Highly Enantioselective Polymerization of β-Butyrolactone by a Bimetallic Magnesium Catalyst: An Interdependent Relationship Between Favored and Unfavored Enantiomers

Herein, we report that (S,S)-prophenolMg2(μ-OnBu)(THF)2 ((S,S)-1, prophenol = (S,S)-2,6-bis[2-(hydroxydiphenylmethyl)pyrrolidin-1-ylmethyl]-4-methylphenol) is a highly enantioselective (kR/kS = 140) precatalyst for ring-opening polymerization of rac-β-butyrolactone (β-BL) to isotactic poly(3-hydroxybutyrate) (i-PHB), a high performance, biodegradable polyester. Precatalyst (S,S)-1 polymerizes (R)-β-BL with an inversion of stereochemistry to (S)-PHB with a m% (percentage of adjacent linkages with a meso configuration) of 98% at 41% conversion and Tm of 165 °C under a variety of conditions. Complex (S,S)-1 demonstrates unique polymerization kinetics, as it does not polymerize the preferred enantiomer, (R)-β-BL, alone. Mechanistic studies revealed that (S)-β-BL is needed to convert (S,S)-1 into the active enantioselective polymerization catalyst. To the best of our knowledge, (S,S)-1 produces i-PHB with the highest degree of isotacticity observed from a polymerization of rac-β-BL. This study informs the design and understanding of future enantioselective and earth-abundant metal catalysts for ring-opening polymerization of β-lactones.

Atactic, Isotactic, and Syndiotactic Methylated Polyhydroxybutyrates: An Unexpected Series of Isomorphic Polymers

Polyhydroxyalkanoates (PHAs), such as poly[(R)-3-hydroxybutyrates] [(R)-P3HB], are produced by bacteria and are promising alternatives to nondegradable polyolefin plastics, but their semicrystallinity and high melting points are only maintained at high tacticity, which are commonly seen in other semicrystalline polymers like isotactic polypropylene (iPP). We herein report a class of synthetic PHAs, cis-poly(3-hydroxy-2-methylbutyrate)s (cis-PHMBs), that exhibit tacticity-independent semicrystallinity. The syndiotactic, isotactic, and even atactic PHMBs all share high melting points (Tm > 170 °C) and nearly identical crystal structures. The isomorphism of these polymers across three different tacticities has allowed access to iPP-like, high-performance PHMB without the requirement of high tacticity.

Highly Isoselective Ring-Opening Polymerization of rac-β-Butyrolactone: Access to Synthetic Poly(3-hydroxybutyrate) with Polyolefin-like Material Properties

We report the highly isoselective ring-opening polymerization (ROP) of racemic β-butyrolactone (β-BL) using in situ-generated catalysts based on Y[N(SiHMe₂)₂]₃(THF)₂ and salan-type pro-ligands. The catalyst system produces isotactic poly(3-hydroxybutyrate) (PHB) with record productivity (TOF up to 32 000 h^-1) and the highest isoselectivity (P_m up to 0.89) in ROP of β-BL achieved to date. In contrast to bacterial PHB, the chemically synthesized PHB has beneficial material properties, such as increased melt processing window attributed to a lowered melting temperature (T_m ≈ 140 °C) and drastically reduced brittleness. The produced PHB showed an elongation at break of 392%, thus demonstrating promising polyolefin-like thermomechanical material properties.

Installing Controlled Stereo-Defects Yields Semicrystalline and Biodegradable Poly(3-Hydroxybutyrate) with High Toughness and Optical Clarity

Stereo-defects present in stereo-regular polymers often diminish thermal and mechanical properties, and hence suppressing or eliminating them is a major aspirational goal for achieving polymers with optimal or enhanced properties. Here, we accomplish the opposite by introducing controlled stereo-defects to semicrystalline biodegradable poly(3-hydroxybutyrate) (P3HB), which offers an attractive biodegradable alternative to semicrystalline isotactic polypropylene but is brittle and opaque. We enhance the specific properties and mechanical performance of P3HB by drastically toughening it and also rendering it with the desired optical clarity while maintaining its biodegradability and crystallinity. This toughening strategy of stereo-microstructural engineering without changing the chemical compositions also departs from the conventional approach of toughening P3HB through copolymerization that increases chemical complexity, suppresses crystallization in the resulting copolymers, and is thus undesirable in the context of polymer recycling and performance. More specifically, syndio-rich P3HB (sr-P3HB), readily synthesized from the eight-membered meso-dimethyl diolide, has a unique set of stereo-microstructures comprising enriched syndiotactic [rr] and no isotactic [mm] triads but abundant stereo-defects randomly distributed along the chain. This sr-P3HB material is characterized by high toughness (U_T = 96 MJ/m³) as a result of its high elongation at break (>400%) and tensile strength (34 MPa), crystallinity (T_m = 114 °C), optical clarity (due to its submicron spherulites), and good barrier properties, while it still biodegrades in freshwater and soil.

Advances in the Synthesis of Chemically Recyclable Polymers

The development of modern society is closely related to polymer materials. However, the accumulation of polymer materials and their evolution in the environment causes not only serious environmental problems, but also waste of resources. Although physical processing can be used to reuse polymers, the properties of the resulting polymers are significantly degraded. Chemically recyclable polymers, a type of polymer that degrades into monomers, can be an effective solution to the degradation of polymer properties caused by physical recycling of polymers. The ideal chemical recycling of polymers, i. e., quantitative conversion of the polymer to monomers at low energy consumption and repolymerization of the formed monomers into polymers with comparable properties to the original, is an attractive research goal. In recent years, significant progress has been made in the design of recyclable polymers, enabling the regulation of the "polymerization-depolymerization" equilibrium and closed-loop recycling under mild conditions. This review will focus on the following aspects of closed-loop recycling of poly(sulfur) esters, polycarbonates, polyacetals, polyolefins, and poly(disulfide) polymer, illustrate the challenges in this area, and provide an outlook on future directions.

Living ring-opening polymerization of β-butyrolactone initiated by mononuclear zirconium compounds containing sterically hindered N,O-chelate and anionic dimethylamide ligands†

The ring-opening polymerization of β-lactones into polyhydroxyalkanoates (PHA), biodegradable polymers with high molecular weight and narrow polydispersity, is of significant interest. The mononuclear zirconium compound containing sterically hindered N,O-chelate and anionic dimethylamide ligands was used as an initiator for the polymerization of β-butyrolactone (BBL), resulting in polyhydroxylbutyrate (PHB) with a number-average molecular weight of 12 000 g mol⁻¹. Kinetic studies demonstrate a first-order dependence on β-butyrolactone (BBL) concentration at room temperature, accompanied by narrow molecular weight distributions (ca. 1.03–1.07), indicating a well-controlled living polymerization.

The ring-opening polymerization of β-lactones into polyhydroxyalkanoates (PHA), biodegradable polymers with high molecular weight and narrow polydispersity, is of significant interest.

Synthesis and characterization of rare earth/lithium complexes stabilized by ethylenediamine-bridged bis(phenolate) ligands and their activity in catalyzing amidation reactions

Rare earth/lithium complexes stabilized by ethylenediamine-bridged bis(phenolate) ligands have been synthesized and characterized. In addition to five rare earth/lithium amides isolated as major complexes, two other rare earth/lithium complexes bearing two phenolate ligands were also isolated. The activities of rare earth/lithium amides in catalyzing the amidation of aldehydes and amines were studied, which revealed that the yttrium/lithium complex was highly active for a wide range of substrates, generating 58 examples of amides in 42-99% yields under mild conditions (i.e., room temperature, 3-hour reaction time, additive-free). More importantly, this is the first example of rare earth-based catalysts capable of catalyzing the amidation of primary aliphatic amines.

Synthesis of N-Methyl-o-phenylenediamine-Bridged Bis(phenolato) Lanthanide Alkoxides and Their Catalytic Performance for the (Co)Polymerization of rac-Butyrolactone and l-Lactide

A series of lanthanide alkoxo complexes supported by ONNO salalen ligands were synthesized and characterized. A one-pot reaction of LH₂ (L = (2-O-C₆H₂-^tBu₂-3,5)CH═N-C₆H₄-N(CH₃)CH₂(2-O-C₆H₂-^tBu₂-3,5)) with LnCp₃(THF) in a 1:1 molar ratio followed by the addition of 1 equiv of ROH (R = Bn, ⁱPr, and CF₃CH₂), afforded the dimeric lanthanide alkoxo complexes [LLn(μ-OCH₂Ph)]₂ [Ln = Lu (1), Yb (2), Sm (3), Nd (4)], [L²Yb(μ-OⁱPr)]₂ (5), and [L²Yb(μ-OCH₂CF₃)]₂ (6) in good isolated yields. All these lanthanide complexes were characterized by elemental analysis and FT-IR spectroscopy. In addition, complex 1 has been characterized by NMR spectroscopy. Single-crystal X-ray diffraction analysis of complexes 1, 2, 5, and 6 showed that these lanthanide alkoxo complexes are dimeric in the solid state. Complexes 1-6 showed good activity toward the homopolymerization of rac-butyrolactone (rac-BBL) to give atactic PHB, and ionic radii of central metals have profound influence on the polymerization. The polymerization behavior of l-lactide (l-LA) initiated by complex 2 was also explored. The kinetic study revealed that the polymerizations of rac-BBL and l-LA initiated by salalen lanthanide akoxide are first order for both the monomer and the initiator concentrations. Furthermore, it was found that complexes 1 and 2 showed good activity in the copolymerization of l-LA and rac-BBL, affording gradient copolymers.

N-Oxides amplify catalyst reactivity and isoselectivity in the ring-opening polymerization of rac-β-butyrolactone

N-Oxides can amplify the performance of a lanthanum aminobisphenolate catalyst in the ring-opening polymerization (ROP) of rac-β-butyrolactone (rac-BBL) to unprecedented levels (TOF/P_m; At RT: 1900 h^-1/0.73, At -30 °C: 200 h^-1/0.82). Experiments and computations establish donor electronics control catalyst activity, while donor sterics control catalyst deactivation.

The versatile roles of neutral donor ligands in tuning catalyst performance for the ring-opening polymerization of cyclic esters

The use of neutral donor ligands is an effective strategy to modify catalyst structure and performance in the synthesis of sustainable polymers through the ring-opening polymerization (ROP) of cyclic esters.

Cyclic (Alkyl)(amino)carbene Lanthanide Amides: Synthesis, Structure, and Catalytic Selective Hydrosilylation of Alkenes

The first examples of cyclic (alkyl)(amino)carbene (CAAC) lanthanide (Ln) complexes were synthesized from the reaction of CAAC with Yb[N(SiMe₃)₂]₂ and Eu[N(SiMe₃)₂]₂(THF)₂ (THF = tetrahydrofuran). The structures of (CAAC)Yb[N(SiMe₃)₂]₂ (2) and (CAAC)Eu[N(SiMe₃)₂]₂(THF) (3) were determined by X-ray diffraction analysis. Density functional theory calculations of 2 revealed the predominantly ionic bond between the Ln ion and CAAC. Complex 3 enabled catalytic hydrosilylation of aryl- and silylalkenes with primary and secondary silanes in high yields and Markovnikov selectivity.

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

Low-Symmetry β-Diketimine Aryloxide Rare-Earth Complexes: Flexible, Reactive, and Selective

We report the synthesis, characterization, and reactivity of a new low-symmetry β-diketimine featuring a pendant amino(methyl)phenol donor and its corresponding heteroleptic rare-earth (RE) complexes. This includes the first structurally characterized examples of alcoholysis and insertion from an isolated RE^III amide in a β-diketimine framework. The flexible methylene linkage leads to RE^III complexes with tunable dynamic solution behavior that defines their stoichiometric and catalytic reactivity. The addition of a strong neutral donor ligand, tricyclohexylphosphine oxide, suppresses a prevalent catalyst degradation pathway (base-promoted elimination) and dramatically enhances the catalyst performance in the stereospecific ring-opening polymerization of rac-β-butyrolactone. Our results further demonstrate the importance of ligand reorganization in the stoichiometric and catalytic activity of RE^III ions.

The role of neutral donor ligands in the isoselective ring-opening polymerization of rac-β-butyrolactone

Isoenriched poly-3-hydroxybutyrate (P3HB) is a biodegradable material with properties similar to isotactic polypropylene, yet efficient routes to this material are lacking after 50+ years of extensive efforts in catalyst design. In this contribution, a novel lanthanum aminobisphenolate catalyst (1-La) can access isoenriched P3HB through the stereospecific ring-opening polymerization (ROP) of rac-β-butyrolactone (rac-BBL). Replacing the tethered donor group of a privileged supporting ligand with a non-coordinating benzyl substituent generates a catalyst whose reactivity and selectivity can be tuned with inexpensive achiral neutral donor ligands (e.g. phosphine oxides, OPR₃). The 1-La/OPR₃ (R = n-octyl, Ph) systems display high activity and are the most isoselective homogeneous catalysts for the ROP of rac-BBL to date (0 °C: P_m = 0.8, TOF ∼190 h⁻¹). Combined reactivity and spectroscopic studies provide insight into the active catalyst structure and ROP mechanism. Both 1-La(TPPO)2 and a structurally related catalyst with a tethered donor group (2-Y) operate under chain-end stereocontrol; however, 2-RE favors formation of P3HB with opposite tacticity (syndioenriched) and its ROP activity and selectivity are totally unaffected by added neutral donor ligands. Our studies uncover new roles for neutral donor ligands in stereospecific ROP, including suppression of chain-scission events, and point to new opportunities for catalyst design.

Simple achiral neutral donor ligands modify catalyst structure and function to enable access to isoenriched poly-3-hydroxybutyrate, a biodegradable material with properties similar to isotactic polypropylene.

Lanthanum complexes stabilized by a pentadentate Schiff-base ligand: synthesis, characterization, and reactivity in statistical copolymerization of ε-caprolactone and l-lactide

Developing catalysts that are capable of catalyzing the copolymerization of ε-caprolactone and l-lactide to give random CL/LA copolymers is of great importance. One-pot reaction of La[N(SiMe₃)₂]₃ with 1 equiv. of LH₂ (LH₂ = NH(CH₂CH₂N[double bond, length as m-dash]CHC₆H₂-3,5-^tBu₂-2-OH)₂) in THF at room temperature, followed by protolysis with one equivalent amount of ROH (R = C₆H₂-2,6-^tBu₂-4-CH₃, ^tBu, ⁱPr, Bn, and Et) at 60 °C gave the mono-Schiff-base-ligated lanthanum aryloxide complex LLa(OC₆H₂-2,6-^tBu₂-4-CH₃)(THF) (1), and lanthanum alkyloxide complexes LLaO^tBu(THF) (2), [LLaOⁱPr]₂ (3), [LLaOBn]₂ (4), and [LLaOEt]₂ (5) in 59-69% isolated yields. These lanthanum complexes were capable of initiating the homopolymerization of l-lactide and rac-lactide with extremely high activity, and the copolymerization of ε-caprolactone (ε-CL) and l-lactide (l-LA) to give statistical CL/LA copolymers via a transesterification reaction.

Biodegradable Polyhydroxyalkanoates by Stereoselective Copolymerization of Racemic Diolides: Stereocontrol and Polyolefin-Like Properties

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-8DL^R , 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-8DL^Me with rac-8DL^R (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.

Highly selective redistribution of primary arylsilanes to secondary arylsilanes catalyzed by Ln(CH2C6H4NMe2-o)3@SBA-15

The organometallic–inorganic hybrid materials Ln(CH₂C₆H₄NMe₂-o)₃@SBA-15 (Ln = La, Y) were prepared, which demonstrated extremely high selectivity (>99%) in catalyzing the redistribution of primary arylsilanes to secondary arylsilanes.

Stereosequenced crystalline polyhydroxyalkanoates from diastereomeric monomer mixtures

Stereoselective polymerization of chiral or prochiral monomers is a powerful method to produce high-performance stereoregular crystalline polymeric materials. However, for monomers with two stereogenic centers, it is generally necessary to separate diastereomers before polymerization, resulting in substantial material loss and added energy cost associated with the separation and purification process. Here we report a diastereoselective polymerization methodology enabled by catalysts that directly polymerize mixtures of eight-membered diolide (8DL) monomers with varying starting ratios of chiral racemic (rac) and achiral meso diastereomers into stereosequenced crystalline polyhydroxyalkanoates with isotactic and syndiotactic stereodiblock or stereotapered block microstructures. These polymers show enhanced ductility and toughness relative to polymers of pure rac-8DL, subject to tuning by variation of the diastereomeric ratio and structure of the 8DL monomers.

Yttrium-Mediated Ring-Opening Copolymerization of Oppositely Configurated 4-Alkoxymethylene-β-Propiolactones: Effective Access to Highly Alternated Isotactic Functional PHAs

The ring-opening copolymerization (ROCOP) of functional 4-alkoxymethylene-β-propiolactones (BPL^OR s) by yttrium-bisphenolate complexes was investigated. The ROCOP of equimolar mixtures of BPL^OR s of opposite configurations, namely (R)-BPL^OR1 /(S)-BPL^OR2 [R¹ , R² =OMe, OAllyl, OCH₂ Ph (=OBn), OSiMe₂ tBu (=OTBDMS)], by the syndioselective Y{ONOO^cum }/iPrOH catalyst/initiator system affords P(HB^OR1 -alt-HB^OR2 ) copolymers with high alternation degrees (altern.=89-94 %), as determined by comprehensive kinetic, ¹³ C{¹ H} NMR spectroscopy, MALDI-ToF MS and ESI MS/MS fragmentation studies. The ROCOP of the (R)-BPL^OMe /(S)-BPL^OTBDMS combination, featuring a large difference in the substituents' bulkiness, constitutes the only observed exception to this trend (altern.=64 %). On the other hand, the isoselectivity of the Y{ONNO^Cl }/iPrOH catalyst/initiator system has been exploited to generate, in a one-pot/one-step procedure, original mixtures of isotactic poly(hydroxyalkanoate)s (PHAs). This system efficiently transforms equimolar mixtures of (R)-BPL^OAll /(S)-BPL^OMe into a 1:1 mixture of the corresponding isotactic iso-(R)-PHB^OAll and iso-(S)-PHB^OMe homopolymers; almost no copolymerization defects are observed. This new approach has been extended successfully to the ROCOP of equimolar mixtures of racemic monomers, rac-BPL^OAll /rac-BPL^OMe .