Highly Active Organic Lewis Pairs for the Copolymerization of Epoxides with Cyclic Anhydrides: Metal-Free Access to Well-Defined Aliphatic Polyesters

Choline Halide-Based Deep Eutectic Solvents as Biocompatible Catalysts for the Alternating Copolymerization of Epoxides and Cyclic Anhydrides

Aliphatic polyesters have received considerable attention in recent years due to their biodegradability and biocompatible, mechanical, and thermal properties that can make them a suitable alternative to today's commercialized polymers. The ring-opening copolymerization (ROCOP) of epoxides and cyclic anhydrides is a route to synthesize a diverse array of polyesters that could be useful in many applications. However, the catalysts used rarely consider biocompatible catalysts in the case that any are left in the polymer. To the best of our knowledge, we report the first example of using deep eutectic solvents (DESs) as biocompatible catalysts for this target ROCOP with polymerization activity for at least six diverse monomer pairs. Choline halide salts are active for this polymerization, with dried salts showing polymerization slower than that of those conducted in air. Hydrogen bonding with water is hypothesized to enhance the rate-determining step of epoxide ring opening. While the presence of water improves the rate of polymerization, it also acts as a chain transfer agent, leading to smaller molar mass polymers than intended. Combining the choline halide salts with urea or ethylene glycol hydrogen bond donors in air led to DES catalysts that reacted similarly to the salts exposed to air. However, when generating these DESs in air-free conditions, they showed similar rates of polymerization without a drop in polymer molar mass. The hydrogen bonding provided by urea and ethylene glycol seems to promote the rate increase without serving as a chain transfer agent. Results reported herein display the promising potential of biocompatible catalyst systems for this ROCOP process as well as introducing the use of hydrogen bonding to enhance polymerization rates.

Phosphine-Capped Effects Enable Full-Color Clusteroluminescence in Nonconjugated Polyesters

Full-color luminophores have advanced applications in materials and engineering, but constructing color-tunable clusteroluminescence (CL) from nonconjugated polymers based on through-space interactions remains a huge challenge. Herein, we develop phosphine-capped nonconjugated polyesters exhibiting blue-to-red CL (400-700 nm) based on phosphine-initiated copolymerization of epoxides and cyclic anhydrides, especially P1-0.5TPP, which exhibits red CL (610 nm) with a high quantum yield of 32%. Experiments and theoretical calculations disclose that the phosphine-capped effect in polyesters brings about conformational changes and induces phosphine-ester clusters by through-space (n,π*) interactions. Moreover, CL colors and efficiencies can be easily tailored by types of phosphines, compositions and structures of polyesters, and concentration. Significantly, the role of polymer motions (group, segmental, and chain motions) on CL originating from microregions inside polyesters is revealed. Further, phosphine-capped nonconjugated polyesters are demonstrated to be nonconjugated dyes and fluorescent fibers and are also used for multicolor light-emitting diodes including white light. This work not only provides an engineering strategy based on the end-group effect to prepare full-color clusteroluminogens but also broadens the prospects for material applications.

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.

Mechanistic Insight Into the Reactivity of Frustrated Lewis Pairs: Liquid-State NMR Studies

Frustrated Lewis pairs (FLPs) have been widely investigated as promising catalysts due to their metal-free feature and ability to activate small molecules. Over the last few years, the structure, dynamics and interactions between the Lewis centers and their effects on the reactivity with different substrates have been studied. Nuclear magnetic resonance (NMR) is a powerful tool in studying the reaction intermediates, kinetics and mechanism of frustrated Lewis pairs (FLPs). Various NMR experiments have been applied to precisely determine the association or cooperativity of FLPs and one or two-dimensional spectra were obtained. Herein, insights coming from NMR spectroscopy for FLPs are presented, the structure and reactivity of FLPs in solution are described, and their effects on the kinetics and mechanism of different substrates are also illustrated in this review.

Creating Remarkably Moisture- and Air-Stable Macromolecular Lewis Acid by Integrating Borane within the Polymer Chain: A Highly Active Catalyst for Homo(co)polymerization of Epoxides

Borane-based Lewis acids (LA) play an indispensable role in the Lewis pair (LP) mediated polymerization. However, most borane-based LPs are moisture- and air-sensitive. Therefore, development of moisture and air-stable borane-based LP is highly desirable. To achieve this goal, the concept of "aggregation induced enlargement effects" by chemically linking multiple borane within a nanoscopic confinement was conceived to create macromolecular LA. Accordingly, an extremely moisture and air stable macromolecular borane, namely, PVP-1B featuring poly(4-vinylphenol) backbone, was constructed. The concentration of borane active site is greatly higher than average concentration due to local confinement. Therefore, an enhanced activity was observed. Moreover, the local LA aggregation effects allow its tolerance to air and large amount of chain transfer agent. Consequently, PVP-1B showed remarkable efficiency for propylene oxide (PO) polymerization at 25 °C (TOF=27900 h-1 ). Furthermore, it enables generation of well-defined telechelic poly (CHO-alt-CO2 ) diol (0.6-15.3 kg/mol) with narrow Đs via copolymerizing cyclohexene oxide and CO2 at 80 °C. This work indicates unifying multiple borane within a polymer in a macromolecular level shows superior catalytic performance than constructing binary, bi(multi)functional systems in a molecular level. This paves a new way to make functional polyethers.

Multi-Functional Organofluoride Catalysts for Polyesters Production and Upcycling Degradation

The production and degradation of polyesters are two crucial processes in polyester materials' life cycle. In this work, multi-functional organocatalysts based on fluorides for both processes are described. Organofluorides were developed as catalysts for ring-opening polymerization of lactide (lactone). Compared with a series of organohalides, organofluoride performed the best catalytic reactivity because of the hydrogen bond interaction between F- and alcohol initiator. The Mn values of polyester products could be up to 72 kg mol-1 . With organofluoride catalysts, the ring-opening copolymerization between various anhydrides and epoxides could be established. Furthermore, terpolymerization of anhydride, epoxide, and lactide could be constructed by the self-switchable organofluoride catalyst to yield a block polymer with a strictly controlled polymerization sequence. Organofluorides were also efficient catalysts for upcycling polyester plastic wastes via alcoholysis. Mixed polyester materials could also be hierarchically recycled.

Enabling nonconjugated polyesters emit full-spectrum fluorescence from blue to near-infrared

Near-infrared luminophores have many advantages in advanced applications, especially for structures without π-conjugation aromatic rings. However, the fabrication of red clusteroluminogens from nonconjugated polymers is still a big challenge, let alone the near-infrared clusteroluminogens. Here, we develop nonconjugated luminophores with full-spectrum from blue to near-infrared light (470 ~ 780 nm), based on color phenomenon of nonconjugated polyesters synthesized from the amine-initiated copolymerization of epoxides and cyclic anhydrides. We reveal that amines act as initiators attached to polymer chain ends. The formation of various amine-ester complexes in polyesters induces red to near-infrared light, conceptually, amine-ester complexed clusteroluminescence via intra/inter-chain charge transfer. Significantly, emission colors can be easily tuned by the contents and types of amines, microstructures of polyesters, and their concentration. This work provides a low-cost, scalable platform and strategy for the production of high-efficiency, multicolor luminescent materials.

Ketyl Radical Anion Mediated Radical Polymerization and Anionic Ring-Opening Polymerization to Give Polymers with Low Molecular Weight Distribution

The development of novel polymerization capable of yielding polymers with low molecular weight distribution (Đ) is essential and significant in polymer chemistry, where monofunctional initiator contains only one initiation site in these polymerizations generally. Here, ketyl radical anion species is introduced to develop a novel Ketyl Mediated Polymerization (KMP), which enables radical polymerization at carbon radical site and anionic ring-opening polymerization at oxygen anion site, respectively. Meanwhile, polymerization and corresponding organic synthesis generally couldn't be performed simultaneously in one pot. Through KMP, organic synthesis and polymerization are achieved in one pot, where small molecules (cyclopentane derivates) and polymers with low Đ are successfully prepared under mild condition simultaneously. At the initiation step, both organic synthesis and polymerization are initiated by single electron transfer reaction with ketyl radical anion formation. Cyclopentane derivates are synthesized through 3-3 coupling reaction and cyclization. Polystyrene and polycaprolactone with low Đ and a full monomer conversion are prepared by KMP via radical polymerization and anionic ring-opening polymerization, respectively. This work therefore enables both organic synthesis and two different polymerizations from same initiation system, which saves time, labour, resource and energy and expands the reaction mode and method libraries of organic chemistry and polymer chemistry.

Elucidation of the Alternating Copolymerization Mechanism of Epoxides or Aziridines with Cyclic Anhydrides in the Presence of Halide Salts

Organic halide salts in combination with metal or organic compound are the most common and essential catalysts in ring-opening copolymerizations (ROCOP). However, the role of organic halide salts was neglected. Here, we have uncovered the complex behavior of organic halides in ROCOP of epoxides or aziridine with cyclic anhydride. Coordination of the chain-ends to cations, electron-withdrawing effect, leaving ability of halide atoms, chain-end basicity/nucleophilicity, and terminal steric hindrance cause three types of side reactions: single-site transesterification, substitution, and elimination. Understanding the complex functions of organic halide salts in ROCOP led us to develop highly active and selective aminocyclopropenium chlorides as catalysts/initiators. Adjustable H-bonding interactions of aminocyclopropenium with propagating anions and epoxides create chain-end coordination process that generate highly reactive carboxylate and highly selective alkoxide chain-ends.

Alkali metal carbonate catalyzed copolymerization of anhydrides and epoxides: a simple, efficient and versatile approach to well-defined alternating polyesters

Readily available and economical alkali metal carbonates have been utilized to catalyze the ROAC of PA and various epoxides to afford polyesters with perfectly alternating sequence distribution, controlled molar masses, and moderate dispersity.

Aliphatic Polyesters with White-Light Clusteroluminescence

Single-molecule white-light emission (SMWLE) has many advantages in practical applications; however, the fabrication of SMWLE from nonconjugated luminescent polymers, namely, clusteroluminogens (CLgens), is still a big challenge. Herein, the first example of linear nonconjugated polyesters with SMWLE is reported. Twenty-four kinds of nonconjugated aliphatic polyesters with tunable clusteroluminescence (CL) colors and efficiency were synthesized by the copolymerization of six epoxides and four anhydrides. Experimental and calculation results prove that, at the primary structure level, the balance of structural flexibility and rigidity via adjusting the side-chain length significantly enhances the efficiency of CL without wavelength change. However, altering the chemical structures of the monomer from succinic anhydride to trans-maleic anhydride (MA), cis-MA, and citraconic anhydride (CA), secondary structures of these polyesters change from helix to straight and folding sheet accompanied by gradually red-shifted CL from 460 to 570 nm due to the increase in through-space n-π* interactions, as demonstrated by the computational and experimental results. Then, pure SMWLE with CIE coordination (0.30, 0.32) based on overlapped short-wavelength and long-wavelength CL is achieved in CA-based polyesters. This work not only provides further insights into the emission mechanism of CL but also provides a new strategy to manipulate the properties of CL by regulating the hierarchical structures of CLgens.

A Facile Fabrication of Lysosome-Targeting pH Fluorescent Nanosensor Based on PEGylated Polyester Block Copolymer

A novel lysosome-targeting PEGylated polyester-based fluorescent pH nanosensor is fabricated by the combination of ring-opening copolymerization (ROCOP), side-group modification and subsequent self-assembly. First, a key target amphiphilic copolymer carrier for rhodamine (Rh) pH indicator is synthesized in a facile manner by the ROCOP of phthalic anhydride with allyl glycidyl ether using mPEG-OH and t-BuP₁/Et₃B as the macroinitiator and binary catalyst, respectively. Subsequently, Rh moieties are covalently attached on the polymer chain with controllable grafting degree via an efficient thiol-ene click reaction. Concurrently, the effect of catalyst systems and reaction conditions on the catalytic copolymerization performance is presented, and the quantitative introduction of Rh is described in detail. Owing to its amphiphilic characteristics, the rhodamine-functionalized polyester-based block copolymer can self-assemble into micelles. With the covalent incorporation of Rh moieties, the as-formed micelles exhibit excellent absorption and fluorescence-responsive sensitivity and selectivity towards H⁺ in the presence of various metal cations. Moreover, the as-prepared micelles with favorable water dispersibility, good pH sensitivity and excellent biocompatibility also display appreciable cell-membrane permeability, staining ability and pH detection capability for lysosomes in living cells. This work provides a new strategy for the facile synthesis of novel biocompatible polymeric fluorescent pH nanosensors for the fluorescence imaging of lysosomal pH changes.

Switchable Polymerization Organocatalysis: From Monomer Mixtures to Block Copolymers

One-pot production of sequence-controlled block copolymer from mixed monomers is a crucial but rarely reached goal. Using a switchable Lewis-pair organocatalyst, we have accomplished sequence-selective polymerization from a mixture of O-carboxyanhydride (OCA) and epoxide. Polymerization of the OCA monomer occurs first and exclusively because of its exceedingly high polymerizability. When OCA is fully consumed, alternating copolymerization of epoxide and CO₂ liberated in OCA polymerization is triggered from the termini of the first block. The two polymerizations thus occur in tandem, both in chemoselective fashion, so that a sequence-controlled block polymer with up to 99 % CO₂ conversion is furnished in this one-pot protocol. Calculations and experimental results demonstrate a chemoselective and cooperative mechanism, where the high polymerizability of the OCA monomers guarantees exquisite sequence selectivity and the cooperative decarboxylation partly arose from the stabilization effect by triethylborane, which facilitates the smooth transformation of the chain end from carbonate to alkoxide.

Using Triethylborane to Manipulate Reactivity Ratios in Epoxide-Anhydride Copolymerization: Application to the Synthesis of Polyethers with Degradable Ester Functions

The anionic ring-opening copolymerization (ROCOP) of epoxides, namely of ethylene oxide (EO), with anhydrides (AH) generally produces strictly alternating copolymers. With triethylborane (TEB)-assisted ROCOP of EO with AH, statistical copolymers of high molar mass including ether and ester units could be obtained. In the presence of TEB, the reactivity ratio of EO (rEO), which is normally equal to 0 in its absence, could be progressively raised to values lower than 1 or higher than 1. Conditions were even found to obtain rEO equal or close to 1. Samples of P(EO-co-ester) with minimal compositional drift could be synthesized; upon basic degradation of their ester linkages, these samples afforded poly(ethylene oxide) (PEO) diol samples of narrow molar mass distribution. In other cases where rEO were lower or higher than 1, the PEO diol samples eventually isolated after degradation exhibited a broader distribution of molar masses because of the compositional drift of initial P(EO-co-ester) samples.

Iodine and alkali metal alkoxides: a simple and versatile catalytic system for fully alternating polyester synthesis from phthalic anhydride and epoxides

Regioselective ROCOP of various epoxides with phthalic anhydride using readily available and economical catalysts i.e. I2 in combination with alkali metal alkoxides was used.

Selective ring-opening polymerization of glycidyl ester: a versatile synthetic platform for glycerol-based (co)polyethers

Linear polyglycerol is highly valued for its excellent hydrophilicity and biocompatibility as well as its multihydroxy nature. We report here a convenient route for controlled synthesis of polyglycerol through ring-opening...

Construction of triblock copolyesters via one-step switchable terpolymerization of epoxides, phthalic anhydride and ε-caprolactone using dual urea/organic base catalysts

A cost-effective and efficient system of ureas/organic bases toward the controlled self-switchable copolymerization of epoxide/PA/CL to obtain well-defined triblock polyesters.

From Impossible to Possible: Atom-Economic Polymerization of Low Strain Five-Membered Carbonates

The direct ring-opening polymerization (ROP) of propylene carbonate (PC) only affords oligomers with substantial unidentified by-products, which hinders the efficient utilization of PC. Through detailed studies, for the first time, a careful mechanism involving the in situ release of propylene oxide (PO) from PC decarboxylation is proposed. Further, we report a novel strategy of copolymerization of PC/cyclic anhydrides via in situ capture of the formed intermediates. Results show that PC is successfully transformed into polyesters. Especially for the ring-opening alternating copolymerization (ROAC) of PC/phthalic anhydride (PA), a variety of advantages are manifold: i) slow-release of PO ensuring a perfectly alternating structure; ii) quantitative and fast transformation of PC; iii) visualization of polymerization process by a CO₂ pressure gauge. Of importance, through tandem polymerizations, PC is fully transformed into polyesters and polycarbonates concurrently, thus achieving PC utilization with a high atom-economy.

Altering Chain Flexibility of Aliphatic Polyesters for Yellow-Green Clusteroluminescence in 38 % Quantum Yield

Preparation of non-conjugated polymers with long-wavelength emission and high quantum yield (QY) is still a huge challenge. Herein, we report the first example of linear non-conjugated polyester exhibiting yellow-green clusteroluminescence (CL) and a high QY of 38 %. We discovered that the polyester P3 with balanced flexibility and rigidity showed the longest CL wavelength and highest QY. Systematically photophysical characterization unravel the key role of ester cluster in the CL and the cluster formation via the aggregate of ester units was visualized. Moreover, P3 was demonstrated to be a highly selective, quick-responsive (ca. 1.2 min) and sensitive detector (detection limit is 0.78 μM) for irons owing to the fast disassociation of clusters by irons. This work not only gains further mechanistic insight into CL but also provides a new strategy to design high-efficiency and long-wavelength CL, meanwhile, enlightens the glorious application prospect of luminescent polyester.

Record Productivity and Unprecedented Molecular Weight for Ring-Opening Copolymerization of Epoxides and Cyclic Anhydrides Enabled by Organoboron Catalysts

Producing polyesters with high molecular weight (M_n ) through ring-opening copolymerization (ROCOP) of epoxides with cyclic anhydrides remains a major challenge. Herein, we communicate a metal-free, highly active, and high thermoresistance system for the ROCOP of epoxides with cyclic anhydrides to prepare polyesters (13 examples). The organoboron catalysts can endure a reaction temperature as high as 180 °C for the ROCOP of cyclohexane oxide (CHO) with phthalic anhydride (PA) without the observation of any side reactions. The average M_n of the produced poly(CHO-alt-PA) climbed to 94.5 kDa with low polydispersity (Ð=1.19). Furthermore, an unprecedented turnover number of 9900, equivalent to an efficiency of 7.4 kg of polyester/g of catalyst, was achieved at a feed ratio of CHO/PA/catalyst=20000:10000:1 at 150 °C. Kinetic studies, crystal structure analysis, ¹¹ B NMR spectra, and DFT calculations provided mechanistic justification for the effectiveness of the catalyst system.

Recent trends in biodegradable polyester nanomaterials for cancer therapy

Biodegradable polyester nanomaterials-based drug delivery vehicles (DDVs) have been largely used in most of the cancer treatments due to its high biological performance and wider applications. In several previous studies, various biodegradable and biocompatible polyester backbones were used which are poly(lactic acid) (PLA), poly(ε-caprolactone) (PCL), poly(propylene fumarate) (PPF), poly(lactic-co-glycolic acid) (PLGA), poly(propylene carbonate) (PPC), polyhydroxyalkanoates (PHA), and poly(butylene succinate) (PBS). These polyesters were fabricated into therapeutic nanoparticles that carry drug molecules to the target site during the cancer disease treatment. In this review, we elaborately discussed the chemical synthesis of different synthetic polyesters and their use as nanodrug carriers (NCs) in cancer treatment. Further, we highlighted in brief the recent developments of metal-free semi-aromatic polyester nanomaterials along with its role as cancer drug delivery vehicles.