Polymerization Of Cyclic Esters Research Articles

Bifunctional phosphazene-thiourea/urea catalyzed ring-opening polymerization of cyclic esters

A series of new bifunctional phosphazene-thiourea/urea catalysts have been designed and applied to ring-opening polymerization (ROP) of rac-lactide, δ-valerolactone and ε-caprolactone. The ROPs were promoted efficiently, delivering the corresponding polyesters with controlled molecular weights, narrow molecular weight distributions, well-defined end group fidelities and without undesired epimerization of rac-lactide. Kinetic experiment of rac-LA confirmed the controlled/living nature of the ROP process. The isoselective polymerization of rac-LA was achieved with a highest Pm value of 0.79 (based on Bernouillan statics) under mild conditions. In addition, the ROPs of δ-VL and ε-CL were also smoothly achieved with high conversions and narrow molecular weight distributions. These catalysts exhibit a broad monomer scope of lactones, providing a facile synthesis methodology of metal-free and applicable polyester materials.

DFT Visualization and Experimental Evidence of BHT-Mg-Catalyzed Copolymerization of Lactides, Lactones and Ethylene Phosphates.

Catalytic ring-opening polymerization (ROP) of cyclic esters (lactides, lactones) and cyclic ethylene phosphates is an effective way to process materials with regulated hydrophilicity and controlled biodegradability. Random copolymers of cyclic monomers of different chemical nature are highly attractive due to their high variability of characteristics. Aryloxy-alkoxy complexes of non-toxic metals such as derivatives of 2,6-di-tert-butyl-4-methylphenoxy magnesium (BHT-Mg) complexes are effective coordination catalysts for homopolymerization of all types of traditional ROP monomers. In the present paper, we report the results of density functional theory (DFT) modeling of BHT-Mg-catalyzed copolymerization for lactone/lactide, lactone/ethylene phosphate and lactide/ethylene phosphate mixtures. ε-Caprolactone (ε-CL), l-lactide (l-LA) and methyl ethylene phosphate (MeOEP) were used as examples of monomers in DFT simulations by the Gaussian-09 program package with the B3PW91/DGTZVP basis set. Both binuclear and mononuclear reaction mechanistic concepts have been applied for the calculations of the reaction profiles. The results of calculations predict the possibility of the formation of random copolymers based on l-LA/MeOEP, and substantial hindrance of copolymerization for ε-CL/l-LA and ε-CL/MeOEP pairs. From the mechanistic point of view, the formation of highly stable five-membered chelate by the products of l-LA ring-opening and high donor properties of phosphates are the key factors that rule the reactions. The results of DFT modeling have been confirmed by copolymerization experiments.

Ring-opening (co)polymerization of γ-butyrolactone: a review

With increased environmental concerns and the rising demands for sustainable polymers, e.g., degradable polymers and chemically recyclable polymers, studies on ring-opening polymerization (ROP) of cyclic esters have been developed in recent decades. Biorenewable five-membered γ-butyrolactone (γBL) may be a desirable feedstock for the chemical synthesis of poly(γ-butyrolactone) (PγBL) or for the incorporation of γBL units into polyester chains to modify their properties. Although γBL is traditionally considered to be “nonpolymerizable”, some progress was recently made concerning the ROP of γBL. This mini-review is thus specifically focused on the ROP of γBL and its copolymerization with other cyclic esters. Recent progress in ring-opening (co)polymerization of γ-butyrolactone (γBL) in the presence of various initiating/catalyst systems has been reviewed. Low reaction temperatures and high monomer concentrations are critical for the success of such polymerization, regardless of the different initiating/catalyst systems.

Utilization of Azobenzene Units to Control Zinc-mediated Ring Opening Polymerization of Cyclic Esters

Salicylaldimine Zn(II) complexes bearing azobenzene moieties were prepared to catalyze ring opening po-lymerization of various cyclic esters. The different reactivities of the trans and cis isomers of the zinc complexes provide a facile strategy to control polymerization processes as well as the compositions of the resultant polymers.

Synthesis of poly(conjugated ester)s by ring-opening polymerization of cyclic hemiacetal ester bearing acryl skeleton

Ring-opening polymerization (ROP) of 2,6-dimethyl-5-methylene-1,3-dioxan-4-one (DMDO), a cyclic hemiacetal ester containing an acrylate skeleton, was investigated. Although the ROPs catalyzed by tin(II) 2-ethylhexanoate [Sn(Oct)2] and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) did not yield polymeric products, diphenyl phosphate (DPP) functioned a catalyst for the ROP through acyl scission accompanying with the elimination of acetaldehyde at 50 °C and 80 °C. The resulting polymer was a poly(conjugated ester) that had similar structure to the polymer of α-methylene-β-butyrolactone (MβBL), an α-exomethylene lactone with four-membered ring. Copolymerizations of ε-caprolactone and δ-valerolactone were also performed to yield the corresponding polyesters. The chemoselective main chain scission of the copolymers at the conjugated ester units were achieved by conjugate substitution reaction with benzyl mercaptan. Although the ROP of DMDO left a problem in the control of molecular weight, DMDO exhibited a potential as an easier accessible monomer alternative to MβBL for the preparation of bio- and chemo-degradable polyesters.

ε-Caprolactone Polymerization Catalyzed by Heteropolyacid. Derivation of the Kinetic Equation for Activated Monomer Propagation and Determination of the Rate Constants of Propagation

This study demonstrates the method to distinguish between the active chain end (ACE) and the activated monomer mechanism (AMM) in polymerization of cyclic esters using kinetic analysis. The kinetic...

Tetrylenes based on tri- and tetradentate ONO-, NNO-, NNN-, and ONNO-type ligands: synthesis, structure, and reactivity

The review discusses the results of the authors’ research aimed at the synthesis of stable tetrylenes (germylenes and stannylenes) and the determination of the structure of these compounds, including their oligomeric degree. Substituted dialkanolamines, dialkylenetriamines, aminobisphenols, pyridine and phenanthroline-containing dialcohols, and some other derivatives were used as the ligands for the synthesis. The reactivity of monomeric and dimeric tetrylenes in the reactions proceeding with both the change and the retention of the oxidation state of the metal atom (2+) is discussed in detail, as well as the influence of the oligomeric degree on the reactivity. The use of tetrylenes in synthetically useful reactions as ligands for catalytically active transition metal complexes, as well as initiators for ring-opening polymerization of cyclic esters, is analyzed.

Recent Advances in Amphiphilic Polymer-Oligonucleotide Nanomaterials via Living/Controlled Polymerization Technologies.

Over the past decade, the field of polymer-oligonucleotide nanomaterials has flourished because of the development of synthetic techniques, particularly living polymerization technologies, which provide access to polymers with well-defined architectures, precise molecular weights, and terminal or side-chain functionalities. Various "living" polymerization methods have empowered chemists with the ability to prepare functional polymer-oligonucleotide conjugates yielding a library of architectures, including linear diblock, comb, star, hyperbranched star, and gel morphologies. Since oligonucleotides are hydrophilic and synthetic polymers can be tailored with hydrophobicity, these amphiphilic polymer-oligonucleotide conjugates are capable of self-assembling into nanostructures with different shapes, leading to many high-value-added biomedical applications, such as drug delivery systems, gene regulation, and 3D-bioprinting. This review aims to highlight the main living polymerization approaches to polymer-oligonucleotide conjugates, including ring-opening metathesis polymerization, atom transfer radical polymerization (ATRP), reversible addition-fragmentation transfer polymerization (RAFT), and ring-opening polymerization of cyclic esters and N-carboxyanhydride. The self-assembly properties and resulting applications of polymer-DNA hybrid materials are highlighted as well.

ε-Caprolactone: Activated monomer polymerization; controversy over the mechanism of polymerization catalyzed by phosphorus acids (diarylhydrogen phosphates). Do acids also act as initiators?

Whenever Activated Monomer Mechanism (AMM) is involved in cationic ring-opening polymerization of cyclic esters, giving biodegradable polymers, catalysts and initiators are used. In some studies of AMM polymerization of ε-caprolactone (CL), without external initiator added, it was proposed that protonic catalysts are initiating themselves. We have shown by DOSY and other analytical methods for derivatives of phosphoric acid (claimed to act also as initiators) that these catalysts are not initiating polymerization and are separate from macromolecules. Traces of water are initiators. Understanding the role of catalysts in the bulk polymerization of CL allowed preparation of poly(ε-caprolactone) with Mn up to 140,000, for the first time over 105 in AMM.

Dibutyl phosphate catalyzed commercial relevant ring-opening polymerizations to bio-based polyesters

Organocatalyzed ring-opening polymerization (ROP) of cyclic esters was developed into a stage that demanding translation of the academic success into commercially viable production. An industrial relevant development of the organocatalysis in ROPs required bulk polymerizations at high temperatures up to 180 °C. Organic phosphoric acid was one of the few organocatalysts workable under these conditions that devoid of otherwise commonly appeared transesterification, epimerization, and decarboxylation in the ROPs. A commercial dibutyl phosphate (DBuP), which was inexpensive and available by hundreds of tons as antistatic agent, was firstly evaluated as a catalyst. Well-defined polyesters catalyzed by DBuP were obtained with narrow dispersities (Đm 1.08–1.31) and predicted molecular weights from 2.1 to 55 kg mol−1. Series of homopolymers, diblock copolymers and end-functionalized polymers of ε-caprolactone (CL), δ-valerolactone (VL), trimethylene carbonate (TMC), and l-lactide (LA) were synthesized. 1H NMR, 13C NMR, SEC, and MALDI–ToF MS analyses indicated that the obtained polyesters were exactly the designated ones. NMR titration experiments confirmed that the ROP proceeded a bifunctional activation mechanism. The cytotoxicity test using MTT assay showed biosafety of the polyesters. The experimental results suggested that dibutyl phosphate is a commercial organocatalyst widely useful in all major cyclic ester monomer toward commercial relevant ring-opening polymerizations.

Air-stable di-nuclear yttrium complexes as versatile catalysts for lactide polymerization and copolymerization of epoxides with carbon dioxide or phthalic anhydride

Four novel di-nuclear yttrium catalysts coordinated by bis(benzotriazole iminophenolate) (BiIBTP) ligands have been synthesized and structurally characterized. The one-pot procedure of BiIBTP pro-ligand with [Y(NO3)3.6H2O] (1.0 equiv.) and 2-nitrophenol derivative (1.0 equiv.) in the presence of triethylamine (5.0 equiv.) under refluxing MeOH solution gave bimetallic yttrium nitrophenolate complexes 1–4, [(μ-BiIBTP)Y(L)]2 (BiIBTP = C13CBiIBTP or C83CBiIBTP; L = 2-nitrophenolate or 2,4-dinitrophenolate) with good to high yields. The solid-state structure of 1 and 3–4 exhibits a di-yttrium(III) di-nitrophenolate species with the pentadentate BiIBTP ligand chelating two metal atoms. Versatile catalysis towards ring-opening polymerization of cyclic ester and copolymerization of internal epoxides with carbon dioxide (CO2) or phthalic anhydride (PA) catalyzed by these di-yttrium nitrophenolate complexes was systematically studied. Experimental results revealed that air-stable complexes 3 and 4 were shown to effectively catalyze not only lactide polymerization but also copolymerization of cyclohexene oxide (CHO) with CO2 or PA. Among these compounds, di-Y complex 4 was the most active catalyst for the ring-opening polymerization of l-lactide in combination with 9-anthracenemethanol, providing poly(l-lactide)s having the expected molecular weight and narrow molecular weight distribution. Furthermore, catalyst 3 was capable of copolymerizing CO2 and CHO in the presence of co-catalysts under mild conditions, producing perfectly alternating poly(cyclohexene carbonate)s with the controllable character. In addition, the molecular weight controllable poly(PA-co-CHO)s with high ester linkage contents could be afforded on copolymerizing PA and CHO utilizing di-yttrium complex 3 as the catalyst. This work provides the facilely synthesized di-nuclear yttrium complexes as versatile catalysts for three different types of polymerization catalysis.

Recent progress in the application of group 1, 2 & 13 metal complexes as catalysts for the ring opening polymerization of cyclic esters

This review focuses on recent developments concerned with the use of well-defined main group complexes as (pre-)catalysts for the ROP of cyclic esters to give aliphatic polyesters; factors influencing catalytic activity, selectivity and polymer properties are all discussed.

Rare-earth complexes supported by an ansa-bis(amidinate) ligand with a rigid o-phenylene linker: synthesis, structure, and catalytic activity for polymerization of cyclic esters.

A bis(amidinate) ligand with a rigid o-phenylene linker C6H4-1,2-[NC(tBu)NH(C6H5)]2 (1) was successfully employed for the syntheses of rare-earth amides 2-5 in good yields. The structures of the amides were established by X-ray diffraction analysis and revealed that the ligand framework coordinated to the lanthanide atoms in different fashions depending on the central ion size. Larger lanthanide metal ions La3+ and Nd3+ prefer to form dinuclear complexes with one linked bis(amidinate) ligand as a bridge, while the smaller sized Y3+ and Er3+ favor the formation of mononuclear complexes. These complexes exhibited good catalytic activity for ring-opening polymerization of ε-caprolactone and l-lactide and the mononuclear complexes display higher activity.

Mono-oxo molybdenum(vi) and tungsten(vi) complexes bearing chelating aryloxides: synthesis, structure and ring opening polymerization of cyclic esters.

The mono-oxo aryloxide complexes [M(O)(L1)2] (M = Mo (1·hexane), W(2·2MeCN)) have been prepared from [Mo(O)(Cl)4] or [W(O)(Ot-Bu)4] and two equivalents of the di-phenol 2,2'-ethylidenebis(4,6-di-tert-butylphenol) L1H2, respectively. Use of in situ generated [Mo(O)(Ot-Bu)4] with two equivalents of L1H2 also led to the isolation of 1·2MeCN. In the presence of adventitious oxygen, attempts to generate in situ [Mo(O)(Ot-Bu)4] and reaction with one equivalent of L1H2 afforded the bi-metallic complex [Mo(O)(L1)(μ-O)Li(THF)(MeCN)]2·2MeCN (3·2MeCN). Use of the tetra-phenol α,α,α',α'-tetrakis(3,5-di-tert-butyl-2-hydroxyphenyl)-p-xyleneH4 (L2H4) with [Mo(O)(Oi-Pr)4] led to the isolation of {[Mo(O)]L2}2 (4), whilst the analogous tungsten complex {[W(O)]L2}2 (5) was isolated from the reaction of L2H4 with [W(O)(Ot-Bu)4]. Similar reaction of p-tert-butylcalix[4]areneH4 (L3H4) with [Mo(O)(Oi-Pr)4] afforded ([Mo(O)L3(NCMe)]·3MeCN (6). Modifications of known routes were employed to access complexes [W(Cl)2L3]·3.5MeCN (7·3.5MeCN) and [W(O)L3(NCMe)] (8), whilst use of [WO(Ot-Bu)4] with L3H4 unexpectedly afforded [W(Ot-Bu)2L3]·MeCN (9·MeCN). The molecular crystal structures for 1-9 are reported, and the ability of these complexes to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL), δ-valerolactone (δ-VL) and ω-pentadecalactone (ω-PDL) has been investigated. The molybdenum complexes 1 and 4 were the best performers for ε-Cl and δ-VL, but all complexes exhibited poor control and were also inactive toward the ROP of PDL.

Computational mapping of redox-switchable metal complexes based on ferrocene derivatives.

DFT calculations were used to capture the properties of redox-switchable metal complexes relevant to the ring-opening polymerisation of cyclic esters by varying the metals, donors, linkers, and substituents in both accessible ferrocene oxidation states. A map of this chemical space highlights that modifying the ligand architecture and the metal has a larger impact on structural changes than changing the oxidation state of the ferrocene backbone.

Heterogeneous catalysts based on built-in N-heterocyclic carbenes with high removability, recoverability and reusability for ring-opening polymerization of cyclic esters

Both activity and reusability are critical issues for developing new generation metal-free catalytic systems.

One-Pot Synthesis of Block Copolymers Containing a Polyolefin Block

Polyolefin-containing block copolymers were catalytically synthesized using a postpolymerization modification strategy. A traditional olefin polymerization followed by a tandem hydroformylation/hydrogenation of the olefinic terminated polymer was implemented to yield a hydroxyl terminated polyolefin. The hydroxyl terminated polyolefin was then used as a macroinitiator for the ring-opening polymerization of cyclic esters to yield the corresponding diblock copolymer. This methodology was applied to an array of different polyolefins and two cyclic monomers (e-caprolactone and lactide) showcasing the versatility of the protocol. Most noticeably, isotactic polypropylene was quantitatively incorporated into block copolymers. Additionally, the hydrophobicity of polyethylene films was successfully decreased by the addition of small amounts of a block copolymer.

Binary organocatalytic system for ring-opening polymerization of ε-caprolactone and δ-valerolactone: Synergetic effects for enhanced selectivity

Phosphazenes have attracted considerable attention in the field of ring-opening polymerization (ROP) of cyclic esters. However, the application of phosphazenes as catalyst is subject to a trade-off between activity and selectivity (chain propagation versus transesterification/back-biting reaction, e.g., controlled polymerization). Despite extensive research efforts, the development of a new catalytic system having concurrent high activity and selectivity remains a great challenge. In this work, we report that the binary organocatalytic system comprised of cyclic trimeric phosphazene base (CTPB) and ureas shows both of high activity and selectivity for the ROP of ε-caprolactone (ε-CL) and δ-valerolactone (δ-VL) in the presence of alcohol as initiator. It is found that the introduction of ureas would significantly increase the catalytic selectivity during ROP without sacrificing the catalytic activity of CTPB. Moreover, the polymerizations catalyzed by the title catalytic system exhibit immortal characteristics and thus thousands equivalent of monomer could be converted to polymer efficiently with desired MWs and microstructures. Linear diblock copolymer AB and star copolymers (AB)3 with three identical diblock arms could be readily synthesized by sequential copolymerization using different alcohols as initiators.

Phosphazene Bases as Organocatalysts for Ring-Opening Polymerization of Cyclic Esters.

Over the past several years, organocatalyzed polymerization reactions have attracted considerable attention, and these efforts have led to major advances. A large number of organic compounds have been proven active for the polymerization of a large variety of monomers. In particular, phosphazene bases (PBs) are a family of extremely strong, non-nucleophilic, and uncharged auxiliary bases, and have shown their remarkable potential as organocatalysts for the ring-opening polymerization (ROP) of cyclic monomers. By deprotonation of weak acids or in combination with lithium cation, PBs significantly enhance the nucleophilicity of the initiator/chain-end, thus allowing fast and usually controlled anionic polymerization. In this feature article, the recent advances in phosphazene-catalyzed ROP of cyclic esters are summarized. This review is divided into three sections, including general features, design and synthesis, and catalytic applications. It aims to provide a critical analysis of PB-mediated ROP systems and a useful guide for the further design of organocatalysts applied to polymer synthesis. An outlook is given at the end.

Exploring Oxidation State-Dependent Selectivity in Polymerization of Cyclic Esters and Carbonates with Zinc(II) Complexes

SummaryNeutral zinc alkoxide complexes show high activity toward the ring-opening polymerization of cyclic esters and carbonates, to generate biodegradable plastics applicable in several areas. Herein, we use a ferrocene-chelating heteroscorpionate complex in redox-switchable polymerization reactions, and we show that it is a moderately active catalyst for the ring-opening polymerization of L-lactide, ɛ-caprolactone, trimethylene carbonate, and δ-valerolactone. Uniquely for this type of catalyst, the oxidized complex has a similar polymerization activity as the corresponding reduced compound, but displays significantly different rates of reaction in the case of trimethylene carbonate and δ-valerolactone. Investigations of the oxidized compound suggest the presence of an organic radical rather than an Fe(III) complex. Electronic structure and density functional theory (DFT) calculations were performed to support the proposed electronic states of the catalytic complex and to help explain the observed reactivity differences. The catalyst was also compared with a monomeric phenoxide complex to show the influence of the phosphine-zinc interaction on catalytic properties.