Catalysts For Ring-opening Polymerization Research Articles

Highly Selective O-Phenylene Bisurea Catalysts for ROP: Stabilization of Oxyanion Transition State by a Semiflexible Hydrogen Bond Pocket.

Organocatalyzed ring-opening polymerization (ROP) is a versatile technique for synthesizing biodegradable polymers, including polyesters and polycarbonates. We introduce o-phenylene bisurea (OPBU) (di)anions as a novel class of organocatalysts that are fast, easily tunable, mildly basic, and exceptionally selective. These catalysts surpass previous generations, such as thiourea, urea, and TBD, in selectivity (kp/ktr) by 8 to 120 times. OPBU catalysts facilitate the ROP of various monomers, achieving high conversions (>95%) in seconds to minutes, producing polymers with precise molecular weights and very low dispersities (Đ ≈ 1.01). This performance nearly matches the ideal distribution expected from living polymerization (Poisson distribution). Density functional theory (DFT) calculations reveal that the catalysts stabilize the oxyanion transition state via a hydrogen bond pocket similar to the "oxyanion hole" in enzymatic catalysis. Both experimental and theoretical analyses highlight the critical role of the semirigid o-phenylene linker in creating a hydrogen bond pocket that is tight yet flexible enough to accommodate the oxyanion transition state effectively. These new insights have provided a new class of organic catalysts whose accessibility, moderate basicity, excellent solubility, and unparalleled selectivity and tunability open up new opportunities for controlled polymer synthesis.

Recent Advances in the Titanium-Based Catalysts for Ring-Opening Polymerization.

At present, economic development and daily life cannot be separated from organic synthetic polymers. However, a large number of nondegradable polymers have caused serious pollution to the environment. It is necessary for sustainable development to use biodegradable materials instead of traditional polymers, but it is not yet comparable in performance and cost to the competitor it will replace. Therefore, there is a long way to go to develop effective synthesis methods. Through ring-opening polymerization, some cyclic monomers, such as ε-caprolactone or lactide, can be synthesized into biodegradable polymers, which can not only replace traditional synthetic polymers in some fields but also have applications in drug delivery, surgical consumables, human implant materials, bone materials, etc. Ring-opening polymerization is a potential candidate for solving environmental pollution. For ring-opening polymerization, catalysts are very important, among which titanium catalysts have attracted much attention because of their high efficiency, economy, and nontoxicity. In this paper, the development status of organotitanium compounds as ring-opening polymerization catalysts is reviewed, including the effects of different ligand structures on polymerization behavior and polymer structure, and its development trend is prospected. We hope that this review will be helpful for developing efficient ring-opening polymerization catalysts.

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.

Potassium phenoxides bearing bulky yet flexible substituents as highly active catalysts for ring opening polymerization of biobased macrolactone of ethylene brassylate

Well-controlled and efficient ring opening polymerization of biobased ethylene brassylate is of great challenge due to its stainless ring structure that often results in inferior reactivities as well as the presence of two ester groups in the monomer that often lead to extensive intra-/inter- molecular transesterification side reactions. This report discloses a family of potassium phenoxide complexes that bear bulky yet flexible substituents. The bulky nature of the ligand could suppress undesired transesterification reactions, allowing the polymerization proceed in a well controlled manner, as revealed from the detailed kinetic studies that linear relationships between −ln(1-x) (x denotes the polymer yield) and the polymerization time and between molecular weights of the obtained polymers and polymer yields. The flexible substituents on the ligand allowed more monomer accessing to the active species, which eventually gave rise to TOFs in a range of 3604–3780 h−1, that were much higher than analogues in previous reports. Moreover, systematic evaluation of the polymerization behaviors demonstrated that the catalytic efficiency of the complexes was closely related to the steric nature of the metal center, increased the steric congestion would give rise to much lower monomer conversions. A plausible mechanism was proposed based on the polymerization results, and such a mechanism was further confirmed by NMR monitoring studies.

Machine learning and DFT coupling: A powerful approach to explore organic amine catalysts for ring-opening polymerization reaction

Recently, using well-known data to drive the chemical feature of catalysts for the specified reaction has emerged as a prevalent approach in catalysis science. Amines as essential compounds play crucial roles in living organisms, pharmaceutical, agricultural applications, and various chemical reactions. However, developing a systematic strategy for the screening of amine as an organic catalyst in ring-opening polymerization (ROP) remains a significant challenge. In predicting the catalytic performance of ROP reactions, the proton affinity (PA) of amines serves as a significant indicator, which reflects the basicity of the catalyst and the activity of the ROP reaction. Herein, we conducted a systematical analysis on a dataset of 259 unique amine compounds, involving the application of seven classical machine learning models and 23 molecular descriptors. The predictive model was constructed with molecular descriptors and DFT-calculated PA values. Among the regression results, the Support Vector Machine (SV) model exhibited the optimal performance, with R2, average mean absolute error (MAE) and root mean square error (RMSE) achieving 0.84, 8.38 kJ/mol and 13.77 kJ/mol, respectively. We further employed SHAP (SHapley Additive exPlanations) to evaluate feature significance and provide insights into screening amine catalysts for ROP reaction. The top 10 amines with high PA values are enumerated and discussed. This workflow of coupling machine learning and the DFT approach proves an effective way for screening amine catalysts in ROP reaction.

Comparative Study of Titanium Complexes Bearing 2-(Arylideneamino)phenolates and 2-((Arylimino)methyl)phenolates as Catalysts for Ring-Opening Polymerization of ε‑Caprolactone and L-Lactide

Titanium complexes bearing 2-(arylideneamino)phenolates and 2-((arylimino)methyl)phenolates were synthesized, and their catalytic activities in the polymerization of ε-caprolactone and L-lactide were studied. Among five-membered ring Ti complexes bearing 2-(arylideneamino)phenolates, FCl-Ti exhibited...

Multi-Functional Organofluoride Catalysts for Polyesters Production and Upcycling Degradation.

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

Novel Latent Catalyst for Ring-Opening Polymerization of 1,3-Benzoxazines Triggered by a Dual Ionic/Non-ionic Monomer Partnership

Novel Latent Catalyst for Ring-Opening Polymerization of 1,3-Benzoxazines Triggered by a Dual Ionic/Non-ionic Monomer Partnership

Bayesian-optimization-assisted discovery of stereoselective aluminum complexes for ring-opening polymerization of racemic lactide

Stereoselective ring-opening polymerization catalysts are used to produce degradable stereoregular poly(lactic acids) with thermal and mechanical properties that are superior to those of atactic polymers. However, the process of discovering highly stereoselective catalysts is still largely empirical. We aim to develop an integrated computational and experimental framework for efficient, predictive catalyst selection and optimization. As a proof of principle, we have developed a Bayesian optimization workflow on a subset of literature results for stereoselective lactide ring-opening polymerization, and using the algorithm, we identify multiple new Al complexes that catalyze either isoselective or heteroselective polymerization. In addition, feature attribution analysis uncovers mechanistically meaningful ligand descriptors, such as percent buried volume (%Vbur) and the highest occupied molecular orbital energy (EHOMO), that can access quantitative and predictive models for catalyst development.

Ring-Opening Polymerization of Lactones Catalyzed by Silicon-Based Lewis Acid

Many catalysts containing various elements at their active sites have been reported for the ring-opening polymerization (ROP) of cyclic esters. However, to our knowledge, silicon-based catalysts for ROP have never been reported. Here we report the ROP of cyclic esters and cyclic carbonates catalyzed by the derivatives of bis(perchlorocatecholato)silane (Si(catCl)2), which is a neutral silicon-based Lewis acid recently reported by Greb et al. The catalyst systems show high activity for the ROP of seven- and six-membered ring monomers such as ε-caprolactone, δ-valerolactone, and trimethylene carbonate to produce the polymers with molecular weights up to 32 kg/mol. The matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and nuclear magnetic resonance analysis of the obtained polymers indicates the predominant formation of cyclic polymers.

Ring-Opening Polymerization of rac-Lactide Catalyzed by Octahedral Nickel Carboxylate Complexes

To date, nickel(II) complexes have not been practically investigated as catalysts in ring-opening polymerization (ROP) of lactide to produce biodegradable poly(lactic acid), which is in demand in biomedicine and industry. In this study, carboxylate complexes of nickel(II) containing various N-donor ligands with different nuclearity, metal core rigidity and nature of carboxylate ligands were synthesized and studied by infrared spectroscopy, X-ray diffraction, elemental and thermogravimetric analyses. The obtained complexes were examined in the ROP of the rac-lactide in bulk and in toluene solution with and without the addition of a benzyl alcohol initiator. In the series of complexes studied, the complex [Ni(DBED)2(O2CC(CH3)3)2]·(CH3)3CCO2H (DBED is N,N′-dibenzylethylenediamine) was a syndioselective catalyst and showed the highest catalytic ability in the polymerization without the addition of benzyl alcohol. For this complex, according to 1D DOSY 1H NMR spectroscopy and mass spectrometry with electrospray ionization, polymerization is initiated by a free secondary amine, DBED, leaving the metal’s inner coordination sphere. Based on the experimental data obtained, a comprehensive density functional theory (DFT) study of the ROP pathways including the initiation and first chain growth cycle steps with a detailed description of the intermediates and evaluation of the energy barriers of the steps was carried out. It was shown that one of the key roles in the reaction process is played by carboxylate ligands, which act as proton carriers from the initiator molecule and have a significant influence on the reactivity of the catalytic metal complexes.

(Bipyridine bisphenolate)-aluminum/onium salt pair: a highly active binary catalyst for ring-opening polymerization of lactide with improved thermostability and protic tolerance

A series of bipyridine bisphenolate aluminum complexes with outstanding thermostability and tolerance to protic agents were synthesized and used to catalyze the ROP of LA with assist of onium salt as cocatalyst in epoxide.

A DFT-metadynamics study disclosing key properties of ring-opening polymerization catalysts to produce polyethercarbonate polyols from cyclic ethylene carbonate as part of an emerging CCU technology.

The ring opening polymerization of cyclic carbonates made from epoxide and CO2 to CO2-containing polymers constitutes an emerging technology of particular industrial interest. Considering the reaction of ring-opening polymerization of cyclic ethylene carbonate to produce polyethercarbonate polyols, several types of catalysts were tested experimentally and mechanistic pathways were proposed, but a detailed analysis of structure property relationship including the CO2-liberation pathways is still lacking. This contribution is using computational methods to investigate reported benchmark catalysts with the lead structure AxMyOz (A: alkali metal or alkyl, M: main group element or transition metal) that are particularly approved as effiecient catalysts for industrial purpose. Employing DFT-metadynamics simulations, free energy surfaces (FESs) for the key-steps in the catalytic polymerization of cyclic ethylene carbonate (cEC) are generated. Important structural criteria and characteristics of the catalysts that influence the catalytic performance and (side)reaction pathways are determined. It turns out that less nucleophilicity of the catalyst anion and more labile cations remain major criteria for prohibiting CO2 liberation during polymerization. The key learnings of this contribution currently serve as a basis to develop the next generation of catalysts to bring this emerging carbon capture and use (CCU) technology into industrial application.

Taking the Next Step—Model‐Based Analysis of Robust and Non‐Toxic Zn Catalysts for the Ring Opening Polymerization of Lactide in the Polymer Melt

AbstractIn the field of bioplastics, complete kinetic models are rare but needed for industrial scale‐up. Herein, a complete kinetic model is developed to describe the behavior of robust and nontoxic guanidine carboxy Zn complexes in the lab‐scale ring opening polymerization (ROP) of l‐lactide, mimicking industrial melt polymerization in the presence of a co‐initiator. The model includes inter‐ and intramolecular transesterification and random chain scission, considering the single site activity of the studied catalyst class, based upon the “asme”‐ligand already established in lactide ROP. This allows for the description of reaction rate constants and the prediction of conversion as well as molar mass and dispersity of the obtained polymer. It is further shown that the model can be applied to describe the same characteristics for the faster, next generation catalysts based on the archetype “asme” catalyst. This lays the foundation for a faster catalyst design, targeting the needs of industry in developing fast and nontoxic alternatives for the industrially used toxic ROP catalyst Sn octanoate, bridging the gap between academia and industry.

Highly Active Heterogeneous Double Metal Cyanide Catalysts for Ring-Opening Polymerization of Cyclic Monomers

A series of heterogeneous Zn-Co double metal cyanide (DMC) catalysts were investigated for ring-opening polymerization (ROP) of various cyclic monomers. Notably, inexpensive and commonly used organic solvents such as acetone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide, nitromethane, and 1-methylpyrrolidin-2-one were very effective complexing agents for the preparation of DMC catalysts, showing high catalytic activity for the ROP of propylene oxide, ε-caprolactone, and δ-valerolactone. The chemical structures and compositions of the resultant catalysts were determined using various techniques such as FT-IR, X-ray photoelectron spectroscopy, powder X-ray diffraction, and elemental analysis. α,ω-Hydroxyl-functionalized polyether and polyester polyols with high yields and tunable molecular weights were synthesized in the presence of various initiators to control functionality. Kinetic studies of the ROP of δ-valerolactone were also performed to confirm the reaction mechanism.

Reversible On/Off Switching of Lactide Cyclopolymerization with a Redox-Active Formazanate Ligand.

Redox-switching of a formazanate zinc catalyst in ring-opening polymerization (ROP) of lactide is described. Using a redox-active ligand bound to an inert metal ion (Zn2+) allows modulation of the catalytic activity by reversible reduction/oxidation chemistry at a purely organic fragment. A combination of kinetic and spectroscopic studies, together with mass spectrometry of the catalysis mixture, provides insight in the nature of the active species and the initiation of lactide ring-opening polymerization. The mechanistic data highlight the key role of the redox-active ligand and provide a rationale for the formation of cyclic polymer.

Iron complexes of [2+2] and [6+6] Schiff-base macrocycles derived from 2,2′-oxydianiline and their applications

Reaction of [2+2] and [6+6] Schiff-base macrocycles with FeBr2 are reported, together with preliminary studies of the applications of the iron-containing products. In particular, we have investigated peroxidase-like activity and determination of H2O2, as well as their ability to act as catalysts for ring opening polymerization of cyclic esters.

Aluminium complexes containing indolyl-phenolate ligands as catalysts for ring-opening polymerization of cyclic esters.

A family of aluminium complexes supported by mono-anionic indolyl-phenolate ligands are described. Reactions of indolyl-phenolate based ligand precursors, IndHPhROH, with 1.0 or 0.5 equivalents of AlMe2Cl in toluene afforded aluminium indolyl-phenolate complexes 1-4 and aluminium bis-indolyl-phenolate complexes 5-8 respectively. The molecular structure is reported for 5. Based on the NMR spectroscopic and X-ray crystallographic studies, a 1,3-hydrogen shift could happen from nitrogen to carbon on the five-membered ring of the indolyl group upon reacting with aluminium reagents. These novel aluminium complexes demonstrate catalytic activities toward the ring-opening polymerization of cyclic esters in the presence of alcohol.

Photocleavable Regenerative Network Materials with Exceptional and Repeatable Viscoelastic Manipulability.

The development of solventless system for modulating properties of network materials is imperative for the next generation sustainable technology. Utilization of photostimulation is important owing to its spatial and temporal locality, yet designing photoresponsive network materials exhibiting repeatable and dramatic change in their properties remains a challenge. Here, the authors report a photocleavable regenerative network (PRN) linked with photoresponsive hexaarylbiimidazoles (HABIs) synthesized from narrow dispersity star‐shaped poly(dimethylsiloxane)s (PDMSs) having 2,4,5‐triphenylimidazole end groups. The use of urea anion as a catalyst for ring opening polymerization (ROP) of cyclic siloxane initiated from silanols enables control of molecular weight and dispersity. The rheological measurements for the synthesized PRNs exhibit drastic changes in storage and loss moduli (G′ and G″) upon photoirradiation in the solid state (G′ > G″). This photocontrolled change in viscoelasticity with retaining solidity enables application of PRNs as a remotely‐controlled photo‐melt adhesive and photo‐scissible string. The developed PRNs will enable a wide variety of applications such as industrially important next‐generation sustainable adhesive, sealant, and reversibly‐deformable 3D printing materials with their spatially and temporally local manipulability, solventless handleability, and excellent reversibility.

Towards a selective synthetic route for cobalt amino acid complexes and their application in ring opening polymerization of rac-lactide.

Catalysts based on cobalt amino acids and 2,2 bipyridine (bipy) present an attractive and cost-effective alternative as ring opening polymerization catalysts, yet this system remains underexplored despite the advantageous coordination properties of amino acids and bipy as ligands combined with the variety of accessible oxidation states and coordination geometries of cobalt. Here, metal complexes of type [Co(aa)2(bipy)] with amino acids (aa: glycine, leucine and threonine) as ligands are reported. The complexes were characterized spectroscopically (IR, UV-vis and 1H, 13C NMR for diamagnetic species), and by MS spectrometry and elemental analysis. The data reveal that the 2,2 bipyridine acts as a neutral bidentate donor coordinating to the metal ion through two nitrogen atoms and the amino acid acts as a bidentate ligand coordinating through the carboxylate and amino group forming a stable five membered ring and a pseudo-octahedral geometry around the Co center. The activity of the complexes for the ring opening polymerization (ROP) of rac-lactide is presented. The complexes are effective initiators for the ROP of rac-lactide (Kobs = 9.05 × 10−4 s−1) at 100 : 1 [rac-lactide] : [catalyst] 1 M overall concentration of lactide in toluene at 403 K.