Ring-opening Polymerization Of Cyclic Esters Research Articles

Group IV complexes with nitrogen based ligands in the ring opening polymerization of cyclic esters

The ring opening polymerization of cyclic esters has attracted much interest for the relevance of polyesters as sustainable alternatives to petroleum-based plastics. Since their first discovery, group IV metal complexes have been demonstrated to be active in the different fields of the homogenous catalysis. This brief review focused the light on the ring-opening polymerization of cyclic esters promoted by group IV complexes supported by ligand with anionic nitrogen atoms. The structures and catalytic activity of the complexes are discussed. The aim of this work is to demonstrate how this field of nitrogen-based ligands to support group IV metals deserves to be further investigated to obtain increasingly efficient catalysts in this polymerization reaction, which remains poorly researched.

Ring-Opening Polymerization of Cyclic Esters and Carbonates with (Thio)urea/Cyclopropenimine Organocatalytic Systems.

Organocatalyzed ring-opening polymerization is a powerful tool for the synthesis of a variety of functional, readily degradable polyesters and polycarbonates. We report the use of (thio)ureas in combination with cyclopropenimine bases as a unique catalyst for the polymerization of cyclic esters and carbonates with a large span of reactivities. Methodologies of exceptionally effective and selective cocatalyst combinations were devised to produce polyesters and polycarbonates with narrow dispersities (Đ = 1.01-1.10). Correlations of the pKa of the various ureas and cyclopropenimine bases revealed the critical importance of matching the pKa of the two cocatalysts to achieve the most efficient polymerization conditions. It was found that promoting strong H-bonding interactions with a noncompetitive organic solvent, such as CH2Cl2, enabled greatly increased polymerization rates. The stereoselective polymerization of rac-lactide afforded stereoblock poly(lactides) that crystallize as stereocomplexes, as confirmed by wide-angle X-ray scattering.

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers.

The use of multimetallic complexes is a rapidly advancing route to enhance catalyst performance in the ring-opening polymerization of cyclic esters and ethers. Multimetallic catalysts often outperform their monometallic analogues in terms of reactivity and/or polymerization control, and these improvements are typically attributed to "multimetallic cooperativity". Yet the origins of multimetallic cooperativity often remain unclear. This review explores the key factors underpinning multimetallic cooperativity, including metal-metal distances, the flexibility, electronics and conformation of the ligand framework, and the coordination environment of the metal centers. Emerging trends are discussed to provide insights into why cooperativity occurs and how to harness cooperativity for the development of highly efficient multimetallic catalysts.

Ring-Opening Polymerization of rac-β-Butyrolactone Promoted by New Tetradentate Thioether-Amide Ligand-Type Zinc Complexes.

In this work, thioether-amide ligands featuring a combination of hard amide groups with soft donor groups have been employed to develop new zinc catalysts for the ring-opening polymerization of cyclic esters. All complexes were prepared in high yields through alkane elimination reactions with diethyl zinc and characterized using nuclear magnetic resonance (NMR) spectroscopy. Density functional theory (DFT) characterization provided insight into the parameters that influence catalytic activity, such as steric hindrance at the metal center, Lewis acidity and electronic density of thioether-amide ligands. In the presence of one equivalent of isopropanol, all complexes were active in the ring-opening polymerization of rac-β-butyrolactone. Quantitative conversion of 100 monomer equivalents was achieved within 1 h at 80 °C in a toluene solution. Number-average molecular weights increased linearly with monomer conversion; the values were in optimal agreement with those expected, and polydispersity index values were narrow and relatively constant throughout the course of polymerization. The most active complex was also effective in the ring-opening polymerization of ε-caprolactone and L-lactide. To propose a reliable reaction path, DFT calculations were undertaken. In the first step of the reaction, the acidic proton of the alcohol is transferred to the basic nitrogen atom of the amide ligand coordinated to the zinc ion. This leads to the alcoholysis of the Zn-N bond and the formation of an alcoholate derivative that starts the polymerization. In subsequent steps, the reaction follows the classical coordination-insertion mechanism.

Quinolyl/pyridyl-amino Li complexes as dual catalysts for the ring-opening polymerization of cyclic esters and degradation toward a circular economy approach

Bi- and tetra-lithium quinolyl/pyridyl-amino complexes 1–4 were synthesized and characterized. These lithium complexes were capable of initiating the polymerization of rac-lactice (rac-LA) and ε-caprolactone (ε-CL) with high catalytic activity, respectively, whether in the presence or absence of BnOH. Both number of metal centers and ligand structures are identified as the crucial factors that determine the reactivity of a catalyst in the ROP. Their kinetic analyses indicated that tetrametallic complex 3 showed a more cooperativity comparable to that achieved with bimetallic analogues in the ring-opening polymerization (ROP) of ε-CL, however, number of metal centers has no obvious effect on catalytic activity for the ROP of rac-LA. Polymer end-group analysis by 1H NMR and MALDI-TOF MS support the reaction initiated by Li complexes without or with BnOH switching mechanism from a coordination-insertion to an active monomer in the ROP processes reasonably. The diblock copolymer PCL-b-PLA can be effectively prepared by polymerizing ε-CL firstly followed by addition of rac-LA. However, the formation of PLA-b-PCL or PLA-random-PCL is limited, and the resulting copolymers only contain a short sequence of PCL moieties which produced by the transesterification between PLA and ε-CL, whether the two monomers are added sequentially or simultaneously. Interestingly, these Li complexes not only exhibit a high activity for the polymerization of PLA, but also can fully degrade the resultant PLA into methyl lactate (Me-La) in the presence of MeOH, which is a rare example of making or breaking polymer for chemical recycling.

Alkali Metal Carboxylates: Simple, Efficient, and Industrial Relevant Catalysts for Controlled Polymer Synthesis

Abstract Polymer materials play a pivotal role in shaping modern society. The continuous development of polymer materials can be largely attributed to innovations in polymerization methods, especially in the development of new catalysts. When pursuing high-performance catalysts, it is crucial to consider their cost and safety for practical applications. Alkali metal carboxylates (AMCs) are widely available simple chemicals, and some of them are used as food additives. In the past few years, our group has demonstrated the ability of such simple compounds to catalyze the ring-opening polymerization of cyclic esters, cyclic carbonates, epoxides, and episulfides. Additionally, AMCs have demonstrated their efficacy in facilitating the ring-opening alternating copolymerization of cyclic anhydrides and cyclic ethers, resulting in the production of polyesters in a well-controlled manner. Moreover, AMCs can also catalyze the complicated self-switchable polymerization of a mixture of at least three monomers, yielding block copolymers with well-controlled block components and architectures. This account summarizes these successful examples, offering mechanistic insights and an outlook.

Progress of Ring-Opening Polymerization of Cyclic Esters Catalyzed by Iron Compounds

Aliphatic polyesters such as polylactide (PLA) and polycaprolactone (PCL) have attracted much attention due to their prominent features of environmentally friendly materials with good biocompatibility and biodegradability. The commercialized PLA and PCL materials have been commonly prepared at high temperature using tin octanoate as catalysts: however, a higher temperature of polymerization potentially causes broader dispersity or/and lower molecular weight of the resultant polymers; most importantly, residual tin catalysts in the polymer products may be harmful to mammals and/or other living organisms in the ocean. As an abundant metal in the earth, not only is iron an essential element in the human body but also its compounds play important roles in pharmaceutical and polymerization chemistry. Recently, iron-based compounds have been proved to be efficient promoters of ring-opening polymerization of cyclic esters under mild conditions. Herein the progress of iron compounds used for the ring-opening polymerization (ROP) of cyclic esters is reviewed in order to develop practical catalysts for industrial processes in the future.

Living and Immortal Ring-Opening Polymerization of Cyclic Esters

The limited availability of fossil fuels on the Earth has led researchers to develop new materials that are derived from renewable feedstocks. The polymers produced from the ROP of cyclic esters like (LA and ɛ-CL) are biodegradable, biocompatible, and bioassimilable and thus find major applications in various field. The ROP are catalyzed by the metal-based organometallic catalyst and metal-free organocatalyst. This review exemplifies the living and immortal ROP. The advantage of such polymerization is that they produce polymers with controlled molecular weight distribution. For the immortal ROP, more than one polymer chain grows from the single catalytic site in the presence of chain transfer agents (CTAs), and thus catalyst loading is low, which make the process economically more viable. The nature of CTAs and loading of CTAs with respect to the catalyst is crucial as the catalyst should be effective in the presence of CTAs. The review also discusses functionalized CTAs employed for the polymerization in some instances where functionalized polymers are generated.

Ring-opening polymerization of cyclic esters mediated by zinc complexes coordinated with benzotriazo-based imino-phenoxy ligands

A series of zinc complexes coordinated with benzotriazo-based imino-phenoxy ligands (ZnII(BTPIP)) has been developed for the mediation of ring-opening polymerization (ROP) of ε−caprolactone (ε−CL) and L−lactide (L−LA). In the presence of ZnII(BTPIP), the polymerization generated the PCL with the molecular weights in the range of 9600–17700 g mol−1 and narrow dispersity (Ð) around 1.08 to 1.33 as well as the PLA showing a molecular weight as 12300–14400 g mol−1 with narrow molecular weight distribution (Ð < 1.11). Furthermore, the living characters of the linearly increased molecular weight with monomer conversion and the synthesis of the tri-block copolymer of PEG-b-PCL-b-PLA were achieved. The substituent effect of ZnII(BTPIP) in the ROPs of ε−CL and L−LA was rationalized by the DFT calculations based on the coordination-insertion mechanism.

Recent developments in the UV-visible light-intervened ring-opening polymerization of cyclic esters

Synthesis of biodegradable polyesters using the photoinduced ring-opening polymerization (photoROP) technique in the presence of photoacid generators (PAGs) and photobase generators (PBGs) as catalysts.

Stereoselective synthesis of biodegradable polymers by salen-type metal catalysts

Biodegradable polymers are a promising sustainable alternative to conventional petroleum-based polymers and have attracted recent extensive research interest due to their potential environmental friendliness and sustainability. Among them, aliphatic polyesters and polycarbonates are the most extensively studied ones. The metal-catalyzed ring-opening polymerization (ROP) of cyclic esters and ring-opening copolymerization (ROCOP) of epoxides with anhydrides or CO2 are often considered to be the classic and efficient methods to synthesize stereoregular polymers. Moreover, the versatile salen-type metal complexes have been used to prepare almost all types of biodegradable polymers with excellent stereoselectivity control. Hence, this review focuses on stereoselective synthesis of biodegradable polymers by salen-type metal catalysts developed in the last decade. Aliphatic polyesters from ROP of cyclic esters, ROCOP of epoxides with cyclic anhydrides, and carbonylative polymerization of epoxides, as well as aliphatic poly(thio)carbonate from ROCOP of epoxides with CO2 or COS are discussed in detail. This review highlights the polymerization mechanisms, catalyst characteristics, and factors controlling the stereoselectivity of each polymerization reaction, aiming to provide general rules for the future design of stereoselective catalysts.

Synthesis , Antibacterial Activity of Sm(III) Complex With L-phenylalanine

Synthesis , Antibacterial Activity of Sm(III) Complex With L-phenylalanine

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.

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.

Organobase/urea catalyzed ring opening polymerization of 3‐methyl‐1, 4‐dioxan‐2‐one to prepare chemically recyclable poly(ether ester)

AbstractIt is greatly desirable to develop degradable polymers from easily accessible chemicals. In addition to well‐known degradable aliphatic polyesters, poly(ether‐ester) prepared from 1, 4‐dioxan‐2‐one with perfectly alternative ether and ester bonds is a new type of degradable polymer. Due to the low strain of six‐membered 1, 4‐dioxan‐2‐one ring, it remains a challenge to realize the effective and controllable ring‐opening polymerization (ROP) of 1, 4‐dioxan‐2‐one derived monomers despite several catalysts have been attempted. In recent years, the combination of organobase/(thio)urea binary catalytic system has displayed great activity and versatility in the ROP of cyclic esters. Herein, we studied the ROP of 3‐methyl‐1, 4‐dioxan‐2‐one (MDO) using the combinations of various organobases and ureas as catalysts at relatively low temperature. We found that 1, 8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU)/urea pair showed great overall performance in the ROP of MDO, achieving monomer conversion up to 82% within 7 h at −40 °C. Notably, DBU/urea binary catalytic system achieved higher catalytic activity as well as better control over molecular weights and molecular weight distributions compared to previously reported catalysts. The obtained PMDO can completely depolymerize to recover MDO monomer by simple thermolysis, thus established a close‐loop life cycle.

Zn(II) pyridinyl amine complexes, synthesis and crystal structure studies: A comparative study of the effect of nuclearity and benzoate type on the ring-opening polymerization of cyclic esters

A series of N-(pyridinylmethyl)aniline Zn(II) carboxylate complexes were synthesized and fully characterized by NMR, IR, mass spectroscopy and elemental analysis. The reaction of the N-(pyridin-4-ylmethyl)aniline ligand (L1) with Zn(II) acetate and benzoic acid derivatives promoted the formation of dinuclear species with the formula [Zn(CH3COO)(RCOO)(L1)]2 and [Zn(RCOO)2(L1)]2, whereas the N-(pyridin-3-ylmethyl)aniline ligand (L2) exclusively afforded mononuclear complexes of the form Zn(L1)2(RCOO)2 (where R = C6H5-, 4-Me-C6H4-, 4-F-C6H4-, and 4-Br-C6H4-). At elevated temperatures, the carboxylate complexes are active for the ring-opening polymerization (ROP) of ɛ-caprolactone (ɛ-CL) lactides (rac/L-LA) in bulk and in toluene, yielding polymers with low molecular weights between 2320 g mol−1 and 4980 g mol−1. The mononuclear complexes, which rearranged easily to their active species, were generally more active than their dinuclear counterparts. The electronic properties of the carboxylate co-ligands significantly influenced the catalytic performance of all the complexes. The complexes [Zn(CH3COO)(4-F-C6H4COO)(L1)]2 (3) and Zn(4-Br-C6H4COO)2(L2)2 (6) having electron-withdrawing substituent on the carboxylate co-ligands, gave the most promising results with a conversion of 99% after 24 h with apparent rate constants (kapp) of 18.13 × 10−2 h−1 and 19.11 × 10−2 h−1, respectively.

Ring-opening polymerization of cyclic esters by 3- and 4-pyridinyl Schiff base Zn(II) and Cu(II) paddlewheel complexes: kinetic, mechanistic and tacticity studies

We have previously reported a series of highly active N,O-Amino-phenolate Mg(II) and Zn(II) Schiff base complexes for caprolactone and lactide polymerization (Munzeiwa, Nyamori, and Omondi, 2019). Herein we report carboxylate Zn(II) and Cu(II) pyridinyl Schiff base complexes and in particular, those whose ligands have substituents slightly situated away from the metal center. The complexes were fully characterized and for some fully by single-crystal X-ray diffraction studies. The ligands were confirmed to be monodentate and their complexes have the famous paddlewheel conformation. The carboxylate complexes were found to initiate the polymerization of ɛ-caprolactone and lactides in the presence or absence of alcohol, yielding low molecular weight polymers between 1.27 and 5.65 kg mol ‐1 , with broad Ð between 1.7 and 2.0. The Zn(II) complexes were more reactive than their Cu(II) analogues. Activity of the complexes seems to have been influenced by the electronic properties of the substituents with the highest apparent rate constant, k app of 0.4043 h −1 , being obtained for the complex of the ligand with an electron-withdrawing group (chloro). The complex was used to investigate activity and stereo-selectivity towards the ring-opening polymerization of L -lactide and rac -lactide. Tetrad analysis of the polylactides obtained from homonuclear 1 H and 13 C NMR afforded isotactic PLA with L -LA and heterotactic (Pr = 0.68) stereo-block PLA with rac -LA.

NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l-Lactide.

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe2)4 or Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Co2O3 and MnO2 as inexpensive catalysts for the ring-opening polymerization of cyclic esters

The cheaper and easy to handle catalysts are particularly important for catalytic studies. Herein, inexpensive Co2O3 and MnO2 were used as catalysts for the ring-opening polymerization (ROP) of rac-lactide (rac-LA) and e-caprolactone (e-CL). The polymerization proceeded in a controlled manner with the formation of high molecular weight (Mn) and narrow dispersity (Ð). Heterotactically enriched (Pr up to 0.7) poly(lactic acid) (PLA) was formed during the polymerization process. MALDI-TOF and 1H NMR analyses of low Mn oligomer from rac-LA indicated that the polymerization followed an activated monomer mechanism. The data on the polymerization kinetics showed that the polymerization followed first order kinetics.

Titanium complexes of pyrrolylaldiminate ligands and their exploitation for the ring-opening polymerization of cyclic esters.

A series of six-coordinate titanium complexes 1-6 supported by pyrrolylaldiminate ligands were prepared via the reaction of 2 equivalents of ligands and Ti(OiPr)4 in toluene at 70 °C. The X-ray structure of 2 revealed that the two ligands were κ2-coordinated to the titanium center with the two pyrrole nitrogen atoms in trans positions and the two imine nitrogen atoms in cis positions. All complexes were active initiators for the ring-opening polymerization (ROP) of rac-lactide (rac-LA), ε-caprolactone (ε-CL), and three substituted ε-caprolactones (γ-methyl-ε-caprolactone (γMeCL), γ-ethyl-ε-caprolactone (γEtCL), and γ-phenyl-ε-caprolactone (γPhCL)). Polymerizations of all monomers were well controlled, affording predetermined molar masses and narrow dispersity values. Complex 5 exhibited the highest polymerization activities with rac-LA and ε-CL and its performance was comparable to other highly active six-coordinate titanium complexes reported thus far. Kinetic results revealed a first-order dependency on the monomer concentration, and the rate of polymerization was greatly influenced by the substituent on the imine nitrogen. End-group analysis of the isolated PLA and PCL suggested a coordination-insertion mechanism.