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.

Development of a H-bond donor–Lewis basic anion bifunctional organocatalyst for ring-opening polymerizations

Squaramide–phenolate exemplified one-component H-bond donor–anion bifunctional catalyst for ring-opening polymerizations of various cyclic esters, where the Lewis basic phenolate activated initiator and the H-bond donor squaramide activated monomer.

Grignard reagents as deprotonation agents for oxazoline-amido-phenolate ligands: structural and catalytic implications with the role of halogen ions.

In this study, various halogen-substituted Grignard reagents were assessed as deprotonating agents for the oxazoline-amido-phenolate ligand, leading to the formation of magnesium complexes. The newly synthesized complexes with halogen substituents displayed three distinct coordinative modes, all extensively characterized through crystallographic methods. The introduction of halogen substituents induced changes in the Lewis acid properties of the complexes, thereby impacting their structural attributes and catalytic behavior during the initiation and propagation of ring polymerization of cyclic esters.

Aluminum and titanium complexes bearing 2,6-Bis(2-hydroxyphenyl)pyridines for ring-opening polymerization of lactide, ε-caprolactone and their copolymerization: Effect of a ligand on coordination chemistry and reactivity

In ring-opening polymerization (ROP) of cyclic esters, the rational design of the catalyst is generally applied to solve the task of providing polyester biocompatibility (non-toxicity) and the problem of copolymer homogeneity when using comonomers of different nature (e. g. lactones and lactides). In the present work, we report the synthesis of aluminum and titanium complexes, based on substituted 2,6-bis(2-hydroxyphenyl)pyridine pro-ligands 1 and 2. It was found that the structure of the pro-ligand drastically affect the structure of the reaction product. Pro-ligand 2 with a bulky tert‑butyl group leads to the monomeric complexes 6 (LAlMe) and 7 (LTi(OiPr)2), while the ligands with less bulky groups lead to the oligomeric Al-containing compounds 3 and 4, and in the case of Ti bis-ligand complex 5 is formed. Complexes 6 (in presence of BnOH) and 7 turned out to be active in the ROP of ε-caprolactone and l-lactide and gave PCL and PLA with high molecular masses. Compound 6 was an effective initiator of copolymerization of ε-caprolactone and l-lactide which led to statistical copolymer poly(LA-stat-CL) with comonomer subunits ratio of 1:1.

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.

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects.

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

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.

Intercalation of Ionic Liquids into LDH Structures for Microwave-Accelerated Polymerizations.

Microwave-accelerated ring-opening polymerization (ROP) of cyclic esters catalyzed by ionic liquid (IL) anions, intercalated into layered double hydroxides (LDHs), has been recently described as a fast and environmentally friendly synthetic way to prepare biodegradable polyester/LDH nanocomposites. However, to observe this synergistic catalytic effect between microwaves and IL anions and to achieve a homogeneous structure of the final polymer nanocomposite, the IL anions must be efficiently intercalated inside the LDH structure. Herein, we investigate the effects of various metal compositions of M2+/Al3+ LDHs (M = Mg, Co, and Ca) and different LDH synthetic routes (one-step direct coprecipitation, two-step coprecipitation/anion exchange, and two-step urea/anion exchange) on the intercalation efficiency of trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate IL. The most effective IL anion intercalation was observed for Ca2+/Al3+ LDH prepared using the two-step method consisting of coprecipitation and subsequent anion exchange. After optimization, this synthetic pathway led to the production of LDHs with intercalated IL anions and a reduced amount of intercalated water (<0.6 wt %). The catalytic ability of thus optimized LDH particles was demonstrated on the microwave-assisted ROP of ε-caprolactone, showing rapid progress of polymerization. Within minutes, the polycaprolactones with an average molecular mass in the range of 20 000-50 000 g/mol containing fully delaminated and exfoliated LDH nanoparticles were obtained.

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.

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.

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.

Ring-Opening Polymerization of Cyclic Acetals: Strategy for both Recyclable and Degradable Materials.

To cope with the severe plastic waste crisis, massive efforts have been made to develop sustainable polymer materials whose degradation involves a disposing and decomposing to small molecule (DDM) and/or a chemical recycling to monomer (CRM) process. Polyacetals, a type of pH-responsive polymers, are degradable under acidic conditions, while highly stable under neutral and basic circumstances. As for their synthesis, the cationic ring-opening polymerization (CROP) of cyclic acetals is an elegant and promising approach, though suffering from fatal side reactions and polymerization-depolymerization equilibrium. Recent development in CRM restimulates our interests in the long-forgotten CROP method due to its inherent depolymerization characteristics. In terms of the end-of-life options, polyacetals are recyclable materials with both DDM and CRM potentials. They not only expand the scope of materials for closed-loop recycling, but also help to tune the degradation properties of traditional polyesters and polyolefins. This review aims to discuss the synthesis of various polyacetals by CROP and their degradation properties from the perspectives of (1) polymerization of cyclic acetals, dioxepins, and hemiacetal esters, (2) copolymerization of cyclic acetals with heterocyclic or vinyl monomers, and (3) degradation and recycling properties of the related polymers. This article is protected by copyright. All rights reserved.

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.

Cooperative effects of Schiff base binuclear zinc complexes on the synthesis of aliphatic and semi-aromatic polyesters.

In this paper, we use mono- and bimetallic complexes based on Earth-abundant, cheap and benign zinc for the synthesis of sustainable aliphatic and semi-aromatic polyesters. Tridentate and hexadentate aldimine-thioetherphenolate ligands were used to obtain the desired zinc complexes by the reaction of proligands with opportune equivalents of zinc bis[bis(trimethylsilyl)amide]. The obtained bimetallic complexes 1 and 2 and the monometallic complex 3 were used as catalysts in the Ring-Opening Polymerization (ROP) of landmark cyclic esters, such as ε-caprolactone and lactide, and in the Ring-Opening COPolymerization (ROCOP) of cyclohexene oxide and phthalic anhydride under different reaction conditions. All catalysts were active in these two classes of reactions, showing good control of the polymerization processes. Interestingly, the bimetallic complexes have higher activity compared to their monometallic counterparts, highlighting the cooperation between the two zinc centers.

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.

Pillared MOFs: structure and ring opening polymerization of cyclic esters

Pillared 2- and 3D MOFs have been isolated from reactions of 5-aminoisophthalic acid, 4,4′-bipyridyls and metal (Zn, Co, Mn, Cd) nitrate salts. Their ability to act as catalysts for the ROP of ε-caprolactone and δ-valerolactone has been investigated.

In Situ Initiation of Epoxides: Activated Metal Salt Catalysts for Cyclic Ester Polymerization

In Situ Initiation of Epoxides: Activated Metal Salt Catalysts for Cyclic Ester Polymerization