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Geminal disubstitution of cyclic monomers is an effective strategy to enhance the chemical recyclability of their polymers, but it is utilized for that purpose alone and often at the expense of performance properties. Here we present synergistic use of gem-α,α-disubstitution of available at-scale, bio-based δ-valerolactones to yield gem-dialkyl-substituted valerolactones ([Formula: see text]), which generate polymers that solve not only the poor chemical recyclability but also the low melting temperature and mechanical performance of the parent poly(δ-valerolactone); the gem-disubstituted polyesters ([Formula: see text]) therefore not only exhibit complete chemical recyclability but also thermal, mechanical and transport properties that rival or exceed those of polyethylene. Through a fundamental structure-property study that reveals intriguing impacts of the alkyl chain length on materials performance of [Formula: see text], this work establishes a simple circular, high-performance polyester platform based on [Formula: see text] and highlights the importance of synergistic utilization of gem-disubstitution for enhancing both chemical recyclability and materials performance of sustainable polyesters.

Aliphatic polyamides, or nylons, are typically highly crystalline and thermally robust polymers used in high-performance applications. Nylon 6, a high-ceiling-temperature (HCT) polyamide from ε-caprolactam, lacks expedient chemical recyclability, while low-ceiling temperature (LCT) nylon 4 from pyrrolidone exhibits complete chemical recyclability, but it is thermally unstable and not melt-processable. Here, we introduce a hybrid nylon, nylon 4/6, based on a bicyclic lactam composed of both HCT ε-caprolactam and LCT pyrrolidone motifs in a hybridized offspring structure. Hybrid nylon 4/6 overcomes trade-offs in (de)polymerizability and performance properties of the parent nylons, exhibiting both excellent polymerization and facile depolymerization characteristics. This stereoregular polyamide forms nanocrystalline domains, allowing optical clarity and high thermal stability, however, without displaying a melting transition before decomposition. Of a series of statistical copolymers comprising nylon 4/6 and nylon 4, a 50/50 copolymer achieves the greatest synergy in both reactivity and polymer properties of each homopolymer, offering an amorphous nylon with favorable properties, including optical clarity, a high glass transition temperature, melt processability, and full chemical recyclability.

Two well-known low-ceiling-temperature (LCT) monomers, γ-butyrolactone (γ-BL) toward ring-opening polymerization (ROP) to polyester and cyclohexene toward ring-opening metathesis polymerization (ROMP) to poly(cyclic olefin), are notoriously "nonpolymerizable". Here we present a strategy to render not only polymerizability of both the γ-BL and cyclohexene sites, orthogonally, but also complete and orthogonal depolymerization, through creating an LCT/LCT hybrid, bicyclic lactone/olefin (BiL

Summary This perspective discusses a set of design principles for next-generation kinetically trapped, intrinsically circular polymers (iCPs) that are inherently, selectively, and expediently depolymerizable to their monomer state once their kinetic barriers of deconstruction are overcome, thereby enabling not only the ideal shortest chemical circularity but also tunable performance properties. After describing four elements of the design principles—thermodynamics and kinetics, strategies to overcome trade-offs and unify conflicting properties, predictive modeling, and supply-chain life-cycle assessment and techno-economic analysis, which are illustrated with state-of-the-art examples—it concludes with presenting key challenges and opportunities for sustainable development of iCPs.

Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL), an acrylic polymer bearing a cyclic lactone ring, has attracted increasing interest because it not only is biorenewable but also exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). However, such property enhancement has been limited to resistance to heat and solvent, and mechanically both types of polymers are equally brittle. Here we report the expeditious synthesis of well-defined PMMBL-based ABA tri-block copolymers (tri-BCPs)—enabled by dual-initiating and living frustrated Lewis pairs (FLPs)—which are thermoplastic elastomers showing much superior mechanical properties, especially at high working temperatures (80–130 °C), to those of PMMA-based tri-BCPs. The FLPs consist of a bulky organoaluminum Lewis acid and a series of newly designed bis(imino)phosphine superbases bridged by an alkyl linker, which promote living polymerization of MMBL. Uniquely, such bisphosphine superbases initiate the chain growth from both P-sites concurrently, enabling the accelerated synthesis of tri-BCPs in a one-pot, two-step procedure. The results from mechanistic studies, including the single crystal structure of the dually initiated active species, detailed polymerizations, and kinetic studies confirm the livingness of the polymerization and support the proposed polymerization mechanism featuring the dual initiation and subsequent chain growth from both P-sites of the superbase di-initiator., Biobased poly(γ-methyl-α-methylene-γ-butyrolactone) (PMMBL) has attracted interest because it is biorenewable and exhibits superior properties to petroleum-based linear analog poly(methyl methacrylate) (PMMA). Here the authors report the synthesis of well-defined PMMBL-based ABA tri-block copolymers, enabled by dual-initiating and living frustrated Lewis pairs, which have superior mechanical properties compared to those of PMMA-based tri-BCPs.

Summary Intrinsically recyclable polymers represent a circular economy approach to address plastics problems. However, the design of such circular polymers is challenged by unyielding trade-offs between the monomer’s polymerizability and the polymer’s depolymerizability and performance properties. Here, we introduce a hybrid monomer design strategy that synergistically couples a high ceiling temperature (HCT) sub-structure for high polymerizability and performance properties with a low ceiling temperature (LCT) sub-structure for high depolymerizability and recyclability within the same monomer structure. Thus, structural hybridization between HCT e-caprolactone and LCT γ-butyrolactone led to an offspring [3.2.1]bicyclic lactone, which exhibits both high polymerizability and depolymerizability, otherwise conflicting properties in a typical monomer. The resulting polymer becomes a high-performance material, and thermal transition temperatures are ∼200°C higher and tensile modulus 10× higher than its parent polymers. These results demonstrate that the HCT/LCT hybrid monomer strategy is a powerful approach for designing circular polymers where conflicting properties must be exploited and unified.

Precision synthesis of cyclic polymers with predictable molecular weight and low dispersity is a challenging task, particularly concerning cyclic polar vinyl polymers through a rapid chain-growth mechanism and without high dilution. Harder yet is the precision synthesis of cyclic block copolymers (cBCPs), ideally from comonomer mixtures. Here we report that Lewis pair polymerization (LPP) capable of thermodynamically and kinetically compounded sequence control successfully addressed this longstanding challenge. Thus, LPP of acrylate/methacrylate mixtures under ambient temperature and normal concentration conditions rapidly and selectively affords well-defined cBCPs with high molecular weight (Mn = 247 kg/mol) and low dispersity (Đ = 1.04) in one step. Such cBCPs have been characterized by multiple techniques, including direct structural observation by imaging.

The ability to synthesize well-defined block copolymers (BCPs) from one-pot comonomer mixtures has powerful chemical and practical implications. However, controlling sequences between highly reactive, homologous comonomers such as acrylates during polymerization is challenging. Here we present a Lewis pair polymerization strategy that uniquely utilizes preferential Lewis acid coordination to differentiate between comonomers, distinctive kinetics, and compounded thermodynamic and kinetic differentiation to precisely control sequences and suppress tapering and misincorporation errors, thus achieving well-defined and resolved di- or tri-BCPs of acrylates.

Bacterial polyhydroxyalkanoates (PHAs) are a unique class of biodegradable polymers because of their biodegradability in ambient environments and structural diversity enabled by side-chain groups. However, the biosynthesis of PHAs is slow and expensive, limiting their broader applications as commodity plastics. To overcome such limitation, the catalyzed chemical synthesis of bacterial PHAs has been developed, using the metal-catalyzed stereoselective ring-opening (co)polymerization of racemic cyclic diolides (rac-8DLR , R=alkyl group). In this combined experimental and computational study, polymerization kinetics, stereocontrol, copolymerization characteristics, and the properties of the resulting PHAs have been examined. Most notably, stereoselective copolymerizations of rac-8DLMe with rac-8DLR (R=Et, Bu) have yielded high-molecular-weight, crystalline isotactic PHA copolymers that are hard, ductile, and tough plastics, and exhibit polyolefin-like thermal and mechanical properties.

In typical cyclic polymer synthesis via ring-closure, chain growth and cyclization events are competing with each other, thus affording cyclic polymers with uncontrolled molecular weight or ring size and high dispersity. Here we uncover a mechanism by which Lewis pair polymerization (LPP) operates on polar vinyl monomers that allows the control of where and when cyclization takes place, thereby achieving spatial and temporal control to afford precision cyclic vinyl polymers or block copolymers with predictable molecular weight and low dispersity (≈1.03). A combined experimental and theoretical study demonstrates that cyclization occurs only after all monomers have been consumed (when) via conjugate addition of the propagating chain end to the specific site of the initiating chain end (where), allowing the cyclic polymer formation steps to be regulated and executed with precision in space and time.

This work utilizes frustrated Lewis pairs consisting of tethered bis-organophosphorus superbases and a bulky organoaluminum to furnish the highly efficient synthesis of well-defined triblock copolymers via one-step block copolymerization of lignin-based syringyl methacrylate and n-butyl acrylate, through di-initiation and compounded sequence control. The resulting thermoplastic elastomers (TPEs) exhibit microphase separation and much superior mechanical properties (elongation at break up to 2091 %, tensile strength up to 11.5 MPa, and elastic recovery up to 95 % after 10 cycles) to those of methyl methacrylate-based TPEs. More impressively, lignin-based tri-BCPs can maintain TPEs properties up to 180 °C, exhibit high transparency and nearly 100 % UV shield, suggesting potential applications in temperature-resistant and optical devices.

Plastics have become indispensable in modern life and the material of choice in packaging applications, but they have also caused increasing plastic waste accumulation in oceans and landfills. Although there have been continuous efforts to develop biodegradable plastics, the mechanical and/or transport properties of these materials still need to be significantly improved to be suitable for replacing conventional plastic packaging materials. Here we report a class of biorenewable and degradable plastics, based on copolymers of γ-butyrolactone and its ring-fused derivative, with competitive permeability and elongation at break compared to commodity polymers and superior mechanical and transport properties to those of most promising biobased plastics. Importantly, these materials are designed with full chemical recyclability built into their performance with desired mechanical and barrier properties, thus representing a circular economy approach to plastic packaging materials., Biodegradable plastics usually have poorer mechanical or transport properties compared to conventional plastics. Here the authors show a class of biorenewable and chemically recyclable plastics based on copolymers of γ-butyrolactone and its ring-fused derivative, with competitive properties compared to conventional plastics.

Current practices in the generation and disposal of synthetic polymers are largely unsustainable, causing severe worldwide polymer pollution and enormous materials value loss. To address these dire environmental and economic issues, several research fronts aim to develop sustainable polymers with closed-loop life cycles.

Organocatalyzed cross-coupling of biomass platform chemicals furfural and 5-hydroxymethylfurfual has been utilized to prepare a pair of constitutional isomers of difuranic C11 diols. Polycondensation of the diols with three diacyl chlorides and polyaddition with two diisocyanates produce linear polyesters and polyurethanes, respectively, with furan rings residing in both the polymer backbone and pendent positions. One monomer isomer has a steric advantage, leading to higher molecular weight polymers. Thermal and mechanical properties of the resulting polymers can be tuned in a broad range by varying the monomer pairs. In addition, a thermally reversible cross-linked network has been realized by applying the Diels–Alder reaction between a bis-maleimide cross-linker and the furan rings located at both the backbone and pendent positions. This property, coupled with formation of a significant amount (up to 34%) of stable carbonaceous materials when heating the difuranic polymers to 700 °C, demonstrates some p...

Summary Plastics, used in countless consumer products that our daily lives depend on, have become indispensable materials essential for modern life and the global economy. At the same time, currently unsustainable practices in the production and disposal of plastics continue to deplete our finite natural resources and create severe worldwide environmental consequences. In the search for feasible solutions to these issues, significant recent advances have been made in developing chemically recyclable plastics, which allow for recovery of the building-block chemicals via depolymerization, for repolymerization to virgin-quality plastics, or for creative repurposing into value-added materials. Among such recyclable plastics, polyesters derived from renewable cyclic esters possess real potential to meet these challenges. Hence, this review highlights the plastics derived from common four-, five-, six-, seven-, and eight-membered cyclic esters by covering synthetic strategies, material properties, and, particularly, chemical recyclability. Such studies have culminated a recent discovery of infinitely recyclable plastics with properties of common plastics.

Construction of robust, stereocomplexed (sc) crystalline material, based on a recently discovered infinitely recyclable polymer system, requires blending of enantiomeric polymer chains produced from respective enantiopure, fused six-five bicyclic lactones. Herein, the stereoselective polymerization of the racemic monomer by yttrium catalysts bearing tetradentate ligands is reported, where the tethered donor sidearm switches the heteroselectivity of the catalyst to isoselectivity when it is changed from the β-OMe to β-NMe2 sidearm. The latter catalyst produces an isotactic stereoblock polymer (Pm up to 0.95) that forms the crystalline sc-material with a Tm of up to 171 °C. This sc-material can be fully depolymerized back to rac-monomer in a quantitative yield and purity, thus establishing its circular life cycle.

The upgrading of biomass furaldehydes—furfural (FF) and 5-alkyl (R) substituted furfurals (R = Me, MF; CH2OH, HMF; CH2OMe, MMF)—into C10–14 furoins was achieved by the selective umpolung self-coupl...

Crotonates, unlike their constitutional isomers of methacrylates, are not readily polymerized by conventional radical or anionic polymerization methods. A recent effort in polymerizing biorenewable methyl crotonate (MC) using organic N-heterocyclic carbene (NHC) initiators led to only dimerization. This contribution reports an effective polymerization of MC using Lewis pairs consisting of an NHC or N-heterocyclic olefin (NHO) Lewis base and a group 13 Lewis acid, in particular sterically encumbered methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), producing high-molecular-weight poly(MC) (PMC) with Mn up to 161 kg/mol under ambient temperature and solvent free conditions. Depending on the nature of the Lewis pair and reaction conditions, the polymerization proceeds either catalytically producing lower molecular weight PMC or noncatalytically leading to high-molecular-weight PMC. Investigations into initiation mechanisms have revealed both nucleophilic and basic pathways. The observation of th...

The development of chemically recyclable polymers promises a closed-loop approach towards a circular plastic economy but still faces challenges in structure/property diversity and depolymerization selectivity. Here we report the first successful coordination ring-opening polymerization of 4,5-trans-cyclohexyl-fused γ-butyrolactone (M1) with lanthanide catalysts at room temperature, producing P(M1) with Mn up to 89 kg mol-1 , high thermal stability, and a linear or cyclic topology. The same catalyst also catalyses selective depolymerization of P(M1) back to M1 exclusively at 120 °C. This coordination polymerization is also living, enabling the synthesis of well-defined block copolymer.

Bacterial poly(3-hydroxybutyrate) (P3HB) is a perfectly isotactic, crystalline material possessing properties suitable for substituting petroleum plastics, but high costs and low volumes of its production are impractical for commodity applications. The chemical synthesis of P3HB via ring-opening polymerization (ROP) of racemic β-butyrolactone has attracted intensive efforts since the 1960s, but not yet produced P3HB with high isotacticity and molecular weight. Here, we report a route utilizing racemic cyclic diolide (rac-DL) derived from bio-sourced succinate. With stereoselective racemic catalysts, the ROP of rac-DL under ambient conditions produces rapidly P3HB with perfect isotacticity ([mm] > 99%), high melting temperature (Tm = 171 °C), and high molecular weight (Mn = 1.54 × 105 g mol−1, Đ = 1.01). With enantiomeric catalysts, kinetic resolution polymerizations of rac-DL automatically stops at 50% conversion and yields enantiopure (R,R)-DL and (S,S)-DL with >99% e.e. and the corresponding poly[(S)-3HB] and poly[(R)-3HB] with high Tm = 175 °C., Bacterial poly(3-hydroxybutyrate) possesses physical and mechanical properties suitable for substituting high-performance petroleum plastics but current production is costly and slow. Here the authors produce poly(3-hydroxybutyrate) with similar properties via ring-opening polymerization of bio-derived racemic cyclic diolide.

The living polymerization of conjugated polar alkenes such as methacrylates by a noninteracting, authentic frustrated Lewis pair (FLP) has remained elusive ever since the report on FLP-promoted polymerization in 2010. Here we report that the polymerization of alkyl methacrylates by a FLP system based on a strongly nucleophilic N-heterocyclic olefin (NHO) Lewis base and sterically encumbered but modestly strong Lewis acid MeAl(4-Me-2,6-tBu2-C6H2O)2 is not only rapid but also living. This living polymerization was indicated by the formation of a linear, living chain, capped with NHO/H chain ends, without backbiting-derived cyclic chain ends. The true livingness of this FLP-promoted polymerization has been unequivocally verified by five lines of evidence, including the predicted polymer number-average molecular weight (Mn, up to 351 kg·mol–1) coupled with low dispersity (Đ = 1.05) values; obtained high to quantitative initiation efficiencies; an observed linear increase of polymer Mn vs monomer conversion an...

Summary Conventional thermosets, despite their technological significance in today’s materials economy, present a modern sustainability challenge because of their lack of end-of-life options for recyclability or reprocessability. Emerging covalent adaptable networks (CANs) offer sustainable alternatives to permanently crosslinked materials, but ideal orthogonal working/reprocessing conditions are hardly achievable by the current thermochemical activation mechanism. Here we report a CAN system of additive/catalyst-free, fully reprocessable, crosslinked, tri-block copolymer (tri-BCP) thermoplastic elastomer networks based on acid-anhydride bond exchange operated on a thermomechanical activation mechanism. The unique functionality of the tri-BCP architecture enables self-assembly into inter-linked, hexagonally packed cylinder nanostructures that preclude any productive inter-cylinder bond exchange (and, thus, creep) without cooperative thermal and mechanical (heating and compression) processing conditions.

A bisfuran dibromide has been established as the versatile and common intermediate for the high-yield synthesis of the three important classes of bisfuran monomers for furan-based renewable materials, bisfuran diacids, diols, and diamines. The general synthetic route involves a coupling reaction of 2-methylfuran with a ketone (acetone or cyclohexanone) under acidic conditions and a bromination reaction of the resulting bisfuran dimethyl compound to produce the bisfuran dibromide intermediate. This dibromide intermediate is subsequently converted to the corresponding bisfuran diacid (via oxidation reaction with KMnO4 under basic conditions), bisfuran diol (by hydrolysis reaction under mild basic conditions), and bisfuran diamine (through the Gabriel reaction). The versatility of the bisfuran dibromide intermediate and the effective transformation into the monomers with high to quantitative yield typically without the need for further purification highlight the two attractive features and potential for larg...

This contribution investigates the impact of rigid and flexible difuranic polyols on the resulting polyester (PE) and poly(ester-urethane) (PEU) properties. Three biobased difuranic polyol monomers, 5,5′-bihydroxymethyl furil (BHMF), 5,5′-dihydroxymethyl furoin (DHMF), and bis[5-(hydroxymethyl)furan-2-yl)methyl]adipate (BHFA), all derived from the biomass platform chemical 5-hydroxymethylfurfural (HMF), were employed for the synthesis of a series of new linear and cross-linked PEs as well as amorphous and semicrystalline PEUs. The polycondensations of diols (rigid BHMF and flexible BHFA) with various diacyl chlorides afford linear PEs, whereas the rigid triol (DHMF) reacts with diacyl chlorides to form cross-linked PEs. Among these difuranic PEs, the most intriguing PE is the one containing C═C double bonds, derived from BHFA and fumaryl chloride, which exhibits the unique self-curing ability via the Diels–Alder reaction. Furthermore, the catalyzed polyaddition of BHFA with various diiscyanates produces n...

α-Methylene-γ-butyrolactone (MBL), a naturally occurring and biomass-sourced bifunctional monomer, contains both a highly reactive exocyclic C═C bond and a highly stable five-membered γ-butyrolactone ring. Thus, all previous work led to exclusive vinyl-addition polymerization (VAP) product P(MBL)VAP. Now, this work reverses this conventional chemoselectivity to enable the first ring-opening polymerization (ROP) of MBL, thereby producing exclusively unsaturated polyester P(MBL)ROP with Mn up to 21.0 kg/mol. This elusive goal was achieved through uncovering the thermodynamic, catalytic, and processing conditions. A third reaction pathway has also been discovered, which is a crossover propagation between VAP and ROP processes, thus affording cross-linked polymer P(MBL)CLP. The formation of the three types of polymers, P(MBL)VAP, P(MBL)CLP, and P(MBL)ROP, can be readily controlled by adjusting the catalyst (La)/initiator (ROH) ratio, which is determined by the unique chemoselectivity of the La–X (X = OR, NR2,...

To synthesize the still missing C11 furoins as potential trifunctional difuranic building blocks for biopolymers and biofuels, the route based on the cross-coupling of two biofuraldehydes, furfural (FF) and 5-hydroxymethylfurfural (HMF), has been investigated using organic N-heterocyclic carbene (NHC) catalysts. This NHC-catalyzed coupling reaction in solution gives a statistical mixture of products including homocoupled C10 and C12 furoins 1 and 4, as well as cross-coupled C11 furoins 2 and 3, regardless of the electronics or sterics of the NHC precatalyst used, but a slight preference for cross-coupled products (∼60% 2 + 3 combined) under neat conditions has been achieved. Cross-coupled products 2 and 3 have been isolated in 48.1% yield and further separated into their pure state in 22.2% and 19.9% yield for 2 and 3, respectively. The isolated 2 and 3 have been fully characterized by NMR, HRMS, and single crystal X-ray diffraction. A simple metal-free/in-air oxidation reaction converts these two isomeri...

This contribution presents a full account of experimental and theoretical/computational investigations into the N-heterocyclic carbene (NHC)-catalyzed proton-transfer polymerization (HTP) that converts common dimethacrylates (DMAs) containing no protic groups into unsaturated polyesters. This new HTP proceeds through the step-growth propagation cycles via enamine intermediates, consisting of the proposed conjugate addition-proton transfer-NHC release fundamental steps. This study examines the monomer and catalyst scopes as well as the fundamental steps involved in the overall HTP mechanism. DMAs having six different types of linkages connecting the two methacrylates have been polymerized into the corresponding unsaturated polyesters. The most intriguing unsaturated polyester of the series is that based on the biomass-derived furfuryl dimethacrylate, which showed a unique self-curing ability. Four MeO- and Cl-substituted TPT (1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene) derivatives as methanol insertion products, (Rx)TPT(MeO/H) (R = MeO, Cl; x = 2, 3), and two free carbenes (catalysts), (OMe2)TPT and (OMe3)TPT, have been synthesized, while (OMe2)TPT(MeO/H) and (OMe2)TPT have also been structurally characterized. The structure/reactivity relationship study revealed that (OMe2)TPT, being both a strong nucleophile and a good leaving group, exhibits the highest HTP activity and also produced the polyester with the highest Mn, while the Cl-substituted TPT derivatives are least active and efficient. Computational studies have provided mechanistic insights into the tail-to-tail dimerization coupling step as a suitable model for the propagation cycle of the HTP. The extensive energy profile was mapped out, and the experimentally observed unicity of the TPT-based catalysts was satisfactorily explained with the thermodynamic formation of key spirocyclic species.

Six Na+/montmorillonite (MMT) – supported thiazolium (TM) ionic liquids (ILs), MMT–[TM], which are precatalysts to N-heterocyclic carbene (NHC) catalysts, have been synthesized and employed to catalyze the highly selective condensation of furfural (FF) and 5-hydroxymethylfurfural (HMF) into C10 and C12 furoins, respectively. Among them, five supported [TM] salts carrying the electron-donating group exhibit excellent catalytic activity. Thus, supported NHC catalysts, generated in situ from activation of MMT–[TM] with Et3N, afford C10 and C12 furoins in 97–99% and 97–98% yield, respectively. This condensation reaction is also carried out under solvent-free conditions, and the catalysts can be readily recycled, showing no noticeable loss in their catalytic activity over four consecutive recycles. Thus, these desired features possessed by the current catalyst system have enabled the development of the greener and more environmentally benign process for the upgrading of FF and HMF.

The development of new or more sustainable, active, efficient, controlled, and selective polymerization reactions or processes continues to be crucial for the synthesis of important polymers or materials with specific structures or functions. In this context, the newly emerged polymerization technique enabled by main-group Lewis pairs (LPs), termed as Lewis pair polymerization (LPP), exploits the synergy and cooperativity between the Lewis acid (LA) and Lewis base (LB) sites of LPs, which can be employed as frustrated Lewis pairs (FLPs), interacting LPs (ILPs), or classical Lewis adducts (CLAs), to effect cooperative monomer activation as well as chain initiation, propagation, termination, and transfer events. Through balancing the Lewis acidity, Lewis basicity, and steric effects of LPs, LPP has shown several unique advantages or intriguing opportunities compared to other polymerization techniques and demonstrated its broad polar monomer scope, high activity, control or livingness, and complete chemo- or regioselectivity, as well as its unique application in materials chemistry. These advances made in LPP are comprehensively reviewed, with the scope of monomers focusing on heteroatom-containing polar monomers, while the polymerizations mediated by main-group LAs and LBs separately that are most relevant to the LPP are also highlighted or updated. Examples of applying the principles of the LPP and LP chemistry as a new platform for advancing materials chemistry are highlighted, and currently unmet challenges in the field of the LPP, and thus the suggested corresponding future research directions, are also presented.

International audience; Reductive hydrosilylation of CO2 by catalysts based on environmentally benign metals such as zinc still presents an unmet challenge in terms of catalyst selectivity and activity. Here we show that tetra-coordinated neutral zinc siloxide complexes supported by the beta-diketiminato ligand bearing a pendant amine arm exhibit unprecedentedly high catalytic activity toward the selective reduction of CO2 by a hydrosilane to silyl formate. More significantly, this zinc catalyst system can also effectively catalyze the subsequent reduction of silyl formate into methoxysilane in quantitative yield. To understand the mechanism of the CO2 hydrosilylation and the influence of the side arm, a series of catalytic active species (or intermediates), zinc hydrides, and formates, has been synthesized and structurally characterized; the relevant stoichiometric reactions and kinetic study were performed. DFT calculations were also carried out to determine the reaction profiles for the CO2 hydrosilylation and to explain the relative activity of the zinc catalysts employed in this study. This combined experimental and theoretical study gives insights into the critical effects of the pendant amine donor arm and the size of the alkyl group of the amine on the relative energetics of the species involved in the proposed catalytic cycle.

Ring-opening polymerization (ROP) is a powerful synthetic methodology for the chemical synthesis of technologically important biodegradable aliphatic polyesters from cyclic esters or lactones. However, the bioderived five-membered γ-butyrolactone (γ-BL) is commonly referred as ‘non-polymerizable’ because of its low strain energy. The chemical synthesis of poly(γ-butyrolactone) (PγBL) through the ROP process has been realized only under ultrahigh pressure (20,000 atm, 160 °C) and only produces oligomers. Here we report that the ROP of γ-BL can, with a suitable catalyst, proceed smoothly to high conversions (90%) under ambient pressure to produce PγBL materials with a number-average molecular weight up to 30 kg mol–1 and with controlled linear and/or cyclic topologies. Remarkably, both linear and cyclic PγBLs can be recycled back into the monomer in quantitative yield by simply heating the bulk materials at 220 °C (linear polymer) or 300 °C (cyclic polymer) for one hour, which thereby demonstrates the complete recyclability of PγBL. Bio-derived γ-butyrolactone (γ-BL) is commonly referred to as ‘non-polymerizable’ due to its low strain energy. Now it has been shown that ring-opening polymerization of γ-BL can in fact proceed to high conversions under ambient pressure with a suitable catalyst, producing high-molecular-weight polymers with controlled topologies and complete recyclability.

Two highly efficient and recyclable heterogeneous azolium catalyst systems, one grafted (g) onto the inorganic oxide (Silica) and the other onto the organic polymer [Merrifield’s peptide or chloromethylated polystyrene (PS) resin], have been developed and employed to catalyze quantitative self-coupling (umpolung condensation) reactions of furfural and 5-hydroxymethylfurfural (HMF) into C10 and C12 furoins, respectively. Supported benzimidazolium ([BI]) salts bearing a long-chain alkyl substituent (i.e., C12 dodecyl) on the azolium nitrogen atom, upon activation with a suitable base to generate the corresponding N-heterocyclic carbene (NHC) catalyst, are found to be far more effective catalysts for furaldehyde self-coupling reactions than the analogous catalysts carrying a short-chain alkyl substituent (i.e., C1 methyl). Thus, supported NHC catalysts generated in situ from Silica-g-[BI]-C12 or PS-g-[BI]-C12-benzyl/base afford the C10 and C12 furoins in about 97% and 94% yield, respectively. By adopting a c...

B/N Lewis pairs have been discovered to catalyze rapid epimerization of meso-lactide (LA) or LA diastereomers quantitatively into rac-LA. The obtained rac-LA is kinetically polymerized into poly(L-lactide) and optically resolved D-LA, with a high stereoselectivity k(L)/k(D) of 53 and an ee of 91% at 50.6% monomer conversion, by newly designed bifunctional chiral catalyst 4 that incorporates three key elements (β-isocupreidine core, thiourea functionality, and chiral BINAM) into a single organic molecule. The epimerization and enantioselective polymerization can be coupled into a one-pot process for transforming meso-LA directly into poly(L-lactide) and D-LA.

This work has uncovered the first highly active and efficient Lewis pair polymerization (LPP) system based on N-heterocyclic carbene (NHC)/B(C6F5)3 pairs for converting acrylic monomers into medium- to high-molecular weight polymers. The study has systematically examined steric and electronic effects of three 1,3-dialkyl(Me, iPr, tBu)imidazol-2-ylidene NHCs on the LPP of three classes of acrylic monomers, including linear methyl methacrylate (MMA), cyclic biorenewable γ-methyl-α-methylene-γ-butyrolactone (γMMBL), and difunctional allyl methacrylate (AMA). For MMA polymerization, IiPr is not only the most active (∼3× and ∼120× more active than IMe and ItBu, respectively), but also the most effective NHC, especially under low NHC loading conditions. Kinetic results are consistent with a bimolecular, activated monomer propagation mechanism. In the case of the more reactive γMMBL, the polymerization by NHC/B(C6F5)3 in CH2Cl2 is extremely rapid, with all three NHCs achieving quantitative monomer conversion in 1 min and thus reaching a high turnover frequency of≥48,000 h−1. The molecular weight (MW) of PγMMBL can be tuned by adjusting the [γMMBL]/[NHC] ratio, and thus high MW polymers with relatively narrow MW distributions can be readily synthesized (e.g., from Mn=1.41×105 g mol−1, Đ=1.08 to Mn=4.89×105 g mol−1, Đ=1.20). The LPP by NHC/B(C6F5)3 is completely chemoselective, as demonstrated by the polymerization of AMA, which selectively polymerizes the conjugated vinyl group while leaving the non-conjugated vinyl group in the allyl moiety intact, thanks to its activated monomer propagation mechanism. The resulting PAMA is syndiotactic (rr=83 %), uncross-linked, and soluble in common solvents, thus suitable for further functionalization. This quantitatively chemoselective polymerization by NHC/B(C6F5)3 should provide a facile, yet powerful, approach to functional acrylic polymers.

Two poly(3-hexylthiophene) (P3HT) macromers containing a donor polymer with a polymerizable methacrylate (MA) end group, P3HT-CH2-MA and P3HT-(CH2)2-MA, have been synthesized, and P3HT-(CH2)2-MA has been successfully homopolymerized and copolymerized with methyl methacrylate (MMA) into stereoregular brush polymers and graft copolymers, respectively, using chiral ansa-zirconocene catalysts. Macromer P3HT-CH2-MA is too sterically hindered to polymerize by the current Zr catalysts, but macromer P3HT-(CH2)2-MA is readily polymerizable via either homopolymerization or copolymerization with MMA in a stereospecific fashion with both C2-ligated zirconocenium catalyst 1 and Cs-ligated zirconocenium catalyst 2. Thus, highly isotactic (with mm% ≥ 92%) and syndiotactic (with rr% ≥ 93%) brush polymers, it-PMA-g-P3HT and st-PMA-g-P3HT, as well as well-defined stereoregular graft copolymers with different grafted P3HT densities, it-P(M)MA-g-P3HT and st-P(M)MA-g-P3HT, have been synthesized using this controlled coordination-addition polymerization system under ambient conditions. These stereoregular brush polymers and graft copolymers exhibit both thermal (glass and melting) transitions with Tg and Tm values corresponding to transitions within the stereoregular P(M)MA and crystalline P3HT domains. Acceptor molecules such as C60 can be effectively encapsulated inside the helical cavity of st-P(M)MA-g-P3HT to form a unique supramolecular helical crystalline complex, thus offering a novel strategy to control the donor/acceptor solar cell domain morphology.

In the presence of a N-heterocyclic carbene (NHC) in THF, Br-substituted l-lactide (Br-LA) unexpectedly undergoes exclusive coupling with THF to form a chiral ω-bromo-α-keto-diester. This coupling reaction is completely selective (in a precise 1:1 fashion), readily scalable (>20 g scale), and extremely efficient (with only 50 ppm of NHC loading). Other cyclic ethers and carbonates can also undergo similar coupling with Br-LA, thus offering a class of Br-functionalized chiral diesters with various functions and chain lengths. Combined experimental and computational studies led to a coupling mechanism that proceeds through an anion (bromide)-mediated catalytic cycle, rather than an apparent NHC-catalyzed cycle.

A new polymerization termed proton (H)-transfer polymerization (HTP) has been developed to convert dimethacrylates to unsaturated polyesters. HTP is catalyzed by a selective N-heterocyclic carbene capable of promoting intermolecular Umpolung condensation through proton transfer and proceeds through the step-growth propagation cycles via enamine intermediates. The role of the added suitable phenol, which is critical for achieving an effective HTP, is twofold: shutting down the radically induced chain-growth addition polymerization under HTP conditions (typically at 80-120 °C) and facilitating proton transfer after each monomer enchainment. The resulting unsaturated polyesters have a high thermal stability and can be readily cross-linked to robust polyester materials.

Report herein is an integrated catalytic process for conversion and upgrading of biomass feedstocks into 5,5′-dihydroxymethyl furoin (DHMF), through self-coupling of 5-hydroxymethyl furfural (HMF) via organocatalysis, and subsequently into n-C12H26 alkane fuel via metal–acid tandem catalysis. The first step of the process involves semicontinuous organocatalytic conversion of biomass (fructose, in particular) to the high-purity HMF. N-Heterocyclic carbenes (NHCs) are found to catalyze glucose-to-fructose isomerization, and the relatively inexpensive thiazolium chloride [TM]Cl, a Vitamin B1 analog, catalyzes fructose dehydration to HMF of good purity (>99% by HPLC), achieving a constant HMF yield of 72% over 10 semicontinuous extraction batch runs. Crystallization of the crude HMF from toluene yields the spectroscopically and analytically pure HMF as needle crystals. The second step of the process is the NHC-catalyzed coupling of C6 HMF produced by the semicontinuous process to C12 DHMF; the most effective ...

The first highly active phosphine (P)/borane (B) Lewis pair polymerization is promoted unexpectedly by P-B adducts. The P and B site cooperativity is essential for achieving effective polymerization, as shown by this study examining the reactivity of a library of P/B Lewis pairs toward polymerization of a renewable acrylic monomer.

Kinetic hydrate inhibitors (KHIs) are used in the oil industry to prevent plugging of pipelines by gas hydrates. Cationic molecules with small alkyl groups, such as tetraalkylammonium and alkylimidazolinium salts are known to be synergists for KHI polymers such as N-vinyl lactam polymers. Instead of using separate synergist and polymer molecules, we have investigated the effect on KHI performance of placing the synergist active groups into N-vinyl lactam copolymers. Thus, 3-alkyl-1-vinylimidazolium bromides and tributylammoniumethylacrylate bromide (TBAEABr) monomers and homopolymers and copolymers with N-vinyl lactams were prepared. These were tested for their performance as KHIs with a Structure II-forming gas mixture as well as their ability to prevent tetrahydrofuran hydrate crystal growth inhibition. We conclude that incorporation of the synergist functional groups into N-vinyl lactam polymers does not give a greater benefit as a KHI than using a physical mixture of the synergist molecule and PVCap polymer. However, a cationic TBAEABr homopolymer showed good kinetic inhibition performance when used alone on both THF and SII gas hydrates. This is the first cationic homopolymer KHI with good performance to be reported. Poly(tributylammonium ethylacrylate bromide) (PTBAEABr) was also a better KHI synergist with PVCap than using tetrabutylammonium bromide.

The first effective organopolymerization of the biorenewable "non-polymerizable" γ-butyrolactone (γ-BL) to a high-molecular-weight metal-free recyclable polyester is reported. The superbase tert-Bu-P4 is found to directly initiate this polymerization through deprotonation of γ-BL to generate reactive enolate species. When combined with a suitable alcohol, the tert-Bu-P4 -based system rapidly converts γ-BL into polyesters with high monomer conversions (up to 90 %), high molecular weights (Mn up to 26.7 kg mol(-1) ), and complete recyclability (quantitative γ-BL recovery).

This contribution reports the first example of highly selective reduction of CO2 into CH4 via tandem hydrosilylation with mixed main-group organo-Lewis acid (LA) catalysts [Al(C6F5)3 + B(C6F5)3] {[Al] + [B]}. As shown by this comprehensive experimental and computational study, in this unique tandem catalytic process, [Al] effectively mediates the first step of the overall reduction cycle, namely the fixation of CO2 into HCOOSiEt3 (1) via the LA-mediated C═O activation, while [B] is incapable of promoting the same transformation. On the other hand, [B] is shown to be an excellent catalyst for the subsequent reduction steps 2-4, namely the hydrosilylation of the more basic intermediates [1 to H2C(OSiEt3)2 (2) to H3COSiEt3 (3) and finally to CH4] through the frustrated Lewis pair (FLP)-type Si-H activation. Hence, with the required combination of [Al] and [B], a highly selective hydrosilylative reduction of CO2 system has been developed, achieving high CH4 production yield up to 94%. The remarkably different catalytic behaviors between [Al] and [B] are attributed to the higher overall Lewis acidity of [Al] derived from two conflicting factors (electronic and steric effects), which renders the higher tendency of [Al] to form stable [Al]-substrate (intermediate) adducts with CO2 as well as subsequent intermediates 1, 2, and 3. Overall, the roles of [Al] and [B] are not only complementary but also synergistic in the total reduction of CO2, which render both [Al]-mediated first reduction step and [B]-mediated subsequent steps catalytic.

The successful synthesis of highly syndiotactic polar vinyl polymers bearing the reactive pendant vinyl group on each repeat unit, which is enabled by perfectly chemoselective and highly syndiospecific coordination polymerization of divinyl polar monomers developed through this work, has allowed the construction of robust cross-linked supramolecular stereocomplexes and C60 inclusion complexes. The metal-mediated coordination polymerization of three representative polar divinyl monomers, including vinyl methacrylate (VMA), allyl methacrylate (AMA), and N,N-diallyl acrylamide (DAA) by Cs-ligated zirconocenium ester enolate catalysts under ambient conditions exhibits complete chemoselectivity and high stereoselectivity, thus producing the corresponding vinyl-functionalized polymers with high (92% rr) to quantitative (99% rr) syndiotacticity. A combined experimental (synthetic, kinetic, and mechanistic) and theoretical (DFT) investigation has yielded a unimetallic, enantiomorphic-site-controlled propagation mechanism. Postfunctionalization of the obtained syndiotactic vinyl-functionalized polymers via the thiol-ene click and photocuring reactions readily produced the corresponding thiolated polymers and flexible cross-linked thin-film materials, respectively. Complexation of such syndiotactic vinyl-functionalized polymers with isotactic poly(methyl methacrylate) and fullerene C60 generates supramolecular crystalline helical stereocomplexes and inclusion complexes, respectively. Cross-linking of such complexes affords robust cross-linked stereocomplexes that are solvent-resistant and also exhibit considerably enhanced thermal and mechanical properties compared with the un-cross-linked stereocomplexes.

[EN] This contribution describes the development and demonstration of the ambient-temperature, high-speed living polymerization of polar vinyl monomers (M) with a low silylium catalyst loading (90%) to quantitative efficiencies and a high degree of control over M and MWD (1.07-1.12). An intriguing catalyst "self-repair" feature has also been demonstrated for the current living polymerization system, The work carried out at CSU was supported by the National Science Foundation (NSF-0848845). We thank Boulder Scientific Co. for the research gifts of B(C6F5)3, [Ph3C]+[B(C6F5)4] , and [HN(Me2)Ph]+[B- (C6F5)4]

This contribution reports the first examples of stereoregular (isotactic, it-, and syndiotactic, st-) methacrylate-POSS (polyhedral oligomeric silsesquioxane) hybrid polymers and their derived nanostructured assemblies. The polymerization of methyl methacrylate (MMA) by isospecific and syndiospecific living metallocene catalysts, when end-capped with methacrylisobutyl POSS (MA-POSS) or simultaneously copolymerized with MA-POSS at ambient temperature, readily produces highly stereoregular (94% it- and st-) MA-POSS end-capped PMMA (PMMA-POSS) or statistical copolymers PMMA-co-P(MA-POSS). The MA-POSS incorporation in the it-copolymers ranges from a low 2.6 mol % (20 wt %) to a high, maximum 24 mol % (75 wt %), whereas the incorporation in the st-copolymers is relatively lower with the same comonomer feed ratio due to the formation of a crystalline inclusion complex in which the POSS nanocages are encapsulated within the helical st-PMMA cavity. The it-copolymers with high POSS contents (>20 mol %) show eviden...