Catalytic Lewis Pair Polymerization of Renewable Methyl Crotonate to High-Molecular-Weight Polymers

Synergistic Dual-Cure Reactions for the Fabrication of Thermosets by Chemical Heating

Large composite structures, such as those used in wind energy applications, rely on the bulk polymerization of thermosets on an impressively large scale. To accomplish this, traditional thermoset polymerizations require both elevated temperatures (>100 °C) and extended cure durations (>5 h) for complete conversion, necessitating the use of oversize ovens or heated molds. In turn, these requirements lead to energy-intensive polymerizations, incurring high manufacturing costs and process emissions. In this study, we develop thermoset polymerizations that can be initiated at room temperature through a transformative "chemical heating" concept, in which the exothermic energy of a secondary reaction is used to facilitate the heating of a primary thermoset polymerization. By leveraging a redox-initiated methacrylate free radical polymerization as a source of exothermic chemical energy, we can achieve peak reaction temperatures >140 °C to initiate the polymerization of epoxy-anhydride thermosets without external heating. Furthermore, by employing Trojan horse methacrylate monomers to induce mixing between methacrylate and epoxy-anhydride domains, we achieve the synthesis of homogeneous hybrid polymeric materials with competitive thermomechanical properties and tunability. Herein, we establish a proof-of-concept for our innovative chemical heating method and advocate for its industrial integration for more energy-efficient and streamlined manufacturing of wind blades and large composite parts more broadly.

Topology Controlled All-(Meth)acrylic Thermoplastic Elastomers by Multi-Functional Lewis Pairs-Mediated Polymerization

It remains challenging to synthesize all-(meth)acrylic triblock thermoplastic elastomers (TPEs), due to the drastically different reactivities between the acrylates and methacrylates and inevitable occurrence of side reactions during polymerization of acrylates. By taking advantage of the easy structural modulation features of N-heterocyclic olefins (NHOs), we design and synthesize strong nucleophilic tetraphenylethylene-based NHOs varying in the number (i.e. mono-, dual- and tetra-) of initiating functional groups. Its combination with bulky organoaluminum [iBuAl(BHT)2] (BHT=bis(2,6-di-tBu-4-methylphenoxy)) constructs Lewis pair (LP) to realize the living polymerization of both acrylates and methacrylates, furnishing polyacrylates with ultrahigh molecular weight (Mn up to 2174 kg ⋅ mol-1) within 4 min. Moreover, these NHO-based LPs enable us to not only realize the control over the polymers' topology (i.e. linear and star), but also achieve triblock star copolymers in one-step manner. Mechanical studies reveal that the star triblock TPEs exhibit better mechanical properties (elongation at break up to 1863 % and tensile strength up to 19.1 MPa) in comparison with the linear analogs. Moreover, the presence of tetraphenylethylene group in the NHOs entitled the triblock TPEs with excellent AIE properties in both solution and solid state.

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.

ItOct (ItOctyl) - pushing the limits of ItBu: highly hindered electron-rich N-aliphatic N-heterocyclic carbenes

ItBu (ItBu = 1,3-di-tert-butylimidazol-2-ylidene) represents the most important and most versatile N-alkyl N-heterocyclic carbene available in organic synthesis and catalysis. Herein, we report the synthesis, structural characterization and catalytic activity of ItOct (ItOctyl), C₂-symmetric, higher homologues of ItBu. The new ligand class, including saturated imidazolin-2-ylidene analogues has been commercialized in collaboration with MilliporeSigma: ItOct, 929 298; SItOct, 929 492 to enable broad access of the academic and industrial researchers within the field of organic and inorganic synthesis. We demonstrate that replacement of the t-Bu side chain with t-Oct results in the highest steric volume of N-alkyl N-heterocyclic carbenes reported to date, while retaining the electronic properties inherent to N-aliphatic ligands, such as extremely strong σ-donation crucial to the reactivity of N-alkyl N-heterocyclic carbenes. An efficient large-scale synthesis of imidazolium ItOct and imidazolinium SItOct carbene precursors is presented. Coordination chemistry to Au(i), Cu(i), Ag(i) and Pd(ii) as well as beneficial effects on catalysis using Au(i), Cu(i), Ag(i) and Pd(ii) complexes are described. Considering the tremendous importance of ItBu in catalysis, synthesis and metal stabilization, we anticipate that the new class of ItOct ligands will find wide application in pushing the boundaries of new and existing approaches in organic and inorganic synthesis.

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.

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.

Mechanism of Spatial and Temporal Control in Precision Cyclic Vinyl Polymer Synthesis by Lewis Pair Polymerization

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.

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.

Sequentially bridging anionic addition and ring-opening polymerization by cooperative organocatalysis: Well-defined block copolymers from methacrylates and cyclic esters

Organocatalytic sequential block copolymerization of dissimilar monomers with distinct chemical characteristics is great challenging, usually involving the integration of different polymerization mechanism. In this contribution, we reported the synthesis of...

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.

Lewis Pair Catalyzed Regioselective Polymerization of (E,E)-Alkyl Sorbates for the Synthesis of (AB)n Sequenced Polymers

In this contribution, Lewis pairs (LPs) composed of N-heterocyclic olefins (NHOs) with different steric hindrance and nucleophilicity as Lewis bases (LBs) and Al-based compounds with comparable acidity but different steric hindrance as Lewis acids (LAs) were applied for 1,4-selective polymerization of (E,E)-methyl sorbate (MS) and (E,E)-ethyl sorbate (ES). The effects of steric hindrance, electron-donating ability, and acidity of LPs on MS and ES polymerization were systematically investigated. High catalytic activity and high initiation efficiency can be achieved, leading to the formation of PMS with 100 % 1,4-selectivity, tunable molecular weight (M_w up to 333 kg mol^-1 ), and narrow molecular weight distribution (MWD). Block copolymerization of ES and methyl methacrylate (MMA) was also realized. Meanwhile, this system can be applied to other homologous conjugated diene substrates. Furthermore, simple chemical reactions can efficiently convert PMS to different polymers with strict (AB)_n sequence structures, such as poly(sorbic acid), poly(propylene-alt-methyl acrylate), poly(propylene-alt-acrylic acid), poly(propylene-alt-allyl alcohol), and poly(ethylene-alt-2-butylene).

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.

Copolymers incorporated with β-substituted acrylate synthesized by organo-catalyzed group-transfer polymerization

Various copolymers incorporated with β-substituted acrylates, such as alkyl crotonates (e.g., methyl crotonate (MC), ethyl crotonate (EC), isopropyl crotonate (iPC), and n-butyl crotonate (nBC)) and methyl cinnamate (MCin), were synthesized by group-transfer polymerization (GTP) using a silicon-based Lewis acid catalyst. In addition to β-substituted acrylates, α-substituted acrylates (e.g., methyl methacrylate (MMA) and n-butyl methacrylate (nBMA)) were examined as comonomers. Proton nuclear magnetic resonance (1H NMR) spectroscopy and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) characterizations of the obtained copolymers revealed that each monomer component was incorporated sufficiently. The thermal stabilities of the resulting copolymers were investigated by dynamic mechanical analysis (DMA), indicating that the glass-transition temperature (Tg) of the copolymers can be widely varied over a relatively high-temperature range by selecting the optimal comonomer. More specifically, the Tg values of poly(MC-random-EC) (MC/EC molar ratio = 50/50), poly(MC-random-nBC) (MC/nBC molar ratio = 50/50), poly(MC-random-MCin) (MC/MCin molar ratio = 54/46), and poly(nBC-random-MCin) (nBC/MCin molar ratio = 56/44) were 173, 130, 216, and 167 °C, respectively.

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.

Advances, Challenges, and Opportunities of Poly(γ-butyrolactone)-Based Recyclable Polymers

The discovery and prosperous growth of synthetic polymers have presented both significant advantages and daunting challenges in the last century. To address the issues of environmental pollution and fossil consumption, recyclable, degradable, and/or biobased polymers have been given much attention in the polymer science community. This viewpoint focuses on the emerging fully chemical recyclable poly(γ-butyrolactone)-based polymers. The breakthrough from nonpolymerizable to efficient polymerization is highlighted by the benefits of the development of a series of catalysis for ring-opening polymerization of γ-butyrolactone. Subsequently, the design of γ-butyrolactone derivatives and synthesis of more recyclable polymers are summarized together with the discussions about the structure and property relationship. Finally, the remaining challenges and promising opportunities are suggested in order to provide insights into the further direction for sustainable polymers.

A simple synthesis of [RuCl2(NHC)(p-cymene)] complexes and their use in olefin oxidation catalysis

An operationally simple synthetic route is designed to access the [RuCl₂(NHC)(p-cymene)] family of complexes.

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.

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.

Compounded Sequence Control in Polymerization of One-Pot Mixtures of Highly Reactive Acrylates by Differentiating Lewis Pairs

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.

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

This contribution reports an unprecedentedly efficient dimerization and the first successful polymerization of lignocellulose-based β-angelica lactone (β-AL) by utilizing a selective Lewis pair (LP) catalytic system, thereby establishing a versatile bio-refinery platform wherein two products, including a dimer for high-quality gasoline-like biofuel (C₈ -C₉ branched alkanes, yield=87 %) and a heat- and solvent-resistant acrylic bioplastic (M_n up to 26.0 kg mol^-1 ), can be synthesized from one feedstock by one catalytic system. The underlying reason for exquisite selectivity of the LP catalytic system toward dimerization and polymerization was explored mechanistically.

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.

Lewis pairs polymerization of polar vinyl monomers

The globally increasing demands for polymer materials stimulate the significantly intense attention focused on the Lewis pair polymerization (LPP) of various polar vinyl monomers catalyzed by Lewis pairs (LPs) composed of Lewis acid (LA) and Lewis base (LB). According to the degree of interaction between LA and LB, LPs could be divided into classical Lewis adduct (CLA), interacting Lewis pair (ILP) and frustrated Lewis pair (FLP). Regulation of the Lewis basicity, Lewis acidity, and steric effects of these LPs has a significant impact on the polymer chain initiation, propagation and termination as well as chain transfer reaction during polymerization. Compared with other polymerization strategies, LPP has shown several unique advantages towards the polymerization of polar vinyl monomers such as high activity, control or livingness, mild conditions, and complete chemo- or regioselectivity. We will comprehensively review the recent advances achieved in the LPP of polar vinyl monomers according to the classification of the employed LPs based on different LAs, by highlighting the key polymerization results, polymerization mechanisms as well as the currently unmet challenges and the future research directions of LPP chemistry.

Unique acrylic resins with aromatic side chains by homopolymerization of cinnamic monomers

Cinnamic monomers, which are useful chemicals derived from biomass, contain α,β-unsaturated carbonyl groups with an aromatic ring at the β-position. Homopolymers synthesized by addition polymerization of these compounds are expected to be innovative bio-based polymer materials, as they have both polystyrene and polyacrylate structures. However, polymerization of these compounds by many methods is challenging, including by radical methods, owing to steric hindrance of the substituents and delocalization of electrons throughout the molecule via unsaturated π-bonding. Herein we report that homopolymers of these compounds with molecular weights (Mn) of ~18,100 g mol−1 and controlled polymer backbones can be synthesized by the group-transfer polymerization technique using organic acid catalysts. Additionally, these homopolymers are shown to have high heat resistance comparable to that of engineering plastics. Overall, these findings may open up possibilities for the convenient homopolymerization of cinnamic monomers to produce high-performance polymer materials.

Non-Innocent Methylene Linker in Bridged Lewis Pair Initiators

Deprotonation usually occurs as an unwanted side reaction in the Lewis pair polymerization of Michael acceptors, for which the conjugated addition of the Lewis base to the acid-activated monomer is the commonly accepted initiation mechanism. This has also been reported for B-P-based bridged Lewis pairs (BLPs) that form macrocyclic addition products. We now show that the formerly unwanted deprotonation is the likely initiation pathway in the case of Al-P-based BLPs. In a detailed study of a series of Al-P-based BLPs, using a combination of single-crystal diffraction experiments (X-ray and neutron) and mechanistic investigations (experimental and computational), an active role of the methylene bridge was revealed, acting as a base towards the α-acidic monomers. Additionally, the polymerization studies proved a living behavior combined with significantly high activities, narrow molecular mass distributions, and the possibility of copolymerization.