a process based on step-growth polymerization; thus, molecular weight and structural control are impossible. However, the development of a synthetic strategy for polyurethanes remains a big challenge in designing soft materials. Herein, we demonstrate a synthetic methodology for generating polyurethanes with selectable lengths and termini characteristics. The multistep synthetic process offered the systematic synthesis of high-molecular weight, regioregular, and α,ω-urethane telechelics. Various oligomers with order-made repeating units revealed the effective length of the polymer properties. To demonstrate the scope of our methodology, it was also applied to the synthesis of block co-oligomers, three-armed star oligomers, and miktoarm star co-oligomers. Thus, our method allows the synthesis of high-molecular-weight oligomers with complete structural and molecular weight control, which is of enormous value to materials science; particularly the study and application of structure-property relationships in polyurethanes.

Recent Advances in Polycaprolactones for Anticancer Drug Delivery

Poly(ε-Caprolactone)s are biodegradable and biocompatible polyesters that have gained considerable attention for drug delivery applications due to their slow degradation and ease of functionalization. One of the significant advantages of polycaprolactone is its ability to attach various functionalities to its backbone, which is commonly accomplished through ring-opening polymerization (ROP) of functionalized caprolactone monomer. In this review, we aim to summarize some of the most recent advances in polycaprolactones and their potential application in drug delivery. We will discuss different types of polycaprolactone-based drug delivery systems and their behavior in response to different stimuli, their ability to target specific locations, morphology, as well as their drug loading and release capabilities.

A DFT-metadynamics study disclosing key properties of ring-opening polymerization catalysts to produce polyethercarbonate polyols from cyclic ethylene carbonate as part of an emerging CCU technology

The ring opening polymerization of cyclic carbonates made from epoxide and CO₂ to CO₂-containing polymers constitutes an emerging technology of particular industrial interest. Considering the reaction of ring-opening polymerization of cyclic ethylene carbonate to produce polyethercarbonate polyols, several types of catalysts were tested experimentally and mechanistic pathways were proposed, but a detailed analysis of structure property relationship including the CO₂-liberation pathways is still lacking. This contribution is using computational methods to investigate reported benchmark catalysts with the lead structure A_xM_yO_z (A: alkali metal or alkyl, M: main group element or transition metal) that are particularly approved as effiecient catalysts for industrial purpose. Employing DFT-metadynamics simulations, free energy surfaces (FESs) for the key-steps in the catalytic polymerization of cyclic ethylene carbonate (cEC) are generated. Important structural criteria and characteristics of the catalysts that influence the catalytic performance and (side)reaction pathways are determined. It turns out that less nucleophilicity of the catalyst anion and more labile cations remain major criteria for prohibiting CO₂ liberation during polymerization. The key learnings of this contribution currently serve as a basis to develop the next generation of catalysts to bring this emerging carbon capture and use (CCU) technology into industrial application.

A Biodegradable, Polymer-Supported Oxygen Atom Transfer Reagent

Biodegradable polymers are desirable to mitigate the environmental impact of plastic waste in the environment. Over the past several decades, the development of organocatalytic ring-opening polymerization (OROP) has made the synthesis of many new types of biodegradable polymers possible. In this research article, the first example of an oxygen atom transfer reagent pendant on a biodegradable polymer backbone is reported. The monomers for the polycarbonate backbone are sourced from the biodegradable 2,2-bis(hydroxymethyl) propionic acid molecule, and an iodoaryl group is installed pendant to the cyclic monomer for post-polymerization modification into an iodosylaryl oxygen atom transfer reagent. The key I-O bond is characterized by XPS spectroscopy, and a test reaction to triphenylphosphine demonstrates the ability of the polymer to engage in an oxygen atom transfer reaction with a substrate.

Phosphazene Functionalized Silsesquioxane-Based Porous Polymer as Thermally Stable and Reusable Catalyst for Bulk Ring-Opening Polymerization of ε-Caprolactone

The bulk ring-opening polymerization (ROP) of ε-caprolactone using phosphazene-containing porous polymeric material (HPCP) has been studied at high reaction temperatures (130-150 °C). HPCP in conjunction with benzyl alcohol as an initiator induced the living ROP of ε-caprolactone, affording polyesters with a controlled molecular weight up to 6000 g mol-1 and moderate polydispersity (Ð~1.5) under optimized conditions ([BnOH]/[CL] = 50; HPCP: 0.63 mM; 150 °C). Poly(ε-caprolactone)s with higher molecular weight (up to Mn = 14,000 g mol-1, Ð~1.9) were obtained at a lower temperature, at 130 °C. Due to its high thermal and chemical stability, HPCP can be reused for at least three consecutive cycles without a significant decrease in the catalyst efficiency. The tentative mechanism of the HPCP-catalyzed ROP of ε-caprolactone, the key stage of which consists of the activation of the initiator through the basic sites of the catalyst, was proposed.

PEGylated and functionalized polylactide-based nanocapsules: An overview

Polymeric nanocapsules (NC) are versatile mixed vesicular nanocarriers, generally containing a lipid core with a polymeric wall. They have been first developed over four decades ago with outstanding applicability in the cosmetic and pharmaceutical fields. Biodegradable polyesters are frequently used in nanocapsule preparation and among them, polylactic acid (PLA) derivatives and copolymers, such as PLGA and amphiphilic block copolymers, are widely used and considered safe for different administration routes. PLA functionalization strategies have been developed to obtain more versatile polymers and to allow the conjugation with bioactive ligands for cell-targeted NC. This review intends to provide steps in the evolution of NC since its first report and the recent literature on PLA-based NC applications. PLA-based polymer synthesis and surface modifications are included, as well as the use of NC as a novel tool for combined treatment, diagnostics, and imaging in one delivery system. Furthermore, the use of NC to carry therapeutic and/or imaging agents for different diseases, mainly cancer, inflammation, and infections is presented and reviewed. Constraints that impair translation to the clinic are discussed to provide safe and reproducible PLA-based nanocapsules on the market. We reviewed the entire period in the literature where the term "nanocapsules" appears for the first time until the present day, selecting original scientific publications and the most relevant patent literature related to PLA-based NC. We presented to readers a historical overview of these Sui generis nanostructures.

Organocatalytic [2 + 2] Photopolymerization under Visible Light: Accessing Sustainable Polymers from Cinnamic Acids

Herein, the successful development of a metal-free, solution [2 + 2] photopolymerization of natural cinnamic acid-derived bisolefinic monomers is reported, which is enabled by a strategy based on direct triplet state access via energy transfer catalysis. 2,2'-Methoxythioxanthone has been identified as an effective organic photocatalyst for the [2 + 2] photopolymerization in solution, which can be excited by visible light and activate the biscinnamate monomers via triplet energy transfer. This method features its metal-free conditions, visible light utilization, solution polymerization, and abundant biomass-based feedstock, as well as processable polymer products, which is different from the rigid, insoluble products obtained from solid-state photopolymerization. This solution polymerization method also shows a good compatibility to monomer structures; cinnamic acid-derived bisolefinic monomers with different linkers, including diamine, natural diol, and bisphenol, can all readily undergo [2 + 2] photopolymerization, and be transformed into colorless, sustainable polymers.

Ring-Opening Polymerization of Trimethylene Carbonate with Phosphazene Organocatalyst

Aliphatic polycarbonate (APC) compounds are an important class of biodegradable materials with excellent biocompatibility, good biodegradability, and low toxicity, and the study of these compounds and their modification products aims to obtain biodegradable materials with better performance. In this context, the ring-opening polymerization (ROP) of trimethylene carbonate (TMC) from a low nucleophilic organic superbase of phosphazene (t-BuP₄) as a catalyst and benzyl alcohol (BnOH) as an initiator at room temperature was carefully studied to prepare poly(trimethylene carbonate) (PTMC) which is one of the most studied APC. ¹H NMR and SEC measurements clearly demonstrate the presence of a benzyloxy group at the α-terminus of the obtained PTMC homopolymers while investigation of the polymerization kinetics confirms the controlled/living nature of t-BuP₄-catalyzed ROP of TMC. On the basis of this, the block copolymerization of TMC and δ-valerolactone (VL)/ε-caprolactone (CL) was successfully carried out to give PTMC-b-PCL and PTMC-b-PVL copolymers. Furthermore, PTMC with terminal functionality was also prepared with the organocatalytic ROP of TMC through functional initiators. We believe that the present ROP system is a robust, highly efficient, and practical strategy for producing excellent biocompatible and biodegradable PTMC-based materials.

Organocatalytic Ring-Opening Polymerization of ϵ-Caprolactone with Phosphoramidimidates (PADIs) as a Bifunctional Brønsted Acid Catalyst

In this study, an organocatalytic ring-opening polymerization (ROP) of ϵ-caprolactone (ϵ-CL) has been developed by employing PADIs as a novel and efficient acid/base bifunctional organocatalyst, which could afford metal-free poly(ϵ-caprolactone) with predictable molecular weight and narrow dispersity at a low catalyst loading under mild conditions. NMR and kinetic studies indicate that the ring-opening polymerizations of lactones catalyzed by PADIs proceed in a living and well controlled manner. Moreover, this organic Brønsted acid catalytic system could allow the synthesis of PCL with molecular weight above 60 kg/mol, as well as well-defined star polymers.

Aluminium complexes containing indolyl-phenolate ligands as catalysts for ring-opening polymerization of cyclic esters

A family of aluminium complexes supported by mono-anionic indolyl-phenolate ligands are described. Reactions of indolyl-phenolate based ligand precursors, IndHPhROH, with 1.0 or 0.5 equivalents of AlMe2Cl in toluene afforded aluminium indolyl-phenolate complexes 1-4 and aluminium bis-indolyl-phenolate complexes 5-8 respectively. The molecular structure is reported for 5. Based on the NMR spectroscopic and X-ray crystallographic studies, a 1,3-hydrogen shift could happen from nitrogen to carbon on the five-membered ring of the indolyl group upon reacting with aluminium reagents. These novel aluminium complexes demonstrate catalytic activities toward the ring-opening polymerization of cyclic esters in the presence of alcohol.

Scalable and Room-Temperature Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Active Lithium Tetramethylene-Tethered Bis[N-(N'-butylimidazol-2-ylidene)] N-Heterocyclic Carbene as a Lewis Acid Organocatalyst

The Lewis acid organocatalytic system of lithium tetramethylene-tethered bis[N-(N'-butylimidazol-2-ylidene)] N-heterocyclic carbene (1,4-bisNHC) including lithium benzyloxide and benzyl alcohol has been successfully utilized in the ring-opening polymerization (ROP) of ε-caprolactone (CL) for the first time. The catalytic performance of this organic catalyst in the synthesis of high-molecular-weight polymers was investigated via bulk polymerization using different combinations of tetramethylene-tethered bis[N-(N'-butylimidazolium)] hexafluorophosphate (1,4-bis[Bim][PF₆]), benzyl alcohol (BnOH), and n-butyl lithium (nBuLi) ([1,4-bis[Bim][PF₆]]/[BnOH]/[nBuLi]) with the molar ratios of 0:2:2, 1:1:3, 1:2:3, and 1:2:4. The results showed that the molar ratio of 1:2:3 efficiently and rapidly initiated the bulk ROP of CL at room temperature with a high molar ratio of CL to 1,4-bis[Bim][PF₆] of 3000/1 and produced the highest number of average-molecular-weight (M_n) poly(ε-caprolactone) (103,057 g mol^-1) with the dispersity (D̵) and %conversion of 1.73 and 98% in a short period of time (152 s). From comparative studies, the relative polymerization rates of the bulk ROP of CL with different [1,4-bis[Bim][PF₆]]/[BnOH]/[nBuLi] molar ratios was determined in the following order: 1:2:4 > 1:1:3 > 1:2:3 > 0:2:2. For mechanistic investigation, the bulk ROP mechanism of CL with our organic catalyst was proposed through the intramolecular bis-lithium-carbene interaction pathway for 1,4-bisNHC_1,1,3, 1,4-bisNHC_1,2,3, and 1,4-bisNHC_1,2,4 systems.

Spotting Trends in Organocatalyzed and Other Organomediated (De)polymerizations and Polymer Functionalizations

Organocatalysis has evolved into an effective complement to metal- or enzyme-based catalysis in polymerization, polymer functionalization, and depolymerization. The ease of removal and greater sustainability of organocatalysts relative to transition-metal-based ones has spurred development in specialty applications, e.g., medical devices, drug delivery, optoelectronics. Despite this, the use of organocatalysis and other organomediated reactions in polymer chemistry is still rapidly developing, and we envisage their rapidly growing application in nascent areas such as controlled radical polymerization, additive manufacturing, and chemical recycling in the coming years. In this Review, we describe ten trending areas where we anticipate paradigm shifts resulting from novel organocatalysts and other transition-metal-free conditions. We highlight opportunities and challenges and detail how new discoveries could lead to previously inaccessible functional materials and a potentially circular plastics economy.

Linear Block Copolymer Synthesis

Block copolymers form the basis of the most ubiquitous materials such as thermoplastic elastomers, bridge interphases in polymer blends, and are fundamental for the development of high-performance materials. The driving force to further advance these materials is the accessibility of block copolymers, which have a wide variety in composition, functional group content, and precision of their structure. To advance and broaden the application of block copolymers will depend on the nature of combined segmented blocks, guided through the combination of polymerization techniques to reach a high versatility in block copolymer architecture and function. This review provides the most comprehensive overview of techniques to prepare linear block copolymers and is intended to serve as a guideline on how polymerization techniques can work together to result in desired block combinations. As the review will give an account of the relevant procedures and access areas, the sections will include orthogonal approaches or sequentially combined polymerization techniques, which increases the synthetic options for these materials.

Highly Active Heterogeneous Double Metal Cyanide Catalysts for Ring-Opening Polymerization of Cyclic Monomers

A series of heterogeneous Zn-Co double metal cyanide (DMC) catalysts were investigated for ring-opening polymerization (ROP) of various cyclic monomers. Notably, inexpensive and commonly used organic solvents such as acetone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, and 1-methylpyrrolidin-2-one were very effective complexing agents for the preparation of DMC catalysts, showing high catalytic activity for the ROP of propylene oxide, ε-caprolactone, and δ-valerolactone. The chemical structures and compositions of the resultant catalysts were determined using various techniques such as FT-IR, X-ray photoelectron spectroscopy, powder X-ray diffraction, and elemental analysis. α,ω-Hydroxyl-functionalized polyether and polyester polyols with high yields and tunable molecular weights were synthesized in the presence of various initiators to control functionality. Kinetic studies of the ROP of δ-valerolactone were also performed to confirm the reaction mechanism.

Phenalenyl Radical: Smallest Polycyclic Odd Alternant Hydrocarbon Present in the Graphene Sheet

Phenalenyl, a zigzag-edged odd alternant hydrocarbon unit can be found in the graphene nanosheet. Hückel molecular orbital calculations indicate the presence of a nonbonding molecular orbital (NBMO), which originates from the linear combination of atomic orbitals (LCAO) arising from 13 carbon atoms of the phenalenyl molecule. Three redox states (cationic, neutral radical, and anionic) of the phenalenyl-based molecules were attributed to the presence of this NBMO. The cationic state can undergo two consecutive reductions to result in neutral radical and anionic states, stepwise, respectively. The phenalenyl-based radicals were found as crucial building blocks and attracted the attention of various research fields such as organic synthesis, material science, computation, and device physics. From 2012 onward, a strategy was devised using the cationic state of phenalenyl-based molecules and in situ generated phenalenyl radicals, which created a new domain of catalysis. The in situ generated phenalenyl radicals were utilized for the single electron transfer (SET) process resulting in redox catalysis. This emerging range of applications rejuvenates the more than six decades-old phenalenyl chemistry. This review captures such developments ranging from fundamental understanding to multidirectional applications of phenalenyl-based radicals.

A Cation-Dependent Dual Activation Motif for Anionic Ring-Opening Polymerization of Cyclic Esters

A new organocatalyst for the ring-opening polymerization of lactones has been identified. Under the tested conditions, the anions of 2,2'-bisindole promote fast, living polymerizations (as short as 10 ms) which are selective for chain elongation over transesterification (Đ ≤ 1.1). While structurally related to (thio)urea anion catalysts, anions of 2,2'-bisindole activate the monomer via the counterion rather than through hydrogen bonding. This new activation motif enables modulation of the polymerization rate by 2 orders of magnitude by changing the counterion.

Recent Advances in Poly(α-L-glutamic acid)-Based Nanomaterials for Drug Delivery

Poly(α-L-glutamic acid) (PGA) is a class of synthetic polypeptides composed of the monomeric unit α-L-glutamic acid. Owing to their biocompatibility, biodegradability, and non-immunogenicity, PGA-based nanomaterials have been elaborately designed for drug delivery systems. Relevant studies including the latest research results on PGA-based nanomaterials for drug delivery have been discussed in this work. The following related topics are summarized as: (1) a brief description of the synthetic strategies of PGAs; (2) an elaborated presentation of the evolving applications of PGA in the areas of drug delivery, including the rational design, precise fabrication, and biological evaluation; (3) a profound discussion on the further development of PGA-based nanomaterials in drug delivery. In summary, the unique structures and superior properties enables PGA-based nanomaterials to represent as an enormous potential in biomaterials-related drug delivery areas.

Toughening Poly(l-lactide) Blends: Effectiveness of Sequence-Controlled Six-Arm Star-Branched Block Copolymers of Poly(l-lactide) and Poly(ε-caprolactone)

Well-defined six-arm star-branched bio-degradable block copolymers of l-lactide and ε-caprolactone were prepared using controlled ring-opening polymerization and a sequential monomer addition method using dipentaerythritol as the initiator core and organocatalysts at low temperatures in solution. Sequence of enchainment was changed by reversing the order of monomer addition giving, either, a crystalline PLA block or an amorphous PCL block as the outer segment. Well-defined six-arm poly(ε-caprolactone-b-l-lactide, 6s-PCL-b-PLA) block copolymers were obtained with a range of segment molecular weights. However, in the case of six-arm poly(l-lactide-b-ε-caprolactone, 6s-PLA-b-PCL), disruption of the block structure was observed on account of competing transesterification reactions accompanying a chain-growth reaction. Such sequence-controlled block copolymers showed interesting phase morphologies, as evidenced by differential scanning calorimetry (DSC) studies. 6s-PCL-b-PLA showed two glass-transition temperatures and two melting temperatures characteristic of the amorphous and crystalline blocks. 6s-PCL-b-PLA and 6s-PLA-b-PCL with different segment chain lengths were solution blended (10 wt %) with a commercially sourced PLA. All the blends were highly transparent. The structure and properties of the blend were examined by DSC, measurement of mechanical properties, and scanning electron microscopy. The results show that a phase-separated 6s-PCL-b-PLA copolymer results in two- to three-fold improvement in tensile toughness without the loss of modulus. A possible hypothesis for the mechanism of tensile toughness in the blend has been proposed.

Simple and Efficient Synthesis of Functionalized Cyclic Carbonate Monomers Using Carbon Dioxide

Aliphatic polycarbonates represent an important class of materials with diverse applications ranging from battery electrolytes, polyurethane intermediates, and materials for biomedical applications. These materials can be produced via the ring-opening polymerization (ROP) of six- to eight-membered cyclic carbonates derived from precursor 1,3- and 1,5-diols. These diols can contain a range of functional groups depending on the desired thermal, mechanical, and solution properties. Generally, the ring closure to form the cyclic carbonate requires the use of undesirable and hazardous reagents. Advances in synthetic methodologies and catalysis have enabled the use of carbon dioxide (CO₂) to perform these transformations with a high conversion of diol to cyclic carbonate, yet modest isolated yields due to oligomerization side reactions. In this Letter, we evaluate a series of bases in the presence of p-toluenesulfonyl chloride and the appropriate diol to better understand their effect on the yield and presence of oligomer byproducts during cyclic carbonate formation from CO₂. From this study, N,N-tetramethylethylenediamine (TMEDA) was identified as an optimal base, facilitating the preparation of a diverse array of both six- and eight-membered carbonates from CO₂ within 10 to 15 min. The robust conditions for both, the preparation of the diol precursor, and the TMEDA-mediated carbonate synthesis enabled readily telescoping the two-step reaction sequence, greatly simplifying the process of monomer preparation.

Towards high-performance sustainable polymers via isomerization-driven irreversible ring-opening polymerization of five-membered thionolactones

The development of sustainable polymers that possess useful material properties competitive with existing petroleum-derived polymers is a crucial goal but remains a formidable challenge for polymer science. Here we demonstrate that irreversible ring-opening polymerization (IROP) of biomass-derived five-membered thionolactones is an effective and robust strategy for the polymerization of non-strained five-membered rings-these polymerizations are commonly thermodynamically forbidden under ambient conditions, at industrially relevant temperatures of 80-100 °C. Computational studies reveal that the selective IROP of these thionolactones is thermodynamically driven by S/O isomerization during the ring-opening process. IROP of γ-thionobutyrolactone, a representative non-strained thionolactone, affords a sustainable polymer from renewable resources that possesses external-stimuli-triggered degradability. This poly(thiolactone) also exhibits high performance, with its key thermal and mechanical properties comparing well to those of commercial petroleum-based low-density polyethylene. This IROP strategy will enable conversion of five-membered lactones, generally unachievable by other polymerization methods, into sustainable polymers with a range of potential applications.