Lewis-Pair-Mediated Selective Dimerization and Polymerization of Lignocellulose-Based β-Angelica Lactone into Biofuel and Acrylic Bioplastic

Enabling Closed-Loop Circularity of "Non-Polymerizable" α, β-Conjugated Lactone Towards High-Performance Polyester with the Assistance of Cyclopentadiene

Chemical recycling of polymers to monomers presents a promising solution to the escalating crisis associated with plastic waste. Despite considerable progress made in this field, the primary efforts have been focused on redesigning new monomers to produce readily recyclable polymers. In contrast, limited research into the potential of seemingly "non-polymerizable" monomers has been conducted. Herein, we propose a paradigm that leverages a "chaperone"-assisted strategy to establish closed-loop circularity for a "non-polymerizable" α, β-conjugated lactone, 5,6-dihydro-2H-pyran-2-one (DPO). The resulting PDPO, a structural analogue of poly(δ-valerolactone) (PVL), exhibits enhanced thermal properties with a melting point (Tm) of 114 °C and a decomposition temperature (Td,5%) of 305 °C. Notably, owing to the structural similarity between DPO and δ-VL, the copolymerization generates semi-crystalline P(DPO-co-VL)s irrespective of the DPO incorporation ratio. Intriguingly, the inherent C=C bonds in P(DPO-co-VL)s enable their convenient post-functionalization via Michael-addition reaction. Lastly, PDPO was demonstrated to be chemically recyclable via ring-closing metathesis (RCM), representing a significant step towards the pursuit of enabling the closed-loop circularity of "non-polymerizable" lactones without altering the ultimate polymer structure.

Electron-Poor Butenolides: The Missing Link between Acrylates and Maleic Anhydride in Radical Polymerization

Butenolides are a class of 5-membered lactones that hold great potential as bio-based monomers to replace oil-derived acrylates, of which they are cyclic analogues. Despite this structural resemblance, the reactivity of the unsaturated ester moiety of electron-poor butenolides leans toward that of maleic anhydride, another essential monomer that does not homopolymerize but copolymerizes in a highly alternating fashion with polarized electron-rich comonomers. By studying the reactivity of 5-methoxy and 5-acyloxy butenolides through a combination of kinetics and density functional theory (DFT) experiments, we explain why electron-poor butenolides constitute a missing link between acrylates and maleic anhydride in radical polymerization.

Catenane formation of a cyclic poly(alkyl sorbate) via chain-growth polymerization induced by an N-heterocyclic carbene and ring-closing without extreme dilution

1,3-Di-tert-butylimidazol-2-ylidene (NHCtBu), a typical N-heterocyclic carbene (NHC), was previously found to induce the anionic chain-growth polymerization of ethyl sorbate (ES) in the presence of an aluminum Lewis acid, i.e., methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (MAD), in which the neighboring of α-terminal dienolate with a propagating anion induced cyclization without highly diluted conditions, after monomer depletion, to give the cyclic poly(ES). In this paper, we report that catenane formation occurs by two-step polymerization of ethyl sorbate (ES), in which, after complete monomer (ES) consumption ([ES]₀/[NHCtBu]₀ = 100/1) in toluene followed by purification by reprecipitation, a second addition of ES monomer ([ES]₀/[ NHCtBu]₀ = 20/1) in another pot (in toluene or tetrahydrofuran (THF)) resulted in catenane formation, namely a polycatenane. TEM images of a sample from the second step polymerization in THF revealed particles of polycatenane structure consisting of cyclic poly(ES) with sizes ranging from 200 to 1000 nm, showing that this NHCtBu triggered chain polymerization and successive cyclization without highly diluted conditions enabled us to fabricate the intended polycatenane in the successive two-step polymerization.

Converting Non-strained γ-Valerolactone and Derivatives into Sustainable Polythioesters via Isomerization-driven Cationic Ring-Opening Polymerization of Thionolactone Intermediate

This contribution reports the efficient conversion of γ-valerolactone and its derivatives, abundant but unexplored renewable feedstocks, into sustainable and degradable polythioesters via the establishment of the first isomerization-driven ring-opening polymerizations (IROPs) of corresponding thionolactone intermediates. The key to this success relies on the development of a new simple and robust [Et₃ O]⁺ [B(C₆ F₅ )₄ ]^- cationic initiator which possesses high activity, exclusive selectivity, living nature, and broad scope of thionolactones. A complete inversion of configuration during IROP of enantiopure γ-thionovalerolactone is also disclosed, affording isotactic semicrystalline polythioesters (T_m =87.0 °C) with mechanical property compared well to the representative commodity polyolefins. The formation of a highly crystalline supramolecular stereocomplex with enhanced thermal property (T_m =117.6 °C) has also been revealed.

Zinc-Mediated Allylation-Lactonization One-Pot Reaction to Methylene Butyrolactones: Renewable Monomers for Sustainable Acrylic Polymers with Closed-Loop Recyclability

Despite biomass-derived methylene butyrolactone monomers having great potential in substituting the petroleum-based methacrylates for synthesizing the sustainable acrylic polymers, the possible industrial production of these cyclic monomers is unfortunately not practical due to moderate overall yields and harsh reaction conditions or a time-consuming multistep process. Here we report a convenient and effective synthetic approach to a series of biomass-derived methylene butyrolactone monomers via a zinc-mediated allylation-lactonization one-pot reaction of biorenewable aldehydes with ethyl 2-(bromomethyl)acrylate. Under simple room-temperature sonication conditions, near-quantitative conversions (>90%) can be accomplished within 5-30 min, providing pure products with high isolated yields of 70-80%. Their efficient polymerizations with a high degree of control and complete chemoselectivity were enabled by the judiciously chosen Lewis pair catalyst based on methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) [MeAl(BHT)₂] Lewis acid and 3-diisopropyl-4,5-dimethylimidazol-2-ylidene (I ⁱ Pr) Lewis base, affording new poly(methylene butyrolactone)s with high thermal stability and thermal properties tuned in a wide range as well as pendant vinyl groups for postfunctionalization. Through the development of an effective depolymerization setup (370-390 °C, ca. 100 mTorr, 1 h, a muffle furnace), thermal depolymerizations of these polymers have been achieved with monomer recovery up to 99.8%, thus successfully constructing sustainable acrylic polymers with closed-loop recyclability.

Boron-Based Lewis Pairs Catalyzed Living, Regioselective and Topology-Controlled Polymerization of (E, E)-alkyl Sorbates

It remains as a great challenge to realize living and controlled polymerization of renewable monomers by the boron-based Lewis pairs. Here we employ strong nucleophilic N-heterocyclic olefins (NHOs) or N-heterocyclic carbenes (NHCs) as Lewis bases (LBs), and boron-based compounds as Lewis acids (LAs) to construct LPs for polymerization of alkyl sorbates, including (E, E)-methyl sorbate (MS) and (E, E)-ethyl sorbate (ES). Systematic investigation reveal that the combinations of B(C₆ F₅ )₃ with appropriate acidity and steric hindrance, and strong nucleophilic NHOs promote living and controlled polymerization of alkyl sorbates in 100% 1,4-addition manner, furnishing polymers with predicted molecular weight (M_w up to 56.6 kg/mol) and narrow molecular weight distribution (Đ as low as 1.12). Furthermore, topology analysis shows that NHC1/B(C₆ F₅ )₃ LP produced PMS possessing cyclic structure. This article is protected by copyright. All rights reserved.

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.

One-Step Synthesis of Lignin-Based Triblock Copolymers as High-Temperature and UV-Blocking Thermoplastic Elastomers

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.

Brush-like polymers: design, synthesis and applications

With the development of controlled polymerisation, almost all polymerisation strategies have been successfully transplanted to surface-initiated polymerisation. The resulting polymer brushes have emerged as an effective tool for surface functionalization and modulation of the surface properties of materials. To meet various demands it is possible to tailor a material surface with polymer brushes that have diverse dimensionalities, morphologies and compositions. The crowded environment within polymer brushes as well as the stretched conformation of polymer chains sometimes provide unique physicochemical properties, which lead to the delicate creation of inorganic-organic hybridised nanostructures, anti-fouling coatings, biomedical carriers, and materials for use in lubrication, photonics and energy storage. So far, challenges remain in the high-precision synthesis and topological control needed to realize extended applications of polymer brushes. In this Feature Article, we highlight the topology, potential application prospects and various synthetic protocols, particularly for recently established methods, for the efficient synthesis of polymer brushes, as well as their benefits and limitations.

Dual-initiating and living frustrated Lewis pairs: expeditious synthesis of biobased thermoplastic elastomers

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.

Synchronous Control of Chain Length/Sequence/Topology for Precision Synthesis of Cyclic Block Copolymers from Monomer Mixtures

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 (M_n = 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.

Structure and activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems: application in ring-opening polymerization of lactones

Structure–activity relationship studies of N-heterocyclic olefin and thiourea/urea catalytic systems were performed and applied to ROP of lactones.

Living polymerization of naturally renewable butyrolactone-based vinylidenes mediated by a frustrated Lewis pair

The frustrated Lewis pair composed of an organophosphorus(iii) superbase and a bulky organoaluminum Lewis acid promoted the living/controlled polymerization of naturally renewable butyrolactone-based vinylidenes.

Controlled and efficient polymerization of methyl methacrylate catalyzed by pyridinylidenaminophosphine based Lewis pairs

Developing different synthetic approaches to realize controlled or living polymerization is of great interest to polymer chemists to obtain polymers with defined molecular weight, narrow molecular weight distribution and unambiguous structures.

Broadening the reaction scope of unprotected aldoses via their corresponding nitrones: 1,3-dipolar cycloadditions with alkenes

Condensation reactions of unprotected tetroses and pentoses with hydroxylamines afforded nitrones, which were easily converted to densely functionalized isoxazolidines in the presence of electron-poor alkenes. The 1,3-dipolar cycloaddition occurred with good facial discrimination of the chiral nitrone but under rather low endo/exo control. Stereochemistry of isomers was ascertained by chemical correlation with known derivatives from the literature. Microwave activation appeared as the most efficient reaction mode, affording the expected adducts within several minutes whereas hours were needed under standard heating. Alternatively, the transformation proved also possible under high pressure conditions by using a hand pump system, avoiding any energy source. Although water could not be used as the solvent, leading to hydrolysis of the nitrone substrate, a large variety of organic solvents proved efficient. The method has potential use in the preparation of non-ionic carbohydrate-based amphiphiles.

α-Angelica lactone catalyzed oxidation of benzylic sp3 C-H bonds of isochromans and phthalans

A metal-free organocatalytic system has been developed for highly efficient benzylic C-H oxygenations of cyclic ethers using oxygen as an oxidant. This oxidation reaction utilizes α-angelica lactone as a low cost/low molecular weight catalyst. The optimized reaction conditions allow the synthesis of valued isocoumarins and phthalides from readily available precursors in good yields. Mechanistic studies indicate that the reaction pathway likely involves a radical process via a peroxide intermediate.

Rapid and Scalable Access to Sequence-Controlled DHDM Multiblock Copolymers by FLP Polymerization

An immortal N-(diphenylphosphanyl)-1,3-diisopropyl-4,5-dimethyl-1,3-dihydro-2H-imidazol-2-imine/diisobutyl (2,6-di-tert-butyl-4-methylphenoxy) aluminum (P(NIⁱ Pr)Ph₂ /(BHT)Alⁱ Bu₂ )-based frustrated Lewis pair (FLP) polymerization strategy is presented for rapid and scalable synthesis of the sequence-controlled multiblock copolymers at room temperature. Without addition of extra initiator or catalyst and complex synthetic procedure, this method enabled a tripentacontablock copolymer (n=53, k=4, dp_n =50) to be achieved with the highest reported block number (n=53) and molecular weight (M_n =310 kg mol^-1 ) within 30 min. More importantly, this FLP polymerization strategy provided access to the multiblock copolymers with tailored properties by precisely adjusting the monomer sequence and block numbers.

Binuclear aluminum Lewis acid and its behavior in the polymerization of methyl methacrylate and n-butyl acrylate

Bimetallic organoaluminum vs. monomeric organoaluminum in Lewis pair catalysed MMA and nBA polymerization.

Chiral phosphoric acid catalyzed asymmetric kinetic resolution polymerization of 6-aryl-ε-caprolactones

The first chiral phosphoric acid catalyzed asymmetric kinetic resolution polymerization (AKRP) of 6-aryl-ε-caprolactones has been achieved.