Mechanistic Aspects of the Polymerization of Lactide Using a Highly Efficient Aluminum(III) Catalytic System

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.

A nanoscale MOF-based heterogeneous catalytic system for the polymerization of N-carboxyanhydrides enables direct routes toward both polypeptides and related hybrid materials

Synthetic polypeptides have emerged as versatile tools in both materials science and biomedical engineering due to their tunable properties and biodegradability. While the advancements of N-carboxyanhydride (NCA) ring-opening polymerization (ROP) techniques have aimed to expedite polymerization and reduce environment sensitivity, the broader implications of such methods remain underexplored, and the integration of ROP products with other materials remains a challenge. Here, we show an approach inspired by the success of many heterogeneous catalysts, using nanoscale metal-organic frameworks (MOFs) as co-catalysts for NCA-ROP accelerated also by peptide helices in proximity. This heterogeneous approach offers multiple advantages, including fast kinetics, low environment sensitivity, catalyst recyclability, and seamless integration with hybrid materials preparation. The catalytic system not only streamlines the preparation of polypeptides and polypeptide-coated MOF complexes (MOF@polypeptide hybrids) but also preserves and enhances their homogeneity, processibility, and overall functionalities inherited from the constituting MOFs and polypeptides.

Polymeric encapsulation of a ruthenium(ii) polypyridyl complex: from synthesis to in vivo studies against high-grade epithelial ovarian cancer

The in vitro to in vivo translation of metal-based cytotoxic drugs has proven to be a significant hurdle in their establishment as effective anti-cancer alternatives. Various nano-delivery systems, such as polymeric nanoparticles, have been explored to address the pharmacokinetic limitations associated with the use of these complexes. However, these systems often suffer from poor stability or involve complex synthetic procedures. To circumvent these problems, we report here a simple, one-pot procedure for the preparation of covalently-attached Ru-polylactide nanoparticles. This methodology relies on the ring-opening polymerization of lactide initiated by a calcium alkoxide derivative formed from calcium bis(trimethylsilyl amide) and a hydroxyl-bearing ruthenium complex. This procedure proceeds with high efficiency (near-quantitative incorporation of Ru in the polymer) and enables the preparation of polymers with varying molecular weights (2000-11000 Da) and high drug loadings (up to 68% w/w). These polymers were formulated as narrowly dispersed nanoparticles (110 nm) that exhibited a slow and predictable release of the ruthenium payload. Unlike standard encapsulation methods routinely used, the release kinetics of these nanoparticles is controlled and may be adjusted on demand, by tuning the size of the polymer chain. In terms of cytotoxicity, the nanoparticles were assessed in the ovarian cancer cell line A2780 and displayed potency comparable to cisplatin and the free drug, in the low micromolar range. Interestingly, the activity was maintained when tested in a cisplatin-resistant cell line, suggesting a possible orthogonal mechanism of action. Additionally, the internalization in tumour cells was found to be significantly higher than the free ruthenium complex (>200 times in some cases), clearly showcasing the added benefit in the drug's cellular permeation and accumulation of the drug. Finally, the in vivo performance was evaluated for the first time in mice. The experiments showed that the intravenously injected nanoparticles were well tolerated and were able to significantly improve the pharmacokinetics and biodistribution of the parent drug. Not only was the nanosystem able to promote an 18-fold increase in tumour accumulation, but it also allowed a considerable reduction of drug accumulation in vital organs, achieving, for example, reduction levels of 90% and 97% in the brain and lungs respectively. In summary, this simple and efficient one-pot procedure enables the generation of stable and predictable nanoparticles capable of improving the cellular penetration and systemic accumulation of the Ru drug in the tumour. Altogether, these results showcase the potential of covalently-loaded ruthenium polylactide nanoparticles and pave the way for its exploitation and application as a viable tool in the treatment of ovarian cancer.

Ion-in-Conjugation: A Promising Concept for Multifunctional Organic Semiconductors

Most organic semiconductors (OSCs) consist of conjugated skeletons with flexible peripheral chains. Their weak intermolecular interactions from dispersion and induction forces result in environmental susceptibilities and are unsuitable for many multifunctional applications where direct exposure to external environments is unavoidable, such as gas absorption, chemical sensing, and catalysis. To exploit the advantages of inorganic semiconductors in OSCs, ion-in-conjugation (IIC) materials are proposed. An IIC material refers to any conjugated material (molecules, polymers, and crystals) in Kekule's structural formula containing stoichiometric ionic states in its conjugated backbone in the electronic ground state. In this review, the definitions, structures, synthesis, properties, and applications of IIC materials are described briefly. Four types of IIC material, including zwitterionic conjugated molecules/polymers, conjugated ionic dyes, π-d conjugated molecules and polymers, and coordinatively doped polymers, are reported. Their applications in gas sensing, humidity sensing, resistive memory devices, and thermal/photo-/electro-catalysis are demonstrated. The challenges and opportunities for future research are also discussed. It is expected that this work will inspire the design of new organic electronic information materials.

One-Pot Catalysis: A Privileged Approach for Sustainable Polymers?

Almost all aspects of daily life involve polymers in some form or the other. However, polymer production is largely based on finite feedstocks. These limitations combined with environmental concerns force us to rethink the strategies for the synthesis of these materials. As an abundant and renewable resource, biomass is composed of a very diverse range of molecules that deserve to be valorized. The development of new methods for transforming biomass into resources suitable for polymer production remains a crucial hurdle on the road to a more sustainable chemical economy. The main challenge is to design efficient and selective transformations of abundant and inexpensive raw materials into innovative polymers. For the chemical industry to meet these challenges, process intensification must play an important role in developing cleaner and more energy-efficient technologies while aiming for safer and more sustainable processes. Catalysis is an important tool to support more sustainable plastics production by being ideally efficient, practical, and versatile. In this regard, the creation of sustainable polymers through one-pot catalysis represents an exciting frontier in materials science.In this Account, we describe some of the published advances for achieving one-pot synthesis of biobased monomers and the resulting (co)polymers. These studies demonstrate that one-pot reactions can produce sustainable materials for a wide range of applications. We show that these new multistep "one-pot" approaches are very promising from an academic and industrial point of view. These synthetic schemes have indeed allowed us to investigate the formation of new polyesters, polypeptides, and poly(meth)acrylates by different polymerization mechanisms. We discuss their efficiency by highlighting their ability to perform multiple (quantitative) synthetic transformations and bond formation steps while bypassing multiple purification procedures at the same time. While enabling the development of novel polymeric structures, we demonstrate that these one-pot procedures can also contribute to reducing the environmental footprint.In light of the growing concerns for sustainable development, these procedures may therefore allow, in the near future, one to prepare sustainable polymeric materials with advanced properties through extremely simplified routes from renewable feedstocks. Among these materials, block and alternating copolymers are unique structures that can exhibit a wide range of properties. While their multistep synthesis remains a demanding process, the one-pot synthesis of these polymers is much more scalable and can create multiblock or alternating copolymers with a wide range of potential sequences. These approaches then give access to materials whose structure and functionality can be designed to suit the need.

Use of group 13 aryloxides for the synthesis of green chemicals and oxide materials

In this work, group 13 metal aryloxides [Al(MesalO)3] (1), [Me2Ga(MesalO)]2 (2), [AlLi3(MesalO)6] (3) and [Me2GaLi(MesalO)2(THF)] (4), were obtained by reaction of methyl salicylate (MesalOH) with group-13 alkyls MMe3 (for M...

Combining high activity with broad monomer scope: indium salan catalysts in the ring-opening polymerization of various cyclic esters

An indium salan-type catalyst shows very high activities in the ring-opening polymerization of various cyclic esters, including β-butyrolactone, γ-butyrolactone, lactide, ε-caprolactone and ε-decalactone.

Iodine and alkali metal alkoxides: a simple and versatile catalytic system for fully alternating polyester synthesis from phthalic anhydride and epoxides

Regioselective ROCOP of various epoxides with phthalic anhydride using readily available and economical catalysts i.e. I2 in combination with alkali metal alkoxides was used.

Ring-Opening Polymerization of ε-Caprolactone Mediated by Di-Zinc Complex Bearing Macrocyclic Thioether-phenolate [OSSO]-type Ligand

A unique example of zinc bromide complexes bearing macrocyclic [OSSO]-type thioetherphenolate ligand (Di-[OSSO]ZnBr) has been successfully explored toward ring-opening polymerization (ROP) of -caprolactone (ε-CL) in the presence of epoxides and...

Controlling polymer stereochemistry in ring-opening polymerization: a decade of advances shaping the future of biodegradable polyesters

This review highlights recent developments in the field of biodegradable polymeric materials intended to replace non-degradable conventional plastics, focusing on studies from the last ten years involving the stereoselective ring-opening polymerization of cyclic esters. This encompasses exciting advances in both catalyst design and monomer scope. Notably, the last decade has seen the emergence of metal-free stereocontrolled ROP for instance, as well as the synthesis and stereocontrolled polymerization of new types of chiral monomers. This study will emphasize recent stereoselective polymerization catalysts and chiral monomers and will focus on stereocontrol quantification, the mechanisms of stereocontrol and their differentiation if reported and studied for a specific catalyst system.

Highly Efficient Synthesis of Poly(silylether)s: Access to Degradable Polymers from Renewable Resources

The design of new materials with tunable properties and intrinsic recyclability, derived from biomass under mild conditions, stands as a gold standard in polymer chemistry. Reported herein are platinum complexes which catalyze the formation of poly(silylether)s (PSEs) at low catalyst loadings. These polymers are directly obtained from dual-functional biobased building blocks such as 5-hydroxymethylfurfural (HMF) or vanillin, coupled with various dihydrosilanes. Access to different types of copolymer architectures (statistical or alternating) is highlighted by several synthetic strategies. The materials obtained were then characterized as low T_g materials (ranging from -60 to 29 °C), stable upon heating (T_-5% up to 301 °C) and resistant towards uncatalyzed methanolysis. Additionally, quantitative chemical recycling of several PSEs could be triggered by acid-catalyzed hydrolysis or methanolysis. These results emphasize the interest of biobased poly(silylether)s as sustainable materials with high recycling potential.

Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive. In contrast, Al/Li and Al/Ca heterocombinations stunted the polymerization rate. Kinetic and computational studies suggest that Al/Mg and Al/Zn cooperativity arises from the close intermetallic proximity facilitating chloride bridging (thus enhancing initiation), which promotes a rigid square pyramidal geometry around the Al center and further increases the available monomer coordination sites. This work also translates the use of ab initio molecular dynamics calculations to ROP, introducing a useful method of investigating catalyst flexibility and revealing that ligand strain and molecular rigidity can enhance heterometallic catalyst performance.

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

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

Catalysts based on Co, amino acids, and 2,2-bipyridine present an attractive and economic alternative in ring opening polymerization, and possess advantageous ligand coordination properties combined with a variety of accessible oxidation states and coordination geometries.

Efficient ternary catalyst system for the copolymerization of lactide, epoxides and CO2: new insights into the cooperative mechanism

Ternary catalyst systems (TCSs) are an emerging type of catalyst for the synthesis of multiblock copolymers of lactide (LA), epoxides and CO₂.

The crucial roles of guest water in a biocompatible coordination network in the catalytic ring-opening polymerization of cyclic esters: a new mechanistic perspective

The ring-opening polymerization (ROP) of cyclic esters/carbonates is a crucial approach for the synthesis of biocompatible and biodegradable polyesters. Even though numerous efficient ROP catalysts have been well established, their toxicity heavily limits the biomedical applications of polyester products. To solve the toxicity issues relating to ROP catalysts, we report herein a biocompatible coordination network, CZU-1, consisting of Zn₄(μ₄-O)(COO)₆ secondary building units (SBUs), biomedicine-relevant organic linkers and guest water, which demonstrates high potential for use in the catalytic ROP synthesis of biomedicine-applicable polyesters. Both experimental and computational results reveal that the guest water in CZU-1 plays crucial roles in the activation of the Zn₄(μ₄-O)(COO)₆ SBUs by generating μ₄-OH Brønsted acid centers and Zn-OH Lewis acid centers, having a synergistic effect on the catalytic ROP of cyclic esters. Different to the mechanism reported in the literature, we propose a new reaction pathway for the catalytic ROP reaction, which has been confirmed using density functional theory (DFT) calculations, in situ diffuse reflectance IR Fourier transform spectroscopy (DRIFTS), and matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-TOF MS). Additionally, the hydroxyl end groups allow the polyester products to be easily post-modified with different functional moieties to tune their properties for practical applications. We particularly expect that the proposed catalytic ROP mechanism and the developed catalyst design principle will be generally applicable for the controlled synthesis of biomedicine-applicable polymeric materials.

Ruthenium-initiated polymerization of lactide: a route to remarkable cellular uptake for photodynamic therapy of cancer

Ruthenium complexes have attracted a lot of attention as potential photosensitizers (PSs) for photodynamic therapy (PDT). However, some of these PSs are unsuitable for PDT applications due to their low cellular uptake, which is possibly the consequence of their relatively low degree of lipophilicity, which prevents them from penetrating into tumor cells. Here, we report the simple one-pot synthesis of ruthenium-containing nanoconjugates from a non-cell-penetrating, non-phototoxic ruthenium(ii) polypyridyl complex (RuOH), by a drug-initiated ring-opening polymerization of lactide through the formation of a zinc initiator. These conjugates were then formulated into nanoparticles by nanoprecipitation and characterized by means of nuclear magnetic resonance spectroscopy (NMR), matrix-assisted laser desorption/ionization – time of flight mass spectrometry (MALDI-TOF MS) and dynamic light scattering (DLS). Finally, their photo-therapeutic activity (λ_exc = 480 nm, 3.21 J cm⁻²) in cancerous human cervical carcinoma (HeLa) and non-cancerous retinal pigment epithelium (RPE-1) cells was tested alongside that of RuOH and their cellular uptake in HeLa cells was assessed by confocal microscopy and inductively coupled plasma – mass spectrometry (ICP-MS). All nanoparticles showed improved photophysical properties including luminescence and singlet oxygen generation, enhanced cellular uptake and, capitalizing on this, an improved photo-toxicity. Overall, this study demonstrates how it is possible to transform a non-phototoxic PDT PS into an active PS using an easy, versatile polymerization technique.

This study shows the transformation of a non-phototoxic PDT photosensitizer into an active photosensitizer using an easy, versatile and generalizable one-pot ruthenium-initiated polymerization technique.

Cyclic Polyester Formation with an [OSSO]-Type Iron(III) Catalyst

The efficient formation of cyclic polyesters from the ring-opening polymerization of lactide, ϵ-caprolactone, and β-butyrolactone catalyzed by a 1,4-dithiabutanedyl-2,2'-bis(4,6-dicumylphenol) [OSSO]-FeCl complex activated with cyclohexene oxide was achieved. The catalyst was very active (initial turnover frequency up to 2718 h^-1 ), robust, and worked with a monomer/Fe ratio up to 10 000. The formation of cyclic polymers was supported by using high-resolution matrix-assisted laser desorption ionization (MALDI) MS, and the average ring size (≈5 kDa for cyclic polylactide) independent of the reaction conditions. A monometallic ring-opening polymerization/cyclization mechanism was proposed from the results of a kinetic investigation.

Novel hybrid aluminium(iii)–catalen complexes as highly active catalysts for lactide polymerisation: towards industrial relevance

A series of novel Al(iii) complexes are prepared that show remarkable activity under the industrially preferred conditions.

Zinc Complexes for PLA Formation and Chemical Recycling: Towards a Circular Economy

A series of Zn^II complexes, based on propylenediamine Schiff bases, have been prepared and fully characterized. X-ray crystallography and NMR spectroscopy identified significant differences in the solid and solution state for the Zn^II species. All complexes have been applied to the ring-opening polymerization of l-lactide with emphasis on industrial conditions. High conversion and good molecular weight control were generally achievable for Zn(A-D)₂ , and high-molecular-weight poly(lactic acid) (PLA) was prepared in 1 min at a 10 000:1:33 [lactide]/[Zn]/[BnOH] loading. The more active Zn^II catalysts were also applied to PLA degradation to alkyl lactate under mild conditions. Zn(A-B)₂ demonstrated high activity and selectivity in this process with PLA being consumed within 1 h at 50 °C. Zn(C-D)₂ were shown to be less active, and these observations can be related to the catalysts' structure and the degradation mechanism. Initial results for the degradation of poly(ethylene terephthalate) and mixed feeds are also presented, highlighting the broader applicability of the systems presented.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

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.

Mechanistic Insight into the Ring-Opening Polymerization of ε-Caprolactone and L-Lactide Using Ketiminate-Ligated Aluminum Catalysts

The reactivity and the reaction conditions of the ring-opening polymerization of ε-caprolactone (ε-CL) and L-lactide (LA) initiated by aluminum ketiminate complexes have been shown differently. Herein, we account for the observation by studying the mechanisms on the basis of density functional theory (DFT) calculations. The calculations show that the ring-opening polymerization of ε-CL and LA are rate-determined by the benzoxide insertion and the C–O bond cleavage step, respectively. Theoretical computations suggest that the reaction temperature of L–LA polymerization should be higher than that of ε-CL one, in agreement with the experimental data. To provide a reasonable interpretation of the experimental results and to give an insight into the catalyst design, the influence of the electronic, steric, and thermal effects on the polymerization behaviors will be also discussed in this study.

Anionic hafnium species: an active catalytic intermediate for the coupling of epoxides with CO2?

A series of hafnium complexes were structurally identified showing high activity (up to 500 h^-1) in the selective alternated copolymerization of epoxides with CO₂ under low pressure.

Al(Salen) Metal Complexes in Stereoselective Catalysis

Salen ligands are a class of Schiff bases simply obtained through condensation of two molecules of a hydroxyl-substituted aryl aldehyde with an achiral or chiral diamine. The prototype salen, or N,N'-bis(salicylidene)ethylenediamine has a long history, as it was first reported in 1889, and immediately, some of its metal complexes were also described. Now, the salen ligands are a class of N,N,O,O tetradentate Schiff bases capable of coordinating many metal ions. The geometry and the stereogenic group inserted in the diamine backbone or aryl aldehyde backbone have been utilized in the past to efficiently transmit chiral information in a variety of different reactions. In this review we will summarize the important and recent achievements obtained in stereocontrolled reactions in which Al(salen) metal complexes are employed. Several other reviews devoted to the general applications and synthesis of chromium and other metal salens have already been published.