First example of N-heterocyclic carbenes as catalysts for living polymerization: organocatalytic ring-opening polymerization of cyclic esters

Recyclable and (Bio)degradable Polyesters in a Circular Plastics Economy

Polyesters carrying polar main-chain ester linkages exhibit distinct material properties for diverse applications and thus play an important role in today's plastics economy. It is anticipated that they will play an even greater role in tomorrow's circular plastics economy that focuses on sustainability, thanks to the abundant availability of their biosourced building blocks and the presence of the main-chain ester bonds that can be chemically or biologically cleaved on demand by multiple methods and thus bring about more desired end-of-life plastic waste management options. Because of this potential and promise, there have been intense research activities directed at addressing recycling, upcycling or biodegradation of existing legacy polyesters, designing their biorenewable alternatives, and redesigning future polyesters with intrinsic chemical recyclability and tailored performance that can rival today's commodity plastics that are either petroleum based and/or hard to recycle. This review captures these exciting recent developments and outlines future challenges and opportunities. Case studies on the legacy polyesters, poly(lactic acid), poly(3-hydroxyalkanoate)s, poly(ethylene terephthalate), poly(butylene succinate), and poly(butylene-adipate terephthalate), are presented, and emerging chemically recyclable polyesters are comprehensively reviewed.

Synergetic binary organocatalyzed ring opening polymerization for the precision synthesis of polysiloxanes

Organocatalytic ring-opening polymerization (ROP) is a versatile method for synthesizing well-defined polymers with controlled molecular weights, dispersities, and nonlinear macromolecular architectures. Despite spectacular advances in organocatalytic ROP, precision synthesis of polysiloxanes remains challenging due to the mismatch in polarity between highly polar initiators and nonpolar monomers and polymers and the difficulty in suppressing the formation of scrambling products via transetherification reactions during ROP of cyclic siloxanes. Here, we describe a binary organocatalytic ROP (BOROP) of hexamethylcyclotrisiloxane (D3) employing organic bases as catalysts and (thio)ureas as cocatalysts. The BOROP of D3 using triazabicyclodecene (TBD) and (thio)ureas generates polydimethylsiloxanes (PDMSs) with narrow dispersity (Mw/Mn < 1.1). Despite the similar basicities of TBD and 1,8-bis(tetramethylguanidino)naphthalene (TMGN), which is known as a proton sponge, a unitary organocatalytic system using TMGN was inactive for the ROP of D3. When the TMGN was paired with acidic urea, the BOROP of D3 yielded PDMSs with narrow dispersity (Mw/Mn < 1.1). Data suggest that the synergetic effect of TMGN and urea is results in an unprecedented activation-deactivation equilibrium between dormant and propagating species. The benefits of the present BOROP system are demonstrated by the formation of PDMS elastomers with more uniform network structures that are highly stretchy and have excellent mechanical properties.

A Facile Preparation of N-Heterocyclic Olefins: Ring-Opening Polymerization of β-Butyrolactone and Frustrated Lewis Pair Reactivity

A direct synthesis of N-heterocyclic olefins (NHOs) and their mesoionic congeners (mNHOs) from N-heterocyclic carbenes and N-aziridinylimines is reported. The reaction provided diverse functionalized (m)NHOs and π-extended analogues. The prepared NHOs initiated the ring-opening polymerization of β-butyrolactone, and insertion of aldehyde and nitrile into an NHO-B(C6 F5 )3 adduct was demonstrated.

Calixarene-Metal Complexes in Lactide Polymerization: The Story so Far

Polylactide synthetic procedures have lately gained attention, possibly due to their biocompatibility and the environmental problems associated with fossil-fuel-based polymers. Polylactides can be obtained from natural sources such as cassava, corn, and sugar beet, and polylactides can be manufactured in a laboratory using a variety of processes that begin with lactic acid or lactide. One of the most effective synthetic pathways is through a Lewis acid catalyzed ring-opening polymerization of lactides to obtain a well-defined polymer. In this regard, calixarenes, because of their easy functionalization and tunable properties, have been widely considered to be a suitable 3D molecular scaffold for new metal complexes that can be used for lactide polymerization. This review summarizes the progress made in applying some metal-calixarene complexes in the ring-opening polymerization of lactide.

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.

Scalable and Room-Temperature Ring-Opening Polymerization of ε-Caprolactone Catalyzed by Active Lithium Tetramethylene-Tethered Bis[N-(N'-butylimidazol-2-ylidene)] N-Heterocyclic Carbene as a Lewis Acid Organocatalyst

The Lewis acid organocatalytic system of lithium tetramethylene-tethered bis[N-(N'-butylimidazol-2-ylidene)] N-heterocyclic carbene (1,4-bisNHC) including lithium benzyloxide and benzyl alcohol has been successfully utilized in the ring-opening polymerization (ROP) of ε-caprolactone (CL) for the first time. The catalytic performance of this organic catalyst in the synthesis of high-molecular-weight polymers was investigated via bulk polymerization using different combinations of tetramethylene-tethered bis[N-(N'-butylimidazolium)] hexafluorophosphate (1,4-bis[Bim][PF₆]), benzyl alcohol (BnOH), and n-butyl lithium (nBuLi) ([1,4-bis[Bim][PF₆]]/[BnOH]/[nBuLi]) with the molar ratios of 0:2:2, 1:1:3, 1:2:3, and 1:2:4. The results showed that the molar ratio of 1:2:3 efficiently and rapidly initiated the bulk ROP of CL at room temperature with a high molar ratio of CL to 1,4-bis[Bim][PF₆] of 3000/1 and produced the highest number of average-molecular-weight (M_n) poly(ε-caprolactone) (103,057 g mol^-1) with the dispersity (D̵) and %conversion of 1.73 and 98% in a short period of time (152 s). From comparative studies, the relative polymerization rates of the bulk ROP of CL with different [1,4-bis[Bim][PF₆]]/[BnOH]/[nBuLi] molar ratios was determined in the following order: 1:2:4 > 1:1:3 > 1:2:3 > 0:2:2. For mechanistic investigation, the bulk ROP mechanism of CL with our organic catalyst was proposed through the intramolecular bis-lithium-carbene interaction pathway for 1,4-bisNHC_1,1,3, 1,4-bisNHC_1,2,3, and 1,4-bisNHC_1,2,4 systems.

PEGylated Polyester Nanoparticles Trigger Adverse Events in a Large Animal Model of Trauma and in Naı̈ve Animals: Understanding Cytokine and Cellular Correlations with These Events

Intravenously infusible nanoparticles to control bleeding have shown promise in rodents, but translation into preclinical models has been challenging as many of these nanoparticle approaches have resulted in infusion responses and adverse outcomes in large animal trauma models. We developed a hemostatic nanoparticle technology that was screened to avoid one component of the infusion response: complement activation. We administered these hemostatic nanoparticles, control nanoparticles, or saline volume controls in a porcine polytrauma model. While the hemostatic nanoparticles promoted clotting as marked by a decrease in prothrombin time and both the hemostatic nanoparticles and controls did not active complement, in a subset of the animals, hard thrombi were found in uninjured tissues in both the hemostatic and control nanoparticle groups. Using data science methods that allow one to work across heterogeneous data sets, we found that the presence of these thrombi correlated with changes in IL-6, INF-alpha, lymphocytes, and neutrophils. While these findings might suggest that this formulation would not be a safe one for translation for trauma, they provide guidance for developing screening tools to make nanoparticle formulations in the complex milieux of trauma as well as for therapeutic interventions more broadly. This is important as we look to translate intravenously administered nanoparticle formulations for therapies, particularly considering the vascular changes seen in a subset of patients following COVID-19. We need to understand adverse events like thrombi more completely and screen for these events early to make nanomaterials as safe and effective as possible.

Polyurethanes based on lignin-derived metabolic intermediate with strong adhesion to metals

Polyurethanes based on lignin-derived 2-pyrone-4,6-dicarboxylic acid (PDC) were successfully synthesized in one-pot, and their thermal, mechanical, and adhesive properties were investigated.

N-Heterocyclic carbene iron complexes catalyze the ring-opening polymerization of lactide

Poly(lactic acid), PLA, which holds great promise as a biodegradable substitute of fossil resource-derived polyolefins, is industrially produced by the ring-opening polymerization of lactide using a potentially harmful tin catalyst. Based on mechanistic insights into the reaction of N-heterocyclic carbene (NHC) iron complexes with carbonyl substrates, we surmised and demonstrate here that such complexes are excellent catalysts for the bulk polymerization of lactide. We show that an iron complex with a triazolylidene NHC ligand is active at lactide/catalyst ratios of up to 10 000 : 1, produces polylactide with relatively high number-average molecular weights (up to 50 kg mol⁻¹) and relatively narrow dispersity (Đ ∼ 1.6), and features an apparent polymerization rate constant k_app of up to 8.5 × 10⁻³ s⁻¹, which is more than an order of magnitude higher than that of the industrially used tin catalyst. Kinetic studies and end-group analyses support that the catalytically active species is well defined and that the polymerization proceeds via a coordination–insertion mechanism. The robustness of the catalyst allows technical grade lactide to be polymerized, thus offering ample potential for application on larger scale in an industrially relevant setting.

Iron(ii) complexes containing a mesoionic triazolylidene ligand are highly efficient catalyst precursors for the ring opening polymerization of lactide to poly(lactic acid), surpassing other iron complexes and also industrially utilized Sn(oct)₂.

Diphenyl phosphate/ethyl diphenylphosphinite as an efficient organocatalytic system for ring-opening polymerization of ε-caprolactone and δ-valerolactone

A novel controlled/living ROP method of ε-CL and δ-VL using ethyl diphenylphosphinite/diphenyl phosphate (EDPP/DPP) organocatalytic system was revealed, which involves the activated monomer mechanism and the reversible chain end deactivation process.

Recent developments on NHC-driven dual catalytic approaches

This article takes an in depth look at the early relay combination of NHC and other catalysts, and the latest progress in dual catalysis, analyzing the breakthroughs and limitations.

N-Heterocyclic Carbene-Catalyzed Chemoselective Monoacylation of 1,n-Linear Diols

This paper discusses the N-heterocyclic carbene (NHC)-catalyzed redox monoacylation of 1,n-linear diols using α-benzoyloxyaldehydes. The reactions afforded monoacylated diols in moderate to good selectivities and chemical yields. Our original NHC bearing a pyridine moiety plays an important role in achieving good chemoselectivities. A wide range of 1,n-linear diols were successfully applied to this reaction.

High pressure as a novel tool for the cationic ROP of γ-butyrolactone

In this study, we report the acid-catalyzed and high pressure assisted ring-opening polymerization (ROP) of γ-butyrolactone (GBL). The use of a dually-catalyzed approach combining an external physical factor and internal catalyst (trifluoromethanesulfonic acid (TfOH) or p-toluenesulfonic acid (PTSA)) enforced ROP of GBL, which is considered as hardly polymerizable monomer still remaining a challenge for the modern polymer chemistry. The experiments performed at various thermodynamic conditions (T = 278–323 K and p = 700–1500 MPa) clearly showed that the high pressure supported polymerization process led to obtaining well-defined macromolecules of better parameters (M_n = 2200–9700 g mol⁻¹; Đ = 1.05–1.46) than those previously reported. Furthermore, the parabolic-like dependence of both the molecular weight (M_W) and the yield of obtained polymers on variation in temperature and pressure at either isobaric or isothermal conditions was also noticed, allowing the determination of optimal conditions for the polymerization process. However, most importantly, this strategy allowed to significantly reduce the reaction time (just 3 h at room temperature) and increase the yield of obtained polymers (up to 0.62 g_PGBL/g_GBL). Moreover, despite using a strongly acidic catalyst, synthesized polymers remained non-toxic and biocompatible, as proven by the cytotoxicity test we performed in further analysis. Additional investigation (including MALDI-TOF measurements) showed that the catalyst selection affected not only M_W and yield but also the linear/cyclic form content in obtained macromolecules. These findings show the way to tune the properties of PGBL and obtain polymer suitable for application in the biomedical industry.

Well-defined poly(γ-butyrolactone) was synthesized with great efficiency via high pressure assisted cationic ROP of hardly polimerizable γ-butyrolactone.

Engineering PEGylated Polyester Nanoparticles to Reduce Complement-Mediated Infusion Reaction

Translation of intravenously administered nanomaterials to the clinic is limited due to adverse infusion reactions. While these reactions are infrequent, with up to 10% prone to experiencing infusion reactions, the reactions can be severe and life-threatening. One of the innate immune pathways, the complement activation pathway, plays a significant role in mediating this response. Nanoparticle surface properties are a relevant design feature, as they control the blood proteins the nanoparticles interact with and allow the nanoparticles to evade the immune reaction. PEGylation of nanosurfaces is critical in improving the blood circulation of nanoparticles and reducing opsonization. Our goal was to understand whether modifying the surface architecture by varying the PEG density and architecture can impact the complement response in vitro. We utilized block copolymers of poly(lactic acid)-b-poly(ethylene glycol) prepared with poly(ethylene glycol) macroinitiators of molecular weights 3400 and 5000 Da. Tracking the complement biomarker C5a, we monitored the impact of changing PEGylation of the nanoparticles. We also investigated how the changing PEG length on the nanoparticle surface impacts further strengthening the stealth properties. Lastly, we determined which cytokines change upon blood incubation with nanoparticles in vitro to understand the extent to which inflammation may occur and the crosstalk between the complement and immune responses. Increasing PEGylation reduced the generation of complement-mediated anaphylatoxin C5a in vitro, with 5000 Da PEG more effectively reducing levels of C5a generated compared to 3400 Da PEG. The insights gathered regarding the impact of PEG density and PEG chain length would be critical in developing stealth nanoparticles that do not lead to infusion reactions upon intravenous administration.

Synergistic Catalysis by Brønsted Acid/Carbodicarbene Mimicking Frustrated Lewis Pair-Like Reactivity

Carbodicarbene (CDC), unique carbenic entities bearing two lone pairs of electrons are well-known for their strong Lewis basicity. We demonstrate herein, upon introducing a weak Brønsted acid benzyl alcohol (BnOH) as a co-modulator, CDC is remolded into a Frustrated Lewis Pair (FLP)-like reactivity. DFT calculation and experimental evidence show BnOH loosely interacting with the binding pocket of CDC via H-bonding and π-π stacking. Four distinct reactions in nature were deployed to demonstrate the viability of proof-of-concept as synergistic FLP/Modulator (CDC/BnOH), demonstrating enhanced catalytic reactivity in cyclotrimerization of isocyanate, polymerization process for L-lactide (LA), methyl methacrylate (MMA) and dehydrosilylation of alcohols. Importantly, the catalytic reactivity of carbodicarbene is uniquely distinct from conventional NHC which relies on only single chemical feature of nucleophilicity. This finding also provides a new spin in diversifying FLP reactivity with co-modulator or co-catalyst.

Advances, Challenges, and Opportunities of Poly(γ-butyrolactone)-Based Recyclable Polymers

The discovery and prosperous growth of synthetic polymers have presented both significant advantages and daunting challenges in the last century. To address the issues of environmental pollution and fossil consumption, recyclable, degradable, and/or biobased polymers have been given much attention in the polymer science community. This viewpoint focuses on the emerging fully chemical recyclable poly(γ-butyrolactone)-based polymers. The breakthrough from nonpolymerizable to efficient polymerization is highlighted by the benefits of the development of a series of catalysis for ring-opening polymerization of γ-butyrolactone. Subsequently, the design of γ-butyrolactone derivatives and synthesis of more recyclable polymers are summarized together with the discussions about the structure and property relationship. Finally, the remaining challenges and promising opportunities are suggested in order to provide insights into the further direction for sustainable polymers.

A comparison of zwitterionic and anionic mechanisms in the dual-catalytic polymerization of lactide

Two different polymerization mechanisms for lactide are selectivity addressed to illuminate the respective role of organobase and Lewis acid component.

Visible light-regulated organocatalytic ring-opening polymerization of lactones by harnessing excited state acidity

A metal-free ring-opening polymerization of lactones has been developed using PyOH as a photocatalyst under visible light mediation.

Strategies for the synthesis of block copolymers with biodegradable polyester segments

Oxygenated block copolymers with biodegradable polyester segments can be prepared in one-pot through sequential or simultaneous addition of monomers. This review highlights the state of the art in this area.

Organo-catalyzed/initiated ring opening co-polymerization of cyclic anhydrides and epoxides: an emerging story

This review deals with recent organo-catalyzed/initiated developments of co-polymerization of cyclic anhydrides and epoxides to access polyesters.

TiCl4 Dissolved in Ionic Liquid Mixtures from ab Initio Molecular Dynamics Simulations

To gain a deeper understanding of the TiCl4 solvation effects in multi-component ionic liquids, we performed ab initio molecular dynamics simulations of 1-butyl-3-methylimidazolium [C4C1Im]+, tetrafluoroborate [BF4]−, chloride [Cl]− both with and without water and titanium tetrachloride TiCl4. Complex interactions between cations and anions are observed in all investigated systems. By further addition of water and TiCl4 this complex interaction network is extended. Observations of the radial distribution functions and number integrals show that water and TiCl4 not only compete with each other to interact mainly with [Cl]−, which strongly influences the cation-[BF4]− interaction, but also interact with each other, which leads to the fact that in certain systems the cation-anion interaction is enhanced. Further investigations of the Voronoi polyhedra analysis have demonstrated that water has a greater impact on the nanosegregated system than TiCl4 which is also due to the fact of the shear amount of water relative to all other components and its higher mobility compared to TiCl4. Overall, the polar network of the IL mixture collapses by including water and TiCl4. In the case of [Cl]− chloride enters the water continuum, while [BF4]− remains largely unaffected, which deeply affects the interaction of the ionic liquid (IL) network.

Robust Guanidine Metal Catalysts for the Ring-Opening Polymerization of Lactide under Industrially Relevant Conditions

The increasing awareness of sustainability has led to enormous growth of the demand for bio-based and biodegradable polymers such as poly(lactide) (PLA). In industry, polymerization of lactide is currently carried out using tin catalysts (e. g., tin(II) ethyl hexanoate, Sn(Oct)₂ ). Since the catalyst remains in the polymer, it can accumulate in the soil or in the human body after degradation and cause damage due to its toxicity. Therefore, a search for a suitable substitute for this catalyst has been going on for decades. Guanidine metal complexes prove to be excellent catalysts in the polymerization of lactide. They are not only convincing because of their activity and the synthesis of high molar mass polymers, but also show a high robustness against high temperatures, oxidation as well as residual protic impurities in the monomer. Herein, key zinc and iron guanidine complexes are discussed with respect to their apparent rate constant (k_app ) and rate constant of propagation (k_p ), produced molar masses and the mechanism involved.

Synthesis and synthetic mechanism of Polylactic acid

At present, Polylactic acid (PLA) is one of the most used biodegradable polyesters. The good properties and its biodegradability make that PLA can replace the fossil fuel derived polymers in different applications. PLA can be synthesized by using different methodologies. Among them, the most widely used forms on an industrial scale are the direct polycondensation of Lactic acid and the ring-opening polymerization of cyclic Lactide. The final properties of the obtained PLA are dependent on the used stereoisomers of the raw materials (Lactic acid and/or Lactide) and the conditions employed to polymerize them. Therefore, the comprehension of the synthetic mechanism of PLA is crucial to control the stereoregularity of PLA, which in turn results in an improvement of the polymer properties. So, distinct mechanisms for the synthesis of PLA by ring-opening polymerization using different catalysts systems (organometallic catalysts, cationic catalyst, organic catalyst, bifunctional catalysts) are examined in this review.

Stable abnormal N-heterocyclic carbenes and their applications

Although N-heterocyclic carbenes (NHCs) have been known as ligands for organometallic complexes since the 1960s, these carbenes did not attract considerable attention until Arduengo et al. reported the isolation of a metal-free imidazol-2-ylidene in 1991. In 2001 Crabtree et al. reported a few complexes featuring an NHC isomer, namely an imidazol-5-ylidene, also termed abnormal NHC (aNHCs). In 2009, it was shown that providing to protect the C-2 position of an imidazolium salt, the deprotonation occurred at the C-5 position, affording imidazol-5-ylidenes that could be isolated. Over the last ten years, stable aNHCs have been used for designing a range of catalysts employing Pd(ii), Cu(i), Ni(ii), Fe(0), Zn(ii), Ag(i), and Au(i/iii) metal based precursors. These catalysts were utilized for different organic transformations such as the Suzuki-Miyaura cross-coupling reaction, C-H bond activation, dehydrogenative coupling, Huisgen 1,3-dipolar cycloaddition (click reaction), hydroheteroarylation, hydrosilylation reaction and migratory insertion of carbenes. Main-group metal complexes were also synthesized, including K(i), Al(iii), Zn(ii), Sn(ii), Ge(ii), and Si(ii/iv). Among them, K(i), Al(iii), and Zn(ii) complexes were used for the polymerization of caprolactone and rac-lactide at room temperature. In addition, based on the superior nucleophilicity of aNHCs, relative to that of their nNHCs isomers, they were used for small molecules activation, such as carbon dioxide (CO₂), nitrous oxide (N₂O), tetrahydrofuran (THF), tetrahydrothiophene and 9-borabicyclo[3.3.1]nonane (9BBN). aNHCs have also been shown to be efficient metal-free catalysts for ring opening polymerization of different cyclic esters at room temperature; they are among the most active metal-free catalysts for ε-caprolactone polymerization. Recently, aNHCs successfully accomplished the metal-free catalytic formylation of amides using CO₂ and the catalytic reduction of carbon dioxide, including atmospheric CO₂, into methanol, under ambient conditions. Although other transition metal complexes featuring aNHCs as ligand have been prepared and used in catalysis, this review article summarize the results obtained with the isolated aNHCs.

Two new approaches based on dynamic carboxyl–hydroxyl or hydroxyl–carboxyl transformation for high molecular weight poly(butylene maleate)

Synthesis of high molecular weight maleic acid-based polyesters via a green approach is of great significance but also a huge challenge.